ALBERT

Description: The continuing improvements in accelerator mass spectrometry (AMS) dating technology mean that it is possible to work on ever smaller samples, which in turn, make an ever wider range of sample potentially available for dating. This paper discusses some of the difficulties arising with the interpretation of AMS dates obtained from carbon in iron. The overriding problem is that the carbon, now in chemical combination with the iron, could have come from a variety of sources with very different origins. These are now potentially an iressolvable mixture in the iron. For iron made over the last millennium, there are the additional problems associated with the use of both fossil fuel and biomass fuel in different stages of the iron making, leading to great confusion, especially with authenticity studies.

Description: The dating of selected archaeological and megafaunal sites from the Australian region is reviewed, with emphasis on recent work at some of the oldest sites. Improved chemical procedures with decreased analytical background for 14C analysis, combined with new luminescence dating methods, has confirmed many of the results processed decades ago and significantly increased the maximum age for some others. The oldest occupation horizons in four different regions reliably dated by defendable multi-method results are in the range 42–48,000 calendar years ago, overlapping with the age range for similarly well-dated undisturbed sites containing the youngest extinct megafauna. There is less secure evidence suggesting some archaeology may be earlier and some megafauna may have survived later than this period.

Description: Radiocarbon (14C) produced by cosmogenic processes in the atmosphere reacts rapidly with atomic oxygen to form 14CO. The primary sink for this species is oxidation by the OH radical, the single most important oxidation mechanism for pollutants in the atmosphere. Hence, knowledge of the spatial and temporal distribution of 14CO allows important inferences to be made about atmospheric transport processes and the distribution of OH. Because the chemical lifetime of 14CO against OH attack is relatively short, 1–3 months, its distribution in the atmosphere should show modulations due to changes in 14C production caused by variations in the solar cycle. In this work we present a simple methodology to provide a time series of global 14C production to help interpret time series of atmospheric 14CO measurements covering the whole of solar cycle 23. We use data from neutron monitors, a readily available proxy for global 14C production, and show that an existing 6-year time series of 14CO data from Baring Head, New Zealand, tracks changes in global 14C production at the onset of solar cycle 23.

Description: Several authors have claimed that radiocarbon dates in the Ancient Near East are too early. Herein, a hypothesis that might explain this is presented. Marine degassing of “old” carbon (i.e. 14C-deficient C), induced by upwelling of old subsurface water, has been observed, in modern times, to cause century-scale 14C ages in the surface atmosphere. A review of the Mediterranean Sea post-ice-age circulation concludes that the subsurface waters became very old, primarily due to millennia-long stagnation. It is hypothesized that as the stagnation ended, subsurface waters were brought towards the surface, where they degassed old carbon. Additionally, Anatolian dendrochronology is shown to not contradict the hypothesis.

Description: Direct dating of a human bone fragment from the Chelechol ra Orrak site (western Micronesia) has yielded one of the earliest dates for Palau thus far. This date compares well with recently collected paleoenvironmental evidence and radiocarbon dates on Babeldaob Island suggesting that settlement of the Palauan archipelago took place much earlier than previously thought.

Description: The paper “The effects of possible contamination on the radiocarbon dates of the Dead Sea Scrolls I: castor oil” by Rasmussen et al. (2001) is discussed. Detailed analysis of the extant dates of the Dead Sea Scrolls suggests that the pretreatment of the samples was adequate. Errors and omissions in the paper are discussed and the implications of the experiment of Rasmussen et al. (2001) are questioned.

Description: Two Vertisol soil profiles under xeric soil moisture regimes, located at Qedma and Akko, Israel, were investigated and compared to a profile under ustic moisture regime, located in Hyderabad, India. Samples were taken in complete successive 2 cm thin layers down to about 180 cm depth or more. Organic and inorganic carbon were analyzed with regard to 13C and 14C concentrations. While all soils have radiocarbon ages of several thousand years BP, the depth distributions reveal substantial differences between the soil carbon dynamics. 14C and, less pronounced, δ13C clearly reflect the pedoturbation process. Further, its strength is found to be related to mainly soil moisture regime, then clay content and land use. In one soil, a change of growing from C4 to C3 crops in the past can be concluded from the δ13C depth distribution.

Description: Consistently large differences occur in the calibrated 14C ages of stratigraphically associated shell and charcoal samples from Kilometer 4, an Archaic Period archaeological site located on the extreme south coast of Peru. A series of nine shell and charcoal samples were collected from a Late Archaic Period (~6000–4000 BP) sector of the site. After calibration, the intercepts of the charcoal dates were ~100–750 years older than the paired shell samples. Due to the hyper-arid conditions in this region that promote long-term preservation of organic material, we argue that the older charcoal dates are best explained by people using old wood for fuel during the Middle Holocene. Given this “old wood” problem, marine shell may actually be preferable to wood charcoal for dating archaeological sites in coastal desert environments as in southern Peru and Northern Chile.

Description: In this report we present dating of archaeological samples performed since 1995 in the Radiocarbon Laboratory of the Rudjer Bošković Institute. Several samples from the period before 1995, which were not included in our previous data lists, are given as well. Sample preparation, proportional counter technique, and processing of data are essentially the same as reported earlier (Srdoč et al. 1971, 1979; Obelić 1989). The quality assurance and quality control system according to ISO 17025 has been improved within the IAEA TC Regional Project on Quality Control and Quality Assurance for Nuclear Analytical Techniques. The laboratory participated in 14C intercomparison studies (Horvatinčić et al. 1990; Krajcar Bronić et al. 1995; Bryant et al. 2001; Radiocarbon 2001). We use Oxalic Acid I as modern standard, and anthracite and marble as background standards.

Description: We discus here the prehistoric settlement of the central-south Brazilian coast, and, more specifically, 1 old radiocarbon date obtained for a costal shellmound, as well as its implications concerning the chronology attributed to the settlement process. The accelerator mass spectrometry (AMS) technique was used to determine the 14C age of charcoal from a shellmound on the southern coast of Rio de Janeiro. The resulting age was 7860 ± 80 BP, an unexpected result that reinforces 2 similar previously obtained dates for the same region. Brazilian archaeologists, however, have questioned those 2 dates, because they would predate by some 2000 yr the antiquity consensually accepted for the settlement of the central-south Brazilian littoral.

Description: The extent of business support for democracy in Latin America may determine whether the transitions from authoritarian rule since the 1980s lead to the consolidation of democratic regimes. While some analysts see business support for democracy as firm, others regard its as precarious, and still others believe it is contingent upon whether elected governments contest the economic interests or political dominance of the bourgeoisie. The difficulty of generalizing about the political orientations of business underscores the need for well-documented case studies. This study focuses on Peru, a country where democracy has been at grave risk, and it offers insight into the underlying forces and conditions that determine whether business supports or undermines democratic governance.

Description: In “The Allocation of Resources by Interest Groups: Lobbying, Litigation and Administrative Regulation,” (hereafter referred to as LLAR), John and Rui de Figueiredo make an important contribution to our understanding of how interest groups choose between lobbying and litigation strategies in the regulation game. Their work demonstrates the value of formally modeling the regulation game by distinguishing between lobbying and litigation. Drawing upon my own related work, in this brief comment I will focus upon some of the implications of formal models of lobbying and litigation for our understanding of how regulatory incentives are affected by judicial review and alternative statutory regimes. I hope to atleast suggest that in addition to illuminating many crucial issues in political science—such as the theory of lobbying and theories of political disadvantage—the sort of approach taken by the de Figueiredos has great significance for the analysis of some fundamental issues in administrative law and public law more generally.

Description: This paper examines the relationship between interest group membership, source of authorization, and meeting closure in federal advisory committees. Using observed correlations, it seeks to identify the ultimate source of “inappropriate influence” wielded in advisory committees. The worst offenders in recent years are those committees created jointly by Congress and agencies, and especially certain committees in USDA and DOC. However, these findings may be unique to the last six years, during which the number of closed meetings has tripled.

Description: The Center for Responsive Politics reports that U.S. lobbying expenditures substantially exceed interest group campaign contributions, without including the lobbying that is not required to be reported to the government. Although it has grown in Europe, particularly with respect to the European Union, lobbying is less important than in the United States. Bennedsen and Feldmann (BF) provide an important and insightful explanation for the difference in terms of the institutional structure of governments. They present a model of informational lobbying in client politics where an interest group provides information to a majority-rule (three-member) legislature. The legislature chooses the scale of a program whose benefits can be distributed among legislative districts. The legislative agenda setter has a vote buying problem and allocates benefits to one other legislator to obtain her vote. BF compare legislatures operating with and without a confidence procedure that allows the agenda setter to tie passage of its proposal the continuation of the government. This commentary considers the method for comparing these two institutions, assesses the implications of the theory, and considers future research related to the theory.

Description: This article examines the roles of multinational corporations and the European Union (EU) in structuring global competition around wireless standardization. It analyzes the realities of global competition in information and communications technology (ICT) markets from a more liberal-strategic viewpoint than the subsidy-based industry support promulgated by strategic trade theorists in the 1980s and 1990s. According to a liberal-strategic trade perspective, public actors try to tweak the rules of the world economy to structure global competition in ways that enhance job creation, overall competitiveness in high-technology sectors, and domestic welfare, rather than being primarily concerned about import competition. The story of the European approach to global standardization and competition—and the strategic use of international standards bodies by multinational corporations—primarily represents an aggressive outward-oriented strategy. European actors pursued a globally oriented strategy in the European Telecommunications Standards Institute (ETSI) with the objective of aligning Europe with market and policy developments in the rapidly growing markets of the Asia—Pacific region. By downplaying the importance of import competition, often stressed by strategic trade theorists a liberal-strategic approach to the ICT industry focuses on the prospect of cutting-edge innovations based on a coherent industry strategy that looks at the creation of internationally competitive technologies in the longer-term rather than at incremental change and current import competition pressure.

Description: eBay provides an online auction venue for remote and anonymous members of its online community to realize gains from trade. As a venue it never sees the items sold, verifies the item listings, handles settlements, or represents the buyer or seller. Despite the associated market imperfections and incentive problems, over five million auctions are active on an average day. Trading is based on trust among members of the eBay community, and trust is supported by a multilateral reputation mechanism based on member feedback. eBay supplements the reputation mechanism with rules and policies that mitigate incentive problems, reduce transactions costs, and support trust among members and between members and the company. Reputations and the rules and policies provide a private ordering of eBay's community. This paper examines this private ordering in the context of the company's strategy and in the shadow of the public order.

Description: This paper analyzes how the structure of the legislature affects interest groups' incentives to lobby. Lobbying is modelled as the strategic provision of information by an interest group to a multi-member legislature, and the effectiveness of lobbying lies in the ability of information to change the winning policy coalitions. We show that with a long enough time horizon for policymakers, the distinguishing feature between the U.S. Congress and European parliamentary systems—the vote of confidence procedure—reduces an agenda setter's willingness to change policy coalitions, and thus significantly lowers the incentives for interest group lobbying.

Description: The vast majority of papers written about interest groups' political influence focuses on the role of money in politics. Business and interest groups' participation in campaign finance, in the form of hard and soft money, has been the subject of hundreds of theoretical and empirical studies. Moreover, with the recent congressional moves to reform campaign finance laws, campaign finance studies have received a prominent position in public discourse.

Description: I wish to point out some statistical errors in the uncertainty calculations for the radiocarbon age of the Vinland Map (Donahue et al. 2002). First, the authors state that the 14C measurements of samples with chemical treatments B, C, D, and E “… result in consistent radiocarbon ages”, and that “… two (of these analyses) are 1.5 sigma, and the others are one sigma or less, from the weighted average.” In fact, the overall scatter of the data show that it is improbable that these data are statistically coherent, largely because analysis J21C lies 2.1 sigma from the weighted mean.

Description: We have observed A14C concentrations in the northern hemisphere temperate region in the bomb pulse period, using cross-dated tree ring samples. The tree-ring samples were taken from one 70-year-old and two 50-year-old red pines (Pinus densiflora) on Mt Chiak, Korea and from a 50-year-old red pine (Pinus densiflora) on Mt Kyeryong, Korea. Twenty-two tree-ring samples from four red pines ranging from 1950 to 2000 AD were pretreated to obtain holo-cellulose, combusted to CO2 by an element analyzer (EA) and converted to graphite for δ14C measurement using the accelerator mass spectrometry (AMS) facility at Seoul National University. Our results for δ14C showed good agreement with those measured by other researchers at similar latitudes. The observed steady decrease of δ14C from 1965 to 2000 AD is described by a single exponential function with a lifetime τ = 15.99 ± 0.43 yr. This lifetime is similar to that of the high-latitude region in Europe.

Description: In order to estimate the apparent age of seawater (R) and the corresponding local offset from the global marine radiocarbon calibration curve (δR) on the far south coast of Peru for 2 periods in the past, 6 pairs of associated marine shell and unburned wood samples from archaeological excavations at Loreto Viejo were 14C dated. Three pairs from about cal AD 1280–1380 indicated larger and more variable δR estimates than have been obtained for other periods in nearby regions, suggesting that δR may vary considerably over space and/or time. Three pairs from about 1870–1680 cal BC yielded consistent shell dates, but only one reasonable terrestrial date and δR estimate, probably due to stratigraphic mixing in antiquity. The one early δR estimate falls slightly outside the range of the later ones, suggesting either still greater spatial variability in δR, or some temporal variability.

Description: Fewer than 20 radiocarbon dates have been obtained from Paleolithic sites on the Korean Peninsula. It is still unknown how and when Korean Middle Paleolithic stone industries developed, despite the handful of dates older than 40,000 BP obtained from some sites. A lower boundary for the Korean Upper Paleolithic of approximately 30,000 BP can be inferred from the few dates associated with stone blade industries. 14C dates associated with microlithic industries of 24,000 BP are considered too old in light of evidence from other areas of East Asia. Most such assemblages are post-Last Glacial Maximum in age. Improved understanding of the Korean Paleolithic sequence will depend ultimately on the further accumulation of 14C dates, as well as the application of alternative dating techniques and attention to the reconstruction of site formation process.

Description: The authors study the radiocarbon dating of a relic believed to be the tablet that was placed on the cross of Jesus Christ at the time of his crucifixion.

Description: This study presents the results of carbon and nitrogen isotopic analyses of six human skeletons excavated from the Tochibara rockshelter (Nagano, Japan). The human skeletons were reported to be accompanied by “Oshigata-mon” type pottery dating to the Earliest Jomon period (8900 BP ≃ 6600 BP). A radiocarbon determination from charcoal associated with the human remains was reported to be 8650 ± 180 BP (GaK-1056). However, the depositional context of human skeletons was uncertain because they were recovered by excavations that were dug by prescribed levels. Our results indicated that these skeletons date to the Earliest Jomon period; the 14C determinations place these remains between 8260 ± 100 BP (TERRA-b030799ab38) and 8580 ± 100 BP (TERRA-b011300a35). This coincides with the archaeological evidence that these specimens are some of the oldest Jomon skeletal materials. Furthermore, δ13C and δ15N values provide evidence for the first reconstruction of the diet of an inland Earliest Jomon population. Although the distribution of data indicated a possibility that they had exploited small amounts of seafood, the isotopic data point to this group having relied heavily on a terrestrial ecosystem based on C3 plants.

Description: The first meeting of the IntCal04 working group took place at Queen's University Belfast from April 15 to 17, 2002. The participants are listed as co-authors of this report. The meeting considered criteria for the acceptance of data into the next official calibration dataset, the importance of including reliable estimates of uncertainty in both the radiocarbon ages and the cal ages, and potential methods for combining datasets. This preliminary report summarizes the criteria that were discussed, but does not yet give specific recommendations for inclusion or exclusion of individual datasets.

Description: Existing data and theory do not support a recent assertion that upwelling of old carbon has led to systematically 100–300 yr too old radiocarbon ages for the Mediterranean region. Similarly, the prehistoric tree-ring record produced over 3 decades by the Aegean Dendrochronology Project is shown to provide robust, well-replicated data, contrary to a recent unfounded assertion. 14C and dendrochronology provide an accurate and precise chronometric framework for the Mediterranean region.

Description: Geomorphology, clay mineral composition, and radiocarbon dates from Muttukadu to Marakkanam estuaries and the tidal zone along the east coast of Tamil Nadu, India, have been used to reconstruct coastal evolution between approximately 4500 and 1100 B P. Formation of alternate oyster beds with intervening tidal clay units indicate fluctuation in the sea level may be a consequence of changes in the Mid-Holocene sedimentation pattern and coastal configuration. 14C dates from Muttukadu indicate a rapid relative sea-level rise (RSL) subsequent to 3500 BP and tidal flat sedimentation between 3475 and 3145 BP. Marine conditions along the east coast area returned around 1900 B P. Comparison of dates with other sites, e.g. Muttukadu, Mammallapuram, and Marakkanam, points toward short removal of marine conditions, ample sediment supplies in the tidal zones, and neotectonic activity. Reactivation of the north–south trending fault line occurred not earlier than approximately 1050 B P. Our study indicates that Middle to Late Holocene coastal sedimentation and the chronology of the tidal zone formation have been strongly influenced by local factors. These have provided considerable scope for internal reorganization with changing coastal processes.

Description: The discovery of the Iwajuku site in Japan is the beginning of the study of the first Paleolithic cultures in the region. In this paper we examine the timing of the earliest colonization of southern Japan, especially focusing on the areas of Kyushu, Shikoku, and the Ryukyu archipelago. Osteological studies have proposed the ultimate origin of these western Japanese Paleolithic populations in Southeast Asia. If this hypothesis is correct, Native Americans may be remotely related to the populations of this region. Greater attention to data from areas such as Japan is necessary to understand the timing and nature of New World colonization.

Description: Ever since José de Acosta's prescient speculation, in 1590, that Native Americans were descended from “savage hunters” who had followed game animals across a land bridge from northeastern Asia into northwestern America (Acosta 1604), most serious scholars have assumed that this was the migration route. The main point of dispute has been the date when the ancestral Asians made the crossing. After many nineteenth-century claims of the discovery of stone tools or bones of “early man” failed to withstand scientific scrutiny, a conservative reaction set in, embodied by the hyper-skeptical Aleš Hrdlička of the Smithsonian Institution. Hrdlička dismissed all claims of a human presence in the Americas prior to about 5000 years ago.

Description: The process of calibrating radiocarbon determinations onto the calendar scale requires the setting of a specific statistical model for the calibration curve. This model specification will bear fundamental importance for the resulting inference regarding the parameter of interest—namely, in general, the calendar age associated to the sample that has been 14C-dated.Traditionally, the 14C calibration curve has been modelled simply as the piece-wise linear curve joining the (internationally agreed) high-precision calibration data points; or, less frequently, by proposing spline functions in order to obtain a smoother curve.We present a model for the 14C calibration curve which, based on specific characteristics of the dating method, yields a piece-wise linear curve, but one which rather than interpolating the data points, smooths them. We show that with this specific model if a piece-wise linear curve is desired, an underlying random walk model is implied as covariance structure (and vice versa). Furthermore, by making use of all the information provided by the calibration data in a comprehensive way, we achieve an improvement over current models by getting more realistic variance values for the calibration curve.

Description: We have constructed a detailed chronological description of soil formation and its environments with data obtained on radiocarbon ages, palynology, and pedology of the Holocene buried soils in the forest steppe of western and central Siberia. We studied a number of Holocene sections, which were located in different geomorphic situations. Radiocarbon dating of materials from several soil horizons, including soil organic matter (SOM), wood, peat, charcoal, and carbonates, revealed three climatic periods and five stages of soil formation in the second part of the Holocene. 14C ages of approximately 6355 BP, 6020 BP, and 5930 BP showed that the longest and most active stage is associated with the Holocene Climatic Optimum, when dark-grey soils were formed in the forest environment. The conditions of birch forest steppe favored formation of chernozem and associated meadow-chernozem and meadow soils. Subboreal time includes two stages of soil formation corresponding to lake regressions, which were less intense than those of the Holocene Optimum. The soils of that time are chernozem, grassland-chernozem, and saline types, interbedded with thin peat layers 14C dated to around 4555 B P, 4240 BP and 3480 BP, and 3170 B P. Subatlantic time includes two poorly developed hydromorphic paleosols formed within inshore parts of lakes and chernozem-type automorphic paleosol. The older horizon was formed during approximately 2500–1770 BP, and the younger one during approximately 1640–400 B P. The buried soils of the Subatlantic time period also attest to short episodes of lake regression. The climate changes show an evident trend: in the second part of the Atlantic time period it was warmer and drier than at present, and in the Subboreal and Subatlantic time periods the climate was cool and humid.

Description: We present here the results of dating of 80 archaeological and paleoenvironmental samples from Argentina and Uruguay, processed between 1986 and 1988 by M A Albero and M A Gonzalez. Series of samples and single samples are grouped by province and then by locality or archeological site, from north to south. See sample location maps for details.Procedures for sample pretreatment, counting, statistical analysis, and age calculation were essentially the same as previously described by Albero and Angiolini (1985). Results are reported as conventional 14C dates in years before AD 1950. They are corrected for isotopic fractionation. 14C contents of some paleoenvironmental samples are expressed in percent modern carbon (pMC).

Description: Soil organic matter (SOM), leaf litter, and root material of an Ultisol from the tropical rainforest of Kakamega, Kenya, were analyzed for stable carbon (δ13C) and nitrogen (δ15N) isotopic values as well as total organic carbon (TOC) and total nitrogen (TN) contents in order to determine trends in SOM decomposition within a very well-developed soil under tropical conditions. In addition, we quantified mineralogy and chemistry of the inorganic soil fraction. Clay mineralogical variation with depth was small and the abundance of kaolin indicates intense weathering and pedoturbation under humid tropical conditions. The soil chemistry was dominated by silica, aluminium, and iron with calcium, potassium, and magnesium as minor constituents. The relative depletion of base cations compared with silica and aluminium is an indicator for intense weathering and leaching conditions over long periods of time. Depth profiles of δ13C and δ15N showed a distinct enrichment trend down profile with a large (average 13δC = 5.0 and average 15δN= 6.3) and abrupt offset within the uppermost 10–20 cm of the soil. Isotopic enrichment with depth is commonly observed in soil profiles and has been attributed to fractionation during decomposition. However, isotopic offsets within soil profiles that exceed 3 are usually interpreted as a recent change from C4 to C3 dominated vegetation. We argue that the observed isotopic depth profiles along with data from mineralogy and chemistry of the inorganic fraction from the Kakamega Forest soil are a result of intense weathering and high organic matter turnover rates under humid tropical conditions.

Description: Correcting the 14C age of a sample for fractionation is straightforward if the measured carbon was derived entirely from the atmosphere, either directly or through chemical and/or biological reactions that originated with atmospheric carbon. This correction is complicated in the case of gastropods that incorporate carbon from limestone or secondary carbonate (e.g. soil carbonate) during shell formation. The carbon isotopic composition of such gastropod shells is determined by fractionation, as well as mixing of carbon from sources with different isotopic values. Only the component of shell carbonate derived from atmospheric carbon should be corrected for fractionation. In this paper, the author derives a new expression for correcting the measured 14C activity of gastropod shells for fractionation, and describe an iterative approach that allows the corrected 14C activity and the fraction of shell carbonate derived from atmospheric carbon to be determined simultaneously.

Description: Located in the Ziway-Shala Basin of the Main Ethiopian Rift, Lake Langano is part of an asymmetric half-graben, defined by a series of north-northeast-trending faults in the tectonically active zone of the rift. A 15-m deep succession of organic homogeneous muds, silts, bioclastic sands, and pyroclastic layers was cored in 1994. The definition of a certified radiocarbon chronology on these deposits required the indispensable establishment of modern hydrological and geochemical balances. The isotopic contents of the total dissolved inorganic carbon (TDIC) of surface water clearly show the influence of a deep CO2 rising along the main fault crossing the lake basin. The 5.8 pMC disequilibrium existing in 1994 with the atmosphere likely produces the aging of authigenic materials developing at the lake surface. However, with a mean residence time of ~15 years, this apparent 14C aging of Lake Langano water still integrates the 14C produced by the nuclear tests in the 1960s. Reconstructing the natural 14C activity of the lake TDIC allows for the quantification of the deep CO2 influence, and for the correction of AMS-14C datings performed along the core. The correction of the AMS-14C chronology defined on Lake Langano allows for a better understanding of paleohydrological changes at a regional scale for at least the last 12,700 cal BP.

Description: One of the largest sources of uncertainty in radiocarbon dating stems from the sample pretreatment procedures used to minimize contamination. A major source of carbon contamination in charcoal from archaeological sites is humic substances carried by groundwater. Here we present a method, independent of 14C dating itself, to evaluate the effectiveness of the cleaning procedure of charcoal. Raman spectra of mixtures of humic substances (HS) and laboratory prepared charcoal indicate that Raman spectroscopy can be used as a semi-quantitative measure of the amount of humic substances associated with archaeological charcoal. Raman spectral analysis of archaeological charcoal samples subjected to different cleaning regimes supports this contention. Such measurements can provide quality control for charcoal preparation procedures and may assist in the interpretation of carbon-dating results.

Description: Coeval shell and charcoal from Santa Catarina State, Brazil, differ systematically in 14C content, indicating a reservoir effect in marine samples. For modern samples (AD 1939–2000) and archeological samples (2500–1595 BP), the mean 14C age difference between marine and atmospheric carbon is 220 ± 20 years, the marine carbon being older. For three samples dated AD 1939–1944, a distinct reservoir correction of 510 ± 10 years is also observed. The ages of archeological shell samples from Jabuticabeira may be corrected by subtracting 220 years from the apparent 14C ages.

Description: MNCs are increasingly facing global environmental issues demanding coordinated market and non-market strategic responses. The home country institutional context and individual company histories can create divergent pressures on strategy for MNCs based in different countries; however, the location of MNCs in global industries and their participation in ‘global issues arenas’ create issue-level fields within which strategic convergence might also be expected. This paper analyzes the responses of oil MNCs to climate change and finds that local context influenced initial corporate reactions, but that convergent pressures predominate as the issue matures.

Description: This paper uses newly available data from the 1995 Lobby Disclosure Act to assess the argument that PAC contributions are used to gain access to legislators. First, we find a much stronger connection between lobbying and campaign contributions than previous statistical research has revealed—groups that have both a lobbyist and a PAC account for 70 percent of all interest group expenditures and 86 percent of all PAC contributions. Second, we find that groups that engage in relatively large amounts of lobbying-and therefore presumably have a high demand for access—allocate their campaign contributions differently than groups that do not. Groups that emphasize lobbying pay more attention to members' positions of power inside Congress, and less attention to members' electoral circumstances, than other groups. Groups that emphasize lobbying also appear to be more bipartisan and less ideological than other groups, giving more equally to both parties and more broadly across the ideological spectrum.

Description: One of the central concerns about American policy making institutions is the degree to which political outcomes can be influenced by interested parties. While the literature on interest group strategies in particular institutions—legislative, administrative, and legal—is extensive, there is very little scholarship which examines how the interdependencies between institutions affects the strategies of groups. In this paper we examine in a formal theoretical model how the opportunity to litigate administrative rulemaking in the courts affects the lobbying strategies of competing interest groups at the rulemaking stage. Using a resource-based view of group activity, we develop a number of important insights about each stage that cannot be observed by examining each one in isolation. We demonstrate that lobbying effort responds to the ideology of the court, and the responsiveness of the court to resources. In particular, (1) as courts become more biased toward the status quo, interest group lobbying investments become smaller, and may be eliminated all together, (2) as interest groups become wealthier, they spend more on lobbying, and (3) as the responsiveness of courts to resources decreases, the effect it has on lobbying investments depends on the underlying ideology of the court.

Description: Analyses of corporate governance problems in China's state sector have mainly focused on administrative interference from state agencies. So far the influence of Communist party institutions has received little attention. Although the influence of ideology has diminished greatly, the Chinese Communist party continues to monitor and control economic actors at every level of the state sector. This article shows that the institutional structure through which the party executes its monitoring and control functions has a corrosive effect on the day-to-day governance of the vast majority of state enterprises. The party's management structure aggravates the inadequate monitoring of managerial performance, weakens managerial incentives, and amplifies insufficient corporate transparency, thereby allowing state asset managers to carve out informal spheres of autonomy. These spheres of autonomy create opportunities for insider control, economic corruption, and the illicit privatization of state assets. Effective and sustainable privatization and corporate governance reforms in China's state sector will thus require the party to substantially diminish its authority over state sector executives.

Description: The most intractable and protracted transatlantic trade conflict of the last decade was over bananas, which grow neither on the European nor on the North American continent. Our explanation of the conflict emphasizes the determining role of the domestic politics of the EU and the United States. It was driven not only by the extreme divergence of preferences of Brussels' and Washington's domestic constituencies, rooted in the competitive position of competing banana industries, but also, and critically, by the institutional configuration of (agricultural) trade policymaking on either side of the Atlantic. The EU agricultural trade policy process is characterized by a division of labor that favors agricultural over wider trading interests, sectoral segmentation, and sector-specific issue-linkage. The U.S. trade policy process is characterized by the Congress's growing reassertion of its trade policy prerogatives, the growing institutionalization of firms' access to the trade policy bureaucracy, and the growing volume and role of corporate campaign donations. The combined effect of these different policy process traits has been to facilitate the capture of banana trade policy by highly organized, particularistic, and predominantly trading interests. Although neither the WTO nor the transatlantic trading relationship ultimately “slipped” over bananas, the conflict provides scant reason for optimism concerning the future of this relationship or indeed of the multilateral international trading system, at least in as far as the latter depends on good EU-U.S. relations.

Description: This article tests two hypotheses about post-communist business associations. The first predicts weak business associations which are presented with insurmountable collective action problems by the flattened civil society inherited from totalitarianism. According to this hypothesis, no business associations are inherited from the previous regime, and associations are confronted with difficult-to-organize latent groups of large numbers of new small enterprises. The second hypothesis, as proposed by Mancur Olson, predicts strong business associations benefiting from the collective action advantages of the communist economic structure which was composed of small numbers of large enterprises. The hypotheses are tested with case studies of Poland's five most influential business associations. The conclusion is that the flattened civil society hypothesis is best borne out by the evidence. This suggests that, in other countries, political factors, rather than the standard communist economic structure, are more likely to explain the persistence of industrial super lobbies.

Description: When I was a boy, my family would occasionally receive tall baskets filled with fruits, nuts and jams, most often around the holidays. These gifts from business acquaintances of my father were not meant to be inducements for him to break the law; rather, they were little niceties intended to maintain ongoing relationships. Today, when my wife and I are invited to dinner, we usually bring flowers or a bottle of wine as a gift. This is not some crass attempt on our part to ensure that sanitary conditions are maintained during meal preparation; it is only a symbol of our appreciation for the kindness of our hosts. Not for a moment do I believe that we would be ostracized should we go to dinner engagements empty-handed, nor would my father have punished nongivers. As such, these gift exchanges can be seen as epiphenomena: they symbolize underlying relationships, but they do not constitute relationships.

Description: Administrative agencies in the United States have developed highly formalized and complex processes for public participation in rulemaking, especially in areas of social regulation such as the environment and workplace safety and health. This case study considers the significance of participation in formal rulemaking processes by connecting the quality of participation to the strategic possibilities in litigation between private interests and regulatory agencies. Specifically, the strategic possibilities of the leading interest groups engaged in the Occupational Safety and Health Administration's major “Lockout/Tagout” rulemaking illustrate how legal resources are created through the development of evidence and claims in hearings. Written and oral presentations, apparently aimed directly at persuading the agency, indirectly affect agency deliberations by increasing the possibility that courts will constrain agency decisionmaking, thus creating opportunities for negotiated alternatives. The case ultimately serves as a paradigmatic example of how bargaining arises at the micro level of policy systems that are infused with broader legal structures.

Description: Electro-osmotic flow is a convenient mechanism for transporting fluid in microfluidic devices. The flow is generated through the application of an external electric field that acts on the free charges that exist in a thin Debye layer at the channel walls. The charge on the wall is due to the particular chemistry of the solid-fluid interface and can vary along the channel either by design or because of various unavoidable inhomogeneities of the wall material or because of contamination of the wall by chemicals contained in the fluid stream. The channel cross-section could also vary in shape and area. The effect of such variability on the flow through microfluidic channels is of interest in the design of devices that use electro-osmotic flow. The problem of electro-osmotic flow in a straight microfluidic channel of arbitrary cross-sectional geometry and distribution of wall charge is solved in the lubrication approximation, which is justified when the characteristic length scales for axial variation of the wall charge and cross-section are both large compared to a characteristic width of the channel. It is thereby shown that the volume flux of fluid through such a microchannel is a linear function of the applied pressure drop and electric potential drop across it, the coefficients of which may be calculated explicity in terms of the geometry and charge distribution on the wall. These coefficients characterize the 'fluidic resistance' of each segment of a microfluidic network in analogy to the electrical 'resistance' in a microelectronic circuit. A consequence of the axial variation in channel properties is the appearance of an induced pressure gradient and an associated secondary flow that leads to increased Taylor dispersion limiting the resolution of electrophoretic separations. The lubrication theory presented here offers a simple way of calculating the distortion of the flow profile in general geometries and could be useful in studies of dispersion induced by inhomogeneities in microfluidic channels.

Description: We examine a spiralling slender inviscid liquid jet which emerges from a rapidly rotating orifice. The trajectory of this jet is determined using asymptotic methods, and the stability using a multiple scales approach. It is found that the trajectory of the jet becomes more tightly coiled as the Weber number is decreased. Unstable travelling wave modes are found to grow along the jet. The breakup length of the jet is calculated, showing good agreement with experiments.

Description: We present experimental measurements of velocity and temperature fields in horizontal planes crossing a cylindrical Rayleigh-Bénard convection cell in steady rotation about its vertical axis. The range of dimensionless rotation rates Ω is from zero to 5 × 104 for a Rayleigh number R = 3.2 × 108. The corresponding range of convective Rossby numbers is ∞ 〉 Ro 〉 0.06. The patterns of velocity and temperature and the flow statistics characterize three basic flow regimes. For Ro ≫ 1, the flow is dominated by vortex sheets (plumes) typical of turbulent convection without rotation. The flow patterns for Ro ∼ 1 are cyclone-dominated, with anticyclonic vortices rare. As the Rossby number continues to decrease, the number of anticyclonic vortex structures begins to grow but the vorticity PDF in the vicinity of the top boundary layer still shows skewness favouring cyclonic vorticity. Velocity-averaging near the top of the cell suggests the existence of a global circulation pattern for Ro ≫ 1.

Description: This paper discusses a potential-vorticity-conserving approach to modelling nonlinear internal gravity waves in a rotating Boussinesq fluid. The focus of the work is on the pseudo-plane motion (motion in the x, z-plane), for which we present a broad range of numerical results. In this case there are two material coordinates, the density and the y-component of the velocity in the inertial frame of reference, which are related to the x and z displacements of fluid particles relative to a reference configuration. The amount of potential vorticity within a fluid region bounded by isosurfaces of these material coordinates is proportional to the area within this region, and is therefore conserved as well. Two new potentials, defined in terms of the displacements and combining the vorticity and density fields, are introduced as new dependent variables. These potentials entirely govern the dynamics of internal gravity waves for the linearized system when the basic state has uniform potential vorticity. The final system of equations consists of three prognostic equations (for the potential vorticity and the Laplacians of the two potentials) and one diagnostic equation, of Monge-Ampère type, for a third potential. This diagnostic equation arises from the nonlinear definition of potential vorticity. The ellipticity of the Monge-Ampère equation implies both inertial and static stability. In three dimensions, the three potentials form a vector, whose (three-dimensional) Laplacian is equal to the vorticity plus the gradient of the perturbation density. Numerical simulations are carried out using a novel algorithm which directly evolves the potential vorticity, in a Lagrangian manner (following fluid particles), without diffusion. We present results which emphasize the way in which potential vorticity anomalies modify the characteristics of internal gravity waves, e.g. the propagation of internal wave packets, including reflection, refraction, and amplification. We also show how potential vorticity anomalies may generate internal gravity waves, along with the subsequent 'geostrophic adjustment' of the flow to a 'balanced' wave-less state. These examples, and the straightforward extension of the theoretical and numerical approach to three dimensions, point to a direct and accurate means to elucidate the role of potential vorticity in internal gravity wave interactions. As such, this approach may help a better understanding of the observed characteristics of internal gravity waves in the oceans.

Description: In classical WKB theory the only way wave energy density, as a surrogate for wave action density, can increase or decrease along a ray is as a result of the ray focusing or widening. This occurs when the group velocity is divergent. There are particular regions, however, where the wave can resonantly exchange action with another wave mode with approximately the same wavenumbers; a situation known as Landau-Zener transition, mode conversion, linear (adiabatic) resonance, etc. This effect invalidates locally the underlying assumption of WKB theory that no scattering of energy occurs between WKB wave modes. In this paper this effect is investigated theoretically for free long baroclinic Rossby waves in a two-layer planetary geostrophic model of the ocean with a purely zonal topography, here taken as a Gaussian ridge. The waves are excited along the east coast by an unspecified wavemaker at a fixed frequency Ω. In the computation considered, mode conversion is found to occur principally near the ridge's top and on the eastern flank. The predictions of mode conversion theory are tested against the results of direct numerical simulations. This shows excellent agreement, both for the locations of mode conversion points, and for the amplitude of the transmitted and converted WKB wave modes.

Description: The theoretical work reported herein studies the free-surface profile, the flow structure, and the pressure distribution of a finite-amplitude solitary wave on shallow water with uniform vorticity. The kinematic problem for the stream function is formulated employing the vertical coordinate and the free surface as the independent variables of the Poisson equation with variable coefficients that are functions of the Hamiltonian of the rotational solitary wave. The exact solution of the boundary-value kinematic problem for the stream function is derived in the form of a power series complemented by a recurrence relation. The dynamic problems for the Hamiltonian and the free surface are solved globally in the Boussinesq-Rayleigh approximation. To find angles enclosed by the branches of the solution at critical points and points of bifurcation the surface streamline is also treated locally by an exact topological solution. The complete analysis of the four-dimensional Hamiltonian maps presented in §4 specifies critical values of the Froude number and the vorticity for five flow regimes: the emergence of the solitary wave, the flow separation near the bottom, the flow separation near the crest, the critical regime for an instability, and the formation of a limiting configuration. The streamlines of the recirculating flow are obtained as a single-eddy bifurcation that preserves continuity of all derivatives on the boundary streamline. The eddy separated near the crest forms the limiting configuration by blocking the upstream current. The results are compared with weakly nonlinear theory, with numerical simulations and with field observations with satisfactory agreement.

Description: Computational experiments based on direct numerical simulation of wall-bounded flow reveal that turbulence production can be suppressed by the action of a transverse travelling wave. Flow visualizations show that the near-wall flow structure is altered substantially, compared to other turbulence control techniques, leading to a large amount of shear stress reduction (i.e., more than 30%). The travelling wave can be induced by a spanwise force that is confined within the viscous sublayer, it has its maximum at the wall, and decays exponentially away from it. We demonstrate the robustness of this approach, and its application in salt water using arrays of electromagnetic tiles that can produce the required travelling wave excitation. We also study corresponding results from spanwise oscillations using a similar force, which also leads to large drag reduction. Although the turbulence statistics for the two approaches are similar, the near-wall structures appear to be different: in the spanwise oscillatory excitation there is a clear presence of wall-streaks whereas in the travelling wave excitation these streaks have disappeared. From the fundamental point of view, the new finding of this work is that appropriate enhancement of the streamwise vortices leads to weakening of the streak intensity, as measured by the normal vorticity component, and correspondingly substantial suppression of turbulence production. From the practical point of view, our findings provide guidance for designing different surface-based actuation techniques including piezoelectric materials, shape memory alloys, and electro-magnetic tiles.

Description: The effects of thermal modulation with time on the thermocapillary instability of a thin horizontal fluid layer with a deformable free surface are investigated on the basis of linear stability theory. First, a sinusoidal heating with a mean component is applied at the lower wall, corresponding to boundary conditions either in the form of prescribed temperature or heat flux. For finite-wavelength convection the thermal modulation exerts a strong effect, giving rise to a family of looped regions of instability corresponding to alternating synchronous or subharmonic responses. In the case of prescribed heat flux, the critical curve consists of significantly fewer loops than in the case of prescribed temperature. Thermal modulation with moderate modulation amplitude tends to stabilize the mean basic state, and optimal values of frequency and amplitude of modulation are determined to yield maximum stabilization. However, large-amplitude modulation can be destabilizing. A basic state with zero mean is then considered and the critical Marangoni number is obtained as a function of frequency. The effects of modulation are also investigated in the long-wavelength limit. For the case of prescribed temperature, the modulation does not affect the onset of the long-wavelength mode associated with the mean basic state and a purely oscillating basic state is always stable with respect to long-wavelength disturbances. For the case of prescribed heat flux both at the wall and free surface, by contrast, thermal modulation exerts a significant effect on the onset of convection from a mean basic state and long-wavelength convection can occur even for a purely oscillating basic state. The modulation can be stabilizing or destabilizing, depending on the frequency.

Description: We present a mathematical model for the drainage of a surfactant-stabilized foam lamella, including capillary, Marangoni and viscous effects and allowing for diffusion, advection and adsorption of the surfactant molecules. We use the slender geometry of a lamella to formulate the model in the thin-film limit and perform an asymptotic decomposition of the liquid domain into a capillary-static Plateau border, a time-dependent thin film and a transition region between the two. By solving a quasi-steady boundary-value problem in the transition region, we obtain the flux of liquid from the lamella into the Plateau border and thus are able to determine the rate at which the lamella drains. Our method is illustrated initially in the surfactant-free case. Numerical results are presented for three particular parameter regimes of interest when surfactant is present. Both monotonic profiles and those exhibiting a dimple near the Plateau border are found, the latter having been previously observed in experiments. The velocity field may be uniform across the lamella or of parabolic Poiseuille type, with fluid either driven out along the centreline and back along the free surfaces or vice versa. We find that diffusion may be negligible for a typical real surfactant, although this does not lead to a reduction in order because of the inherently diffusive nature of the fluid-surfactant interaction. Finally, we obtain the surprising result that the flux of liquid from the lamella into the Plateau border increases as the lamella thins, approaching infinity at a finite lamella thickness.

Description: A rigid-plastic Cosserat model for slow frictional flow of granular materials, proposed by us in an earlier paper, has been used to analyse plane and cylindrical Couette flow. In this model, the hydrodynamic fields of a classical continuum are supplemented by the couple stress and the intrinsic angular velocity fields. The balance of angular momentum, which is satisfied implicitly in a classical continuum, must be enforced in a Cosserat continuum. As a result, the stress tensor could be asymmetric, and the angular velocity of a material point may differ from half the local vorticity. An important consequence of treating the granular medium as a Cosserat continuum is that it incorporates a material length scale in the model, which is absent in frictional models based on a classical continuum. Further, the Cosserat model allows determination of the velocity fields uniquely in viscometric flows, in contrast to classical frictional models. Experiments on viscometric flows of dense, slowly deforming granular materials indicate that shear is confined to a narrow region, usually a few grain diameters thick, while the remaining material is largely undeformed. This feature is captured by the present model, and the velocity profile predicted for cylindrical Couette flow is in good agreement with reported data. When the walls of the Couette cell are smoother than the granular material, the model predicts that the shear layer thickness is independent of the Couette gap H when the latter is large compared to the grain diameter dp. When the walls are of the same roughness as the granular material, the model predicts that the shear layer thickness varies as (H/dp)1/3 (in the limit H/dp ≫ 1) for plane shear under gravity and cylindrical Couette flow.

Description: A theory is developed for the speed and structure of steady-state non-dissipative gravity currents in rotating channels. The theory is an extension of that of Benjamin (1968) for non-rotating gravity currents, and in a similar way makes use of the steady-state and perfect-fluid (incompressible, inviscid and immiscible) approximations, and supposes the existence of a hydrostatic 'control point' in the current some distance away from the nose. The model allows for fully non-hydrostatic and ageostrophic motion in a control volume V ahead of the control point, with the solution being determined by the requirements, consistent with the perfect-fluid approximation, of energy and momentum conservation in V, as expressed by Bernoulli's theorem and a generalized flow-force balance. The governing parameter in the problem, which expresses the strength of the background rotation, is the ratio W = B/R, where B is the channel width and R = (g′ H)1/2/f is the internal Rossby radius of deformation based on the total depth of the ambient fluid H. Analytic solutions are determined for the particular case of zero front-relative flow within the gravity current. For each value of W there is a unique non-dissipative two-layer solution, and a non-dissipative one-layer solution which is specified by the value of the wall-depth ho. In the two-layer case, the non-dimensional propagation speed c = cf(g′ H)-1/2 increases smoothly from the non-rotating value of 0.5 as W increases, asymptoting to unity for W → ∞. The gravity current separates from the left-hand wall of the channel at W = 0.67 and thereafter has decreasing width. The depth of the current at the right-hand wall, ho, increases, reaching the full depth at W = 1.90, after which point the interface outcrops on both the upper and lower boundaries, with the distance over which the interface slopes being 0.881R. In the one-layer case, the wall-depth based propagation speed Froude number c0 = cf(g′ ho)-1/2 = 21/2, as in the non-rotating one-layer case. The current separates from the left-hand wall of the channel at Wo ≡ B/Ro = 2-1/2, and thereafter has width 2-1/2 Ro, where Ro = (g′ho)1/2 / f is the wall-depth based deformation radius.

Description: Three-dimensional velocity distributions of a turbulent flow in the core region of a square duct at ReH = 1.2 × 105 are measured using holographic particle image velocimetry (HPIV). Spatial filtering of the 136 × 130 × 128 velocity vector maps enables calculation of subgrid-scale (SGS) stresses and parameters based on the filtered velocity gradients, such as the filtered strain-rate tensor and vorticity vector. Probability density functions (p.d.f.) of scalar parameters characterizing eigenvalue structures confirm that the most probable strain-rate topology is axisymmetric extension, and show that the most probable SGS stress state is axisymmetric contraction. Conditional sampling shows that high positive SGS dissipation occurs preferentially in regions with these preferred strain-rate and stress topologies. High negative SGS dissipation (backscatter) occurs preferentially in regions of axisymmetric contracting SGS stress topology, but is not associated with any particular strain-rate topology. The nonlinear model produces the same trends but tends to overpredict the likelihood of the preferred stress state. Joint p.d.f.s of relative angles are used to investigate the alignments of the SGS stress eigenvectors relative to the vorticity and eigenvectors associated with eddy viscosity and similarity/nonlinear models. The results show that the most extensive SGS stress eigenvector is preferentially aligned at 32° to the most contracting strain-rate eigenvector. This alignment trend persists, with some variations in angle and peak probability, during conditional samplings based on the SGS dissipation rate, vorticity and strain-rate magnitudes. The relative alignment of the other two stress and strain-rate eigenvectors has a bimodal behaviour with the most contracting and intermediate stress eigenvectors 'switching places': from being aligned at 32° to the most extensive strain-rate eigenvector to being parallel to the intermediate strain-rate eigenvector. Conditional sampling shows that one of the alignment configurations occurs preferentially in regions of high vorticity magnitude, whereas the other one dominates in regions where the filtered strain-rate tensor has axisymmetric contracting topology. Analysis of DNS data for isotropic turbulence at lower Re shows similar trends. Conversely, the measured stress eigenvectors are preferentially aligned with those of the nonlinear model. This alignment persists in various regions of the flow (high vorticity, specific flow topologies, etc). Furthermore, the alignment between the strain-rate and nonlinear model tensors also exhibits a bimodal behaviour, but the alignment angle of both configurations is 42°. Implications of alignment trends on SGS dissipation are explored and conditions for high backscatter are identified based on the orientation of the stress eigenvectors. Several dynamical and kinematical arguments are presented that may explain some of the observed preferred alignments among tensors. These arguments motivate further analysis of the mixed model, which shows good alignment properties owing to the dominance of the Leonard stress on the alignments. Nevertheless, the data also show that the mixed model produces some unrealistic features in probability distributions of SGS dissipation, and unphysical eigenvector alignments in selected subregions of the flow.

Description: Journal of Fluid Mechanics, vol. 443 (2001), pp. 69–99

Description: It is demonstrated that the growth of the mixing zone generated by Rayleigh-Taylor instability can be greatly retarded by the application of rotation, at least for low Atwood number flows for which the Boussinesq approximation is valid. This result is analysed in terms of the effect of the Coriolis force on the vortex rings that propel the bubbles of fluid in the mixing zone.

Description: In this study, the limiting maximum drag-reduction asymptote for the moment coefficient of a rotating disk in a surfactant solution was obtained analytically. The analysis, which was based on the logarithmic velocity profile of turbulent pipe flow in the surfactant solution, was carried out using momentum integral equations of the boundary layer, and the moment coefficient results agreed with experimental results for maximum drag reduction in surfactant solution. Additionally, flow visualization was performed using the tracer and the tuft techniques, which revealed that the direction of flow of surfactant solution on the disk was turned towards the circumferential direction and the amplitude of the circular vortex on the rotating disk was reduced by addition of surfactant solution. The experimental results for flow angle on a rotating disk can be explained well with the analytical results.

Description: In decaying two-dimensional Navier-Stokes turbulence, Batchelor's similarity hypothesis fails due to the existence of coherent vortices. However, it is shown that decaying two-dimensional turbulence governed by the Charney-Hasegawa-Mima (CHM) equation ∂-∂t(∇2 φ - λ2 φ) + J (φ, ∇2 φ) = D, where D is a damping, is described well by Batchelor's similarity hypothesis for wave numbers k ≪ λ (the so-called AM regime). It is argued that CHM turbulence in the AM regime is a more 'ideal' form of two-dimensional turbulence than is Navier-Stokes turbulence itself.

Description: We study the steady three-dimensional flow field and bed topography in a channel with sinusoidally varying width, under the assumptions of small-amplitude width variations and sufficiently wide channel to neglect nonlinear effects and sidewall effects. The aim of the work is to investigate the role of width variations in producing channel bifurcation in braided rivers. We infer incipient bifurcation in cases where the growth of a central bar leads to planimetric instability of the channel, i.e. when the given infinitesimal width perturbation is enhanced. Results of the three-dimensional model suggest that the equilibrium bottom profile mainly consists of a purely longitudinal component, uniformly distributed over the cross-section, which induces deposition at the wide section and scour at the constriction, and of a transverse component in the form of a central bar (wide sections) and scour (constrictions), with longitudinal wavelength equal to that of width variations. A comparison between the results of the three-dimensional model and those obtained by means of a two-dimensional depth-averaged approach shows that the transverse component is mainly related to three-dimensional effects. Theoretical findings display a satisfactory agreement with results of flume experiments. Transverse variations are responsible for the planimetric instability of the channel; we find that in the range of values of Shields stress typical of braided rivers, the incipient bifurcation is enhanced as the width ratio of the channel increases.

Description: The linear stability of the impulsively started flow through a pipe of circular cross-section is studied at high Reynolds number R. A crucial non-dimensional time of O(R7/9) is identified at which the disturbance acquires internal flow characteristics. It is shown that even if the disturbance amplitude at this time is as small as O(R-22/27) the subsequent evolution of the perturbation is nonlinear, although it can still be followed analytically using a multiple-scales approach. The amplitude and wave speed of the nonlinear disturbance are calculated as functions of time and we show that as t → ∞, the disturbance evolves into the long-wave limit of the neutral mode structure found by Smith and Bodonyi in the fully developed Hagen-Poiseuille flow, into which our basic flow ultimately evolves. It is proposed that the critical amplitude found here forms a stability boundary between the decay of linear disturbances and 'bypass' transition, in which the fully developed state is never attained.

Description: A model for the deformation of thin viscous sheets of arbitrary shape subject to arbitrary loading is presented. The starting point is a scaling analysis based on an analytical solution of the Stokes equations for the flow in a shallow (nearly planar) sheet with constant thickness T0 and principal curvatures k1 and k2, loaded by an harmonic normal stress with wavenumbers q1 and q2 in the directions of principal curvature. Two distinct types of deformation can occur: an 'inextensional' (bending) mode when /L3(k1q22 + k2q12)/ ≪ ∈, and a 'membrane' (stretching) mode when /L3(k1q22 + K2q12)/ ≫ ∈, where L ≡ (q12 + q22)t/2 and ∈ = To / L ≪ 1. The scales revealed by the shallow-sheet solution together with asympotic expansions in powers of ∈ are used to reduce the three-dimensional equations for the flow in the sheet to a set of equivalent two-dimensional equations, valid in both the inextensional and membrane limits, for the velocity U of the sheet midsurface. Finally, kinematic evolution equations for the sheet shape {metric and curvature tensors} and thickness are derived. Illustrative numerical solutions of the equations are presented for a variety of buoyancy-driven deformations that exhibit buckling instabilities. A collapsing hemispherical dome with radius L deforms initially in a compressional membrane mode, except in bending boundary layers of width ∼ (∈ L)1/2 near a clamped equatorial edge, and is unstable to a buckling mode which propagates into the dome from that edge. Buckling instabilities are suppressed by the extensional flow in a sagging inverted dome (pendant drop), which consequently evolves entirely in the membrane mode. A two-dimensional viscous jet falling onto a rigid plate exhibits steady periodic folding, the frequency of which varies with the jet height and extrusion rate in a way similar to that observed experimentally.

Description: Laboratory experiments were performed in which an intrusive gravity current was observed using shadowgraph and particle tracking methods. The intrusion was generated in a two-layer fluid with a sharp interface by mixing the fluid behind a vertical lock gate and then suddenly withdrawing the gate from the tank. The purpose of the experiments was to determine the structure of the velocity field inside the intrusion and the stability characteristics of the interface. Soon after the removal of the lock gate, the front of the intrusive gravity current travelled at a constant speed close to the value predicted by theory for an energy-conserving gravity current. The observed structure of the flow inside the intrusion can be divided into three regions. At the front of the intrusion there is an energy-conserving head region in which the fluid velocity is nearly uniform with speed equal to the front speed. This is followed by a dissipative wake region in which large billows are present with their associated mixing and in which the fluid velocity is observed to be non-uniform and have a maximum speed approximately 50% greater than the front speed. Behind the wake region is a tail region in which there is very little mixing and the velocity field is nearly uniform with a speed slightly faster than the front speed.

Description: The singular effects of steady large-scale external strain on the viscous wake generated by a rigid body and the overall flow field are analysed. In an accelerating flow strained at a positive rate, the vorticity field is annihilated owing to positive and negative vorticity either side of the wake centreline diffusing into one another and the volume flux in the wake decreases with downwind distance. Since the wake disappears, the far-field flow changes from monopolar to dipolar. In this case, the force on the body is no longer proportional to the strength of the monopole, but is proportional to the strength of the far field dipole. These results are extended to the case of strained turbulent wakes and this is verified against experimental wind tunnel measurements of Keffer (1965) and Elliott and Townsend (1981) for positive and negative strains. The analysis demonstrates why the total force acting on a body may be estimated by adding the viscous drag and inviscid force due to the irrotational straining field. Applying the analysis to the wake region of a rigid body or a bubble shows that the wake volume flux decreases even in uniform flows owing to the local straining flow in the near-wake region. While the wake volume flux decreases by a small amount for the flow over streamline and bluff bodies, for the case of a clean bubble the decrease is so large as to render Betz's (1925) drag formula invalid. To show how these results may be applied to complex flows, the effects of a sequence of positive and negative strains on the wake are considered. The average wake width is much larger than in the absence of a strain field and this leads to diffusion of vorticity between wakes and the cancellation of vorticity. The latter mechanism leads to a net reduction in the volume flux deficit downstream which explains why in calculations of the flow through groups of moving or stationary bodies the wakes of upstream bodies may be ignored even though their drag and lift forces have a significant effect on the overall flow field.

Description: The effect of random wave fields on passive tracer spatial variations is studied. We derive a closed-form expression for the spatial autocorrelation function (or power spectrum) of the tracer fluctuations that is quantitatively accurate so long as wave field nonlinearities are small. The theory is illustrated for the case of long internal gravity waves in the ocean. We find that even if the (rear face of the) spectrum of the advecting velocity field is a pure power law, the tracer spectrum has two separate power law subranges. Most important to oceanographic applications, in the larger scale subrange, the effective horizontal compressibility of the wave velocity field becomes a dominant factor of the tracer variations. In such cases, the concentration spectrum becomes approximately proportional to the spectrum of the wave potential energy. The latter, which decays with increasing wavenumber much more rapidly than that known for two-dimensional eddy turbulence, is confirmed by satellite observations in wave-dominated ocean regions. As an additional confirmation of the theory, we demonstrate the occurrence of spectral peaks at wavenumbers corresponding to the semi-diurnal tide frequency.

Description: We consider the problem of a thin liquid layer falling down an inclined plate that is subjected to non-uniform heating. The plate temperature is assumed to be linearly distributed and both directions of the temperature gradient with respect to the flow are investigated. The film flow is not only influenced by gravity and mean surface tension, but in addition by the thermocapillary force acting along the free surface. The coupling of thermocapillary instability and surface-wave instabilities is studied for two-dimensional disturbances. Applying the long-wave theory, a nonlinear evolution equation is derived. When the plate temperature is decreasing in the downstream direction, linear stability analysis exhibits a film stabilization, compared to a uniformly heated film. In contrast, for increasing temperature along the plate, the film becomes less stable. Numerical solution of the evolution equation indicates the existence of permanent finite-amplitude waves of different kinds. The shape of the waves depends mainly on the mean flow and the mean surface tension, but their amplitudes and phase speeds are influenced by thermocapillarity.

Description: The results of an experimental study of stability, receptivity and transition of the flat-plate laminar boundary layer at Mach 3 are discussed. With a relatively low free-stream disturbance level (∼ 0.1%), spectra, growth rates and amplitude distributions of naturally occurring boundary layer waves were measured using hot wires. Physical (mass-flux) amplitudes in the boundary layer and free stream are reported and provide stability and receptivity results against which predictions can be directly compared. Comparisons are made between measurements of growth rates of unstable high-frequency waves and theoretical predictions based on a non-parallel, mode-averaging stability theory and receptivity assumptions; good agreement is found. In contrast, it was found that linear stability theory does not account for the measured growth of low-frequency disturbances. A detailed investigation of the disturbance fields in the free stream and on the nozzle walls provides the basis for a discussion of the source and the development of the measured boundary layer waves. Attention is drawn to the close matching in streamwise wavelengths for instability waves and the free-stream acoustic disturbances. It was also found that a calibration of the hot wire in the free stream yields a double-peak boundary layer disturbance amplitude distribution, as has been found by previous investigators, which is not consistent with the predictions of linear stability theory. This double peak was found to be an experimental anomaly which resulted from assumptions that are frequently made in the free-stream calibration procedure. A single-peak amplitude distribution across the boundary layer was established only when the hot-wire voltage was calibrated against the mean boundary layer profile. Finally, the late stages of transition, at a higher Reynolds number with a higher free-stream disturbance level, were explored. Calibrated amplitude levels are provided at locations where nonlinearities are first detected and where the mean boundary layer profile is first observed to depart from the laminar similarity solution. A qualitative discussion of the character of ensuing nonlinearities is also included.

Description: The gravity-driven flow of non-neutrally buoyant suspensions is shown to be unstable to spanwise perturbations when the shearing motion generates a density profile that increases with height. The instability is simply due to having heavier material over light - a Rayleigh-Taylor-like instability. The wavelength of the fastest growing disturbance is on the order of the thickness of the suspension layer. The parameters important to the problem are the angle of inclination of the layer relative to gravity, the relative density difference between the particles and the fluid, the ratio of the particle size to the thickness of the layer and the bulk volume fraction of particles. The instability is illustrated for a range of these parameters and shown to be most pronounced at intermediate values thereof. This instability mechanism may play an important role in pattern formation in multiphase flows.

Description: We investigate magnetic Taylor-Couette flow in the presence of an imposed axial magnetic field. First we calculate nonlinear steady axisymmetric solutions and determine how their strength depends on the applied magnetic field. Then we perturb these solutions to find the critical Reynolds numbers for the appearance of wavy modes, and the related wave speeds, at increasing magnetic field strength. We find that values of imposed magnetic field which alter only slightly the transition from circular-Couette flow to Taylor-vortex flow, can shift the transition from Taylor-vortex flow to wavy modes by a substantial amount. The results are compared to those for onset in the absence of a magnetic field.

Description: For many years there has been debate regarding why shock wave reflection off a solid surface has allowed regular reflection to persist beyond the incidence angles where it becomes theoretically impossible. Theory predicts that at some limiting angle the reflection point will move away from the wall and Mach reflection will occur. Previous studies have suggested that the paradox could be due to the presence of the growing viscous boundary layer immediately behind the point of reflection, and some numerical studies support this view. This paper takes the approach of establishing an experimental facility in which the theoretical assumptions regarding the surface of reflection are met, i.e. that the reflecting surface is perfectly smooth, perfectly rigid, and adiabatic. This is done by constructing a bifurcated shock tube facility in which a shock wave is split into two plane waves that are then allowed to reflect off each other at the trailing edge of wedge. The plane of symmetry between the waves then acts as the perfect reflection surface. Through a careful set of visualization experiments, and the use of multivariate analysis to take account of the uncertainty in shock Mach number, triple-point trajectory angle, and slightly different shock wave arrival times at the trailing edge, the current work shows that the transition from one type of reflection to the other does indeed occur at the theoretical value. Conventional tests of reflection off a solid wall show significantly different transition results. Furthermore, it is also shown that the thermal boundary layer plays an important role in this regard. It is thus confirmed that viscous and thermal effects are the reason for the paradox. Reasons are also suggested for the counter-intuitive behaviour of the reflected shock wave angle.

Description: The morning glory is a meteorological phenomenon which occurs in northern Australia and takes the form of a series of roll clouds. The morning glory is generated by the interaction of nocturnal seabreezes over Cape York Peninsula and propagates in a south-westerly direction over the Gulf of Carpentaria. In the present work, it is shown that the morning glory can be modelled by the resonant flow of a two-layer fluid over topography, the topography being the mountains of Cape York Peninsula. In the limit of a deep upper layer, the equations of motion reduce to a forced Benjamin-Ono equation. In this context, resonant means that the underlying flow velocity of the seabreezes is near a linear long-wave velocity for one of the long-wave modes. The morning glory is then modelled by the undular bore (simple wave) solution of the modulation equations for the Benjamin-Ono equation. This modulation solution is compared with full numerical solutions of the forced Benjamin-Ono equation and good agreement is found when the wave amplitudes are not too large. The reason for the difference between the numerical and modulation solutions for large wave amplitude is also discussed. Finally, the predictions of the modulation solution are compared with observational data on the morning glory and good agreement is found for the pressure jump due to the lead wave of the morning glory, but not for the speed and half-width of this lead wave. The reasons for this are discussed.

Description: The stability of fluid flow past a membrane of infinitesimal thickness is analysed in the limit of zero Reynolds number using linear and weakly nonlinear analyses. The system consists of two Newtonian fluids of thickness R* and HR*, separated by an infinitesimally thick membrane, which is flat in the unperturbed state. The dynamics of the membrane is described by its normal displacement from the flat state, as well as a surface displacement field which provides the displacement of material points from their steady-state positions due to the tangential stress exerted by the fluid flow. The surface stress in the membrane (force per unit length) contains an elastic component proportional to the strain along the surface of the membrane, and a viscous component proportional to the strain rate. The linear analysis reveals that the fluctuations become unstable in the long-wave (α → 0) limit when the non-dimensional strain rate in the fluid exceeds a critical value Λt, and this critical value increases proportional to α2 in this limit. Here, α is the dimensionless wavenumber of the perturbations scaled by the inverse of the fluid thickness R*-1, and the dimensionless strain rate is given by Λt = (+γ* R* η* / *), where η* is the fluid viscosity, * is the tension of the membrane and -γ* is the strain rate in the fluid. The weakly nonlinear stability analysis shows that perturbations are supercritically stable in the α → 0 limit.

Description: Laboratory experiments are described which investigate the dynamics of a vortex dipole moving in water towards a planar sloping beach inclined at an angle α to the horizontal. Results are compared with those of a vortex ring model first developed by Peregrine (1996). The vortices separate as they travel up the beach, eventually moving in opposite directions and nearly parallel to the shoreline. The ranges of conditions examined are 3° ≤ α ≤ 45°, 1 × 103 ≤ Re ≤ 6 × 103 and 0.05 ≤ Fr ≤ 0.14, where Re and Fr are the on-slope Reynolds number and the Froude number of the vortices, respectively. The minimum distance from the shoreline reached by the vortices and their along-shore speed are in general agreement with the predictions when, respectively, Ri* 〈 3 (where Ri* is the non-dimensional initial distance of the vortices from the shore-line) and Re ≳ 1500. The vortex ring model is likely to have useful applications to the study of the dynamics of the near-shore zone.

Description: A transition scenario initiated by streamwise low- and high-speed streaks in a flat-plate boundary layer is studied. In many shear flows, the perturbations that show the highest potential for transient energy amplification consist of streamwise-aligned vortices. Due to the lift-up mechanism these optimal disturbances lead to elongated streamwise streaks downstream, with significant spanwise modulation. In a previous investigation (Andersson et al. 2001), the stability of these streaks in a zero-pressure-gradient boundary layer was studied by means of Floquet theory and numerical simulations. The sinuous instability mode was found to be the most dangerous disturbance. We present here the first simulation of the breakdown to turbulence originating from the sinuous instability of streamwise streaks. The main structures observed during the transition process consist of elongated quasi-streamwise vortices located on the flanks of the low-speed streak. Vortices of alternating sign are overlapping in the streamwise direction in a staggered pattern. The present scenario is compared with transition initiated by Tollmien-Schlichting waves and their secondary instability and by-pass transition initiated by a pair of oblique waves. The relevance of this scenario to transition induced by free-stream turbulence is also discussed.

Description: This paper reports on an experimental investigation to determine the structure and mean flow quantities of round zero-net-mass-flux (ZNMF) jets. These jets are generated by a piston oscillating in a cavity behind a circular orifice. Several different flow patterns were observed with dye flow visualization and a parameter map of these was generated. Cross-correlation digital particle image velocimetry was used to measure instantaneous two-dimensional in-plane velocity fields in a plane containing the orifice axis. These velocity fields are used to investigate the existence of a self-preserving velocity profile in the far field of the ZNMF jet. The mean flow quantities and turbulent statistics of the ZNMF jets were compared with measurements for 'equivalent' continuous jets in the same apparatus. Phase-averaged velocity measurements were obtained in the near field of the ZNMF jets and were used to determine the radial entertainment. The out-of-plane vorticity fields were also investigated to gain an understanding of the mechanisms responsible for the difference in spreading rate of ZNMF jets compared to conventional continuous jets. A conceptual model of the ZNMF jet structure in the near field for Strouhal numbers much less than one is proposed that explains the observed behaviour of these ZNMF jets.

Description: Numerical finite-difference results from the full axisymmetric incompressible Navier-Stokes equations are presented for the problem of the slow axial motion of a disk particle in an incompressible, rotating fluid in a long cylindrical container. The governing parameters are the Ekman number, E = v*/(Ω* a*2), Rossby number, Ro = W* /(Ω* a*), and the dimensionless height of the container, 2H (the scaling length is the radius of the particle, a*; Ω* is the container angular velocity, W* is the particle axial velocity and v* the kinematic viscosity). The study concerns the flow field for small values of E and Ro while H E is of order unity, and hence the appearance of a free Taylor column (slug) of fluid 'trapped' at the particle is expected. The numerical results are compared with predictions of previous analytical approximate studies. First, developed (quasi-steady-state) cases are considered. Excellent agreement with the exact linear (Ro = O) solution of Ungarish and Vedensky (1995) is obtained when the computational Ro = 10-4. Next, the time-development for both an impulsive start and a start under a constant axial force is considered. A novel unexpected behaviour has been detected : the flow field first attains and maintains for a while the steady-state values of the unbounded configuration, and only afterwards adjusts to the bounded container steady state. Finally, the effects of the nonlinear momentum advection terms are investigated. It is shown that when Ro increases then the dimensionless drag (scaled by μ*a*W*) decreases, and the Taylor column becomes shorter, this effect being more pronounced in the rear region (μ* is the dynamic viscosity). The present results strengthen and extend the validity of the classical drag force predictions and therefore the issue of the large discrepancy between theory and experiments (Maxworthy 1970) concerning this force becomes more acute.

Description: Digital particle image velocimetry (DPIV) measurements of the velocity field under breaking waves in the laboratory are presented. The region of turbulent fluid directly generated by breaking is too large to be imaged in one video frame and so an ensemble-averaged representation of the flow is built up from a mosaic of image frames. It is found that breaking generates at least one coherent vortex that slowly propagates downstream at a speed consistent with the velocity induced by its image in the free surface. Both the kinetic energy of the flow and the vorticity decay approximately as t-1. The Reynolds stress of the turbulence also decays as t-1 and is, within the accuracy of the measurements, everywhere negative, consistent with downward transport of streamwise momentum. Estimates of the momentum flux from waves to currents based on the measurements of the Reynolds stress are consistent with earlier estimates. The implications of the measurements for breaking in the field are discussed. Based on geometrical optics and wave action conservation, we suggest that the presence of the breaking-induced vortex provides an explanation for the suppression of short waves by breaking. Finally, in Appendices, estimates of the majority of the terms in the turbulent kinetic energy budget are presented at an early stage in the evolution of the turbulence, and comparisons with independent acoustical measurements of breaking are presented.

Description: Consider the three-dimensional flow of a viscous Newtonian fluid upon an arbitrarily curved substrate when the fluid film is thin as occurs in many draining, coating and biological flows. We drive the lubrication model of the dynamics of the film expressed in terms of the film thickness. The comprehensive model accurately includes the effects of the curvature of the substrate, via a physical multiple-scale approach, and gravity and inertia, via more rigorous centre manifold techniques. This new approach theoretically supports the use of the model over a wide range of parameters and provides a sound basis for further development of lubrication models. Numerical simulations exhibit some generic features of the dynamics of such thin fluid films on substrates with complex curvature: we here simulate a film thinning at a corner, the flow around a torus, and draining of a film down a cylinder. The last is more accurate than other lubrication models. The model derived here describes well thin-film dynamics over a wide range of parameter regimes.

Description: This article presents an analytical and experimental study of magnetohydrodynamic Rayleigh-Bénard convection in a large aspect ratio, 20 : 10 : 1, rectangular box. The test fluid is a eutectic sodium potassium Na22K78 alloy with a small Prandtl number of Pr ≈ 0.02. The experimental setup covers Rayleigh numbers in the range 103 〈 Ra 〈 8 × 104 and Chandrasekhar numbers 0 ≤ Q ≤ 1.44 × 106 or Hartmann numbers 0 ≤ M ≤ 1200, respectively. When a horizontal magnetic field is imposed on a heated liquid metal layer, the electromagnetic forces give rise to a transition of the three-dimensional convective roll pattern into a quasi-two-dimensional flow pattern in such a way that convective rolls become more and more aligned with the magnetic field. A linear stability analysis based on two-dimensional model equations shows that the critical Rayleigh number for the onset of convection of quasi-two-dimensional flow is shifted to significantly higher values due to Hartmann braking at walls perpendicular to the magnetic field. This finding is experimentally confirmed by measured Nusselt numbers. Moreover, the experiments show that the convective heat transport at supercritical conditions is clearly diminished. Adjacent to the onset of convection there is a significant region of stationary convection with significant convective heat transfer before the flow proceeds to time-dependent convection. However, in spite of the Joule dissipation effect there is a certain range of magnetic field intensities where an enhanced heat transfer is observed. Estimates of the local isotropy properties of the flow by a four-element temperature probe demonstrate that the increase in convective heat transport is accompanied by the formation of strong non-isotropic time-dependent flow in the form of large-scale convective rolls aligned with the magnetic field which exhibit a simpler temporal structure compared to ordinary hydrodynamic flow and which are very effective for the convective heat transport.

Description: We consider the slow motion of viscous fluid completely filling a rectangular container. The motion is generated by the combined action of differential wall temperatures and the linear motion of the lid. If the relevant Reynolds and Péclet numbers and the lid speed are all small enough, the velocity field will be governed by an inhomogeneous biharmonic equation. In this approximation the temperature field, unaffected by the fluid motion, drives, at least in part, the fluid velocity field. Of interest here are the relative effects of buoyancy and lid motion. It is shown that the field, suitably scaled, depends on the dimensionless depth and lid speed alone. The mixed convection problem is solved for two pairs of wall heating protocols by sequentially solving, by an eigenfunction expansion method, up to four biharmonic problems. We present streamline patterns and quantitative data on the relative effects of lid motion on the buoyancy-driven fields in these containers.

Description: A nonlinear equation based on the hydrodynamic equations is solved analytically using perturbation expansions to calculate the flow field of a steady isothermal, compressible and laminar gas flow in either a circular or a planar microchannel. The solution takes into account slip-flow effects explicitly by utilizing the classical velocity-slip boundary condition, assuming the gas properties are known. Consistent expansions provide not only the cross-stream but also the streamwise evolution of the various flow parameters of interest, such as pressure, density and Mach number. The slip-flow effect enters the solution explicity as a zero-order correction comparable to, though smaller than, the compressible effect. The theoretical calculations are verified in an experimental study of pressure-driven gas flow in a long microchannel of sub-micron height. Standard micromachining techniques were utilized to fabricate the microchannel, with integral pressure microsensors based on the piezoresistivity principle of operation. The integrated microsystem allows accurate measurements of mass flow rates and pressure distributions along the microchannel. Nitrogen, helium and argon were used as the working fluids forced through the microchannel. The experimental results support the theoretical calculations in finding that acceleration and non-parabolic velocity profile effects were found to be negligible. A detailed error analysis is also carried out in an attempt to expose the challenges in conducting accurate measurements in microsystems.

Description: We present a numerical study of the downstream evolution (mechanical and thermal) of vortex-jet cores whose velocity and temperature fields far from the axis match a family of inviscid and non-conducting vortices. The far-velocity field is rotational, except for a particular case which corresponds to the well-known Long's vortex. The evolution of the vortex core depends on both the conditions at a certain upstream station, characterized by the dimensionless value of the velocity at the axis, and a dimensionless swirling parameter L defined as the ratio of the values of the azimuthal and axial velocities outside the vortex core. This numerical study, based on the quasi-cylindrical approximation (QC) of the Navier-Stokes equations, determines the conditions under which the vortex evolution proceeds smoothly, eventually reaching an asymptotic self-similar behaviour as described in the literature (Fernández-Feria, Fernández de la Mora & Barrero 1995; Herrada, Pérez-Saborid & Barrero 1999), or breaks in a non-slender solution (vortex breakdown). In particular, the critical value L= Lb(a) beyond which vortex breakdown occurs downstream is a function of a dimensionless parameter a characterizing the axial momentum of the vortex jet at an initial upstream station. It is found numerically that for very large values of a this vortex breakdown criterion tends to an asymptote which is precisely the value L=L* predicted by the self-similar analysis, and beyond which a self-similar structure of the vortex core does not exist. In addition, the computation of the total temperature field provides useful information on the physical mechanisms responsible for the thermal separation phenomenon observed in Ranque-Hilsch tubes and other swirling jet devices. In particular, the mechanical work of viscous forces which gives rise to an intense loss of kinetic energy during the initial stages of the evolution has been identified as the physical mechanism responsible for thermal separation.