bluuebunny/crossref_metadata_2025 huggingface dataset:
Calculation of turbulence-driven secondary motion in non-circular ducts
Authors: Demuren A. O. , Rodi W.
Published: April 2006

Experiments on and calculation methods for flow in straight non-circular ducts involving turbulence-driven secondary motion are reviewed. The origin of the secondary motion and the shortcomings of existing calculation methods are discussed. A more refined model is introduced, in which algebraic expressions are derived for the Reynolds stresses in the momentum equations for the secondary motion by simplifying the modelled Reynolds-stress equations of Launder, Reece & Rodi (1975), while a simple eddy-viscosity model is used for the shear stresses in the axial momentum equation. The kinetic energy k and the dissipation rate ε of the turbulent motion which appear in the algebraic and the eddy-viscosity expressions are determined from transport equations. The resulting set of equations is solved with a forward-marching numerical procedure for three-dimensional shear layers. The model, as well as a version proposed by Naot & Rodi (1982), is tested by application to developing flow in a square duct and to developed flow in a partially roughened rectangular duct investigated experimentally by Hinze (1973). In both cases, the main features of the mean-flow and the turbulence quantities are simulated realistically by both models, but the present model underpredicts the secondary velocity while the Naot-Rodi model tends to overpredict it.
On the interaction of compliant coatings with boundary-layer flows
Authors: Gad-El-Hak Mohamed , Blackwelder Ron F. , Riley James J.
Published: April 2006

The interactions of compliant coatings with laminar, transitional and turbulent boundary layers are investigated. A 2 m long flat plate is towed in the range of speeds of 20–140 cm/s in an 18 m water channel using a carriage riding on an oil film. Isotropic and anistropic compliant coatings are used to cover about 20% of the working Plexiglas surface. The compliant material used is a viscoelastic plastisol gel produced by heating a mixture of polyvinyl chloride resin, a plasticizer and a stabilizer, and allowing them to gel. The shear modulus of rigidity of the plastisol was varied by changing the percentage of PVC in the mix. Anisotropy is introduced by placing the plastisol on a rubber surface having longitudinal grooves scaled with the low-speed streaks in the turbulent boundary layer. The most pronounced effect of the surface compliance in a turbulent boundary layer is a hydroelastic instability in the form of a spanwise wave structure on the compliant surface. The compliant-surface deformation was measured using a novel remote optical technique. The onset speed of the hydroelastic instability waves depends on the thickness and the modulus of rigidity of the plastisol. Their wavelength, wave speed and amplitude are found to depend on these plastisol parameters as well as on the towing speed. In a laminar boundary layer with freestream speeds of over twice the corresponding onset velocity for the turbulent case, no similar instability is observed.
The effects of curvature on the flow field in rapidly rotating gas centrifuges
Authors: Wood Houston G. , Jordan Jo Ann , Gunzburger Max D.
Published: April 2006

The effects of curvature on the fluid dynamics of rapidly rotating gas centrifuges are studied. A governing system of a linear partial differential equation and boundary conditions is derived based on a linearization of the equations for viscous compressible flow. This system reduces to the Onsager pancake model if the effects of curvature are neglected. Approximations to the solutions of the governing equations with and without curvature terms are obtained via a finite-element method. Two examples are considered: first where the flow is driven by a thermal gradient at the wall of the centrifuge, and then for the flow being driven by the introduction and removal of mass through the ends of the centrifuge. Comparisons of the results obtained show that, especially for the second example, the inclusion of the terms due to curvature in the model can have an appreciable effect on the solution.
Rotating flow past disks and cylindrical depressions
Authors: Boyer Don L. , Davies Peter A. , Holland William R.
Published: April 2006

The flow of a rotating fluid over isolated three-dimensional topographic features has been investigated. Laboratory experiments have been conducted with a rotating water channel to determine the characteristics of the flow over and around truncated cylindrical disks and depressions located on the upper and lower bounding surfaces of the channel. The geometry of the rotating channel allowed the beta effect to be modelled and cases of beta-plane westward, beta-plane eastward and f-plane flows were studied. Flow patterns revealed by the use of an electrolytic precipitation technique are presented, and the flow behaviour is quantified in terms of a characteristic speed of the flow through a circular cylinder circumscribing the topography in the vicinity of the channel midplane. Case studies are presented for a range of values of Rossby number, Ekman number, beta parameter, and cylinder height-to-radius ratio. The vorticity equation and associated boundary conditions are discussed for the cases studied in the laboratory and appropriate numerical solutions are obtained. The laboratory and numerical experiments demonstrate the character of the horizontal steering of fluid by the topographic features as a function of the system parameters. Comparisons between laboratory and numerical experiments are presented and shown to be in good agreement.
On the excitation of long nonlinear water waves by a moving pressure distribution
Authors: Akylas T. R.
Published: April 2006

A study is made of the wave disturbance generated by a localized steady pressure distribution travelling at a speed close to the long-water-wave phase speed on water of finite depth. The linearized equations of motion are first used to obtain the large-time asymptotic behaviour of the disturbance in the far field; the linear response consists of long waves with temporally growing amplitude, so that the linear approximation eventually breaks down owing to finite-amplitude effects. A nonlinear theory is developed which shows that the generated waves are actually of bounded amplitude, and are governed by a forced Korteweg-de Vries equation subject to appropriate asymptotic initial conditions. A numerical study of the forced Korteweg-de Vries equation reveals that a series of solitons are generated in front of the pressure distribution.
Contaminant dispersion in some time-dependent laminar flows
Authors: Jimenez C. , Sullivan P. J.
Published: April 2006

Time-dependent flows occur naturally as in pulsatile blood flow and in tidal estuaries and comprise many of the man-made flows of practical importance. A knowledge of the rate of mixing of a contaminant substance in such time-dependent flows is of paramount interest in, for example, the injection of a chemical substance in blood flow, the discharge of outfalls in estuaries, and the mutual contamination length of two feed fluids when switching from one feed line to another as part of a manufacturing process. This paper presents a study of contaminant spread in two specific and well-defined flows and provides a basis for the interpretation of contaminant mixing in the more complex flow situations that normally prevail.

An extension of the probabilistic formulation of the streamwise dispersion of contaminant molecules given in Dewey & Sullivan (1982) is used to study time-dependent laminar flows between parallel plates and in tubes wherein the flows are homogeneous in the streamwise direction. The two flows considered in detail are oscillating flows and impulsively started flows. In impulsively started flows it is shown that, although the basic dispersive mechanism acts in much the same way as described by Taylor (1953), the start-up effects on the dispersion can be quite prolonged and very significantly reduce the streamwise spread of contaminant over that which is observed in the fully developed flow. In oscillatory flows, unlike the situation presented by Taylor (1953) in which a diminished value of molecular diffusivity κ increases the contaminant cloud axial growth rate in a tube of radius a, it is found that optimal streamwise contaminant spread results from a value of κ that depends upon kinematic viscosity v and frequency ω. The streamwise cloud-variance growth rate is explored over the full range of the two parameters γ2 = ωa2/κ and λ2 = ωa2/2v for all time, and it is shown that a global maximum results when γ ≈ 2π and λ ≈ 2.
On the use of an annulus to study mixed-layer entrainment
Authors: Deardorff J. W. , Yoon S.-C.
Published: April 2006

An annulus covered by a rotating screen (or surface) which imparts stress and turbulence to the upper of two fluid layers has been shown by Scranton & Lindberg (1983) and Jones & Mulhearn (1983) to produce mixed-layer entrainment that is dependent on annulus curvature and geometry. This general result is confirmed here, where photographs of the mixed-layer interface indicate that turbulent entrainment rapidly diminishes away from the outer wall. The problem is explained as a damping of the turbulence within the bulk of the mixed layer by inertial stability associated with angular momentum increasing rapidly with radius except very close to the outer wall. Through hot-film-anemometer measurements, a shallow turbulent Ekman layer is found to exist immediately underneath the rotating screen, having a depth of order $ 0.4u_{*}r/\overline{v}$, where u* is the friction velocity, $\overline{v}$ is the mean circumferential flow speed within the layer and r is the radius. For r of order 1 m, this depth is typically only a small fraction of the mixed-layer depth determined from the density profile.

When the stress input is placed near the inner wall, the mean-flow radial profile is much more nearly irrotational and the turbulence-damping problem becomes minimal. The entrainment is then much more uniform with radius, and local interfacial scaling can be used to relate the entrainment rate to one or more local Richardson numbers containing a turbulent velocity scale rather than u* or $\Delta\overline{v}$, where Δ signifies the change across the interface. Although u* and $\Delta\overline{v}$ are proper ‘external’ scales for entrainment in a horizontally homogeneous mixed layer that is turbulent throughout, a local internal turbulent velocity scale can alternatively be used. Entrainment rates thus scaled are presented and compared with Turner's (1973) locally scaled entrainment.
Investigating cavity/wake dynamics for a circular cylinder by measuring noise spectra
Authors: Fry S. A.
Published: April 2006

Noise spectra are measured at different distances downstream of a cavitating circular cylinder. The effects of velocity and cavitation number on noise spectra are presented. Together with flow observation the noise measurements allow the cavity development to be described and related to cavity-collapse intensity.
On instabilities arising during sedimentation of two-component mixtures of solids
Authors: Weiland R. H. , Fessas Y. P. , Ramarao B. V.
Published: April 2006

Experimental evidence is presented which shows that both uniform and differential sedimentation are unstable flows for relatively concentrated fluid suspensions containing particles of two different densities or sizes. The instability takes the form of a new type of fingering phenomenon which develops as a result of the two kinds of particles segregating laterally from each other.
Stresses developed by dry cohesionless granular materials sheared in an annular shear cell
Authors: Savage S. B. , Sayed M.
Published: April 2006

Experimental results obtained during rapid shearing of several dry, coarse, granular materials in an annular shear cell are described. The main purpose of the tests was to obtain information that could be used to guide the theoretical development of constitutive equations suitable for the rapid flow of cohesionless bulk solids at low stress levels. The shear-cell apparatus consists of two concentric disk assemblies mounted on a fixed shaft. Granular material was contained in an annular trough in the bottom disk and capped by a lipped annular ring on the top disk. The bottom disk can be rotated at specified rates, while the top disk is loaded vertically and is restrained from rotating by a torque arm connected to a force transducer. The apparatus was thus designed to determine the shear and normal stresses as functions of solids volume fraction and shear rate.

Tests were performed with spherical glass and polystyrene beads of nearly uniform diameters, spherical polystyrene beads having a bimodal size distribution and with angular particles of crushed walnut shells. The particles ranged from about ½ to 2 mm in size. At the lower concentrations and high shear rates the stresses are generated primarily by collisional transfer of momentum and energy. Under these conditions, both normal and shear stresses were found to be proportional to the particle density, and the squares of the shear rate and particle diameter. At higher concentrations and lower shear rates, dry friction between particles becomes increasingly important, and the stresses are proportional to the shear rate raised to a power less than two. All tests showed strong increases in stresses with increases in solids concentrations. The ratio of shear to normal stresses showed only a weak dependence upon shear rate, but it increased with decreasing concentration. At the very highest concentrations with narrow shear gaps, finite-particle-size effects became dominant and differences in stresses of as much as an order of magnitude were observed for the same shear rate and solids concentration.
Numerical calculation of decaying isotropic turbulence using the LET theory
Authors: Mccomb W. D. , Shanmugasundaram V.
Published: April 2006

The local-energy-transfer (LET) theory (McComb 1978) was used to calculate freely decaying turbulence for four different initial spectra at low-to-moderate values of microscale Reynolds numbers (Rλ up to about 40). The results for energy, dissipation and energy-transfer spectra and for skewness factor all agreed quite closely with the predictions of the well-known direct-interaction approximation (DIA: Kraichnan 1964). However, LET gave higher values of energy transfer and of evolved skewness factor than DIA. This may be related to the fact that LET yields the k−5/3 law for the energy spectrum at infinite Reynolds number.

The LET equations were then integrated numerically for decaying isotropic turbulence at high Reynolds number. Values were obtained for the wavenumber spectra of energy, dissipation rate and inertial-transfer rate, along with the associated integral parameters, at an evolved microscale Reynolds number Rλ of 533. The predictions of LET agreed well with experimental results and with the Lagrangian-history theories (Herring & Kraichnan 1979). In particular, the purely Eulerian LET theory was found to agree rather closely with the strain-based Lagrangian-history approximation; and further comparisons suggested that this agreement extended to low Reynolds numbers as well.
Lunate-tail swimming propulsion as a problem of curved lifting line in unsteady flow. Part 1. Asymptotic theory
Authors: Cheng H. K. , Murillo Luis E.
Published: April 2006

The asymptotic theory of a high-aspect-ratio wing in an incompressible flow is generalized to an oscillating lifting surface with a curved centreline in the domain where the reduced frequency based on the half-span is of order unity. The formulation allows applications to lightly loaded models of lunate-tail swimming and ornithopter flight, provided that the heaving displacement does not far exceed the mean wing chord. The analysis include the quasi-steady limit, in which the crescent-moon wing problem considered earlier by Thurber (1965) is solved and several aerodynamic properties of swept wings are explained.

Among the important three-dimensional and unsteady effects are corrections for the centreline curvature and for the spanwise components of the locally shed vortices. Comparison of the lift distributions obtained for model lunate tails with data computed from the doublet-lattice method (Albano & Rodden 1969) lends support to the asymptotic theory.
Organized structures in a reattaching separated flow field
Authors: Troutt T. R. , Scheelke B. , Norman T. R.
Published: April 2006

Spanwise structures in a two-dimensional reattaching separated flow were studied using multisensor hot-wire anemometry techniques. The results of these measurements strongly support the existence and importance of large-scale vortices in both the separated and reattached regions of this flow. Upstream of reattachment, vortex pairings are indicated and the spanwise structures attain correlation scales closely comparable to previously measured mixing-layer vortices. These large-scale vortices retain their organization far downstream of the reattachment region. However, pairing interactions appear to be strongly inhibited in this region. It is suggested that large-scale vortex dynamics are primarily responsible for some of the important time-averaged features of this flow. Notably, the reduction of turbulence energy in the reattachment region and the slow transition of the mean flow downstream of reattachment are attributed to effects associated with these vortices.
Kinematic dynamo problem in a linear velocity field
Authors: Zel'Dovich Ya. B. , Ruzmaikin A. A. , Molchanov S. A. , Sokoloff D. D.
Published: April 2006

A magnetic field is shown to be asymptotically (t → ∞) decaying in a flow of finite conductivity with v = Cr, where C = Cζ(t) is a random matrix. The decay is exponential, and its rate does not depend on the conductivity. However, the magnetic energy increases exponentially owing to growth of the domain occupied by the field. The spatial distribution of the magnetic field is a set of thin ropes and (or) layers.
Simulation of Taylor-Couette flow. Part 1. Numerical methods and comparison with experiment
Authors: Marcus Philip S.
Published: April 2006

We present a numerical method that allows us to solve the Navier-Stokes equation with boundary conditions for the viscous flow between two concentrically rotating cylinders as an initial-value problem. We use a pseudospectral code in which all of the time-splitting errors are removed by using a set of Green functions (capacitance matrix) that allows us to satisfy the inviscid boundary conditions exactly. For this geometry we find that a small time-splitting error can produce large errors in the computed velocity field. We test the code by comparing our numerically determined growth rates and wave speeds with linear theory and by comparing our computed torques with experimentally measured values and with the values that appear in other published numerical simulations. We find good agreement in all of our tests of the numerical calculation of wavy vortex flows. A test that is more sensitive than the comparison of torques is the comparison of the numerically computed wave speed with the experimentally observed wave speed. The agreements between the simulated and measured wave speeds are within the experimental uncertainties; the best-measured speeds have fractional uncertainties of less than 0.2%.
Instabilities of convection rolls with stress-free boundaries near threshold
Authors: Busse F. H. , Bolton E. W.
Published: April 2006

The stability properties of steady two-dimensional solutions describing convection in a horizontal fluid layer heated from below with stress-free boundaries are investigated in the neighbourhood of the critical Rayleigh number. The region of stable convection rolls as a function of the wavenumber α and the Rayleigh number R is bounded towards higher α by the monotonic skewed varicose instability, while towards low wavenumbers stability is limited by the zigzag instability or by the oscillatory skewed varicose instability. Only for a limited range of Prandtl numbers, 0·543 < P < ∞, does a finite domain of stability exist. In particular, convection rolls with the critical wavenumber αc are always unstable.
Self-consistent effective-medium parameters for oceanic internal waves
Authors: Dewitt R. J. , Wright Jon
Published: April 2006

In this paper we apply a formalism introduced in a previous paper to write down a self-consistent set of equations for the functions that describe the near-equilibrium time behaviour of random oceanic internal waves. These equations are based on the direct-interaction approximation. The self-consistent equations are solved numerically (using the Garrett-Munk spectrum as input) and the results are compared to parameters obtained in the weak-interaction approximation (WIA). The formalism points out that an extra parameter that is implicitly vanishingly small in the WIA has a significant effect on decay rates when computed self-consistently. We end by mentioning possible future self-consistent calculations that would improve upon our own.
Evolution of the flow field associated with a streamwise diffusing vortex
Authors: Pearson C. Frederick , Abernathy F. H.
Published: April 2006

The evolution of a shear flow with an imbedded streamwise vortex is considered. An idealized model for the vortical structure is used; the vortex is assumed to be of infinite extent in the stream direction, and to be a potential vortex (vortex filament) turned on at time zero, and subsequently allowed to diffuse under the action of viscosity. The ambient flow is taken to be, initially, a linear shear profile; the flow then evolves under the joint action of viscosity and convection induced by the vortex. Boundaries are assumed to be infinitely removed from the vortex core. A similarity variable is found which reduces the equation for the induced streamwise velocity perturbation to an ordinary differential equation, which is easily solved numerically. The vortex Reynolds number, circulation/viscosity, is found to be of prime importance. Calculated velocity profiles are presented.
Pressure spectra in turbulent free shear flows
Authors: George William K. , Beuther Paul D. , Arndt Roger E. A.
Published: April 2006

Spectral models for turbulent pressure fluctuations are developed by directly Fourier transforming the integral solution to the Poisson equation for a homogeneous constantmean-shear flow. The turbulence-turbulence interaction is seen to possess the well-knownk−7/3inertial subrange and to dominate the high-wavenumber region. The turbulence–mean-shear contribution is seen to be dominant in the energy-containing range and falls off as$k^{-\frac{11}{3}}$in the inertial subrange. The subrange constants and the mean-square pressure fluctuation are evaluated using a spectral model for the velocity. A spectral analysis of the velocity contamination of a pressure probe is also presented. Results are compared with spectral measurements with a static-pressure probe in the mixing layer of an axisymmetric jet.
The effect of rigid rotation on the finite-amplitude stability of pipe flow at high Reynolds number
Authors: Akylas T. R. , Demurger J.-P.
Published: April 2006

A theoretical study is made of the stability of pipe flow with superimposed rigid rotation to finite-amplitude disturbances at high Reynolds number. The non-axisymmetric mode that requires the least amount of rotation for linear instability is considered. An amplitude expansion is developed close to the corresponding neutral stability curve; the appropriate Landau constant is calculated. It is demonstrated that the flow exhibits nonlinear subcritical instability, the nonlinear effects being particularly strong owing to the large magnitude of the Landau constant. These findings support the view that a small amount of extraneous rotation could play a significant role in the transition to turbulence of pipe flow.
Turbulent airflow over water waves-a numerical study
Authors: Al-Zanaidi M. A. , Hui W. H.
Published: April 2006

Turbulent airflow over a Stokes water-wave train of small amplitude is studied numerically based on the two-equation closure model of Saffman & Wilcox (1974) together with appropriate boundary conditions on the wave surface. The model calculates, instead of assuming, the viscous sublayer flow, and it is found that the energy transfer between wind and waves depends significantly on the flow being hydraulically rough, transitional or smooth. Systematic computations have yielded a simple approximate formula for the fractional rate of growth per radian
\[
\zeta = \delta_{\rm i}\frac{\rho}{\rho_{\rm w}}\left(\frac{U_{\lambda}}{c}-1 \right)^2,
\]
with δi = 0.04 for transitional or smooth flow and δi = 0.06 for rough flow, where ρ is density of air, ρw that of water, Uλ wind speed at one wavelength height and c the wave phase velocity. This formula is in good agreement with most existing data from field experiments and from wave-tank experiments. In the case of waves travelling against wind, the corresponding values are δi = −0.024 for transitional and smooth flow, and δi = −0.04 for rough flow.
Magnetohydrodynamic Flows and Turbulence: a report on the Fourth Beer-Sheva Seminar
Authors: Branover H. , Mond M. , Pierson E. S.
Published: April 2006

This paper is a summary of the Fourth Beer-Sheva Seminar on Magnetohydrodynamic (MHD) Flows and Turbulence held in Israel during 27 February–2 March 1984 with 67 participants from 13 countries. Reviews and contributed papers were presented on laminar and turbulent single-phase and two-phase MHD flows, turbulent and two-phase flows without magnetic fields, and applications of MHD in power generation, in nuclear fission and fusion and in metallurgy.
The upstream scale of flow development in curved circular pipes
Authors: Olson D. E. , Snyder B.
Published: April 2006

For steady airflows at moderate Dean number (100 [les ] κ [les ] 500) axial- and transversevelocity profiles were delineated by anemometry in the entry region of curved circular pipes, having curvature ratios R/a of 4.66 or 8. Uniform inlet velocity profiles were found to evolve downstream over two inviscid length scales. Axial profiles develop on the lengthscale a, the pipe radius. The upstream influence extends at least 6a into a bend. Within this region axial development is largely inviscid and apparently independent of the radius of curvature R. Secondary currents develop on the scale (aR)½. Their initial growth is consistent with a model of vorticity transport based on streamline curvature, but instead of progressing monotonically to an asymptotic value, this swirl first overshoots, then subsides approximately 2(aR)½ downstream from the inlet. Hawthorne's model of streamline twist appears to account qualitatively for this dampening.
Observations of rapidly flowing granular-fluid materials
Authors: Hanes Daniel M. , Inman Douglas L.
Published: April 2006

The rapid shearing of a mixture of cohesionless glass spheres and air or water was studied in an annular, parallel-plate shear cell designed after Savage (1978). Two types of flow were observed. In the first type of flow the entire mass of the granular material was mobilized. At high shear rates the shear and normal stresses were found to be quadratically dependent upon the mean shear rate (at constant volume concentration), in general agreement with the observations of Bagnold (1954) and Savage & Sayed (1984), and the ‘kinetic’ theory of Jenkins & Savage (1983). The stresses were found to be weakly dependent on the volume concentration up to approximately 0.5, and strongly dependent above this concentration. For flows in which water was the interstitial fluid, the ratio of the shear stress to the normal stress was slightly higher (than in air), and the stresses at lower shear rates were found to be more nearly linearly related to the shear rate. It is suggested that these effects are contributed to by the viscous dampening of grain motions by the water. The second type of flow was distinguished by the existence of an internal boundary above which the granular material deformed rapidly, but below which the granular material remained rigidly locked in place. The thickness of the shearing layer was measured to be between 5 and 15 grain diameters. The stress ratio at the bottom of the shearing layer was found to be nearly constant, suggesting the internal boundary is a consequence of the immersed weight of the shearing grains, and may be described by a Coulomb yield criterion. A scaled concentration is proposed to compare similar data obtained using different-sized materials or different apparatus. An intercomparison of the two types of flow studied, along with a comparison between the present experiments and those of Bagnold (1954) and Savage & Sayed (1984), suggests that the nature of the boundaries can have a significant effect upon the dynamics of the entire flow.
Some observations of the subcritical transition in plane Poiseuille flow
Authors: Nishioka Michio , Asai Masahito
Published: April 2006

The subcritical transition in plane Poiseuille flow (generated in a long wind channel of rectangular cross-section) was studied experimentally. Cylinder-generated vortical disturbances were introduced into the parabolic flow (in the test section) or the inlet flow. The parabolic flow was also disturbed by a controlled periodic jet from a wall orifice. On the basis of the three kinds of observations, we come to the conclusion that the minimum transition Reynolds number is about 1000 and the related threshold intensity (of the external disturbance triggering the transition) is comparable to the maximum intensity of u-fluctuations in fully turbulent channel flows.
New derivations of Darwin's theorem
Authors: Yih Chia-Shun
Published: April 2006

Two new derivations of Darwin's theorem on the equality of the added mass for translation of a body moving in an ideal fluid of infinite extent and the drift mass are given. The first is based on the idea of time lag, used by Rayleigh (1876), Ursell (1953), and Longuet-Higgins (1953) to study fluid drift. The second is truly elementary, relying only on the concept of continuity and Newton's second law of motion. A geometrical interpretation of the result in the first derivation is given, and a few examples are provided.
Effect of initial conditions on two-dimensional free shear layers
Authors: Dziomba B. , Fiedler H. E.
Published: April 2006

The influence of periodic perturbations on the development of two-dimensional free shear layers generated by a splitter plate was investigated in cases where the ratios of the two velocities u1 and u2 either side of the splitter plate were such that 0 < u1/u2 < 1. Investigations were carried out in both a suction and a blower wind tunnel. Results show that even very weak periodic perturbations caused by the wind tunnel may cause significant nonlinear spreading in the downstream development of the shear layer, a behaviour which is also observed when the shear layer is deliberately excited. Other things being equal, the effect of the disturbance is greater when flow separation at the splitter plate is turbulent than when it is laminar.

No self-induced feedback frequencies were measured in the test section. All tonal components that were detected in the flow could be traced to external sources.

The influence of trailing-edge thickness on the shear-layer development is found to become significant when it exceeds 50% of the sum of boundary-layer displacement thickness at the point of separation. As the trailing edge becomes thicker, the range over which the shear layer is self-similar is shifted farther downstream. This behaviour may be crucial for predicting the evolution of shear layers in high-speed flows having thin boundary layers at separation.

The momentum thickness criterion for estimating the development length of the flow as suggested by Bradshaw is shown to be insufficient for two-stream layers, where additional parameters, e.g. the trailing-edge geometry, have to be taken into account. Discrepancies between previously published observations of shear layers, as well as the considerable scatter in reported measurements, may therefore, to a large extent, be attributable to contamination of the experimental facility.
Visualization of the viscous flow induced by a round jet
Authors: Zauner Erwin
Published: April 2006

The flow induced by a laminar round glycerol jet emerging from a plane wall is investigated. The flow field (jet region as well as outer flow field) is visualized by means of very small bubbles suspended in the fluid. For slender jets the observed flow pattern disagrees with both potential-flow theory and Squire's ‘exact’ solution, but it is in excellent agreement with results of an asymptotic analysis by Schneider (1981). At a downstream distance that depends exponentially on the square of the Reynolds number, the jet develops into a toroidal eddy as predicted by Schneider (1985). In an apparatus of reasonable size the eddy can be observed only in a very limited Reynolds-number regime.
A numerical study of quasi-geostrophic flow over isolated topography
Authors: Verron J. , Le Provost C.
Published: April 2006

An extensive set of numerical simulations is performed to synthesize the behaviour of a barotropic flow over isolated topography on an f-plane and on a β-plane. The model is based on the quasi-geostrophic vorticity equation, where the dissipation terms have been retained. The use of open boundary conditions. following the method described by Orlanski (1976), allows detailed simulation of time-dependent flows over long periods.

On the f-plane, the ultimate solution is always characterized by a typical vorticity field with an anticyclonic vortex trapped over the topography, but different transient regimes occur, related to the importance of advection versus topography effect: direct advection of the positive vortex for strong flows; eddy interactions and double-vortex-structure appearance for weaker flows; oscillatory regimes with topographic trapped-waves generation for very strong vorticity-interaction cases.

On the β-plane, and for prograde flows, the situation is complicated by a Rossby wave pattern extending mainly downstream but also having an upstream component corresponding to zonal waves. For retrograde flows the obstacle does not excite Rossby waves but a transient response with zonal waves whose lifetime depends on the nonlinearity.
Effect of rotation on isotropic turbulence: computation and modelling
Authors: Bardina Jorge , Ferziger J. H. , Rogallo R. S.
Published: April 2006

This paper uses numerical simulation to analyse the effects of uniform rotation on homogeneous turbulence. Both large-eddy and full simulations were made. The results indicate that the predominant effect of rotation is to decrease the rate of dissipation of the turbulence and increase the lengthscales, especially those along the axis of rotation. These effects are a consequence of the reduction, due to the generation of inertial waves, of the net energy transfer from large eddies to small ones. Experiments are also influenced by a more complicated interaction between the rotation and the wakes of the turbulence-generating grid which modifies the nominal initial conditions in the experiment. The latter effect is accounted for in simulations by modifying the initial conditions. Finally, a two-equation model is proposed that accounts for the effects of rotation and is able to reproduce the experimental decay of the turbulent kinetic energy.
Some numerical experiments on developing laminar flow in circular-sectioned bends
Authors: Humphrey J. A. C. , Iacovides H. , Launder B. E.
Published: April 2006

The paper reports numerical solutions to a semi-elliptic truncation of the Navier–Stokes equations for the case of developing laminar flow in circular-sectioned bends over a range of Dean numbers. The ratios of bend radius to pipe radius are 7:1 and 20:1, corresponding with the configurations examined experimentally by Talbot and his co-workers in recent years. The semi-elliptic treatment facilitates a much finer grid than has been possible in earlier studies. Numerical accuracy has been further improved by assuming radial equilibrium over a thin sublayer immediately adjacent to the wall and by re-formulating the boundary conditions at the pipe centre.

Streamwise velocity profiles at Dean numbers of 183 and 565 are in excellent agreement with laser-Doppler measurements by Agrawal, Talbot & Gong (1978). Good, albeit less complete, accord is found with the secondary velocities, though the differences that exist may be mainly due to the difficulty of making these measurements. The paper provides new information on the behaviour of the streamwise shear stress around the inner line of symmetry. Upstream of the point of minimum shear stress, our numerical predictions display a progressive shift towards the result of Stewartson, Cebici & Chang (1980) as the Dean number is successively raised. Downstream of the minimum, however, in contrast with the monotonic approach to an asymptotic level reported by Stewartson, the numerical solutions display a damped oscillatory behaviour reminiscent of those from Hawthorne's (1951) inviscid-flow calculations. The amplitude of the oscillation grows as the Dean number is raised.
Theoretical pressure–strain term, experimental comparison, and resistance to large anisotropy
Authors: Weinstock J. , Burk Stephen
Published: April 2006

Although models of the pressure–strain term explain many features of nearly uniform homogeneous shear flows, a discrepancy remains (Leslie 1980). It is suggested that the discrepancy is caused by use of an empirical expression for the fluctuation part of the pressure–strain term, the part usually denoted by ϕij,1. The discrepancy is eliminated by replacement of the empirical ϕij,1 with a recent theoretical expression. Relatedly, the Launder, Reece & Rodi (1975) model for the mean-field part ϕij,2 is shown to be a good approximation for both a strongly and weakly sheared flow. This model of ϕij,2 when combined with the theoretical ϕij,1 is found to provide an explanation for experiments of both Champagne, Harris & Corrsin (1970) and Harris, Graham & Corrsin (1977). Full correction requires that deviations from local isotropy be accounted for. Special emphasis is given to a theoretical demonstration that the pressure–strain term does not cause a retun to isotropy but, rather, it resists large anisotropy – a weaker effect.
On the global instability of free disturbances with a time-dependent nonlinear viscous critical layer
Authors: Gajjar J. , Smith F. T.
Published: April 2006

A theoretical study is made of the global nonlinear growth or decay, in space and time, of an unsteady non-neutral disturbance/wavepacket when a time-dependent nonlinear viscous critical layer is present. The basic flow considered is a steady quasiparallel channel flow, boundary layer or liquid-layer flow at high Reynolds number. The unsteadiness with regard to the critical layer shows itself less in the internal dynamics than in the relatively slow movement of the layer across the flow, the temporal and spatial rate of movement discussed being sufficient to affect the nonlinear viscous balance in the layer. This greatly reduces the mean-flow distortions produced. The disturbance amplitude, in contrast, responds nonlinearly on faster time- and space-scales, both inside and outside the critical layer. These slower- and faster-scale properties inside the critical layer and outside, i.e. globally, are coupled together in general. The work addresses first the structure and nonlinear evolution equations for the growing or decaying free disturbance and the critical layer. But preliminary analysis in special cases suggests, among other things, the significant result that previous nonlinear studies based on quasineutral assumptions give unstable subcritical threshold amplitudes, above which increasingly fast disturbance growth takes place globally.
A unified bar–bend theory of river meanders
Authors: Blondeaux P. , Seminara G.
Published: April 2006

A two-dimensional model of flow and bed topography in sinuous channels with erodible boundaries is developed and applied in order to investigate the mechanism of meander initiation. By reexamining the problem recently tackled by Ikeda, Parker & Sawai (1981), a previously undiscovered ‘resonance’ phenomenon is detected which occurs when the values of the relevant parameters fall within a neighbourhood of certain critical values. It is suggested that the above resonance controls the bend growth, and it is shown that it is connected in some sense with bar instability. In fact, by performing a linear stability analysis of flow in straight erodible channels, resonant flow in sinuous channels is shown to occur when curvature ‘forces’ a ‘natural’ solution represented by approximately steady perturbations of the alternate bar type. A comparison with experimental observations appears to support the idea that resonance is associated with meander formation.
Weakly dispersive nonlinear gravity waves
Authors: Miles John , Salmon Rick
Published: April 2006

The equations for gravity waves on the free surface of a laterally unbounded inviscid fluid of uniform density and variable depth under the action of an external pressure are derived through Hamilton's principle on the assumption that the fluid moves in vertical columns. The resulting equations are equivalent to those of Green & Naghdi (1976). The conservation laws for energy, momentum and potential vorticity are inferred directly from symmetries of the Lagrangian. The potential vorticity vanishes in any flow that originates from rest; this leads to a canonical formulation in which the evolution equations are equivalent, for uniform depth, to Whitham's (1967) generalization of the Boussinesq equations, in which dispersion, but not nonlinearity, is assumed to be weak. The further approximation that nonlinearity and dispersion are comparably weak leads to a canonical form of Boussinesq's equations that conserves consistent approximations to energy, momentum (for a level bottom) and potential vorticity.
On the stability of pipe-Poiseuille flow to finite-amplitude axisymmetric and non-axisymmetric disturbances
Authors: Sen P. K. , Venkateswarlu D. , Maji S.
Published: April 2006

The stability of fully developed pipe-Poiseuille flow to finite-amplitude axisymmetric and non-axisymmetric disturbances has been studied using the equilibrium-amplitude method of Reynolds & Potter (1967). In both the cases the least-stable centre-modes were investigated. Also, for the non-axisymmetric case the mode investigated was the one with azimuthal wavenumber equal to one. Many higher-order Landau coefficients were calculated, and the Stuart-Landau series was analysed by the Shanks (1955) method and by using Padé approximants to look for the existence of possible equilibrium states. The results show in both cases that, for each value of the Reynolds number R, there is a preferred band of spatial wavenumbers α in which equilibrium states are likely to exist. Moreover, in both cases it was found that the magnitude of the minimum threshold amplitude for a given R decreases with increasing R. The scales of the various quantities obtained agree very well with those deduced by Davey & Nguyen (1971).
Sonographie bei Appendizitis und deren Differentialdiagnosen
Authors: Gritzmann  , Hübner
Published: April 2006

Die klinische Diagnostik erreicht Treffsicherheiten von 50–70% in der Diagnostik der akuten Appendizitis. Die Sonographie ist das erste bildgebende Verfahren bei Verdacht auf Appendizitis. Die Sonographie führt zu einer deutlichen Verminderung der negativen Appendektomien. Von erfahrenen Untersuchern werden Treffsicherheiten von mehr als 90% erreicht. Die sonographischen Zeichen der akuten Appendizitis werden präsentiert. Als wesentliche Differentialdiagnosen gelten infektiöse Ileozökitis, rechtsseitige Divertikulitis, Netzpathologien und Appendagitis, Erkrankungen des inneren weiblichen Genitales wie Adnexitis, rupturierte oder stielgedrehte Ovarialzysten, extrauterine Graviditäten. Urologischerseits steht die Harnleiterkolik durch ein Steinleiden im Vordergrund. Tiefreichende Cholezystitiden oder Pankreasnekrosen, Psoasblutungen und Bauchdeckenhämatome sind ebenfalls möglich. Eine Appendizitis kann sonographisch weitgehend ausgeschlossen werden, wenn die Appendix in gesamter Länge normal dargestellt werden kann, oder eine andere Diagnose als eindeutige Ursache der Schmerzen erhoben werden kann. Mukozelen zeigen sonographisch eine echoarme zwiebelschalenartige Struktur.
Striated drifting auroral kilometric radiation bursts: Possible stimulation by upward traveling EMIC waves
Authors: Menietti J. D. , Mutel R. L. , Santolik O. , Scudder J. D. , Christopher I. W. , Cook J. M.
Published: April 2006

We investigate waves observed by the Cluster wideband instrument (WBD) during an orbital conjunction with the Polar spacecraft. During this perigee pass, Polar was at the upper extent of the auroral kilometric radiation (AKR) source region in the Southern Hemisphere nightside auroral region. Cluster was located at higher altitude above this region and observed AKR with clear signatures of ordered fine structure striations (rain). Using electron particle data observed by HYDRA on board Polar, we have modeled the electron distribution function within the AKR source region. This distribution function is unstable to a number of low‐frequency wave modes and supports EMIC waves propagating along the magnetic field line. These waves appear to be feasible to stimulate the growth of AKR, producing the ordered fine structure observed by WBD on board Cluster, but further analysis will be important, especially plasma simulations.
Equatorial distributions of the plasma sheet ions, their electric and magnetic drifts, and magnetic fields under different interplanetary magnetic field Bz conditions
Authors: Wang Chih‐Ping , Lyons Larry R. , Weygand James M. , Nagai Tsugunobu , McEntire Richard W.
Published: April 2006

To understand the nightside plasma sheet structure under different interplanetary magnetic field (IMF) Bz conditions, we have investigated statistically the equatorial distributions of ions and magnetic fields from Geotail when the IMF has been continuously northward or southward for shorter or longer than 1 hour. A dawn‐dusk density (temperature) asymmetry with higher density (temperature) on the dawn (dusk) side is seen in the near‐Earth plasma sheet during northward IMF, resulting in roughly dawn‐dusk symmetric pressure. As southward IMF proceeds, the density asymmetry weakens while the temperature asymmetry maintains, resulting in higher pressure on the dusk side. The plasma sheet is relatively colder and denser near the flanks than around midnight. The flux distributions show that the density asymmetry is due to ions <∼3 keV, and the temperature asymmetry is due to ions above thermal energy. The perpendicular flow shows that ions divert around the Earth mainly through the dusk side in the inner plasma sheet because of westward diamagnetic drift. The magnetic fields indicate that field lines are more stretched during southward IMF. Ions' electric and magnetic drift paths evaluated from the observations show that for thermal energy ions, magnetic drift is as important as electric drift. Comparison of the distributions of the observed phase space density with the evaluated drift paths at different energies indicates that the electric and magnetic drift transport is responsible for the observed dawn‐dusk asymmetries in the plasma sheet structure.