bluuebunny/crossref_metadata_2025 huggingface dataset:
Differences in beech (Faguscrenata) regeneration between two types of Japanese beech forest and along a snow gradient
Authors: Shimano Koji
Published: April 2006

Differences in beech (Fagus crenata) regeneration were quantitatively investigated using power function analysis for the size–class (diameter at breast height, DBH) distribution and juvenile‐to‐canopy tree (J/C) ratio along a snow gradient throughout Japan. In snowy areas, all species combined, as well as F. crenata alone, showed constant regeneration, with parameter b≈−1.6 for the power function y=axb (x=DBH, y=density), which is related to the DBH–class distribution. The good fit of the data to the function suggests that beech regenerates constantly with self‐thinning patch dynamics. Parameter a, which indicates the abundance of small trunks, was high. Furthermore, the mean J/C ratio was ≈8, i.e., each parent beech tree produced eight juveniles. These results suggest that beech regenerates constantly with gap dynamics in snowy beech forests on the Japan Sea side of Japan (snowy). However, the fit of F. crenata was lower and nonsignificant in some forests in less snowy areas, despite the high fit of all species combined. In these areas, the mean of a was low, and b was often near zero for F. crenata regressions. These results suggest that the abundance of beech was low, and self‐thinning was not evident because of the initial low abundance. Moreover, the mean J/C ratio was <1.0, suggesting that juvenile density was lower than that of canopy trees. Thus, the regeneration of F. crenata on the Pacific Ocean side of Japan (less snowy) is rather sporadic. Less snowy conditions may promote seed desiccation, predation of beechnuts and seedlings, and water stress. Lower F. crenata density may also reduce predator satiation and wind pollination.
A Novel Method for the Quantitative Evaluation of Lumbar Spinal Stenosis
Authors: Zheng Fengyu , Farmer James C. , Sandhu Harvinder S. , O'Leary Patrick F.
Published: April 2006

Study design

A retrospective study of computed tomography (CT) myelographic images in patients with degenerative lumbar spinal stenosis (LSS).

Objectives

To introduce a new technique for the quantitative evaluation of LSS.

Background

Advances in hardware and software technology now permit inexpensive digitalization of radiological images, and enable methodologies for quantifying space available for neural elements in spinal canal. However, a valid method with quantitative evaluation of spinal stenosis in living patients has not been developed yet.

Methods and materials

Preoperative CT myelographic scans of 50 patients with degenerative LSS were collected for retrospective investigation. The patients subsequently underwent lumbar decompressive surgery. They included scans from thoracic vertebra 12 (T12) to sacrum (S1), in which each segment was scanned through both the vertebral body and disk. All CT scan films were digitized using a high-resolution digital camera. ImageTool™ software was used to measure three parameters: cross-sectional area of dural sac at disk level (A), cross-sectional area of spinal canal at midpedicular level (B), and cross-sectional area of vertebral body (C). The dural sac canal ratio (DSCR) was calculated as A/ B×100%. Low DSCR implied severe dural sac compression with a high degree of stenosis. The spinal canal vertebral ratio (CVR) was also calculated as B/ C×100%. Low CVR implied a low baseline of canal capacity for neural elements. They were calculated from T12 to S1.

Results

The study consisted of 26 male and 24 female patients, with an average age of 68.4 (35–97) years. A total of 295 segments were evaluated, of which 118 (40%) were surgically decompressed. There were wide ranges of canal cross-sectional areas (140–475 mm2) and dural sac cross-sectional area (54–435 mm2). Male patients had a slightly larger canal cross-sectional area than female patients at each level. The mean CVR was found decreased from T12 (26.1%) to L4 (18.3%). This was higher in female than in male patients, especially from T12 to L2 ( P < 0.01). There were significant correlations between spinal canal and dural sac cross-sectional area ( r = 0.55, P < 0.001), and also between CVR and DSCR ( r = 0.31, P < 0.001). Of the levels decompressed, 82% was performed from the level L2 to L5, in which there was no significant difference in canal cross-sectional area and CVR between decompression and nondecompression ( P > 0.05). There was a good correspondence between decreasing mean DSCR and increasing percentile of levels decompressed.

Conclusion

DSCR represents a useful method for the quantitative diagnosis of lumbar spinal canal stenosis. ImageTool™ software is a useful tool in measuring spinal morphometry.
Molecular imprinted polymer with cloned bacterial protein template enriches authentic target in cell extract
Authors: Zhao Zhuo , Wang Chunhong , Guo Minjie , Shi Linqi , Fan Yunge , Long Yi , Mi Huaifeng
Published: April 2006

Here we describe a new method for preparing a protein‐imprinted polymer with a cloned bacterial protein template, which recognizes/adsorbs authentic target protein present at a relatively low level in cell extract. In this work, cloned pig cyclophilin 18 (pCyP18) was used as a template. The template protein was selectively assembled with memory molecules from their library, which consists of numerous limited length polymer chains with randomly distributed recognition sites and immobilizing sites. These assemblies of protein and memory molecules were adsorbed by porous polymeric beads and immobilized by cross‐linking polymerization. After removing the template, binding sites that were complementary to the target protein in size, shape and the position of recognition groups were exposed, and their confirmation was preserved by the cross‐linked structure. The synthesized imprinted polymer was used to adsorb authentic pCyP18 from cell extract, and its proportional content was enriched 300 times.
The chaperone function of cyclophilin 40 maps to a cleft between the prolyl isomerase and tetratricopeptide repeat domains
Authors: Mok Danny , Allan Rudi K. , Carrello Amerigo , Wangoo Kiran , Walkinshaw Malcolm D. , Ratajczak Thomas
Published: April 2006

Cyclophilin 40 (CyP40), an immunophilin cochaperone present in steroid receptor‐Hsp90 complexes, contains an N‐terminal peptidylprolyl isomerase (PPIase) domain separated from a C‐terminal Hsp90‐binding tetratricopeptide repeat (TPR) domain by a 30‐residue linker. To map CyP40 chaperone function, CyP40 deletion mutants were prepared and analysed for chaperone activity. CyP40 fragments containing the PPIase domain plus linker or the linker region and the adjoining TPR domain retained chaperone activity, whilst individually, the catalytic and TPR domains were devoid of chaperoning ability. CyP40 chaperone function then, is localized within the linker that forms a binding cleft with potential to accommodate non‐native substrates.
Inhibition of 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide (MTT) endocytosis by ouabain in human endothelial cells
Authors: Trevisi Lucia , Pighin Isabella , Bazzan Sara , Luciani Sisto
Published: April 2006

3‐(4,5‐Dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide (MTT) uptake and reduction is widely used to evaluate cell proliferation and viability. MTT is taken up by the cells through endocytosis. We find that ouabain (1–200 nM) inhibits MTT reduction in human umbilical vein endothelial cells (HUVEC) without affecting cell viability. Ouabain does not inhibit MTT reduction when cell lysates substituted for the intact cells. Disruption of caveolae by cholesterol depletion, completely prevents the effect of ouabain. Treatment of HUVEC with Src inhibitor 4‐amino‐5‐(4‐chlorophenyl)‐7‐(t‐butyl)pyrazolo[3,4‐d]pyrimidine partially abrogates the inhibitory effect of ouabain. The data suggest that ouabain interaction with caveolar Na/K‐ATPase inhibits MTT endocytosis through the activation of signaling proteins such as Src kinase.
The benefits of expediting government gold sales
Authors: Henderson Dale W. , Salant Stephen W. , Irons John S. , Thomas Sebastian
Published: April 2006

Additional gold can be made available either by mining at high cost (approximately $250 per ounce in 1997 dollars) or by mobilizing government stocks at zero cost. Governments own massive above‐ground stocks but loan out only a small percentage of these stocks. Making all government gold available for private uses immediately through some combination of sales and loans maximizes total welfare from private uses, a consequence of the first welfare theorem. We simulate a calibrated version of our model to quantify the effects of liquidating government stocks on alternative dates. If governments sell immediately rather than never, total welfare increases by $340 billion; if they make an unanticipated sale in 20 years, $105 billion of that amount is lost. By depressing prices, such sales benefit depletion and service users but injure private owners of stocks above and below‐ground. However, the injury to above‐ground stock owners is more than offset by the benefits to service users—often the same individuals. Mine owners would be the principal losers; however, they could be compensated (twice over) from government sales revenue without any need for tax increases.
Transonic flows of Bethe—Zel'dovich—Thompson fluids
Authors: Cramer M. S. , Tarkenton G. M.
Published: April 2006

Bethe–Zel'dovich–Thompson fluids are ordinary, single-phase fluids in which the fundamental derivative of gasdynamics is negative over a finite range of temperatures and pressures. We examine the steady transonic flow of these fluids over two-dimensional thin wings and turbine blades. The free-stream state is taken to be in the neighbourhood of one of the zeros of the fundamental derivative. It is shown that a modified form of the classical transonic small-disturbance equation governs such flows. Critical Mach number estimates are provided which take into account the non-monotone variation of the Mach number with pressure predicted in previous investigations. Critical Mach numbers well over 0.95 are predicted for conventional airfoil sections. Numerical solutions reveal substantial reductions in the strength of the compression shocks occurring in supercritical flows. Further new results include the prediction of expansion and compression shocks in the same flow and compression bow shocks in flows with subsonic free streams.
Stratified flow past a sphere
Authors: Lin Q. , Lindberg W. R. , Boyer D. L. , Fernando H. J. S.
Published: April 2006

The flow of a linearly stratified fluid past a sphere is considered experimentally in the Froude number Fi, Reynolds number Re, ranges 0.005 ≤ Fi ≤ 20 and 5 ≤ Re ≤ 10000. Flow visualization techniques and density measurements are used to describe the rich range of characteristic flow phenomena observed. These different flow patterns are mapped on a detailed Fi against Re flow regime diagram. In most instances the flow patterns were found to be very different from those observed in homogeneous fluids. Vortex shedding characteristics, for example, were found to be dramatically affected by the presence of stratification. Where possible, the results are compared with available analytical and numerical models.
An experiment on boundary mixing. Part 2 The slope dependence at small angles
Authors: Salmun H. , Phillips O. M.
Published: April 2006

Experiments of the type described by Phillips et al. (1986) were performed using different slopes with the aim of examining the slope dependence of the buoyancy and volume transports, particularly at small slopes. The new observations confirmed the general flow patterns described for experiments conducted at a fixed slope, but a reconsideration of the local balances suggest that the buoyancy flux at small slopes θ is proportional to $(\kappa N)^{\frac{3}{2}}\sin\theta $, where κ is the turbulent diffusivity. and the volume flux associated with the overall convergence flow is linear with depth and proportional to (κ3/2/N½h) sin θ, where h is the thickness of the pycnocline. These differ in their dependence on the slope suggested (but not tested) in Phillips et al., and are generally consistent with measurements over a range of slopes from 7.1° to 23.5°.
On the origin of wave patterns in fluidized beds
Authors: Göz M. F.
Published: April 2006

In this report, a model designed for the description of the flow of two miscible phases in a fluidized bed is discussed. Apart from basic problems of modelling accurately such multi-phase flows, little analytical progress had been achieved in the investigation of a certain standard model based on the theory of interacting continua. It turns out, however, that the model under consideration can be investigated with the help of bifurcation theory. In particular, the methods of the theory of bifurcation with symmetry can be applied owing to the symmetries of the system.

In general, a stationary homogeneous state exists in fluidized beds which can become unstable when the physical parameters of the system are varied. Then pattern formation takes place, e.g. in the form of one- and/or two-dimensional waves, bubbles, or convection patterns; also turbulent behaviour has been observed.

In order to understand the occurrence of wave patterns and other phenomena as an inherent feature of the system, a finite, but periodically continued two-dimensional bed is investigated. While this suppresses certain boundary effects, it gives us thorough insight into the principal behaviour of this complicated system.

In particular, it allows us not only to perform easily a linear stability analysis of the basic state of uniform fluidization, but also to conclude that bifurcation of travelling waves occurs when this state becomes unstable. Well-known patterns like vertical and oblique travelling waves (OTW) of the form $u(x,y,t) = \tilde{u}(x-\omega t\pm ky)$, k [ges ] 0, are discovered. Owing to symmetry, the existence of standing travelling waves (STW) of the form $u(x,y,t) = \tilde{u}(x-\omega t, y)$ is also expected, but regrettably no mathematically rigorous proof of this last conjecture is presently available.

Bubble formation can also be approached via the instability of a vertical travelling plane wave train to transverse perturbations. Then a secondary stationary bifurcation to another kind of standing travelling waves takes place. This scenario is also in agreement with experimental observations. In addition, the occurrence of bifurcations of higher order, which lead to more and more complex wave patterns and are to be found on the route to turbulence, can be deduced.
Direct evaluation of aeroacoustic theory in a jet
Authors: Bridges James , Hussain Fazle
Published: April 2006

This paper provides a unique, detailed evaluation of basic aeroacoustic theory applied to low-Mach-number (M = 0.08) cold jets. In contrast to most prior studies comparing theoretical predictions of jet noise with experimental results, our comparison uses a relatively complete knowledge of the flow field and employs vortex sound theory — an acoustic analogy which is shown to be insensitive to those aspects of the flow field about which our knowledge is incomplete. The primary result is that the measured sound field directivity of vortex ring pairing in circular jets is very similar to that predicted by theory: a stationary, axisymmetric, lateral quadrupole. This directivity is very unlike the monotonic polar dependence found in time-average measures of jet noise fields and unlike the directivity found in similar excited jet experiments. Although not perfect, the agreement between experiment and theory here is satisfyingly close in comparison to the discrepancies found by Huerre & Oighton (1983). Our result also proves that pairing of purely axisymmetric coherent structures is not the dominant sound source in low-Mach-number jets and that vortex asymmetry must be an essential aspect of the vortex motions which produce noise in such jets.
Longitudinal shear-induced diffusion of spheres in a dilute suspension
Authors: Acrivos A. , Batchelor G. K. , Hinch E. J. , Koch D. L. , Mauri R.
Published: April 2006

We present a calculation of the hydrodynamic self-diffusion coefficient of a tagged particle in a dilute mono-dispersed suspension of small neutrally buoyant spheres undergoing a steady simple shearing motion. The displacement of the tagged particle parallel to the longitudinal or streamwise direction resulting from a ‘collision’ with one other particle is calculated on the assumption that inertia and Brownian motion effects are negligible. Summing over different pairs leads to a logarithmically divergent integral for the diffusivity which is rendered finite by allowing for the cutoff due to the occasional presence of another pair of particles. The longitudinal shearinduced self-diffusion coefficient is thus found to be 0.267a2γ{cln c−1 + O(c)], where γ denotes the applied shear rate, a is the radius of the spheres and c their volume concentration.
An analytic model of the generation of surface gravity waves by turbulent air flow
Authors: Van Duin Cornelis A. , Janssen Peter A. E. M.
Published: April 2006

Turbulent air flow over a surface gravity wave of small amplitude is studied on the basis of a family of first-order closure models, of which the eddy viscosity model and Prandtl's mixing-length model are members. Results are obtained by the method of matched asymptotic expansions in three layers. The problem is modelled by taking into account the combined effects of turbulence and molecular viscosity, which accommodates a proper imposition of the boundary conditions at the wave surface. The detailed structure of the various wave-induced field variables throughout the flow is then investigated. In addition, it is found that the growth rate of the waves by wind depends on the turbulence model. In particular, the more sensitively the mixing length depends on the shear in the mean air flow, the higher the growth rate. The validity of the results we obtain is restricted to small drag coefficient and small phase speed. Comparisons are made with other theoretical studies and with recent laboratory and field observations.
Over-reflection and shear instability in a shallow-water model
Authors: Takehiro Shin-Ichi , Hayashi Yoshi-Yuki
Published: April 2006

The characteristics of shallow-water waves in a linear shear flow are studied, and the relationship between waves and unstable modes is examined. Numerical integration of the linear shallow-water equations shows that over-reflection occurs when a wave packet is incident at the turning surface. This phenomenon can be explained by the conservation of momentum as discussed by Acheson (1976). The unstable modes of linear shear flow in a shallow water found by Satomura (1981) are described in terms of the properties of wave propagation as proposed by Lindzen and others. Ripas's (1983) theorem, which is the sufficient condition for stability of flows in shallow water, is also related to the wave geometry. The Orr mechanism, which is proposed by Lindzen (1988) as the primary mechanism of wave amplification, cannot explain the over-reflection of shallow-water waves. The amplification of these waves occurs in the opposite sense to that of Orr's solution.
Two-dimensional patterns in Rayleigh-Taylor instability of a thin layer
Authors: Fermigier M. , Limat L. , Wesfreid J. E. , Boudinet P. , Quilliet C.
Published: April 2006

We study experimentally and theoretically the evolution of two-dimensional patterns in the Rayleigh—Taylor instability of a thin layer of viscous fluid spread on a solid surface. Various kinds of patterns of different symmetries are observed, with possible transition between patterns, the preferred symmetries being the axial and hexagonal ones. Starting from the lubrication hypothesis, we derive the nonlinear evolution equation of the interface, and the amplitude equation of its Fourier components. The evolution laws of the different patterns are calculated at order two or three, the preferred symmetries being related to the non-invariance of the system by amplitude reflection. We also discuss qualitatively the dripping at final stage of the instability.
Cellular Stokes flow induced by rotation of a cylinder in a closed channel
Authors: Hellou Mustapha , Coutanceau Madeleine
Published: April 2006

The evolution of the cellular structure of the two-dimensional creeping flow induced by a rotating circular cylinder set in the centre of a rectangular channel is studied numerically and experimentally when the aspect ratio A increases from 1 to 7. In the calculations, depending on the value of A, either only series in terms of polar coordinates, or both matched polar and Cartesian coordinates series are employed to represent the stream function and an efficient least-squares method, very easy to program, is selected to satisfy some of the boundary conditions. For the experiments, a special technique which visualizes intermittently the paths of solid tracers during long times of exposure permits us to observe the fluid motion in the whole domain, even in the regions where the velocities are very small. An excellent measure of agreement between the numerical and experimental results is found. Thus it is clearly shown how, in the region beyond the rotating flow directly driven by the cylinder, the two main corner cells visualized at A = 1, develop with increasing A and then coalesce, to finally merge and give rise to a single central cell. This central cell develops in its turn, tending finally to the unbounded channel reference cell, after passing through a maximum length however. Owing to the very high precision of the calculations, many details of the flow development have been clearly shown, in particular the periodicity, with increasing A, of all the different phases, progressively inducing a succession of cells. The prediction that the angle of separation of the fluid boundaries of the cells tends towards the theoretical limit of 58.61° when the aspect ratio becomes large is also confirmed.
Nonlinear spatial equilibration of an externally excited instability wave in a free shear layer
Authors: Hultgren Lennart S.
Published: April 2006

A two-dimensional disturbance evolving from a strictly linear, finite-growth-rate instability wave with nonlinear effects first becoming important in the critical layer is considered. The analysis is carried out for a general weakly non-parallel mean flow using matched asymptotic expansions. The flow in the critical layer is governed by a nonlinear vorticity equation which includes a spatial-evolution term. As in Goldstein & Hultgren (1988), the critical layer ages into a quasi-equilibrium one and the initial exponential growth of the instability wave is converted into a weak algebraic growth during the roll-up process. This leads to a next stage of evolution where the instability-wave growth is simultaneously affected by mean-flow divergence and nonlinear critical-layer effects and is eventually converted to decay. Expansions for the various streamwise regions of the flow are combined into a single composite formula accounting for both shear-layer spreading and nonlinear critical-layer effects and good agreement with the experimental results of Thomas & Chu (1989), Freymuth (1966), and C.-M. Ho & Y. Zohar (1989, private communication) is demonstrated.
Flow in a centrifugal spectrometer
Authors: Dahlkild A. A. , Amberg G. , Greenspan H. P.
Published: April 2006

Rotational flow through narrow axial channels is considered in connection with a proposed technique to sort and separate particles according to sedimentation velocities. Nonlinear and linear axisymmetric flow through two channels connected by a slot in the vertical wall is studied numerically. A linearized formulation for the three-dimensional flow through a circumferentially blocked channel, with arbitrary positioning of the inlets and outlets, is examined analytically. Both approaches indicate that to have a sharp criteria for fractionation, the vertical shear layers on the channel walls must overlap. Otherwise, Coriolis effects, accompanying a strong azimuthal motion, make the sorting less precise. Results of an exploratory experiment with a simple two-stage machine demonstrate the feasibility of the basic process for simultaneous and continuous separation and fractionation.
On the multifractal properties of the energy dissipation derived from turbulence data
Authors: Aurell E. , Frisch U. , Lutsko J. , Vergassola M.
Published: April 2006

Various difficulties can be expected in trying to extract from experimental data the distribution of singularities, the f(α) function, of the energy dissipation. One reason is that the multifractal model of turbulence implies a dependence of the viscous cutoff on the singularity exponent. It is an open question if current hot-wire probes can resolve the scales implied by exponents a significantly less than 1.

Two exactly soluble models are used to show how spurious scaling can occur, due to finite Reynolds number effects. In the Gaussian model the true velocity signal is replaced by independent Gaussian random variables. The dissipation, defined as the square of the difference of successive variables, has trivial scaling in so far as all the moments of spatial averages of the dissipation behave asymptotically as a uniform dissipation. Still, contamination by subdominant terms requires that scaling exponents for high-order moments be identified over an increasingly large range of scales. If the available range is limited by the Reynolds number, scaling exponents for high orders will be systematically underestimated and spurious intermittency will be inferred. Burgers’ model is used to highlight further problems. At finite Reynolds numbers, regions with no small-scale activity (away from shocks) have a residual dissipation which contributes a spurious point (α = 1,f(α) = 1). In addition, when the f(α) function is obtained by Legendre transform techniques, convex hull effects generate an entire spurious segment.

Finally, Burgers’ model also indicates that the relation between exponents of structure functions and exponents of local dissipation moments, which goes back to Kolmogorov's (1962) work, leads to an inconsistency for structure functions of low positive order.
Quasi-periodic and chaotic flow regimes in a thermally driven, rotating fluid annulus
Authors: Read P. L. , Bel M. J. , Johnson D. W. , Small R. M.
Published: April 2006

Results are presented from analyses of high-precision time series of measurements of temperature and total heat transport obtained in a high-Prandtl-number (Pr = 26) fluid contained in a rotating, cylindrical annulus subject to a horizontal temperature gradient. Emphasis is placed on regions of parameter space close to the onset of irregular and/or chaotic behaviour. Two distinct transitions from oscillatory to apparently chaotic flow have been identified. The first occurs in an isolated region of parameter space at moderate to high Taylor number in association with a transition to a lower azimuthal wavenumber, in which a quasi-periodic (m = 3) amplitude vacillation (on a 2-torus) gives way to a low-dimensional (D ∼ 3) chaotically modulated vacillation at very low frequency (apparently organized about a 3-torus). The spatial structure of the chaotic flow exhibits the irregular growth and decay of azimuthal sidebands suggestive of a nonlinear competition between adjacent azimuthal wavenumbers. The other main transition to aperiodic flow occurs at high Taylor number as the stability parameter Θ is decreased, and is associated with the onset of ‘structural vacillation’. This transition appears to be associated with the development of small-scale instabilities within the main m = 3 baroclinic wave pattern, and exhibits a route to chaos via intermittency. The nature of the apparent chaos in these two aperiodic regimes is discussed in relation to possible mechanisms for deterministic chaos, apparatus limitations, and to previous attempts to model nonlinear baroclinic waves using low-order spectral models.
Almost symmetric solitary eddies in a two-layer ocean
Authors: Sutyrin G. G. , Dewar W. K.
Published: April 2006

An asymptotic theory of two-dimensional planetary solitary eddies is presented. Previous studies in one-and-a-half layer models have discovered special classes of radially symmetric structure which are associated with eddies of permanent form. We generalize these studies by including an active lower layer and by considering the effects of azimuthal structure. Accordingly, we stress two main results; namely, (i) permanent-form two-layer eddies with essentially arbitrary radial structure exist, provided that the eddy includes a weak imbedded dipolar asymmetry and an appropriate counter-rotating deep flow, and (ii) fluid trapped under an eddy in Taylor columns can significantly affect eddy properties if the trapped fluid possesses non-trivial potential vorticity.

The structural permanency in our solutions arises from a balance between nonlinear steepening, driven by the continuity equation, and planetary dispersion. The structural asymmetries affect eddy propagation, either by dipole interaction within the layer (as occurs in modons) or by pressure forces acting between layers. The primary role of the deep counter-rotating flow is to balance the net upper-layer transport. The interesting layer-layer interaction, however, involves higher-order dynamics and is sensitive to the continuity of the potential-vorticity field. In general, these eddies trap fluid both in the upper thermocline and in the lower layer.

The dominance of oceanic anticyclones over cyclones is relatively well known. A main conclusion of this study is that the class of long-lived anticyclones is considerably broader than previously realized. This may help explain the observed bias toward anticyclonic eddies. A second conclusion is that estimates of material transport by eddies may need to account for the movement of fluid outside the main bowl of the eddies.
Free-surface cusps associated with flow at low Reynolds number
Authors: Jeong Jae-Tack , Moffatt H. K.
Published: April 2006

When two cylinders are counter-rotated at low Reynolds number about parallel horizontal axes below the free surface of a viscous fluid, the rotation being such as to induce convergence of the flow on the free surface, then above a certain critical angular velocity Ωc, the free surface dips downwards and a cusp forms. This paper provides an analysis of the flow in the neighbourhood of the cusp, via an idealized problem which is solved completely: the cylinders are represented by a vortex dipole and the solution is obtained by complex variable techniques. Surface tension effects are included, but gravity is neglected. The solution is analytic for finite capillary number [Cscr ], but the radius of curvature on the line of symmetry on the free surface is proportional to exp (−32π[Cscr ]) and is extremely small for [Cscr ] [gsim ] 0.25, implying (in a real fluid) the formation of a cusp. The equation of the free surface is cubic in (x, y) with coefficients depending on [Cscr ], and with a cusp singularity when [Cscr ] = ∞.

The influence of gravity is considered through a stability analysis of the free surface subjected to converging uniform strain, and a necessary condition for the development of a finite-amplitude disturbance of the free surface is obtained.

An experiment was carried out using the counter-rotating cylinders as described above, over a range of capillary numbers from zero to 60; the resulting photographs of a cross-section of the free surface are shown in figure 13. For Ω < Ωc, a rounded crest forms in the neighbourhood of the central line of symmetry; for Ω > Ωc, the downward-pointing cusp forms, and its structure shows good agreement with the foregoing theory.
Bursts and the law of the wall in turbulent boundary layers
Authors: Morrison J. F. , Subramanian C. S. , Bradshaw P.
Published: April 2006

The bursting mechanism in two different high-Reynolds-number boundary layers has been analysed by means of conditional sampling. One boundary layer develops on a smooth, flat plate in zero pressure gradient; the other, also in zero pressure gradient, is perturbed by a rough-to-smooth change in surface roughness and the new internal layer has not yet recovered to the local equilibrium condition at the measurement station. Sampling on the instantaneousuvsignal in the logarithmic region confirms the presence of two related structures, ‘ejections’ and ‘sweeps’ which, in the smooth-wall layer, appear to be responsible for most of the turbulent energy production, and to effect virtually all that part of the spectral energy transfer that is universal. Ejections show features similar to those of Falco's ‘typical eddies’ while sweeps appear to be inverted ejections moving down towards the wall. The inertial structures associated with ejections show attributes of the true universal motion (Townsend's ‘attached’ eddies) of the inner layer and these are therefore identified as ‘bursts’. In the outer layer, these become ‘detached’ from the wall. The large-scale structures associated with sweeps also appear to be ‘detached’ eddies (‘splats’), but these induce low-wave-number inactive motion near the wall and this is not universal even though the sweep itself is. Neither ejections nor sweeps detected in the rough-to-smooth layer are near a condition of energy equilibrium. The relation of ejections and sweeps to the law of the wall and other accepted laws is discussed.
Parallel flow in Hele-Shaw cells
Authors: Zeybek M. , Yortsos Y. C.
Published: April 2006

We consider the parallel flow of two immiscible fluids in a Hele-Shaw cell. The evolution of disturbances on the fluid interfaces is studied both theoretically and experimentally in the large-capillary-number limit. It is shown that such interfaces support wave motion, the amplitude of which for long waves is governed by a set of KdV and Airy equations. The waves are dispersive provided that the fluids have unequal viscosities and that the space occupied by the inner fluid does not pertain to the Saffman-Taylor conditions (symmetric interfaces with half-width spacing). Experiments conducted in a long and narrow Hele-Shaw cell appear to validate the theory in both the symmetric and the non-symmetric cases.
On turbulent separation in the flow past a bluff body
Authors: Neish A. , Smith F. T.
Published: April 2006

The basic model problem of separation as predicted by the time-mean boundary-layer equations is studied, with the Cebeci-Smith model for turbulent stresses. The changes between laminar and turbulent flow are investigated by means of a turbulence ‘factor’ which increases from zero for laminar flow to unity for the fully turbulent regime. With an attached-flow starting point, a small increase in the turbulence factor above zero is found to drive the separation singularity towards the trailing edge or rear stagnation point for flow past a circular cylinder, according to both computations and analysis. A separated-flow starting point is found to produce analogous behaviour for the separation point. These findings lead to the suggestion that large-scale separation need not occur at all in the fully turbulent regime at sufficiently high Reynolds number; instead, separation is of small scale, confined near the trailing edge. Comments on the generality of this suggestion are presented, along with some supporting evidence from other computations. Further, the small scale involved theoretically has values which seem reasonable in practical terms.
The energy decay in self-preserving isotropic turbulence revisited
Authors: Speziale Charles G. , Bernard Peter S.
Published: April 2006

The assumption of self-preservation permits an analytical determination of the energy decay in isotropic turbulence. Batchelor (1948), who was the first to carry out a detailed study of this problem, based his analysis on the assumption that the Loitsianskii integral is a dynamic invariant – a widely accepted hypothesis that was later discovered to be invalid. Nonetheless, it appears that the self-preserving isotropic decay problem has never been reinvestigated in depth subsequent to this earlier work. In the present paper such an analysis is carried out, yielding a much more complete picture of self-preserving isotropic turbulence. It is proven rigorously that complete self-preserving isotropic turbulence admits two general types of asymptotic solutions: one where the turbulent kinetic energy K ∼ t−1 and one where K ∼ t−α with an exponent α > 1 that is determined explicitly by the initial conditions. By a fixed-point analysis and numerical integration of the exact one-point equations, it is demonstrated that the K ∼ t−1 power law decay is the asymptotically consistent high-Reynolds-number solution; the K ∼ t−α decay law is only achieved in the limit as t → ∞ and the turbulence Reynolds number Rt vanishes. Arguments are provided which indicate that a t−1 power law decay is the asymptotic state toward which a complete self-preserving isotropic turbulence is driven at high Reynolds numbers in order to resolve an O(R1½) imbalance between vortex stretching and viscous diffusion. Unlike in previous studies, the asymptotic approach to a complete self-preserving state is investigated which uncovers some surprising results.
The slow-drift motion of arrays of vertical cylinders
Authors: Emmerhoff O. J. , Sclavounos P. D.
Published: April 2006

The large-amplitude rectilinear ‘slow-drift’ oscillation of a floating body constrained by a weak restoring force in random waves is considered. The free-surface flow is approximated by a perturbation series expansion for a small slow-drift velocity and wave steepness. A model slow-drift equation of motion is derived, the time-dependent slow-drift excitation force and wave damping coefficient are defined and the complete series of free-surface problems governing their magnitude are formulated. The free-surface problem governing the wave-drift damping coefficient in monochromatic waves is studied and an explicit solution is obtained for a vertical circular cylinder of infinite draught. This solution is extended for arrays of vertical circular cylinders by employing an exact interaction theory. The wave-drift damping coefficient is evaluated for configurations of interest in practice and an expression is derived for the steady drifting velocity of an unconstrained body in regular waves.
Solitary internal waves with oscillatory tails
Authors: Akylas T. R. , Grimshaw R. H. J.
Published: April 2006

Solitary internal waves in a density-stratified fluid of shallow depth are considered. According to the classical weakly nonlinear long-wave theory, the propagation of each long-wave mode is governed by the Korteweg–de Vries equation to leading order, and locally confined solitary waves with a ‘sech’ profile are possible. Using a singular-perturbation procedure, it is shown that, in general, solitary waves of mode n > 1 actually develop oscillatory tails of infinite extent, consisting of lower-mode short waves. The amplitude of these tails is exponentially small with respect to an amplitude parameter, and lies beyond all orders of the usual long-wave expansion. To illustrate the theory, two special cases of stratification are discussed in detail, and the amplitude of the oscillations at the solitary-wave tails is determined explicitly. The theoretical predictions are supported by experimental observations.
Linear and nonlinear stability of the Blasius boundary layer
Authors: Bertolotti F. P. , Herbert Th. , Spalart P. R.
Published: April 2006

Two new techniques for the study of the linear and nonlinear instability in growing boundary layers are presented. The first technique employs partial differential equations of parabolic type exploiting the slow change of the mean flow, disturbance velocity profiles, wavelengths, and growth rates in the streamwise direction. The second technique solves the Navier–Stokes equation for spatially evolving disturbances using buffer zones adjacent to the inflow and outflow boundaries. Results of both techniques are in excellent agreement. The linear and nonlinear development of Tollmien–Schlichting (TS) waves in the Blasius boundary layer is investigated with both techniques and with a local procedure based on a system of ordinary differential equations. The results are compared with previous work and the effects of non-parallelism and nonlinearly are clarified. The effect of nonparallelism is confirmed to be weak and, consequently, not responsible for the discrepancies between measurements and theoretical results for parallel flow. Experimental uncertainties, the adopted definition of the growth rate, and the transient initial evolution of the TS wave in vibrating-ribbon experiments probably cause the discrepancies. The effect of nonlinearity is consistent with previous weakly nonlinear theories. White nonlinear effects are small near branch I of the neutral curve, they are significant near branch II and delay or event prevent the decay of the wave.
Instabilities of three-dimensional viscous falling films
Authors: Joo S. W. , Davis S. H.
Published: April 2006

A long-wave evolution equation is used to study a falling film on a vertical plate. For certain wavenumbers there exists a two-dimensional strongly nonlinear permanent wave. A new secondary instability is identified in which the three-dimensional disturbance is spatially synchronous with the two-dimensional wave. The instability grows for sufficiently small cross-stream wavenumbers and does not require a threshold amplitude; the two-dimensional wave is always unstable. In addition, the nonlinear evolution of three-dimensional layers is studied by posing various initial-value problems and numerically integrating the long-wave evolution equation.
The leading edge of an oil slick, soap film, or bubble stagnant cap in Stokes flow
Authors: Harper J. F.
Published: April 2006

Trace impurities often collect on the upstream side of an obstacle in the surface of flowing liquid. The transition from practically free surface to surface sufficiently clogged to be treated as stationary can be quite sharp. The viscous flow underneath is nonlinearly coupled to the convective mass transfer of surface-active material. For two-dimensional flow at high Reynolds number the first observations were due to Thoreau, Langton and Reynolds over 100 years ago, and the theory was given by Harper & Dixon in 1974. If the whole problem is considered from a frame of reference moving with the stream instead of fixed to the downstream surface film, the solution refers to the leading edge of a slowly spreading oil slick.

The present work gives the theory corresponding to Harper & Dixon's for low Reynolds numbers (Stokes flow), for which there is a very simple leading approximation near the transition for a soluble surfactant, and a more complicated one, which can still be found exactly, for an insoluble surfactant which spreads onto clear liquid by surface diffusion. In both cases the surface remains flat: the ridge often observed is not a Stokes flow phenomenon.

The results are used to clarify the circumstances in which Savic's stagnant-cap approximation is useful for a bubble rising in a viscous liquid: the rear stagnation point now plays the role of the obstacle in the surface, and the flow near the surface transition can be treated locally as if it were two-dimensional instead of axisymmetric.
Fixed-flux convection in a tilted slot
Authors: Cessi Paola , Young W. R.
Published: April 2006

We study fixed-flux convection in a long, narrow slot which is inclined to the horizontal. (Gravity is in the vertical direction, and horizontal is perpendicular to this.) Because of the fixed-flux boundary conditions the convective modes have much larger lengthscales in the along-slot direction than in the transverse direction. In the case of a horizontal slot this disparity in scales has been previously exploited to obtain an amplitude equation for the single mode which first becomes unstable as the Rayleigh number is increased above critical. When the slot is tilted we show that there is a distinguished limit in which there are two active modes in the slightly supercritical regime. This new limit is when the horizontal wavenumber, the supercriticality, and the tilt of the slot away from vertical, are all small. A modification of the well-known expansion for fixed flux convection in a horizontal slot leads to a coupled system of partial differential equations for the amplitudes of the two modes.

Numerical solution of this system suggests that all initial conditions eventually evolve into one of the two states, both of which consist of a single, steady roll in the cavity. The states are distinguished by the direction of circulation of the roll, and by the buoyancy fields, which are quite different in the two cases.
Transition to turbulence in a rotating channel
Authors: Finlay W. H.
Published: April 2006

Direct numerical simulation is used to determine the flows that occur as the Reynolds number, Re, is increased in a plane channel undergoing system rotation about a spanwise axis. (Plane Poiseuille flow occurs for zero rotation rate and low Re.) A constant system rotation speed of 0.5, non-dimensionalized with respect to the bulk streamwise velocity and channel full width, is used throughout. The spectral numerical method solves the three-dimensional, time-dependent, incompressible Navier-Stokes equations using periodic boundary conditions in the streamwise and spanwise directions. On increasing the Reynolds number above the temporally periodic wavy vortex regime, near Re = 4Rec (Rec = 88.6 is the critical Re for development of vortices), a second temporal frequency, ω2, occurs in the flow that corresponds to slow, constant, spanwise motion of the vortices, superposed on the much faster, constant, streamwise motion of the wavy vortex waves. Curiously, ω2 is always frequency locked with the wavy vortex frequency ω1 for the parameter range explored, although the locking ratio varies. At the slightly higher Re of 4.1 Rec, ω2 is replaced by a new frequency ω′2 that corresponds to a modulation of the wavy vortices like that seen in modulated wavy Taylor vortex flow. However, unlike the Taylor-Couette geometry, the modulation frequency here can become frequency locked with the wavy vortex frequency. Increasing Re further to Re = 4.2 Rec results in the appearance of a second incommensurate modulation frequency ω3, yielding a quasi-periodic three-frequency flow, although there are only two frequencies (ωω′2 and ω3) present in the reference frame moving with the travelling wave associated with ω1. At still higher Re (Re = 4.5 Rec), weak temporal chaos occurs. This flow is not turbulent however. Calculations of the instantaneous largest Lyapunov exponent, λ(t), and the spatial structure of small perturbations to the flow show that the chaos is driven by spanwise shear instability of the streamwise velocity component. At the highest Re of 6.7 Rec considered, quasi-coherent turbulent boundary layer structures occur as transient, secondary streamwise-oriented vortices in the viscous sublayer near the inviscidly unstable (high-pressure) wall. Calculations of λ(t) and the spatial structure of small perturbations to the flow show that the coherent structures are not caused by the local growth of small disturbances to the flow.
Secondary motion of fully developed oscillatory flow in a curved pipe
Authors: Sudo K. , Sumida M. , Yamane R.
Published: April 2006

Experimental and numerical studies were made of the secondary flow induced in fully developed oscillatory laminar flow in a curved circular pipe. Photographs of traces of nylon particles suspended in water were taken systematically with Womersley numbers α = 5.5 ∼ 28 and oscillatory Dean numbers D = 40 ∼ 500. The secondary flow velocity component and the location of the vortex eye were obtained from the photographs. The experimental results were checked with the numerical ones and the variations of the secondary flow pattern with the Dean and Womersley numbers were analysed based on both results. These results suggest that secondary flows can be classified into five patterns.