ALBERT

Description: This paper includes determinations of archeological and geological samples from different sites in central Italy performed at the Ente per le Nuove Tecnologie l'Energia e l'Ambiente (ENEA) Radiocarbon Laboratory. This laboratory has been in operation since 1985 at the ENEA Bologna Research Center.

Description: This is Radiocarbon's annual list of active radiocarbon laboratories and personnel known to us. Conventional beta-counting facilities are listed in Part I, and accelerator mass spectrometry (AMS) facilities are listed in Part II. Laboratory code designations, used to identify published dates, are given to the left of the listing. (See p 515 for a complete list of past and present lab codes.)

Description: Radiocarbon dates on samples aimed to date the settlement of Iceland are given together with comments by the laboratory, since many of the results and descriptions given by Sveinbjörnsdóttir et al. (2004) in Radiocarbon, together with new results, are in error. The intention of this paper is to present correct dates and further relevant information regarding samples used earlier and to discuss possible complications inherent in the method of Sveinbjörnsdóttir et al. (2004). Examples are given of how critical the collection, treatment, and interpretation of samples may be. An age difference between birch charcoal and grains for a site is expected due to various reasons. If the difference amounts up to ∼100 yr, as reported by Sveinbjörnsdóttir et al. (2004), it must only to a small degree be due to biological age. Reference to an excavation report, details regarding stratigraphy, and discussions of the risk for displacement and contamination are missing in their paper. A final evaluation of the time for settlement should not be done until more research is completed and other possible or earlier suggested or even dated sites are discussed. A summary is given of the research on the island and volcanic effects on the 14C activity of the atmospheric CO2, especially over Iceland.

Description: Despite extensive research on political activity on the part of corporations, clear and consistent findings remain elusive. We identify three reasons for this failure. First, most of the empirical literature on corporate political activity simply studies the wrong phenomena by examining political action committees rather than lobbying more generally. Second, the literature studies an excessively narrow sample of organizations that might engage in lobbying, focusing almost always on extremely large corporations, which inevitably attenuates variance on many of the variables hypothesized to influence engagement in political activity. And third, prior work is rarely attentive to the diversity of corporate activities, narrowly conceptualizing vital aspects of the business context that might influence decisions to engage in political activity. Based on this critique, we develop and test new models of corporate political activity, finding that the diversity of the economic context within which firms work and firm size matter a great deal, if in ways somewhat different from those reported in prior work.

Description: This paper explores the East-West dichotomy of outsourcing in the European Union in the context of its 2004 eastward enlargement. The purpose of the study is to shed light on the connection between outsourcing and the causal logic of regional integration. The conventional view is that the transfer of business operations from Western Europe to low-cost locations to the east represents a process of outsourcing West-European jobs which deprives the EU core of growth opportunities to the exclusive benefit of the new members from Eastern Europe. This analysis posits the systemic functions of EU outsourcing as a mechanism of economic homogenization in the regional market along its three principal dimensions: investment, commodity trade, and labor mobility. At the macro-level, outsourcing complements capital movements and trade, and acts as a substitute for labor mobility. Keeping labor mobility “down” is the main value added of EU outsourcing. Empirically, its relevance to the regional market is established in an input-output framework of relationships with indicators of economic convergence (homogenization effects) and labor mobility (substitution effects) in the EU. Positive correlations with indices of business synchronization and weak negative correlations with measures of labor supply and wages suggest that outsourcing fits well both with strategies fostering market integration and those counterbalancing the politically sensitive labor mobility in the EU. There is no significant evidence to suggest that, at the aggregate level, outsourcing has independent substitution effects with regard to unemployment rates and wages in Western Europe. The geographic expansion of EU integration, therefore, is not a proxy for losses of social welfare in the West. The paper concludes that as the cost efficiency and resource allocation functions of outsourcing facilitate the homogenizing dynamics of regional integration, it is likely to become increasingly subsumed under EU-level regulation and monitoring in a trade-off between the regional interest and domestic sectoral concerns.

Description: This study examines the relationships between deregulation, business strategy (low cost, differentiation, and scope), size, and firm performance in the U.S. airline industry based on archival data for the Major, National, and Large Regional air carriers in the U.S. from 1972 to 1995. Cross-sectional time series regression analysis shows that deregulation had a significant impact on the strategic choices made by airlines. Results also support a significant relationship between business strategy and firm performance. Further, the study found that firm size moderates the environment-business strategy relationship and the business strategy-firm performance relationship, thereby supporting the salience of firm size as a contingency variable in strategy studies.

Description: Compensation hypothesis, which has established a link between trade openness of countries and levels of government spending, has been widely accepted in the literature on trade policy and international globalization. However, the nature of the distribution effects produced by trade is likely to determine the existence of more or less redistribution demands from the median voter, and therefore government growth. In this paper I hypothesize that the effects of trade openness on redistribution demands are not homogeneous between countries, and I argue that they depend both on the type-of-factor endowment of the economy and the size of the sectors more likely to be affected by trade. I test this hypothesis with ISSP data for 23 countries, both with a country level and an individual level analysis. The results show that redistribution demands issued from trade openness of the median voter of a country are largely conditional on GDP per capita and size of potential loser sectors such as manufacturing: while trade has a negative effect on pro-redistribution preferences in “poor” and/or in “low manufacturing” countries; it positively affects pro-redistribution preferences in “rich” and/or in “high manufacturing” countries. Additionally, I empirically observe that the size of the loser sector plays a more important mediating role than the type-of-factor endowment of the economy.

Description: Democratic consolidation was the top priority of re-democratized Argentina and Brazil. Regional integration was also part of this goal from two perspectives: from the outside, through a treaty that diminished the scope for political manoeuvring by the military and increased international support for the incumbent administrations, and; from within, through encouragement of a proactive role for business in integration that would give it democratic legitimacy, while, at the same time, exercising democratic practices. Argentine and Brazilian political classes expected to combine these two aspects but soon had to face business reluctance. Government-business relations in the construction of Mercosur reflected government attempts to balance the trade-off between the approaches from without and from within. Although business was largely excluded from the strategic formulation of integration, in a democratic context, governments have to accommodate societal interests. This occurred through a significant overlap between powerful business interests and the executive's plans. The achievement of integration helped consolidate democracy and the choices made by political elites drove forward the democratic process.

Description: This paper focuses on the use of the radiocarbon content of marine shells collected along the Portuguese coast as a proxy for the intensity of coastal upwelling off of Portugal. Differences in the 14C ages of closely associated marine mollusk shells and terrestrial material (charcoal or bones) from several Portuguese archaeological contexts seem to be significant throughout the Holocene. ΔR values range from 940 ± 50 to −160 ± 40 14C yr. Five of these values are significantly higher than the modern value (250 ± 25 14C yr), while the remaining values are lower. The modern value was calculated by measuring the 14C content of live-collected, pre-bomb marine mollusk shells. This value is in accordance with an active upwelling of strong intensity that currently occurs off of Portugal. Some primary observations based on data presented here can be made: i) during the Holocene important changes have occurred in the ocean reservoir effect off the Portuguese coast; ii) these fluctuations may be correlated with regional oceanographic changes, namely with changes in the strength of coastal upwelling; and iii) these changes suggest some sort of variability of the climatic factors forcing coastal upwelling off of Portugal.

Description: Dating ice samples from glaciers via radiocarbon is a challenge that requires systematic investigations. This work describes an approach for extraction and accelerator mass spectrometry (AMS) 14C analysis of the particulate organic carbon (POC) fraction in glacier ice samples. Measurements were performed at VERA (Vienna Environmental Research Accelerator) on ice samples obtained mainly from the non-temperated ablation zone of the Grenzgletscher (Grenz Glacier) system (Monte Rosa Massif, Swiss Alps). The samples were obtained from 2 sampling sites situated roughly on a common flow line. The sample masses used were between 0.3 and 1.4 kg of ice, yielding between 18 and 307 μg of carbon as POC. The carbon contamination introduced during sample processing varied between 5.4 and 33 μg C and originated mainly from the quartz filters and the rinsing liquids used in processing. Minimum sample sizes for successful graphitization of CO2 in our laboratory could be reduced to

Description: In the journal Radiocarbon, Hall et al. (2005:383) claim that 35-CS-9, located in Bandon Ocean Wayside State Park on the southern Oregon coast, is one of the few Oregon coast sites “that includes sediments and artifacts dating to the early Holocene and possibly to the late Pleistocene.” Their claim for an early Holocene or late Pleistocene human occupation rests on a single radiocarbon date of 11,000 ± 140 BP (12,710–12,680 cal BP) taken from charcoal found at least 20 cm below the nearest artifact. Although Hall et al. compile various kinds of geoarchaeological evidence to support this claim, their case is not convincing. While we applaud aspects of their analyses, the inferences they have drawn are not substantiated by the evidence they present. We agree that 35-CS-9 is a significant site but believe claims for the antiquity of its human use have been exaggerated.

Description: This contribution reports the first set of samples and the creation of an internal reference material at the recently opened Radiocarbon Laboratory at the National Autonomous University of Mexico (UNAM). Samples for the initial measurements were selected from archaeological and dating projects on Teotihuacán, one of the largest and best-studied Mesoamerican urban and ceremonial centers. The 14C dates were compared to results obtained by 2 other laboratories in order to assess the results obtained at UNAM and validate the adopted methodology. As part of the quality assurance protocol, an internal reference material was created that consists of charred wood from the Teotihuacán site with a 14C activity in the value range expected for samples from Mesoamerican archaeological sites. Results from 7 analyses have a mean of 1750 ± 16 BP (80.43 ± 0.16 pMC).

Description: Moss et al. (2006) provided comments and criticisms of our recent paper in this journal (Hall et al. 2005). We can appreciate the need for promoting vigorous dialogue among those interested in the research of early sites along the New World Pacific Margin and thus welcome their intervention; however, we are compelled to respond because they raise several points that require clarification and introduce a critical error that must be corrected.

Description: Isotope tracer studies, particularly radiocarbon measurements, play a key role in biological, nutritional, and environmental research. Accelerator mass spectrometry (AMS) is now the most sensitive detection method for 14C, but AMS is not widely used in kinetic studies of humans. Part of the reason is the expense, but costs would decrease if AMS were used more widely. One component in the cost is sample preparation for AMS. Biological and environmental samples are commonly reduced to graphite before they are analyzed by AMS. Improvements and mechanization of this multistep procedure is slowed by a lack of organized educational materials for AMS sample preparation that would allow new investigators to work with the technique without a substantial outlay of time and effort. We present a detailed sample preparation protocol for graphitizing biological samples for AMS and include examples of nutrition studies that have used this procedure.

Description: The accelerator mass spectrometry facility at Seoul National University (SNU-AMS) began functioning in December 1998 and was first reported at the Vienna AMS conference in October 1999 and at the 17th International Radiocarbon Conference in Israel in June 2000. At the Vienna conference, we reported our accelerator system (Kim et al. 2000) and details of the basic sample preparation system (Lee et al. 2000), such as the combustion line to produce CO2; the catalytic reduction line for the graphitization of CO2; and the pretreatment procedures for wood, charcoal, and peat samples. The recent progress of the AMS facility (Kim et al. 2001) and the extension of the sample pretreatment system to iron and bone samples were reported at the 17th International Radiocarbon Conference (Cheoun et al. 2001). In the meantime, extensive testing of accuracy and reproducibility has been carried out, and ∼1000 unknown archaeological and geological samples have been measured every year. In this report, the archaeological, geological, and environmental data carried out in 1999 are presented in terms of yr BP.

Description: Oceanic uptake and transport of bomb radiocarbon as 14CO2 created by atmospheric nuclear weapons testing in the 1950s and 1960s has been a useful diagnostic for determining the carbon transfer between the ocean and atmosphere. In addition, the distribution of 14C in the ocean can be used as a tracer of oceanic circulation. Results obtained on samples collected in the Gulf of Alaska in the summer of 2002 provide a direct comparison with results in the 1970s during GEOSECS and in the early 1990s during WOCE. The open gyre values are 20–40% lower than those documented in 1991 and 1993 (WOCE), although the general trends as a function of latitude are reproduced. Surface values are still significantly higher than pre-bomb levels (∼ −105% or lower). In the central gyre, we observe Δ14C values that are lower in comparison to GEOSECS (stn 218) and WOCE P16/P17 to a density of ∼26.8 σt. This observation is consistent with the overall decrease in surface Δ14C values and reflects the erosion of the bomb-14C transient. We propose that erosion of the bomb-14C transient is accomplished by entrainment of low-14C water via vertical exchange within the Gulf of Alaska and replenishment of surface and subther-mocline waters with waters derived from the far northwest Pacific.

Description: Radiocarbon ages of 8 decadal tree rings and 66 single-yr tree rings have been measured with a highly accurate liquid scintillation counting (LSC) system (0.2% error) after synthesizing 10.5 g of benzene for each α-cellulose sample produced from tree rings of Choukai Jindai cedar in Japan (39°N). The 14C ages were between 2449 and 2539 14C yr BP for the 21 samples. From the wiggle-matching of the data set using the IntCal04 (Reimer et al. 2004) calibration data in OxCal v 3.10 (Bronk Ramsey 2005), the estimated age of the outer edge of the Choukai tree rings was 477.5 BC (±12.5 yr) with a confidence level of 95.5%; hence, the Choukai tree rings range from 2757 to 2437 cal BP. The age indicates an improved eruption date of the Choukai Volcano. The statistical errors at 1 σ are approximately ± 10 and ± 7 14C yr for the 5-yr data and the decadal data from the single-yr measurements, respectively. For the interval between 2580 and 2520 cal BP, it is statistically significant that the Choukai 14C ages are ∼16 14C yr older on average than both the IntCal04 and QL German oak (∼50°N) data sets. The ∼2.0% offset is informative for the study of regional offset in the Far East.

Description: Radiocarbon analysis was performed by liquid scintillation counting (LSC) and accelerator mass spectrometry (AMS) to assess whether the content of biological components in hydrocarbon fuels could be derived. Different fuel mixtures were prepared containing bioethanol, fossil ethanol, and fossil gasoline. The specific 14C activity of these mixtures was obtained from LSC measurements and directly related to the concentration of carbon originating from the bioethanol (biocarbon). The results were checked via standardized carbon dating procedures and AMS. A good linear correlation exists between the fuel mixture's specific 14C activity and the concentration of biocarbon. Also, the biocarbon fraction of the fuel mixture (the ratio biocarbon : total carbon) and the normalized fraction of biocarbon (%M) showed good linear correlation. Therefore, both relations provide a possibility to quantitatively determine a fuel's biocarbon content by 14C analysis. When the sample composition is known (e.g. resolved by gas chromatography-mass spectroscopy [GC-MS] and nuclear magnetic resonance [NMR]), the amount of particular biological components in a fuel sample can be derived subsequently. For mixtures of bioethanol, fossil ethanol, and gasoline with bioethanol contents in the range of 0.5–2% m/m, it was found that errors in the normalized fraction of biocarbon (%M) were in the range of 25–10%, respectively. For samples with a higher bioethanol content (up to pure bioethanol), the errors in %M were

Description: The isotopic composition of ancient wood has the potential to provide information about past environments. We analyzed the δ13C, δ18O, and δ2H of cellulose of conifer trees from several cross-sections at each of 9 sites around the Great Lakes region ranging from ∼4000 to 14,000 cal BP. Isotopic values of Picea, Pinus, and Thuja species seem interchangeable for δ18O and δ2H comparisons, but Thuja appears distinctly different from the other 2 in its δ13C composition. Isotopic results suggest that the 2 sites of near-Younger Dryas age experienced the coldest conditions, although the Gribben Basin site near the Laurentide ice sheet was relatively dry, whereas the Liverpool site 500 km south was moister. The spatial isotopic variability of 3 of the 4 sites of Two Creeks age shows evidence of an elevation effect, perhaps related to sites farther inland from the Lake Michigan shoreline experiencing warmer daytime growing season temperatures. Thus, despite floristic similarity across sites (wood samples at 7 of the sites being Picea), the isotopes appear to reflect environmental differences that might not be readily evident from a purely floristic interpretation of macrofossil or pollen identification.

Description: High freshwater inputs into Scottish sea lochs (fjords) combined with the restricted exchange between sea loch basin water and coastal Atlantic water masses are likely to result in reduced regional marine radiocarbon reservoir ages (R[t]) in these environments. To test this hypothesis, historical, museum-archived shells, collected live on known dates prior to AD 1950 from coastal locations in NW Scotland, were 14C dated to provide a means of determining R(t) and hence the regional deviation (ΔR) from the modeled global surface ocean reservoir age (R). The sea loch data, when combined with 14C dates from the Scottish west coast (Harkness 1983), yield a regional ΔR value of −26 ± 14 yr. The ΔR of sea loch (fjordic) and coastal waters of NW Scotland are statistically different (at a confidence level 〉95%) from the ΔR value of 17 ± 14 yr reported for UK coastal waters (Reimer 2005; data after Harkness 1983) and are in good agreement with the coastal ΔR value of −33 ± 93 yr reported by Reimer et al. (2002). Therefore, it is recommended that a regional ΔR correction of −26 ± 14 yr should be applied to modern (i.e. pre-bomb but not prehistoric) marine 14C dates from the NW coast of Scotland.

Description: Extensive radiocarbon dating of human remains from Neolithic and Bronze Age hunter-gatherer cemeteries in the Cis-Baikal region of Siberia has been undertaken as a part of the multidisciplinary examination of this material conducted by the Baikal Archaeology Project (BAP; http://baikal.arts.ualberta.ca). Due to the large number of analyzed samples, this paper reports the 14C results only in the context of the basic archaeological information about each of the cemeteries. Comprehensive evaluation, analysis, and interpretation of this entire data set will be undertaken in separate publications. In fact, the dates for one such cemetery have already been examined on 2 recent occasions (Weber et al. 2004, 2005).

Description: In this paper, we investigate how to achieve high-accuracy radiocarbon measurements by accelerator mass spectrometry (AMS) and present measurement series (performed on archived CO2) of 14CO2 between 1985 and 1991 for Point Barrow (Alaska) and the South Pole. We report in detail the measurement plan, the error sources, and the calibration scheme that enabled us to reach a combined uncertainty of better than ±3%. The δ13C correction and a suggestion for a span (or 2-point) calibration for the 14C scale are discussed in detail. In addition, we report new, accurate values for the calibration and reference materials Ox2 and IAEA-C6 with respect to Oxl. The atmospheric 14CO2 records (1985–1991) are presented as well and are compared with other existing records for that period. The Point Barrow record agrees very well with the existing Fruholmen (northern Norway) record from the same latitude. The South Pole record shows a small seasonal cycle but with an extreme phase with a maximum on January 1st (±13 days). Together with its generally elevated 14C level compared to the Neumayer record (coastal Antarctica), this makes our South Pole data set a valuable additional source of information for global carbon cycle modeling using 14CO2 as a constraint.

Description: Accurate and reliable dating of paleosols, animal remains, and artifacts is of crucial importance in reconstructing environmental change and understanding the interrelationship between human activities and natural environments. Dating different materials in the same sample can help resolve problems such as soil carbon sources and carbon storage state. Conventional radiocarbon dating of soil (inorganic and organic matter) and accelerator mass spectrometry (AMS) dating of animal remains (fossil bones and teeth) result in different ages for materials from the same sample position in a typical loess section at Xinglong Mountain, Yuzhong County, Gansu Province in NW China. Inorganic matter is ∼3400 yr older than organic matter, 4175 ± 175 cal BP to 3808 ± 90 cal BP. A 1610-yr difference between the 14C ages of fossils (animal bones and teeth) and soil organic matter suggests that a depositional hiatus exists in the studied profile. The varying 14C ages of fossils and soil organic and inorganic matter have important implications for paleoclimate reconstructions from loess sections. It is critical to consider the meaning of the variable 14C ages from different material components from the same sample position in terms of soil organic and inorganic carbon storage, vegetation history reconstruction, archaeology, and the study of ancient civilizations.

Description: We analyzed a sediment core from the equatorial Arabian Sea, chronologically constrained by accurate accelerator mass spectrometry (AMS) radiocarbon dates on selected planktonic foraminiferal species, for paleoproductivity variations corresponding to the variations in the Indian Ocean Equatorial Westerlies (IEW). The IEW in turn are positively correlated to the Southern Oscillation Index (SOI), which is a measure of El Niño, Southwest monsoon (SWM), and east African rainfall (EAR). The productivity data show that Indian and east African rainfalls declined from 35,000 calendar yr BP up to the last glacial maximum (LGM), with the maximum El Niño frequency during the last glacial period. From ∼14,500 to ∼2000 calendar yr BP (i.e. core top), we find strengthening SWM and EAR along with declining El Niño frequency.

Description: Archaeometry is becoming an increasingly important tool in chronological research related to events in the Ancient Near East during the 2nd millennium BCE. This paper is a review of recently published radiometric results in an attempt to establish the probable dating range for one particular event that occurred during the last quarter of that millennium, the end of the Late Bronze Age. The conclusion is that in spite of significant improvements in methodology in recent years, the quantity and quality of radiocarbon data are still insufficient to define the range of that date to much better than a century. It is concluded that the most likely date of the Late Bronze/Iron Age transition (here defined by the arrival of Mycenaean LH IIIC:1b pottery in the Levant) is somewhere in the 8-decade range between ∼1170 to 1100 BCE. A comparative study of archaeological and historical evidence would appear to favor the lower value.

Description: Within the framework of radioecological studies, 90Sr was determined in wheat grains, soil, and deposition samples. The radiochemical purification of 90Y consisted of liquid-liquid extraction by tributyl phosphate (TBP), followed by hydroxide and oxalate precipitations and, if necessary, the removal of thorium by anion exchange chromatography. The procedure proved to be very robust and reliable, having yttrium yields of 92.7 ± 4.6% for 1-kg wheat samples, 90.9 ± 4.2% for 50-g soil samples, and 90.6 ± 3.2% for wet and dry deposition samples. 90Y was determined by Cerenkov counting and proportional counting. By optimizing the Cerenkov counting window, a figure of merit (FOM) of 4750 could be reached using a Quantulus™ 1220 system. Minimum detectable activities were in the range of 10 mBq.

Description: Calibration (using CALIBomb) of radiocarbon measurements made on the enamel of human teeth from people born during the nuclear era typically produce 2 possible age ranges that potentially reflect the period of tooth formation. These ranges correspond to periods before and after the 1963 atmospheric 14C maximum. Further measurements made on the collagen component of the combined dentine and cementum from the roots of the same teeth enable the appropriate age range to be selected. Using this range and the formation times for individual teeth, we estimated the year of birth of the individuals and compared these to the known dates of birth. The results were relatively accurate and confirmed those of a previous study by another research group. The present study demonstrates that it is possible to produce a good estimate of the year of birth from a single tooth.

Description: In recent years, International Political Economy literature on “politics beyond state” has emphasized the role of non-governmental organizations (NGOs) in broader policy processes, both national and international. In addition to their impact on states, NGOs influence the policies of non-state actors such as firms via public and private politics. Dissatisfied with the progress firms have made in response to public regulation, NGOs have sponsored private authority regimes in several issue areas and pushed firms to participate in them. Across the world, the contest between NGOs and firms has provoked substantial behavioral and programmatic change'including widespread participation in these private authority regimes'among firms seeking to escape NGO pressures. Using firm-level data, this paper examines why direct targeting has not led firms in the U.S. forest products sector to participate in an NGO-sponsored private authority regime, the Forest Stewardship Council. This global regime has been adopted widely in Europe, but U.S.-based forestry firms have tended to favor a domestic industry-sponsored regime, the Sustainable Forestry Initiative. Our analysis suggests that the desire of firms to maintain control over their institutional environment in light of hostile relations with NGOs has led US-based firms to favor the Sustainable Forestry Initiative.

Description: In the study of corporate political activity in the United States, scholars have consistently relied on samples comprised entirely or principally of large firms. While scholars have raised the issue of bias in these samples, there have been no systematic examinations of the consequences for causal inference. We address this issue directly by comparing the results of comprehensive models that examine corporate lobbying using both large-firm and randomly-generated samples. We find that while there are some notable differences, they are certainly not so large as to lead us to question fundamentally the results of decades of scholarship. In short, the results generated using a random sample lead to causal inferences largely consistent with those in the theoretical and empirical literature. In particular, firms' resources and interactions with government condition both their decisions to lobby and the level of their activity.

Description: Why do some countries institutionalize a social program compensating the unemployed while others do not? My main argument is that the choice to have an unemployment insurance program is a function of 1) the distribution of unemployment risks within a country and 2) political processes through which demands for insurance are realized. The distribution of industrial-specific risks and workers' employment status are the driving force in shaping workers' demands. In developing countries, these demands are more likely to be realized under democratic regimes. An event history model for 102 developing countries from 1946 to 2000 is used to test the arguments.

Description: This paper examines the motivation and impact of corporate diversification in Chinese listed firms. We find that in local government owned-firms there is a non-linear relationship between the level of firm diversification and state ownership. As state ownership increases from zero, the level of diversification decreases. After state ownership reaches a certain level, the level of diversification increases as state ownership increases. There is no evidence that ownership is related to corporate diversification in non-state-owned firms or central government-owned firms. We also document that diversification is negatively related to firm performance in local government-owned firms. However, there is no evidence that diversification is negatively related to the firm performance in non-state-owned firms or central government-owned firms. Our findings suggest that agency problems are responsible for local government owned-firms taking value-reducing diversification strategies.

Description: The present effort documents the population trends of prograde and retrograde spanwise vortex cores in wall turbulence outside the buffer layer. Large ensembles of instantaneous velocity fields are acquired by particle-image velocimetry in the streamwise-wall-normal plane of both turbulent channel flow at Reτ ≡ u*δ/ ν =570, 1185 and 1760 and a zero-pressure-gradient turbulent boundary layer at Reτ = 1400, 2350 and 3450. Substantial numbers of prograde spanwise vortices are found to populate the inner boundary of the log layer of both flows and most of these vortices have structural signatures consistent with the heads of hairpin vortices. In contrast, retrograde vortices are most prominent at the outer edge of the log layer, often nesting near clusters of prograde vortices. Appropriate Reynolds-number scalings for outer- and inner-scaled population densities of prograde and retrograde vortices are determined. However, the Reτ = 570 channel-flow case deviates from these scalings, indicating that it suffers from low-Re effects. When the population densities are recast in terms of fractions of resolved prograde and retrograde spanwise vortices, similarity is observed for 100 〈 y+ 〈 0.8δ+ in channel flow and in both flows for 100 〈 y+ 〈 0.3δ+ over the Reτ range studied. The fraction of retrograde vortices increases slightly with Reτ beyond the log layer in both flows, suggesting that they may play an increasingly important role at higher Reynolds numbers. Finally, while the overall prograde and retrograde population trends of channel flow and the boundary layer show little difference for y 〈 0.45δ, the retrograde populations differ considerably beyond this point, highlighting the influence of the opposing wall in channel flow. © 2006 Cambridge University Press.

Description: We analyse the topology of vortex breakdown in a closed cylindrical container in the steady domain under variation of three parameters, the aspect ratio of the cylinder, the Reynolds number, and the ratio of the angular velocities of the covers. We develop a general post-processing method to obtain topological bifurcation diagrams from a database of simulations of two-dimensional flows and apply the method to axisymmetric simulations of the flow in the cylinder. Interpreting the diagrams with the aid of bifurcation theory, we obtain complete topological bifurcation diagrams for the rotation ratio in the interval [-0.04, 0.075]. In this narrow range we identify three codimension-3 bifurcation points which act as organising centres for the entire bifurcation diagram. © 2006 Cambridge University Press.

Description: Microstreaming from oscillating bubbles is known to induce vigorous vortex flow. Here we show how to harness the power of bubble streaming in an experiment to achieve directed transport flow of high velocity, allowing design and manufacture of microfluidic MEMS devices. By combining oscillating bubbles with solid protrusions positioned on a patterned substrate, solid beads and lipid vesicles are guided in desired directions without microchannels. Simultaneously, the flow exerts controlled localized forces capable of opening and reclosing lipid membranes. © 2006 Cambridge University Press.

Description: This paper reports on a lattice Boltzmann simulation of laminar mixing in a coaxial microreactor at low Reynolds numbers. The main focus of the study was to compare the effectiveness of a multi-holed baffle plate, a set of 2 × 2 square bars, and a flat-plate arrangement on the enhancement of reagent mixing inside the reactor. It was found that all three mixers increased mixing through the mechanism of generating coherent structures, which in turn increased interface contact between the fluid streams. However, for this particular microreactor, the efficiency of each mixer depended on its ability to generate coherent structures with high helicity and thereby inducing a swirl motion to the flow. © 2006 Cambridge University Press.

Description: We have studied steady flow in a two-dimensional channel in which a section of one wall has been replaced by an elastic membrane under dimensionless longitudinal tension T but possessing no bending stiffness. The dimensionless upstream transmural pressure takes a value Pext, the membrane section is assumed to be long compared with the channel width and its deformation is assumed to remain within the viscous boundary layers. Standard high-Reynolds-number asymptotic methods are applied to arrive at a coupled boundary-layer-membrane problem. A non-zero cross-stream pressure gradient, leading to flow perturbations upstream of the membrane, is included in the analysis. Linearization of the boundary-layer problem yields firstly an analytic solution at non-zero Pext and asymptotically high T. This takes the form of an expansion in T-1 for which the membrane shape and the flow decouple at each order. Extension of this solution branch to smaller values of the tension suggests a singularity at finite tension, where the deformation of the membrane becomes very large. Secondly, when the upstream transmural pressure is zero the trivial solution is valid for all values of the tension. However, we also obtain eigensolutions where the membrane tension plays the role of eigenvalue. There are thus non-trivial solutions of the problem at these particular values of the tension. The nonlinear coupled boundary-layer-membrane problem is then solved numerically. A finite-difference, Keller-box, marching scheme is used, together with a shooting algorithm to satisfy the boundary condition at the downstream end of the membrane. This reveals a variety of different solutions, showing the relation between the two cases captured by the linearized analysis and demonstrating the existence of parameter ranges for which no solutions exist under the specified constraints. Such parameter ranges appear not to exist if the downstream, rather than the upstream, transmural pressure is held constant. The relation to our results of solutions obtained by solving the two-dimensional Navier-Stokes equations directly is discussed. Reasonable agreement between parameters is obtained, once allowance is made for the finite Reynolds number and membrane length in those computations. © 2006 Cambridge University Press.

Description: Horizontally periodic, vertically localized internal wavepackets evolve nonlinearly due only to interactions between the waves and their wave-induced mean flow. The corresponding weakly nonlinear equation that describes the evolution of the amplitude envelope before the onset of parametric subharmonic instability is examined. The results are compared with fully nonlinear numerical simulations and are shown to lie in excellent agreement for over 15 buoyancy periods. Analysis of the equation shows that the evolution is modulationally unstable if the wave frequency exceeds that of waves with the fastest vertical group speed and if the amplitude is sufficiently large. Waves that move close to the fastest vertical group speed are unstable even if their relative amplitude is a tiny fraction of the inverse relative vertical extent of the wavepacket. At late times in the evolution of an unstable wavepacket third-order dispersion terms become non-negligible and act in conjunction with weakly nonlinear effects to retard the vertical advance of the wavepacket as a whole. © 2006 Cambridge University Press.

Description: This paper is motivated by an observation: in the nascent state of vortex breakdown before it develops into a full-grown radial expansion, an initially straight vortex core first swells, and does so even in a straight pipe for no apparent reason. Although this initial swelling may be explained in many ways according to the perspectives chosen, we offer our own interpretation framed solely within vorticity dynamics: the radial swelling as well as the subsequent growth are induced by the azimuthal vorticity gradient decreasing downstream. The negative azimuthal vorticity gradient first appears at start-up and moves eventually into the region where the circulation reaches its steady-state value. The vorticity gradient can become negative without necessarily being accompanied by a sign-switch of the azimuthal vorticity itself. The key point - that the negative azimuthal vorticity gradient induces initial radial swelling and its growth - is demonstrated in two analyses. First, a kinematic analysis results in an equation for the radial velocity where the azimuthal vorticity gradient appears as a source term. Its solution shows, in general and explicitly, that the negative azimuthal vorticity gradient does induce radially outward velocity. Two heuristic examples serve to illustrate this point further. In the second analysis, using the equation of motion in the streamline coordinates, the negative azimuthal vorticity gradient is shown to diverge the meridional streamlines radially. A numerical simulation using a modified vortex filament method not only corroborates this role of the azimuthal vorticity gradient in initiating and promoting the radial expansion, but also adds details to track the formation process. Both analyses and simulation support our interpretation that the initial radial swelling and its subsequent growth are induced by the negative azimuthal vorticity gradient. © 2006 Cambridge University Press.

Description: The speed of a fluid intrusion propagating along a sharp density interface is predicted using conservation of mass, momentum and energy. For the special case in which the intrusion density equals the depth-weighted mean density of the upper and lower layers, the theory of Holyer & Huppert (J. Fluid Mech., vol. 100, 1980, p. 739) predicts that the intrusion occupies one-half the total depth, its speed is one-half the interfacial long-wave speed and the interface ahead of the intrusion remains undisturbed. For all other intrusion densities, the interface is deflected vertically by a long wave that travels ahead of the intrusion and thereby changes the local upstream conditions. In these cases, the conservation equations must be matched to an exact solution of the two-layer shallow water equations, which describe the spatial evolution of the nonlinear wave. We obtain predictions for the intrusion speed that match closely with experiments and numerical simulations, and with a global energy balance analysis by Cheong, Keunen & Linden (J. Fluid Mech., vol. 552, 2006, p. 1). Since the latter does not explicitly include the energetics of the upstream wave, it is inferred that the energy carried by the wave is a small fraction of the intrusion energy. However, the new more detailed model also shows that the kinematic influence of the upstream wave in changing the level of the interface is a critical component of the flow that has previously been ignored. © 2006 Cambridge University Press.

Description: It is shown for the first time that the gradient diffusion hypothesis often adopted for thermal dispersion heat flux in heat transfer within porous media can be derived from a transport equation for the thermal dispersion heat flux based on the Navier-Stokes and energy equations. The transport equation valid for both thermal equilibrium and non-equilibrium cases is mathematically modelled so that all unknown spatial correlation terms, associated with redistribution and dissipation of the dispersion heat flux, are expressed in terms of determinable variables. The unknown coefficients are determined analytically by considering of macroscopically unidirectional flow through a tube as treated by Taylor. Taylor's expression for the dispersion has been generated from the transport equation. Both laminar and turbulent flow cases are investigated to obtain two distinct limiting expressions for low- and high-Péclet-number regimes. The results obtained for the Taylor diffusion problem are translated to the case of heat and fluid flow in a packed bed, to obtain the corresponding expressions for the axial dispersion coefficient in a packed bed. The resulting expression for the high-Péclet-number case agrees well with the empirical formula, validating of the present transport analysis. © 2006 Cambridge University Press.

Description: Long-surface-wave instability in dense granular flows down inclined planes is analysed using recently proposed three-dimensional constitutive equations. A full linear stability analysis of the local governing equations is performed and compared to previous experimental results obtained with glass beads. We show that the proposed rheology is able to capture all the features of the instability quantitatively. In particular, it predicts well the behaviour and scaling for the cutoff frequency of the instability observed in the experiments. This result gives strong support for the three-dimensional rheology proposed and suggests new terms in the Saint-Venant equations used to describe free-surface granular flows. © 2006 Cambridge University Press.

Description: When a viscoelastic fluid blob is stretched out into a thin horizontal filament, it sags and falls gradually under its own weight, forming a catenary-like structure that evolves dynamically. If the ends are brought together rapidly after stretching, the falling filament tends to straighten by rising. These two effects are strongly influenced by the elasticity of the fluid and yield qualitatively different behaviours from the case of a purely viscous filament analysed previously (Teichman & Mahadevan, J. Fluid Mech. vol. 478, 2003, p. 71). Starting from the bulk equations for the motion of a viscoelastic fluid, we derive a simplified equation for the dynamics of a viscoelastic filament and analyse this equation in some simple settings to explain our observations. © 2006 Cambridge University Press.

Description: A new description of two-dimensional continuous free-surface flows in Lagrangian coordinates is proposed. It is shown that the position of a fluid particle in such flows can be represented as a fixed point of a transformation in ℝ2. Components of the transformation function satisfy the linear Euler-type continuity equation and can be expressed via a single function analogous to an Eulerian stream function. Fixed-point iterations lead to a simple recursive representation of a solution satisfying the Lagrangian continuity equation. Expanding the unknown function in a small-perturbation asymptotic expansion we obtain the complete asymptotic formulation of the problem in a fixed domain of Lagrangian labels. The method is then applied to the classical problem of a regular wave travelling in deep water, and the fifth-order Lagrangian asymptotic solution is constructed, which provides a much better approximation of steep waves than the corresponding Eulerian Stokes expansion. In contrast with early attempts at Lagrangian regular-wave expansions, the asymptotic solution presented is uniformly valid at large times. © 2006 Cambridge University Press.

Description: We investigate flow in a tube driven by axial and radial wall oscillations that are periodic in time. Furthermore, the walls undergo periodic circumferential oscillations, the amplitude of which increases with axial distance along the tube; this wall motion results in a new mechanism for the generation of axial flows within the tube. The axial steady-streaming flows that arise as a result of circumferential oscillations are opposed to those due to the axial and radial pulsations, and are potentially as significant. We then consider the uptake of a passive solute through the walls of such a tube into an adjacent medium in which the solute diffuses and is consumed at a constant rate. The tube ends are open to well-mixed fluid containing the solute. The effect of the streaming flows on the solute flux into the tube is determined. The solute disperses along the tube due to the interaction between advection and transverse diffusion, and the time-mean solute distribution throughout the tube is determined for a wide range of parameters. © 2006 Cambridge University Press.

Description: We have analysed the structure of the irrotational flow near the minimum radius of an axisymmetric bubble at the final instants before pinch-off. The neglect of gas inertia leads to the geometry of the liquid-gas interface near the point of minimum radius being slender and symmetric with respect to the plane z = 0. The results reproduce our previous finding that the asymptotic time evolution for the minimum radius, Ro(t), is τ ∝ Ro2 √-1n Ro2, τ being the time to breakup, and that the interface is locally described, for times sufficiently close to pinch-off, by f(z,t)/Ro(t) = 1 - (6ln Ro) -1 (z/Ro)2. These asymptotic solutions correspond to the attractor of a system of ordinary differential equations governing the flow during the final stages before pinch-off. However, we find that, depending on initial conditions, the solution converges to the attractor so slowly (with a logarithmic behaviour) that the universal laws given above may hold only for times so close to the singularity that they might not be experimentally observed. © 2006 Cambridge University Press.

Description: Experiments are reported on the hydrodynamic performance of a flexible fin. The fin replicates some features of the pectoral fin of a batoid fish (such as a ray or skate) in that it is actuated in a travelling wave motion, with the amplitude of the motion increasing linearly along the span from root to tip. Thrust is found to increase with non-dimensional frequency, and an optimal oscillatory gait is identified. Power consumption measurements lead to the computation of propulsive efficiency, and an optimal efficiency condition is evaluated. Wake visualizations are presented, and a vortex model of the wake near zero net thrust is suggested. Strouhal number effects on the wake topology are also illustrated. © 2006 Cambridge University Press.

Description: Well-resolved large-eddy simulations (LES) are performed in order to investigate flow phenomena and turbulence structure of the boundary layer along a supersonic compression ramp. The numerical simulations directly reproduce an available experimental result. The compression ramp has a deflection angle of β = 25 °. The mean free-stream Mach number is M∞ = 2.95. The Reynolds number based on the incoming boundary-layer thickness is Reδ0 = 63 560 in accordance with the reference experiment. These simulations overcome deficiencies of earlier direct numerical simulations (DNS) and LES in terms of ramp-deflection angle, Reynolds number and spanwise size of the computational domain which is required for capturing the essential flow phenomena. The filtered conservation equations for mass, momentum and energy are solved with a high-order finite-difference scheme. The effect of subgrid scales is modelled by the approximate deconvolution model. About 18.5 × 106 grid points are used for discretizing the computational domain. To obtain mean flow and turbulence structure the flow is sampled 1272 times over 703 characteristic time scales of the incoming boundary layer. Statistical data are computed from these samples. An analysis of the data shows good agreement with the experiment in terms of mean quantities such as shock position, separation and reattachment location, skin-friction and surface-pressure distributions, and turbulence structure. The computational data confirm theoretical and experimental results on fluctuation amplification across the interaction region. In the wake of the main shock a shedding of shocklets is observed. The temporal behaviour of the coupled shock-separation system agrees well with experimental data. Unlike previous DNS the present simulation data provide indications of a large-scale shock motion. Also, evidence for the existence of three-dimensional large-scale streamwise structures, commonly referred to as Görtler-like vortices, is found. © 2006 Cambridge University Press.

Description: The exchange of energy for an inviscid gravity current which is released from a lock and then propagates over a horizontal boundary is considered. Attention is focused on effects due to stratification in the ambient. The investigation uses both a one-layer shallow-water model and Navier-Stokes finite-difference simulations. There is good agreement between these two approaches for the energy of the dense fluid (the current). The results indicate that with respect to the behaviour of energy as a function of time we can distinguish between: (a) currents propagating at supercritical speed (with respect to the fastest internal wave in the ambient), including a nose propagating into an unstratified ambient; and (b) currents propagating at subcritical speed, including the strongest effective stratification for which the density at the base of the ambient is equal to that of the current. The stratification enhances the accumulation of potential energy in the ambient and reduces the energy decay (dissipation) of the two-fluid system. The interaction of the internal waves with the head of the current in the subcritical case has no significant influence on the energy balance of the current. © 2006 Cambridge University Press.

Description: This article examines a reduced form of the 'purely dissipative' model proposed several years ago as a general continuum model for the rheology of non-colloidal particle dispersions, ranging from Stokesian suspensions to non-cohesive granular media. Essential to the model is a positive-definite viscosity tensor η, depending on the history of deformation and providing a crucial restriction on related models for anisotropic fluids and suspensions. In the present treatment, η is assumed to be as an isotropic function of a history-dependent second-rank 'texture' or 'fabric' tensor A. A formula for η(A) borrowed from the analogous theory of linear elasticity, and its subsequent expansion for weak anisotropy provides an explicit expression for the stress tensor in terms of fabric, strain-rate and eight material constants. Detailed consideration is given to the special case of Stokesian suspensions, which represent an intriguing subset of memory materials without characteristic time. For this idealized fluid one finds linear dependence of all stresses, including viscometric normal stress, on present deformation rate, with the provision for an arbitrary fabric evolution ('thixotropy') in unsteady deformations. As a concrete example, a co-rotational memory integral is adopted for A in terms of strain-rate history, and a memory kernel with two-mode exponential relaxation gives close agreement with the rather sparse experimental data on transient shear experiments. In the proposed model, an extremely rapid mode of relaxation is required to mimic the incomplete reversal of stress observed in experiments involving abrupt reversal of steady shearing, supporting the conclusion of others that non-hydrodynamic effects, with breaking of Stokesian symmetry, may be implicated in such experiments. Qualitative comparisons are made to a closely related model, derived from a micro-mechanical analysis of Stokesian suspensions, but also involving non-Stokesian effects. The present analysis may point the way to improved micro-mechanical analysis and to further experiments. Possible extensions of the model to the viscoplasticity of dry and liquid-saturated granular media also are discussed briefly. © 2006 Cambridge University Press.

Description: Transient growth of small disturbances may lead to the initiation of the laminar-turbulent transition process. Such growth in a two-dimensional laminar flow in a channel with a corrugated wall is analysed. The corrugation has a wavy form that is completely characterized by its wavenumber and amplitude. The maximum possible growth and the form of the initial disturbance that leads to such growth have been identified for each form of the corrugation. The form that leads to the largest growth for a given corrugation amplitude, i.e. the optimal corrugation, has been found. It is shown that the corrugation acts as an amplifier for disturbances that are approximately optimal in the smooth channel case but has little effect in the other cases. The interplay between the modal (asymptotic) instability and the transient growth, and the use of the variable corrugation for modulation of the growth are discussed. © 2006 Cambridge University Press.

Description: We present a broad range of measurements of the angular orientation θ0(t) of the large-scale circulation (LSC) of turbulent Rayleigh-Bénard convection as a function of time. We used two cylindrical samples of different overall sizes, but each with its diameter nearly equal to its height. The fluid was water with a Prandtl number of 4.38. The time series θ0(t) consisted of meanderings similar to a diffusive process, but in addition contained large and irregular spontaneous reorientation events through angles Δθ. We found that reorientations can occur by two distinct mechanisms. One consists of a rotation of the circulation plane without any major reduction of the circulation strength. The other involves a cessation of the circulation, followed by a restart in a randomly chosen new direction. Rotations occurred an order of magnitude more frequently than cessations. Rotations occurred with a monotonically decreasing probability distribution p(Δθ), i.e. there was no dominant value of Δθ and small Δθ were more common than large ones. For cessations, p(Δθ) was uniform, suggesting that information of θ0(t) is lost during cessations. Both rotations and cessations have Poissonian statistics in time, and can occur at any θ0. The average azimuthal rotation rate θ̇ increased as the circulation strength of the LSC decreased. Tilting the sample relative to gravity significantly reduced the frequency of occurrence of both rotations and cessations. © 2006 Cambridge University Press.

Description: The flipping motion of a blood platelet convected under the action of a simple shear flow over a substrate is discussed. The platelet is modelled as a rigid oblate spheroid with aspect ratio equal to 0.25 whose axis of revolution is perpendicular to the vorticity of the simple shear flow. The particle motion from a given initial position is computed using a boundary element method for Stokes flow based on the double-layer representation. When the platelet is far from the wall, the motion is described by Jeffery's exact solution. As the platelet approaches the wall, the rate of rotation is reduced significantly when the platelet mid-plane is parallel to wall, and mildly when the mid-plane is perpendicular to the wall. Comparison with laboratory data indicates that wall effects alone do not explain the observed slow rate of rotation. It is proposed that a distributed adhesion force imparts to the particle an effective external force and torque at the nominal point of contact, and these slow down the rate of rotation. The process is demonstrated by computing the motion of an adhering platelet whose lowest point is immobilized under the action of a suitable force and torque. © 2006 Cambridge University Press.

Description: The numerical results obtained by Mougin & Magnaudet (Phys. Rev. Lett. vol. 88, 2002a, 14502) for the flow past a freely moving spheroidal bubble with a prescribed spheroidal shape are processed to analyse the evolution of the forces and torques experienced by the bubble when it rises along a planar zigzag and a circular helix. It is found that, as soon as the wake becomes three-dimensional, a lateral force with a strength comparable with that of the buoyancy force occurs. This force, together with the corresponding torque, drives the horizontal movements of the bubble. The force and torque balances reveal how these wake-induced effects are balanced by added-mass effects to make possible the existence of zigzag and helical motions along which the angle between the velocity and the symmetry axis of the bubble remains small. The evolution of the wake during the zigzag indicates that the sign of the trailing vortices, and thus that of the wake-induced force and torque, is governed by the rotation of the bubble and reveals the sensitivity of the wake dynamics to the changes in the bubble velocity and rotation rate. © 2006 Cambridge University Press.

Description: The evolution of a stratified shear layer with mean shear in the horizontal direction, orthogonal to gravity, is numerically investigated with focus on the structural organization of the vorticity and density fields. Although the Reynolds number of the flow increases with time, facilitating instabilities and turbulence, the bulk Richardson number signifying the level of stratification also increases. Remarkably rich dynamics is found: turbulence; the emergence of coherent core/braid regions from turbulence; formation of a lattice of dislocated vortex cores connected by thin horizontal sheets of collapsed density and vorticity; density-driven intrusions at the edges of the shear layer; and internal wave generation and propagation. Stratification introduces significant vertical variability although it inhibits the vertical velocity. The molecular dissipation of turbulent kinetic energy and of turbulent potential energy are both found to be substantial even in the case with highest stratification, and primarily concentrated in thin horizontal sheets. The simulation data are used to help explain how buoyancy induces the emergence of columnar vortex cores from turbulence and then dislocates these cores to eventually form a lattice of 'pancake' eddies connected by thin sheets with large vertical shear (horizontal vorticity) and density gradient. © 2006 Cambridge University Press.

Description: The motion of an incompressible viscous rotating fluid in an oblate spheroidal container is studied by direct numerical simulation in an appropriate spheroidal coordinate system and in the linear approximation. The behaviour of a few eigenmodes is investigated as a function of the eccentricity e of the container, for Ekman number E=10-5. Viscous effects are shown through internal shear layers, the spatial structure of which strongly depends on the eccentricity. In particular, for the spin-over mode, a resonance occurs around a critical value ec≈0.50, where the decay rate deviates strongly from the predicted theoretical variation. This resonance is discussed in relation to the accidental coincidence between the spin-over frequency and two other frequencies corresponding to the (8,1,5) and (14,1,9) inertial eigenmodes. © 2006 Cambridge University Press.

Description: Two rakes of cross-wire probes were used to capture the two-point velocity statistics in a flow through an axisymmetric sudden expansion. The expansion ratio of the facility is 3, and has a constant geometry. Measurements were acquired at a Reynolds number equal to 54 000, based on centreline velocity and inlet pipe diameter. The two-point velocity correlations were obtained along a plane normal to the flow (r, θ), at eleven downstream step-height positions spanning from the recirculating region, through reattachment, and into the redeveloping region of the flow. Measurements were acquired by means of a flying-hot-wire technique to overcome rectification errors near the outer wall of the pipe where flow recirculations were greatest. A mixed application of proper orthogonal (in radius) and Fourier decomposition (in azimuth) was performed at each streamwise location to provide insight into the dynamics of the most energetic modes in all regions of the flow. This multi-point analysis reveals that the flow evolves from the Fourier-azimuthal mode m = 2 (containing the largest amount of turbulent kinetic energy) in the recirculating region, to m = 1 in the reattachment and redeveloping regions of the flow. An eigenvector reconstruction of the kernel, using the most energetic modes from the decomposition, displays the spatial dependence of the Fourier-azimuthal modes and the characteristics that govern the turbulent shear layer and recirculating regions of the flow. © 2006 Cambridge University Press.

Description: In this paper, we present a new passive control device for form-drag reduction in flow over a two-dimensional bluff body with a blunt trailing edge. The device consists of small tabs attached to the upper and lower trailing edges of a bluff body to effectively perturb a two-dimensional wake. Both a wind-tunnel experiment and large-eddy simulation are carried out to examine its drag-reduction performance. Extensive parametric studies are performed experimentally by varying the height and width of the tab and the spanwise spacing between the adjacent tabs at three Reynolds numbers of Re = u∞h/v = 20 000, 40 000 and 80 000, wher u∞ is the free-stream velocity and h is the body height. For a wide parameter range, the base pressure increases (i.e. drag reduces) at all three Reynolds numbers. Furthermore, a significant increase in the base pressure by more than 30% is obtained for the optimum tab configuration. Numerical simulations are performed at much lower Reynolds numbers of Re = 320 and 4200 to investigate the mechanism responsible for the base-pressure increase by the tab. Results from the velocity measurement and numerical simulations show that the tab introduces the spanwise mismatch in the vortex-shedding process, resulting in a substantial reduction of the vortical strength in the wake and significant increases in the vortex formation length and wake width. © 2006 Cambridge University Press.

Description: The stability of an equilibrium system of two drops suspended from circular holes in a horizontal plate is examined. The drop surfaces are the disconnected axisymmetric surfaces pinned to the edges of the holes. The holes lie in the same horizontal plane and the two drops are connected by a liquid layer that lies above the plate. The total liquid volume is constant. For identical pendant drops pinned to holes of equal radii, axisymmetric perturbations are always the most dangerous. The stability region for two identical drops differs considerably from that for a solitary pendant drop. A bifurcation analysis shows that the loss of stability leads to a continuous transition from a critical system of identical drops to a stable system of axisymmetric non-identical drops. With increasing total protruded liquid volume this system of non-identical drops reaches its own collective stability limit (to axisymmetric perturbations) which gives rise to dripping or streaming from the holes. Critical volumes and heights for non-identical drops have been calculated as functions of the dimensionless hole radius (associated with the Bond number). For unequal hole radii, there are three intervals of the larger dimensionless hole radius, R10, with qualitatively different bifurcation patterns which in turn can depend on the smaller dimensionless hole radius, R10. Loss of stability may occur when the drop suspended from the larger hole reaches its stability limit (to non-axisymmetric perturbations) as a solitary drop or when the system reaches the collective stability limit (to axisymmetric perturbations). Typical situations are illustrated for selected values of R10, and then the basic characteristics of the stability for a dense set of R10 are presented. © 2006 Cambridge University Press.

Description: When the individual particles in an otherwise quiescent suspension of freely suspended spherical particles are acted upon by external couples, the resulting suspension-scale fluid motion is characterized by a non-symmetric state of stress. Viewed at the interstitial scale (i.e. microscopic scale), this coupling between translational and rotational particle motions is a manifestation of particle-particle hydrodynamic interactions and vanishes with the volume fraction φ of suspended spheres. The antisymmetric portion of the stress is quantified by the suspension-scale vortex viscosity μv, different from the suspension's shear viscosity μ. Numerical boundary element method (BEM) simulations of such force-free suspensions of spheres uniformly dispersed in incompressible Newtonian liquids of viscosity μ0 are performed for circumstances in which external couples (of any specified suspension-scale position-dependence) are applied individually to each of the suspended particles in order to cause them to rotate in otherwise quiescent fluids. In the absence of external forces acting on either the spheres or boundaries, such rotations indirectly, through interparticle coupling, cause translational motions of the individual spheres which, owing to the no-slip boundary condition, drag neighbouring fluid along with them. In turn, this combined particle-interstitial fluid movement is manifested as a suspension-scale velocity field, generated exclusively by the action of external couples. Use of this scheme to create suspension-scale particle-phase spin fields Ω and concomitant velocity fields v enables both the vortex and shear viscosities of suspensions to be determined as functions of φ in disordered systems. This scheme is shown, inter alia, to confirm the constitutive equation, Ta=2μvε · [(1/2) ∇ × v - Ω] proposed in the continuum mechanics literature for the linear relation between the antisymmetric stress Ta and the disparity existing between the particle-phase spin rate Ω and half the suspension's vorticity, ∇ × v (with the third-rank pseudotensor ε the permutation triadic). Our dynamically based BEM simulations confirm the previous computations of the Prosperetti et al. group for the dependence of the vortex viscosity upon the solids volume fraction in concentrated disordered suspensions, obtained by a rather different simulation scheme. Moreover, our dynamically based rheological calculations are confirmed by our semi-independent, energetically based, calculations that equate the rates of working (equivalently, the energy dissipation rates) at the respective interstitial and suspension scales. As an incidental by-product, the same BEM simulation results also verify the suspension-scale Newtonian constitutive equation, Ts = μ [∇v + (∇v)], as well as the functional dependence of the shear viscosity μ upon φ found in the literature. © 2006 Cambridge University Press.

Description: The problem of Poiseuille flow in a fluid overlying a porous medium saturated with the same fluid is studied. A careful linear instability analysis is carried out. It is shown that there are three modes of instability, two belong to one eigenvalue and persist in small ranges of parameters, while beyond these parameter ranges a third corresponding to another eigenvalue prevails. These three modes are of different stability characteristics, but are triggered by the shear stress of the Poiseuille flow in the fluid layer. © 2006 Cambridge University Press.

Description: The three-dimensional governing macroscopic equations of the flow field which is developed when an elasto-plastic highly deformable open-cell porous medium whose pores are uniformly filled with liquid and gas is struck head-on by a planar shock wave, are developed using a multiphase approach. The one-dimensional version of these equations is solved numerically using an arbitrary Lagrangian Eulerian (ALE) based numerical code. The numerical predictions are compared qualitatively to experimental results from various sources and good agreements are obtained. This study complements our earlier studies in which we solved, using an ALE-based numerical code, the one-dimensional governing equations of the flow field which is developed when an elasto-plastic flexible open-cell porous medium, capable of undergoing extremely large deformations, whose pores are saturated with gas only, is struck head-on by a planar shock wave. © 2006 Cambridge University Press.

Description: This paper provides a detailed study of turbulent statistics and scale-by-scale budgets in turbulent Rayleigh-Bénard convection. It aims at testing the applicability of Kolmogorov and Bolgiano theories in the case of turbulent convection and at improving the understanding of the underlying inertial-range scalings, for which a general agreement is still lacking. Particular emphasis is laid on anisotropic and inhomogeneous effects, which are often observed in turbulent convection between two differentially heated plates. For this purpose, the SO(3) decomposition of structure functions and a method of description of inhomogeneities are used to derive inhomogeneous and anisotropic generalizations of Kolmogorov and Yaglom equations applying to Rayleigh-Bénard convection, which can be extended easily to other types of anisotropic and/or inhomogeneous flows. The various contributions to these equations are computed in and off the central plane of a convection cell using data produced by a direct numerical simulation of turbulent Boussinesq convection at Ra = 106 and Pr = 1 with aspect ratio A=5. The analysis of the isotropic part of the Kolmogorov equation demonstrates that the shape of the third-order velocity structure function is significantly influenced by buoyancy forcing and large-scale inhomogeneities, while the isotropic part of the mixed third-order structure function 〈(Δ∈)2 Δu〉 appearing in the Yaglom equation exhibits a clear scaling exponent 1 in a small range of scales. The magnitudes of the various low l degree anisotropic components of the equations are also estimated and are shown to be comparable to their isotropic counterparts at moderate to large scales. The analysis of anisotropies notably reveals that computing reduced structure functions (structure functions computed at fixed depth for correlation vectors r lying in specific planes only) in order to reveal scaling exponents predicted by isotropic theories is misleading in the case of fully three-dimensional turbulence in the bulk of a convection cell, since such quantities involve linear combinations of different ℓ components which are not negligible in the flow. This observation also indicates that using single-point measurements together with the Taylor hypothesis in the particular direction of a mean flow to test the predictions of asymptotic dimensional isotropic theories of turbulence or to calculate intermittency corrections to these theories may lead to significant bias for mildly anisotropic three-dimensional flows. A qualitative analysis is finally used to show that the influence of buoyancy forcing at scales smaller than the Bolgiano scale is likely to remain important up to Ra = 109, thus preventing Kolmogorov scalings from showing up in convective flows at lower Rayleigh numbers. © 2006 Cambridge University Press.

Description: We consider the deformation and breakup of a non-Newtonian slender drop in a Newtonian liquid, subject to an axisymmetric extensional flow, and the influence of inertia in the continuous phase. The non-Newtonian fluid inside the drop is described by the simple power-law model and the unsteady deformation of the drop is represented by a single partial differential equation. The steady-state problem is governed by four parameters: the capillary number; the viscosity ratio; the external Reynolds number; and the exponent characterizing the power-law model for the non-Newtonian drop. For Newtonian drops, as inertia increases, drop breakup is facilitated. However, for shear thinning drops, the influence of increasing inertia results first in preventing and then in facilitating drop breakup. Multiple stationary solutions were also found and a stability analysis has been performed in order to distinguish between stable and unstable stationary states. © 2006 Cambridge University Press.

Description: The interaction between the wall and the core region of turbulent channels is studied using direct numerical simulations at friction Reynolds number Reτ ≈ 630. In these simulations the near-wall energy cycle is effectively removed, replacing the smooth-walled boundary conditions by prescribed velocity disturbances with non-zero Reynolds stress at the walls. The profiles of the first- and second-order moments of the velocity are similar to those over rough surfaces, and the effect of the boundary condition on the mean velocity profile is described using the equivalent sand roughness. Other effects of the disturbances on the flow are essentially limited to a layer near the wall whose height is proportional to a length scale defined in terms of the additional Reynolds stress. The spectra in this roughness sublayer are dominated by the wavenumber of the velocity disturbances and by its harmonics. The wall forcing extracts energy from the flow, while the normal equilibrium between turbulent energy production and dissipation is restored in the overlap region. It is shown that the structure and the dynamics of the turbulence outside the roughness sublayer remain virtually unchanged, regardless of the nature of the wall. The detached eddies of the core region only depend on the mean shear, which is not modified beyond the roughness sublayer by the wall disturbances. On the other hand, the large scales that are correlated across the whole channel scale with ULOG = uτ K-1 log (Reτ), both in smooth- and in rough-walled flows. This velocity scale can be interpreted as a measure of the velocity difference across the log layer, and it is used to modify the scaling proposed and validated by del Álamo et al. (J. Fluid Mech., vol. 500, 2004, p. 135) for smooth-walled flows. © 2006 Cambridge University Press.

Description: The objective of the study is first to examine the optimal transient growth of Görtler-Hämmerlin perturbations in swept Hiemenz flow. This configuration constitutes a model of the flow in the attachment-line boundary layer at the leading-edge of swept wings. The optimal blowing and suction at the wall which minimizes the energy of the optimal perturbations is then determined. An adjoint-based optimization procedure applicable to both problems is devised, which relies on the maximization or minimization of a suitable objective functional. The variational analysis is carried out in the framework of the set of linear partial differential equations governing the chordwise and wall-normal velocity fluctuations. Energy amplifications of up to three orders of magnitude are achieved at low spanwise wavenumbers(k ∼ 2000) and large sweep Reynolds number (Re ∼ 2000) Optimal perturbations consist of spanwise travelling chordwise vortices, with a vorticity distribution which is inclined against the sweep. Transient growth arises from the tilting of the vorticity distribution by the spanwise shear via a two-dimensional Orr mechanism acting in the basic flow dividing plane. Two distinct regimes have been identified: for k ≲ 0.25, vortex dipoles are formed which induce large spanwise perturbation velocities; for k ≲ 0.25, dipoles are not observed and only the Orr mechanism remains active. The optimal wall blowing control yields for instance an 80% decrease of the maximum perturbation kinetic energy reached by optimal disturbances at Re = 550 and k = 0.25 The optimal wall blowing pattern consists of spanwise travelling waves which follow the naturally occurring vortices and qualitatively act in the same manner as a more simple constant gain feedback control strategy. © 2006 Cambridge University Press.

Description: The existence and magnitude of slip velocities between deionized water and a smooth glass surface is studied experimentally. Sub-micron fluorescent particles are suspended in water and imaged using total internal reflection velocimetry (TIRV). For water flowing over a hydrophilic surface, the measurements are in agreement with previous experiments and indicate that slip, if present, is minimal at low shear rates, but increases slightly as the shear rate increases. Surface hydrophobicity is observed to induce a small slip velocity, with the slip length reaching a maximum of 96 nm at a shear rate of 1800 s-1. Issues associated with the experimental technique and the interpretation of results are also discussed. © 2006 Cambridge University Press.

Description: The motion of a spherical particle suspended in gravity-driven film flow down an inclined plane is considered in the limit of vanishing Reynolds and Bond numbers where the free-surface deformation is infinitesimal. Taking advantage of the axially symmetry of the boundaries of the flow with respect to the axis that is normal to the wall and free surface and passes through the particle centre, the problem is formulated as a system of one-dimensional integral equations for the first Fourier coefficients of the unknown traction and velocity along the boundary contours in a meridional plane. It is found that the particle translational velocity scaled by the unperturbed velocity evaluated at the particle centre increases monotonically as the particle approaches the free-surface, whereas the corresponding angular velocity of rotation scaled by the unperturbed vorticity evaluated at the particle centre reaches a maximum at a certain intermediate position. The free-surface velocity vector field and deformation are displayed, the force and torque exerted on a spherical particle adhering to the wall are tabulated, and the associated flow pattern is discussed. © 2006 Cambridge University Press.

Description: A numerical treatment of the natural convection and passive dispersion in symmetrically interconnected tilted layers embedded in a rock which is subject to a constant vertical temperature gradient is presented. Such a system is a faithful model of configurations commonly found in the geophysical context. There, flow movements and temperature distributions are closely connected to phenomena of interest such as transport of contaminants and diagenesis. The important case of large thermal conductivity of the rock compared with that of the material filling the layer is discussed in order to show the decisive role of the temperature distribution and the geometrical parameters on the convective flow. The present analysis treats two cases, the fluid-filled layer and the saturated porous layer. Convective flows were calculated for small Rayleigh numbers and the resulting velocity fields were included in the analysis of the transport of a passive contaminant that was initially located where layers connect with each other. Transport of contaminants in the isotropic porous layer was studied by using a model which includes hydrodynamic dispersion terms. How far the tracer transports through the layers and the rate the tracer enters into the system were analysed. The influence of the angle of tilt has also been included. The molecular diffusive Péclet number which relates convective to diffusive species transport is closely associated to a considerable transporting rate, and for the porous layer the hydrodynamic dispersion appears to be an important effect to consider. © 2006 Cambridge University Press.

Description: The problem of irrotational inviscid incompressible free-surface flow is examined in the limit of small Froude number. Since this is a singular perturbation, singularities in the flow field (or its analytic continuation) such as stagnation points, or corners in submerged objects or on rough beds, lead to a divergent asymptotic expansion, with associated Stokes lines. Recent techniques in exponential asymptotics are employed to observe the switching on of exponentially small gravity waves across these Stokes lines. As a concrete example, the flow over astep is considered. It is found that there are three possible parameter regimes, depending on whether the dimensionless step height is small, of the same order, or large compared to the square of the Froude number. Asymptotic results are derived in each case, and compared with numerical simulations of the full nonlinear problem. The agreement is remarkably good, even at relatively large Froude number. This is in contrast to the alternative analytical theory of small step height, which is accurate only for very small steps. © 2006 Cambridge University Press.

Description: Nonlinear travelling waves that are precursors to laminar-turbulent transition and capture the main structures of the turbulent buffer layer have recently been found to exist in all the canonical parallel flow geometries. We study the effect of polymer additives on these 'exact coherent states' (ECS) in the plane Poiseuille geometry, focusing on Reynolds numbers slightly above transition. Many key aspects of the turbulent drag reduction phenomenon are found, including delay in transition to turbulence, drag reduction onset threshold, and diameter and concentration effects. Furthermore, examination of the ECS existence region leads to a distinct prediction, consistent with experiments, regarding the nature of the maximum drag reduction regime: at sufficiently high wall shear rates, viscoelasticity is found to completely suppress the normal (i.e. treamwise-vortex-dominated) dynamics of the near-wall region, suggesting that the maximum drag reduction regime is dominated by a distinct class of flow structures. © 2006 Cambridge University Press.

Description: The three-dimensional flow transition behind a heated cylinder subjected to a horizontal flow (water is used as the working fluid; Pr ≃ 7 at a Reynolds number Re = 85 and a Richardson number Ri = 1.0 manifests itself in the far wake as escaping mushroom-type structures from the upper vortices. The origin of the escaping mushroom-type structures lies in the generation of streamwise vorticity in the near wake, which is described as a cyclic process. In the presence of a spanwise temperature gradient in the near wake, streamwise vorticity is generated, which results from baroclinic vorticity production. Owing to these streamwise vorticity regions, low-speed flow will move upwards at so-called in-plume positions resulting in high- and low-speed streaks in the upper half of the wake. Next, 'transverse' vorticity is generated by the spanwise gradients in the streamwise velocity component, resulting in counter-rotating vortices directly behind the cylinder. These vortices lead to high- and low-temperature regions in the spanwise direction and the process repeats itself. © 2006 Cambridge University Press.

Description: We study the linear temporal hydrodynamic stability in the Rayleigh-Bénard problem for a compressible fluid (a perfect gas) under marginally super-adiabatic conditions, i.e. when the ambient temperature gradient only slightly exceeds the adiabatic gradient and then only within the fluid adjacent to the upper (cold) wall. The onset of convection in this limit demonstrates some unique features which differ qualitatively from those of the familiar Boussinesq approximation. Thus, the ensuing convection is effectively confined to a narrow domain of the fluid close to the upper wall and is characterized by large wavenumbers. Furthermore, these distinct attributes persist with diminishing temperature difference, implying that the prevailing generalized Boussinesq approximation (based on the use of the potential temperature gradient) is non-uniform in the present limit. This non-uniformity is resolved in terms of the small yet significant variations of fluid properties (which are commonly neglected). We comment on the analogy between the present problem and the Taylor-Couette problem for a viscous incompressible fluid within a narrow gap between counter-rotating cylinders. We briefly discuss the potential relevance of the present limit to some recent observations of the onset of convection within near-critical fluids. © 2006 Cambridge University Press.

Description: A generalization of criticality - called secondary criticality - is introduced and applied to finite-amplitude Stokes waves. The theory shows that secondary criticality signals a bifurcation to a class of steady dark solitary waves which are biasymptotic to a Stokes wave with a phase jump in between, and synchronized with the Stokes wave. We find the that the bifurcation to these new solitary waves - from Stokes gravity waves in shallow water - is pervasive, even at low amplitude. The theory proceeds by generalizing concepts from hydraulics: three additional functionals are introduced which represent non-uniformity and extend the familiar mass flux, total head and flow force, the most important of which is the wave action flux. The theory works because the hydraulic quantities can be related to the governing equations in a precise way using the multi-symplectic Hamiltonian formulation of water waves. In this setting, uniform flows and Stokes waves coupled to a uniform flow are relative equilibria which have an attendant geometric theory using symmetry and conservation laws. A flow is then 'critical' if the relative equilibrium representation is degenerate. By characterizing successively non-uniform flows and unsteady flows as relative equilibria, a generalization of criticality is immediate. Recent results on the local nonlinear behaviour near a degenerate relative equilibrium are used to predict all the qualitative properties of the bifurcating dark solitary waves, including the phase shift. The theory of secondary criticality provides new insight into unsteady waves in shallow water as well. A new interpretation of the Benjamin-Feir instability from the viewpoint of hydraulics, and the connection with the creation of unsteady dark solitary waves, is given in Part 2. © 2006 Cambridge University Press.

Description: We investigate the effect of boundary roughness on the dynamical properties of the flow in laminar Ekman boundary layers. The study considers wavy boundaries having both horizontal wavelength and vertical extent comparable in size with the boundary layer width. In the case of flat boundaries, Ekman layers are known to be active, i.e. to affect significantly the dynamics of the mainstream flow. We show how the layer modelling needs to be modified to account for such wavy boundaries. In particular, nonlinear terms enter the laminar description. This model can be linearized in the limit of small Reynolds numbers. The resulting equations are studied using both asymptotic expansions and full numerical simulations. We find that small-scale roughness significantly alters energy dissipation in the boundary layer. This can result in either a reduction or an increase of dissipation, depending on, in particular, the orientation of the mainstream flow with respect to boundary modulation. Agreement is obtained between theoretical and computational results. © 2006 Cambridge University Press.

Description: We consider the direct numerical simulation (DNS) of a homogeneously turbulent flow in combination with a premixed flame. The combustion takes place in the flamelet regime which means that combustion occurs in a very thin layer, called the flame front. The position of the flame front is modelled by means of the G-equation, in which the flame front is represented by an isosurface G0 of a scalar field G(x, t). The flow is described by the Navier-Stokes equations in the low-Mach-number limit, which allows for the inclusion of gas expansion due to the temperature increase by the combustion. The advantage of the low-Mach-number approximation is that efficient numerical methods, used for incompressible flows, can be applied to solve the discretized equations. The calculations are carried out in a box with homogeneous isotropic turbulence. In addition, a uniform mean velocity is imposed with a inflow boundary condition at x = 0. The inflow velocity is adjusted such that the mean position of the flame is stabilized at a fixed position. This allows us to use time averaging to obtain accurate statistics, which are very difficult to obtain when the flame is allowed to propagate. In the y- and z-direction, periodic boundary conditions are applied. The numerical code has been checked with a well-known theoretical result, the so-called Darrieus-Landau instability of a thin flame front. The results show a good agreement between the computed growth rate and the theoretical value when the thickness of the flame front is much smaller than the wavelength of the disturbance. When this condition is not met, the growth rate becomes lower than the theory in agreement with the restriction under which the theory is valid. For the computations in homogeneous turbulence, the results show an increase in the turbulent flame speed with increasing turbulent intensity at the position of the flame front. This is in good agreement with experimental data and theory. The turbulent flame speed shows also an increase as a function of the heat release parameter. This is because disturbances on the flame front, induced by the turbulence, are enhanced by the Darrieus-Landau instability. The budgets of the turbulent kinetic energy and the enstrophy show that the expansion of the gas across the flame front suppresses the turbulence. At higher expansion rates, turbulence in the direction of the mean velocity increases and as a result turbulence becomes strongly anisotropic. The increase is due to two processes. The first is the influence of the Darrieus-Landau instability already mentioned. The second is the baroclinic production of vorticity owing to the flame front density and pressure gradients not being aligned. © 2006 Cambridge University Press.

Description: The stability of fluid through a channel subject to a system rotation of constant rate about the spanwise axis is considered. In contrast to previous studies, a strongly nonlinear bifurcation approach is used to solve for a family of two-dimensional, steady, streamwise-orientated vortex flows. A stability analysis of these flows is also performed. All of the two-dimensional flows considered lose stability to an Eckhaus (streamwise-independent) secondary disturbance in a steady bifurcation to another member of the solution set. This property, given also that lower-order primary and secondary disturbance modes can become unstable, leads to a rich structure of bifurcation relationships between the secondary flows. With increasing Reynolds number, the secondary flow arising from the linear critical point first loses stability to the Eckhaus instability, and then loses stability to a fundamental spanwise mode with small streamwise wavenumber. With a further increase in Reynolds number, the secondary flow then also becomes unstable to a disturbance of subharmonic spanwise and O(1) streamwise wavenumber, and finally (on the upper solution branch) a disturbance of fundamental spanwise and O(1) streamwise wavenumber. Other types of bifurcation for possible tertiary flows are also identified. By superimposing the secondary disturbance onto the secondary flow, visualizations of the possible structure of the bifurcating tertiary flows are obtained. The visualizations show low-speed streaks in the streamwise velocity component lying between a set of staggered vortices for superharmonic bifurcations, and between aligned vortices for subharmonic bifurcations. Excellent qualitative and quantitative agreement is found with previous experimental results and direct-numerical-simulation-based stability studies, and good overall agreement with previous DNS studies was also found. © 2006 Cambridge University Press.

Description: Batchelor (Phys. Fluids, vol. 12, 1969, p. 233) developed a theory of two-dimensional turbulence based on the assumption that the dissipation of enstrophy (mean-square vorticity) tends to a finite non-zero constant in the limit of infinite Reynolds number Re. Here, by assuming power-law spectra, including the one predicted by Batchelor's theory, we prove that the maximum dissipation of enstrophy is in fact zero in this limit. Specifically, as → ∞, the dissipation approaches zero no slower than (In Re)-1/2. The physical reason behind this result is that the decrease of viscosity enhances the production of both palinstrophy (mean-square vorticity gradients) and its dissipation - but in such a way that the net growth of palinstrophy is less rapid than the decrease of viscosity, resulting in vanishing enstrophy dissipation. This result generalizes to a rich class of quasi-geostrophic models as well as to the case of a passive tracer in layerwise-two-dimensional turbulent flows having bounded enstrophy. © 2006 Cambridge University Press.

Description: Unstable modes of a linear shear flow in shallow water on an equatorial β-plane obtained over a wide range of values of a non-dimensional parameter and are interpreted in terms of resonance between neutral waves. The non-dimensional parameter in the system is E≡γ4/(gHβ2), where γ, g, H and β are the meridional shear of basic zonal flow, gravitational constant, equivalent depth and the north-south gradient of the Coriolis parameter, respectively. The value of E is varied within the range -2.50 ≤ log E ≤ 7.50. The problem is solved numerically in a channel of width 5γ/β. The structures of the most unstable modes, and the combinations of resonating neutral waves that cause the instability, change according to the value of E as follows. For log E 〈 2.00, the most unstable modes have zonally non-symmetric structures; the most unstable modes for log E 〈 1.00 are caused by resonance between equatorial Kelvin modes and continuous modes, and those for 1.00 ≤ log E 〈 2.00 are caused by resonance between equatorial Kelvin modes and westward mixed Rossby-gravity modes. The most unstable modes for log E ≥ 2.00 have symmetric structures and are identical with inertially unstable modes. Examinations of dispersion curves suggest that non-symmetric unstable modes for and 1.00 ≤ log E 〈 2.00 and inertially unstable modes for E ≥ 2.00 are the same kind of instability. © 2006 Cambridge University Press.

Description: Film drainage between two drops with viscosity equal to that of the matrix fluid is studied using a numerical method that can capture both the external problem of two touching drops and the inner problem of pressure-driven local film drainage, without assumptions about the dimensions of the film or the use of lubrication approximations. We use a non-singular boundary integral method that has sufficient stability and accuracy to simulate film thicknesses down to and smaller than 10-4 times the undeformed drop radius. After validation of the method we investigate the validity of various results obtained from simple film-drainage models and asymptotic theories. Our results for buoyancy-driven collisions are in agreement with a recently developed asymptotic theory. External-flow-driven collisions are different from buoyancy-driven collisions, which means that the internal circulation inside the drop plays a significant role in film drainage, even for small capillary numbers, as has been recently shown (Nemer et al, Phys. Rev. Lett, vol. 92, 2004, 114501). Despite that, we find excellent correspondence with simple drainage models when considering the drainage time only. © 2006 Cambridge University Press.

Description: In this work we study the classical Landau-Levich problem of dip-coating. While in the clean interface case and in the limit of low capillary numbers it admits an asymptotic solution, its full study has not been conducted. With the help of an efficient numerical algorithm, based on a boundary-integral formulation and the appropriate set of interfacial and inflow boundary conditions, we first study the film thickness behaviour for a clean interface problem. Next, the same algorithm allows us to investigate the response of this system to the presence of soluble surface active matter, which leads to clarification of its role in the flow dynamics. The main conclusion is that pure hydrodynamical modelling of surfactant effects predicts film thinning and therefore is not sufficient to explain the film thickening observed in many experiments. © 2006 Cambridge University Press.

Description: Numerical simulations are used to investigate the effect of aspect ratio on the wake topology and hydrodynamic performance of thin ellipsoidal flapping foils. The study is motivated by the quest to understand the hydrodynamics of fish pectoral fins. The simulations employ an immersed boundary method that allows us to simulate flows with complex moving boundaries on fixed Cartesian grids. A detailed analysis of the vortex topology shows that the wake of low-aspect-ratio flapping foils is dominated by two sets of interconnected vortex loops that evolve into distinct vortex rings as they convect downstream. The flow downstream of these flapping foils is characterized by two oblique jets and the implications of this characteristic on the hydrodynamic performance are examined. Simulations are also used to examine the thrust and propulsive efficiency of these foils over a range of Strouhal and Reynolds numbers as well as pitch-bias angles. © 2006 Cambridge University Press.

Description: Experiments are described which measured concentration fields in liquid-phase strong transverse jets over the Reynolds-number range 1.0 × 103 ≤ Re j ≤ 20 × 103. Laser-induced-fluorescence measurements were made of the jet-fluid-concentration fields at a jet-to-freestream velocity ratio of Vr = 10. The concentration-field data for far-field (x/dj = 50) slices of the jet show that turbulent mixing in the transverse jet is Reynolds-number dependent over the range investigated, with a scalar-field PDF that evolves with Reynolds number. A growing peak in the PDF, indicating enhanced spatial homogenization of the jet-fluid concentration field, is found with increasing Reynolds number. Comparisons between transverse jets and jets discharging into quiescent reservoirs show that the transverse jet is an efficient mixer in that it entrains more fluid than the ordinary jet, yet is able to effectively mix and homogenize the additional entrained fluid. Analysis of the structure of the scalar field using distributions of scalar increments shows evidence for well-mixed plateaux separated by sharp cliffs in the jet-fluid concentration field, as previously shown in other flows. Furthermore, the scalar field is found to be anisotropic, even at small length scales. Evidence for local anisotropy is seen in the scalar power spectra, scalar microscales, and PDFs of scalar increments in different directions. The scalar-field anisotropy is shown to be correlated to the vortex-induced large-scale strain field of the transverse jet. These experiments add to the existing evidence that the large and small scales of high-Schmidt-number turbulent mixing flows can be linked, with attendant consequences for the universality of small scales of the scalar field for Reynolds numbers up to at least Re = 20 × 104. © 2006 Cambridge University Press.

Description: The injection of liquid from a central well into a partially saturated aquifer of finite thickness is described using similarity solutions. The solutions illustrate that injection leads to a growing zone around the source in which the fluid fills the whole depth of the aquifer. Beyond this zone, the current adjusts to the depth in the far field as the fluid slumps under gravity. The radial extent of the zone in which the aquifer is fully flooded depends on the ratio of the buoyancy-driven flow speed to the pressure-driven flow speed associated with the injection. New laboratory experiments, using a model porous medium, support the model predictions in the case of an initially unsaturated layer. The analysis is then developed to allow for a fully saturated aquifer, containing fluid of lower density than the injectate, and a further class of similarity solutions is developed. Again, these are shown to be consistent with new laboratory experiments. In concluding, we briefly consider how the results may be combined, to explore the self-similar dynamics of a relatively dense fluid injected into an aquifer which is partially saturated with fluid of smaller density. © 2006 Cambridge University Press.

Description: We perform direct numerical simulations (DNS) of homogeneous turbulence subject to periodic shear - S = Smax sin(ωt), where ω is the forcing frequency and Smax is the maximum shear. The lattice Boltzmann method (LBM) is employed in our simulations and a periodic body force is introduced to produce the required shear. We find that the turbulence behaviour is a strong function of the forcing frequency. There exists a critical frequency - ωcr/Smax ≈0.5 - at which the observed behaviour bifurcates. At lower forcing frequencies (ω 〈 ωcr), turbulence is sustained and the kinetic energy grows. At higher frequencies, the kinetic energy decays. It is shown that the phase difference between the applied strain and the Reynolds stress decreases monotonically from π in the constant shear case to π/2, in very high frequency shear cases. As a result, the net turbulence production per cycle decreases with increasing frequency. In fact, at ω/Smax ≥ 10, decaying isotropic turbulence results are recovered. The frequency-dependence of anisotropy and Reynolds stress budget are also investigated in detail. It is shown that inviscid rapid distortion theory (RDT) does not capture the observed features: it predicts purely oscillatory behaviour at all forcing frequencies. Second moment closure models do predict growth at low frequencies and decay at high frequencies, but the critical frequency value is underestimated. The challenges posed by this flow to turbulence closure modelling are identified. © 2006 Cambridge University Press.

Description: The problem of low-gravity isothermal capillary flow along interior corners that are rounded is revisited analytically in this work. By careful selection of geometric length scales and through the introduction of a new geometric scaling parameter ̄Tc, the Navier-Stokes equation is reduced to a convenient ∼ O(1) form for both analytic and numeric solutions for all values of corner half-angle α and corner roundedness ratio γ for perfectly wetting fluids. The scaling and analysis of the problem captures much of the intricate geometric dependence of the viscous resistance and significantly reduces the reliance on numerical data compared with several previous solution methods and the numerous subsequent studies that cite them. In general, three asymptotic regimes may be identified from the large second-order nonlinear evolution equation: (I) the 'sharp-corner' regime, (II) the narrow-corner 'rectangular section' regime, and (III) the'thin film' regime. Flows are observed to undergo transition between regimes, or they may exist essentially in a single regime depending on the system. Perhaps surprisingly, for the case of imbibition in tubes or pores with rounded interior corners similarity solutions are possible to the full equation, which is readily solved numerically. Approximate analytical solutions are also possible under the constraints of the three regimes, which are clearly identified. The general analysis enables analytic solutions to many rounded-corner flows, and example solutions for steady flows, perturbed infinite columns, and imbibing flows over initially dry and prewetted surfaces are provided. © 2006 Cambridge University Press.

Description: Two parameters are introduced that uniquely characterize the state of a third-order symmetric tensor. We show that the proposed parameters arise from the uniform metric in the matrix space; thus the joint PDF of these parameters can be used to determine the geometrical statistics of any third-order symmetric tensor. We use this joint PDF to describe the states of the subgrid-scale stress, which is of central interest in large-eddy simulation. Direct numerical simulation of forced isotropic turbulence is used in our a priori tests. With the proposed parameterization we can also assess the most probable flow configuration at the scales of motion just above the Kolmogorov scale. We test four different subgrid-scale models in terms of how well they predict the structure, or state, of the subgrid-scale stress. It is found that models based on truncated Taylor series do not produce an adequate distribution of states, even if augmented by a turbulent viscosity term. On the other hand, models based on the scale-similarity assumption predict a distribution of states that is close to actual. © 2006 Cambridge University Press.

Description: We consider the interaction of free-stream disturbances with the leading edge of a body and its effect on the transition point. We present a method which combines an asymptotic receptivity approach, and a numerical method which marches through the Orr-Sommerfeld region. The asymptotic receptivity analysis produces a three-deck eigensolution which in its far downstream limiting form produces an upstream boundary condition for our numerical parabolized stability equation (PSE). We discuss the advantages of this method compared to existing numerical and asymptotic analysis and present results which justify this method for the case of a semi-infinite flat plate, where asymptotic results exist in the Orr-Sommerfeld region. We also discuss the limitations of the PSE and comment on the validity of the upstream boundary conditions. Good agreement is found between the present results and the numerical results of Haddad & Corke (1998). © 2006 Cambridge University Press.

Description: In the present work, we study the transverse vortex-induced vibrations of an elastically mounted rigid cylinder in a fluid flow. We employ a technique to accurately control the structural damping, enabling the system to take on both negative and positive damping. This permits a systematic study of the effects of system mass and damping on the peak vibration response. Previous experiments over the last 30 years indicate a large scatter in peak-amplitude data (A*) versus the product of mass-damping (α), in the so-called 'Griffin plot'. A principal result in the present work is the discovery that the data collapse very well if one takes into account the effect of Reynolds number (Re), as an extra parameter in a modified Griffin plot. Peak amplitudes corresponding to zero damping (A*α=0), for a compilation of experiments over a wide range of (Re)=500-33 000, are very well represented by the functional form A*α=0 = f(Re) = log(0.41 Re0.36. For a given (Re), the amplitude A* appears to be proportional to a function of mass-damping, A* ∝ g (α), which is a similar function over all Re. A good best-fit for a wide range of mass-damping and Reynolds number is thus given by the following simple expression, where A* = g(α) f (Re): A* = (1 - 1.12α + 0.30α2) log (0.41 Re0.36). In essence, by using a renormalized parameter, which we define as the 'modified amplitude', A*M = A*/A*α=0, the previously scattered data collapse very well onto a single curve, g(α), on what we refer to as the 'modified Griffin plot'. There has also been much debate over the last three decades concerning the validity of using the product of mass and damping (such as α) in these problems. Our results indicate that the combined mass-damping parameter (α) does indeed collapse peak-amplitude data well, at a given Re, independent of the precise mass and damping values, for mass ratios down to m* = 1. © 2006 Cambridge University Press.