ALBERT

Description: Experimental work to investigate plane Couette flow has been performed in the Reynolds number range of 750 ≤ Re (=hUb/(2v)) ≤ 5000 or 50 ≤ Re* (=hu*/v) ≤ 253 where Ub, u* and h are moving wall speed, friction velocity and channel half-height, respectively. The low-Reynolds-number effect on the wall friction coefficient Cf, mean velocity profile and statistical turbulence quantities is discussed in relation to the turbulent Poiseuille flow properties. Since the shear stress is constant in Couette flow, the flow is free from the effect of shear stress gradient and the Reynolds number effect therefore can be seen explicitly, uncontaminated by this effect. A flow region diagram is given to show how the low-Reynolds-number effect penetrates into the wall region. The area of the buffer region is contracted by the low-Reynolds-number effect when Re* ≤ 150, so that the additive constant B of the log law decrease as Re* decreases. Also, Cf has a larger value than in Poiseuille flow in the low Re* range. The log-law area in Couette flow is 2-3 times as wide as that in Poiseuille flow. The defect law is Re*-dependent and the non-dimensional velocity gradient at the core, Rs = (dU1/ dx2(h/u*), increases from 3 to 4.2 as Re* increases from 50 to 253. The peak value of streamwise turbulence intensity u1p + has a constant value of 2.88 but decreases sharply as Re* reduces below 150. The large longitudinal vortices extending the entire height of the channel are shown to be sustained in Couette flow that is oscillating around their average position. This causes a slow fluctuation with large amplitude in the streamwise velocity component. These vortices make the Couette flow three-dimensional and the skin friction coefficient varies 20% sinuously in the spanwise direction, for example. Also, the zero-crossing time separation of streamwise velocity auto-correlation R11(τ) becomes longer as τ =40h/Ub, which is 3 times as long as that in Poiseuille flow. © 2005 Cambridge University Press.

Description: Turbulence quantities have been measured for a low-Reynolds-number fully developed two-dimensional channel flow subjected to system rotation. Turbulence intensities, Reynolds shear stress, correlation coefficient, skewness and flatness factors, four-quadrant analysis, autocorrelation coefficient and power spectra are investigated. According to the dimensional analysis, the relevant parameters of this flow are the Reynolds number Re* = u* D/v and the Coriolis parameter Rc = Ω v/u*2 for the wall region, and Re* and Ω D/u* for the turbulent core-region. The existence of a Coriolis region where turbulence intensities are defined by a new variable yc* = y/δc has been clarified on the pressure side in the rotating channel flow. The amount of turbulent kinetic energy transported by the Coriolis term is extremely small compared to the production term in the transport equation of Reynolds normal stress. However, the Coriolis term makes a large contribution to Reynolds shear stress transport on the pressure side of the channel. It is caused by the strong ejection which occurs periodically on the pressure side even though the ejection frequency is low. The strong ejection is conjectured to be caused by a large-scale longitudinal structure like a roll cell on the pressure side of the channel. © 2005 Cambridge University Press.

Description: Theoretical and experimental studies have been performed on fully developed two-dimensional turbulent channel flows in the low Reynolds number range that are subjected to system rotation. The turbulence is affected by the Coriolis force and the low Reynolds number simultaneously. Using dimensional analysis, the relevant parameters of this flow are found to be Reynolds number Re* = u* D/v (u* is the friction velocity, D the channel half-width) and Ωv/u2* is the angular velocity of the channel) for the inner region, and Re* and ΩD/u* for the core region. Employing these parameters, changes of skin friction coefficients and velocity profiles compared to non-rotating flow can be reasonably well understood. A Coriolis region where the Coriolis force effect predominates is shown to exist in addition to conventional regions such as viscous and buffer regions. A flow regime diagram that indicates ranges of these regions as a function of Re* and Ωv/u2* is given from which the overall flow structure in a rotating channel can be obtained. Experiments have been made in the range of 56 ≤ Re* ≤ 310 and -0.0057 ≤ Ωv/u2* ≤ 0.0030 (these values correspond to Re = 2Um D/v from 1700 to 10000 and rotation number Ro = 2*/D/Um up to 0.055 ; Um is bulk mean velocity). The characteristic features of velocity profiles and the variation of skin friction coefficients are discussed in relation to the theoretical considerations.

Description: Similarity laws of mean velocity profiles and turbulence characteristics of Couette-Poiseuille turbulent flow (C-P flow) have been studied experimentally. The global parameters of C-P flow are the Reynolds number Re* and the dimensionless shear stress gradient μ and flow parameter β. The effects of these parameters on the turbulence structure have also been considered in the wall region and turbulent core region, respectively. In the wall region, the wall law varies greatly with μ but slightly with Re*. Typically, the additive constant B of the logarithmic law (or Van Driest damping factor A+) is shown to depend only on μ. Turbulence characteristics are also strongly influenced by μ, but not much by Re*. Because the relation μ = -Re* holds in plane Poiseuille flow and Re* has little effect on the similarity laws for C-P flows, the low-Reynolds-number effect on the additive constant and turbulence quantities for plane Poiseuille flow can be attributed to the μ effect. In the turbulent core region, however, there is a great difference in the defect law of the velocity profile and the distribution of turbulence intensity between Poiseuille (P)- and Couette (C)-types flows. For P-type flow, an effective friction velocity u*e and a new coordinate η = y -hs are recommended for the universal profile, where y = hs = δp is the position of τ = 0 and δp is considered to be appropriate as a characteristic length scale of turbulence. For C-type flow, a different effective friction velocity u*c, the characteristic length scale 2h and the wall coordinate y are preferred. The turbulence activity away from the wall is extremely high for μ 〉 0 and low for μ 〈 0. A strong sweep event plays a dominant role in the Reynolds shear stress when 0 〈 μ 〈 50, whereas strong ejection from the near-wall region prevails in the case of negative μ with a small absolute value. © 2004 Cambridge University Press.

Description: An analytical study is made of a flow with swirling motion through a bend in a pipe with circular cross-section. The intensity of the swirl and the axial velocity distribution may change along a curved path according to the conservation law of angular momentum that is the basic principle used in this analysis. The analysis deals with the variation of angular-momentum flux components of the flow along the pipe. Approximate inviscid solutions indicate that, depending on the inlet conditions of the flow and the configuration of the bend, different types of swirling motion appear in the bend. The sign of a constant C appearing in the analysis is a governing parameter determining the flow type. Comparisons are made with the experimental results of other investigators. Finally, examples of swirling motion appearing in a real curved duct are discussed. © 1987, Cambridge University Press. All rights reserved.

Description: Swirling flow through a pipe is a highly complex turbulent flow and is still challenging to predict. An experimental investigation is performed to obtain systematic data about the flow and to understand its physics. A free-vortex-type swirling flow is introduced in a long straight circular pipe. The swirling component decays downstream as a result of wall friction. The velocity distributions are continuously changing as they approach fully developed parallel flow. The swirl intensity £2, defined as a non-dimensional angular momentum flux, decays exponentially. The decay coefficients, however, are not constant as conventionally assumed, but depend on the swirl intensity. The wall shear stresses are measured by a direct method and, except in a short inlet region, are a function only of the swirl intensity and the Reynolds number. The velocity distributions and all Reynolds stress components are measured at various axial positions in the pipe. The structure of the tangential velocity profile is classified into three regions: core, annular and wall regions. The core region is characterized by a forced vortex motion and the flow is dependent upon the upstream conditions. In the annular region, the skewness of the velocity vector is noticeable and highly anisotropic so that the turbulent viscosity model does not work well here. The tangential velocity is expressed as a sum of free and forced vortex motion. In the wall region the skewness of the flow becomes weak, and the wall law modified by the Monin-Oboukhov formula is applicable. Data on the microscale and the spectrum are also presented and show quite different turbulence structures in the core and the outer regions. © 1991, Cambridge University Press. All rights reserved.

Description: Observational studies have shown that Eurasian snow-cover anomalies during winter-through-spring seasons have a great effect on anomalies in atmospheric circulation and climate in the following summer season through snow albedo feedback (Hahn and Shukla, 1976; Dey and Bhanu Kumar, 1987). Morinaga and Yasunari (1987) have revealed that large-scale snow-cover extent over central Asia in late winter, which particularly has a great effect on the circulation over Eurasia in the following season, is closely related to the Eurasian pattern circulation (Wallace and Gutzler, 1981) in the beginning of winter. Some atmospheric general circulation models (GCM) have suggested that not only the albedo effect of the snow cover but also the snow-hydrological process are important in producing the atmospheric anomalies in the following seasons (Yeh and others, 1984; Barnett and others, 1988). However, more quantitative evaluations of these effects have not yet been examined. For example, it is not clear to what extent atmospheric anomalies are explained solely by snow-cover anomalies. Spatial and seasonal dependencies of these effects are supposed to be very large. Relative importance of snow cover over Tibetan Plateau should also be examined, particularly relevant to Asian summer monsoon anomalies. Moreover, these effects seem to be very sensitive to parameterizations of these physical processes (Yamazaki, 1988). This study focuses on these problems by using some versions of GCMs of the Meteorological Research Institute. The results include the evaluation of total snow-cover feedbacks as part of internal dynamics of climatic change from 12-year GCM integration, and of the effect of anomalous snow cover over Eurasia in late winter on land surface conditions and atmospheric circulations in the succeeding seasons.

Description: An Antarctic Circumpolar Wave (ACW) is simulated by a global coupled ocean-atmosphere model. Time-longitude diagrams of anomalies in sea-surface temperature (SST) and sea-surface salinity (SSS) show that anomalies propágale eastward, taking 20-30 years to encircle the pole. The time taken is 2-3 times longer than indicated by observations, due to the relatively slow speed of the modelled Antarctic Circumpolar Current (ACC). High-SSS anomalies correspond to high-SST anomalies and high-density anomalies, and thus to low sea-surface height anomalies, indicating that salinity IS a dominant factor for dynamics with in the Southern Ocean and is indispensable for understanding the mechanism of the ACW. Sea-ice formation is suppressed southward of warm, saline surface-water regions. High sea-ice concentration anomalies correspond to thick sea-ice anomalies. Empirical orthogonal function analyses of SSTanomalies for both model and observation show that the dominant mode in the Southern Ocean has a spatial pattern closely related to El Niño activity. Sea-level pressure (SLP) anomalies propagate eastward with the ACW. High SLP anomalies in the atmosphere correspond to low-density anomalies 111 the ocean. The ACC has clockwise geostrophic velocity anomalies over high-density anomaly regions with upwelling. Both heat and salt are transported from the deep layer to the surface layer by upwelling. This could suppress sea-ice formation directly. Anomalous horizontal advection of heat and salt by geostrophic velocity anomalies in the ACC appears to influence the anomalies in SST, SSS and sea ice.

Description: Observational studies have shown that Eurasian snow-cover anomalies during winter-through-spring seasons have a great effect on anomalies in atmospheric circulation and climate in the following summer season through snow albedo feedback (Hahn and Shukla, 1976; Dey and Bhanu Kumar, 1987). Morinaga and Yasunari (1987) have revealed that large-scale snow-cover extent over central Asia in late winter, which particularly has a great effect on the circulation over Eurasia in the following season, is closely related to the Eurasian pattern circulation (Wallace and Gutzler, 1981) in the beginning of winter.Some atmospheric general circulation models (GCM) have suggested that not only the albedo effect of the snow cover but also the snow-hydrological process are important in producing the atmospheric anomalies in the following seasons (Yeh and others, 1984; Barnett and others, 1988).However, more quantitative evaluations of these effects have not yet been examined. For example, it is not clear to what extent atmospheric anomalies are explained solely by snow-cover anomalies. Spatial and seasonal dependencies of these effects are supposed to be very large. Relative importance of snow cover over Tibetan Plateau should also be examined, particularly relevant to Asian summer monsoon anomalies. Moreover, these effects seem to be very sensitive to parameterizations of these physical processes (Yamazaki, 1988).This study focuses on these problems by using some versions of GCMs of the Meteorological Research Institute. The results include the evaluation of total snow-cover feedbacks as part of internal dynamics of climatic change from 12-year GCM integration, and of the effect of anomalous snow cover over Eurasia in late winter on land surface conditions and atmospheric circulations in the succeeding seasons.