bluuebunny/crossref_metadata_2025 huggingface dataset:
Sedimentological and fluid-dynamic implications of the turbulent bursting phenomenon in geophysical flows
Authors: Jackson Roscoe G.
Published: April 2006

The bursting process in turbulent boundary layers provides new insight on turbulence phenomena, mechanics of sedimentation, and genesis of bedforms in natural geophysical flows. Recent visualization experiments suggest that the turbulent boundary layer can be divided into an inner zone, whose essential characteristics scale with inner (wall) variables, and an outer zone, whose properties scale with the fluid-dynamic variables of the entire flow. The inner zone is distinguished by (i) a viscous sublayer displaying spanwise alternations of high-and low-speed streaks and (ii) episodic disruption by lift-ups of low-speed streaks. Oscillatory growth and breakup stages of the Stanford model of bursting characterize the turbulent structure of the outer zone. The burst cycle exists in turbulent boundary layers of all natural flows except perhaps (i) open-channel flows in the upper part of the upper flow regime and (ii) wind-generated surface waves.

Fluid motions described as kolks and boils in incompressible open-channel flows correspond to the oscillatory growth stage andthe late oscillatory growthand breakup stages, respectively, of the Stanford model of bursting. Supporting evidence includes (i) close similarity of gross fluid motions, (ii) equivalent scaling of boils and bursts, and (iii) intensification of boils and bursts in adverse pressure gradients and over rough beds. McQuivey's (1973) turbulence measurements show that the Eulerian integral time scaleTEscales with the same outer variables asboilperiodicity and burst periodicity. It is hypothesized thatTEequals the mean duration of bursts at a point in the flow.

Bedforms governed by the turbulent structure of the inner zone (microforms) cannot form if the sublayer is disrupted by bed roughness. The conditions for the existence of two common microforms and their spacings scale with the inner variables. Grain roughness increases the vertical intensity of the turbulence (by enhancing lift-ups) within the inner zone, thereby explaining textural differences between the coarse ripple and fine ripple bed stages of Moss (1972).

Mesoforms respond to the fluid-dynamical regime in the outer zone and scale with the outer variables. The mean spacing of dunelike large-scale ripples in equilibrium open-channel flows is proportional to the boundary-layer thickness and equals the length scale formed by the product of the free-stream velocity and the boil period.

Strong upward flow in a burst provides the vertical anisotropy in the turbulence which is needed to suspend sediment. Bursting promotes the entrain-ment of more and coarser sediment than tractive forces alone can accomplish.
Selective withdrawal from a layered fluid
Authors: Bryant P. J. , Wood I. R.
Published: April 2006

The selective withdrawal of a layered fluid from a reservoir has interesting properties which are not present if the stratification is continuous. The problem investigated is that of finding the ratios of the discharges from each flowing layer when the total discharge from all layers is regulated at the outlet. It is shown that the ratios are determined by the requirement that the flow be smooth at critical points in the flow, each critical point being a point at which the long-wave velocity on one of the interfaces is zero. If there are too many critical points, the flow is over-determined and becomes unsteady. It follows therefore that if the external conditions change slowly, such as in the slow draining of a reservoir, the flow alternates between intervals in which the flow ratios also change slowly and intervals in which the flow ratios oscillate unsteadily. An experiment is described in which the theoretical conclusions were tested for two-layer flow. Good agreement was obtained between theory and experiment.
Heat diffusion as a source of aerodynamic sound
Authors: Kempton A. J.
Published: April 2006

The paper examines the role of heat diffusion as an internal noise source in aeroengines and as a source of noise in the mixing of hot jets. We consider a number of model problems and find that the sound induced by unsteady heat transfer can show an unusually weak dependence on the mean flow velocityU, the intensity scaling asU3in three dimensions. At low enough velocities diffusion effects will overwhelm other noise sources, but we have failed in our search for a significant practical situation in which we can prove that sound generated by diffusion clearly dominates over that excited by unsteady aerodynamic forces; they are sometimes comparable.

We examine the possibility that diffusive monopole sources feature in the noise of hot jets using model problems in the linear case and using dimensional analysis in the nonlinear case, and conclude that no significant monopole exists when the specific heats are constant. But they are not constant at low frequencies when, for example, heat flows into and out of vibrational energy modes; then an important monopole source is present. This source shows an unusually complicated scale effect.
The effective thermal conductivity of sheared suspensions
Authors: Nir Avinoam , Acrivos Andreas
Published: April 2006

Formal expressions are derived for the effective thermal conductivity Kij of randomly dispersed suspensions undergoing shear. These are then evaluated for the cases of dilute suspensions of cylinders and of spheres when the bulk motion is a simple shear, the Péclet number Pe is large, and the particle Reynolds number is small enough for inertia effects to be negligible. It is shown that as Pe → ∞ the presence of shear can significantly affect the O(ϕ) contribution to Kij (ϕ being the volume fraction of the solids), which becomes independent of k, the thermal conductivity of the suspended material. This results from the presence of regions of closed streamlines surrounding each particle which, for sufficiently large Pe, attain an isothermal state and therefore act as regions of infinite conductivity.
Transient combined laminar free convection and radiation in a rectangular enclosure
Authors: Larson D. W. , Viskanta R.
Published: April 2006

The mass, momentum and energy-transfer equations are solved to determine the response of a rectangular enclosure to a fire or other high-temperature heat source. The effects of non-participating radiation, wall heat conduction, and laminar natural convection are examined. The results indicate that radiation dominates the heat transfer in the enclosure and alters the convective flow patterns significantly. At a dimensionless time of 5·0 the surface of the wall opposite a vertical heated wall has achieved over 99% of the hot-wall temperature when radiation is included but has yet to change from the initial temperature for pure convection in the enclosure. At the same time the air at the centre of the enclosure achieves 33% and 13% of the hot-wall temperature with and without radiation, respectively. For a hot upper wall the convection velocities are not only opposite in direction but an order of magnitude larger when radiation transfer between the walls is included.
The fluid mechanics of the semicircular canals
Authors: Van Buskirk W. C. , Watts R. G. , Liu Y. K.
Published: April 2006

A mathematical model for the unsteady fluid-dynamic response of the semicircular canals is developed. The endolymph is assumed to be an incompressible Newtonian fluid and the presence and effects of both the utricle and the cupula are specifically accounted for. A first approximate solution is obtained using a singular perturbation method. It is shown that the canal can be modelled as a heavily damped, second-order system which behaves as an angular-velocity meter. A comparison of the model response with experimental results is made; fairly good agreement is found.
Time-dependent laminar incompressible flow through a spherical cavity
Authors: Gillani N. V. , Swanson W. M.
Published: April 2006

A finite-difference numerical method for the solution of the unsteady flow of a viscous incompressible fluid through axisymmetric circular ducts of variable axial geometry is developed and applied to the flow in a spherical-cavity geometry approximating the human aortic valve. The presence and motion of the valve leaflets are considered only as long as they can be assumed to present negligible impedance to the flow. The numerical solution is based on the vorticity/streamfunction approach, and is carried out for the systolic acceleration phase of the heart beat. A hybrid-mesh design consisting of a fine cell structure in the region close to the solid walls and a coarser grid in the core region is used. An experimental flow-visualization study in an acrylic model of the spherical cavity shows good agreement with the numerical simulation. An early separation of flow occurs at the entrance to the cavity, and an annular eddy grows in the wake until it occupies most of the cavity. The use of the hybrid mesh also makes possible the simulation of fine secondary-flow features in the cavity under peak-flow conditions.
The stress field of slender particles oriented by a non-Newtonian extensional flow
Authors: Goddard J. D.
Published: April 2006

An analysis is presented of the creeping motion around a flow-oriented slender particle in a material medium subject to a uniaxial extension in the far field. A general quasi-steady rheological model is adopted, of a kind representing isotropic (Noll) fluids subject to time-independent velocity gradients, or isotropic solids subject to time-independent strain fields. The analysis is based on the premise of a shear-dominated motion in the near field, which is joined asymptotically to the extension-dominated motion in the far field. For axisymmetric particles, and to the order of terms in slenderness considered here, the far-field perturbation due to the particle can be represented as a distributed coaxial line force in a transversely isotropic medium whose strength is governed by the structure of the near-field rheology.

On the basis of the results for a single particle, a formula is derived for the stress contribution due to the presence of oriented slender fibres in dilute suspension in a non-Newtonian fluid. For certain simple rheological models exhibiting a strong shear-thinning behaviour, the particle contribution to tensile stress is greatly diminished relative to the Newtonian case, as was predicted by an earlier rudimentary treatment (Goddard 1975). The present analysis is thought to be highly promising for applications to general composite materials.
Liquid-metal magnetohydrodynamics with strong magnetic fields: a report on Euromech 70
Authors: Hunt J. C. R. , Moreau R.
Published: April 2006

This paper is a summary of the first Euromech Colloquium to be held on Magnetohydrodynamics (MHD). It was organized in conjunction with the Centre National de la Recherche Scientifique and held at Grenoble from 16–19 March 1976 with 60 participants from 10 countries present. Papers were presented on laminar and turbulent MHD duct flows; heat transfer and two-phase flows in MHD; the effects of magnetic fields on instabilities and turbulence; the motion of and forces on solid objects in MHD flows; flow-measurement methods, and applications of MHD in the metallurgical industries, in sodium technology and in liquid-metal power generation. Our main conclusion is that there are many industrial applications of the existing body of research findings in MHD, but that quite new research problems have arisen as a result of the new applications, and that these need investigation. MHD lives!
Experiments on a viscous fluid flow between concentric rotating spheres
Authors: Wimmer Manfred
Published: April 2006

Some experimental results on incompressible viscous fluid flow in the gap between two concentric rotating spheres are discussed. The flow field in the spherical gap has been studied qualitatively by flow visualization (photographs) and quantitatively by measurements by the hot-wire technique. For a wide range of Reynolds numbers, the friction torque was measured for several gap widths and a relatively simple method of determining the torque theoretically is given. At higher Reynolds numbers instabilities appear. Their different behaviour for relatively small and large gap widths is demonstrated. For the larger gap widths, the different appearance of the Taylor–Görtler vortices, the reason for their generation, their regimes of existence as well as their influence on the friction torque are thoroughly treated. Detailed information is given on the new effect of the dependence of the wavelength of the vortices on the Reynolds number.
Flow of vapour in a liquid enclosure
Authors: Plesset Milton S. , Prosperetti Andrea
Published: April 2006

A solution is developed for the flow of a vapour in a liquid enclosure in which different portions of the liquid wall have different temperatures. It is shown that the vapour pressure is very nearly uniform in the enclosure, and an expression for the net vapour flux is deduced. This pressure and the net vapour flux are readily expressed in terms of the temperatures on the liquid boundary. Explicit results are given for simple liquid boundaries: two plane parallel walls at different temperatures and concentric spheres and cylinders at different temperatures. Some comments are also made regarding the effects of unsteady liquid temperatures and of motions of the boundaries. The hemispherical vapour cavity is also discussed because of its applicability to the nucleate boiling problem.
Heat transfer from a hot film in reversing shear flow
Authors: Pedley T. J.
Published: April 2006

The two-dimensional thermal boundary layer over a finite hot film embedded in a plane insulating wall, with a shear flow over it which reverses its direction, is analysed approximately using methods similar to those previously developed for viscous boundary layers (Pedley 1976). The heat transfer from the film is calculated both for uniformly decelerated and for oscillatory wall shear, and application is made to predict the response of hot-film anemometers actually used to measure oscillatory velocities in water and blood. The results predict that the velocity amplitude measured on the assumption of a quasi-steady response will depart from the actual amplitude at values of the frequency parameter St greater than about 0·3 (St = ΩX0/U0, where Ω = frequency, U0 = mean velocity, X0 = distance of hot film from the leading edge of the probe). This is in good agreement with experiment. So too is the shape of the predicted anemometer output as a function of time throughout a complete cycle, for cases when the response is not quasi-steady. However, there is a significant phase lead between the predicted and the experimental outputs. Various possible reasons for this are discussed; no firm conclusions are reached, but the most probable cause lies in the three-dimensionality of the velocity and temperature fields, since the experimental hot films are only about 2·5 times as broad as they are long, and are mounted on a cylinder not a flat plate.
Fluctuating forces on a rigid circular cylinder in confined flow
Authors: Richter A. , Naudascher E.
Published: April 2006

The fluctuating lift and drag acting on a long, rigidly supported circular cylinder placed symmetrically in a narrow rectangular duct were investigated for various blockage percentages over a wide range of Reynolds numbers around the critical value. The data obtained permit a full assessment of the effect of confinement on the mean-drag coefficient, the root-mean-square values of both the drag and the lift fluctuations, the Strouhal number of the dominant vortex shedding, and the Reynolds number marking transition from laminar to turbulent flow separation. Besides experimental information on a subject on which little is known so far, the paper provides a basis for the deduction of better correction procedures concerning the effects of blockage.
Sizes and shapes of salt fingers near the marginal state
Authors: Chen C. F. , Sandford R. D.
Published: April 2006

Experiments were carried out to investigate the onset, size and shape of fingers in a salt–sugar two-component system. The results on the critical Rayleigh number ratio for the marginal state agree well with the prediction of the linear stability theory of Stern (1960). The theory also predicts that the fingers at the marginal state should be as wide as the layer thickness. In our experiments, in a layer of approximately 3·5 cm deep, the fingers, when they appeared, were always long and narrow, with typical lateral dimensions of approximately 1 mm.
On the spin-up and spin-down of a rotating fluid. Part 1. Extending the Wedemeyer model
Authors: Weidman Patrick D.
Published: April 2006

The Wedemeyer model describing the spin-up of a fluid in a rotating cylinder is generalized to include the case of spin-down. Attention is focused on spin-up and spin-down at a finite (constant) acceleration for small Ekman numbers En. It is found that when the full nonlinear Ekman suction is included for spin-up from rest, the characteristics near the propagating wave front intersect, thus yielding an O(1) velocity discontinuity for the inviscid model. This anomalous behaviour is limited to only a small range of Rossby numbers and does not appear in any of the spin-down solutions. Transient spin-down velocity profiles in the O(E1/4Ω) shear layer at the cylindrical wall are calculated for the quasi-steady flow which occurs for sufficiently small decelerations. Results for impulsive spin-up and spin-down between infinite parallel plates are presented and compared with the asymptotic solutions given by Greenspan & Weinbaum and Benton. Finally, characteristic spin-up and spin-down times are computed for both the contained cylinder and the infinite-parallel-plates geometry.
Diffusion of passive-scalar and magnetic fields by helical turbulence
Authors: Kraichnan Robert H.
Published: April 2006

Computer simulations of fluid-element trajectories in mirror-symmetric and maximally helical turbulence are used to evaluate Moffatt's (1974) formulae for the magnetic diffusivity η(t) and the coefficient κ(t) of the alpha-effect. The passive-scalar diffusivity κ(t) and the mean response functions of scalar and magnetic field wave-vector modes are also computed. The velocity field is normal, stationary, homogeneous and isotropic with spectrum $E(k) = \frac{3}{2}v^2_0\delta(k - k_0)$ and time correlation exp [−1/2ω2/0(t-t)2]. The cases ω0 = O (frozen turbulence), ω = vo k0 and ω0 = 2v0 k0 are followed to t = 4/v0 k0. In the ω0 > 0 cases with maximal helicity, κ(t) and a(t) approach steady-state values of order vo/k0 and v0, respectively. They behave anomalously for ω0 = 0. In the mirror-symmetric: cases, q(t) and κ(t) differ very little from each other. At all the ω0 values, is bigger in the helical than in the mirror-symmetric case. The difference is marked for ω0 = 0. The simulation results imply that κ(t) becomes negative in non-normal mirror-symmetric turbulence with strong helicity fluctuations that persist over several correlation lengths and times. The computations of response functions indicate that asymptotic expressions for these functions, valid for k [Lt ]; k0, retain good accuracy for k ∼ k0. The mean-square magnetic field is found to grow exponentially, and its kurtosis also grows apidly with t, indicating rapid development of a highly intermittent distribution of magnetic field.
Precursor shock waves at a slow—fast gas interface
Authors: Abd–El–Fattah A. M. , Henderson L. F. , Lozzi A.
Published: April 2006

This paper presents experimental data obtained for the refraction of a plane shock wave at a carbon dioxide–helium interface. The gases were separated initially by a delicate polymer membrane. Both regular and irregular wave systems were studied, and a feature of the latter system was the appearance of bound and free precursor shocks. Agreement between theory and experiment is good for regular systems, but for irregular ones it is sometimes necessary to take into account the effect of the membrane inertia to obtain good agreement. The basis for the analysis of irregular systems is one-dimensional piston theory and Snell's law.
The acoustic field of sources in shear flow with application to jet noise: convective amplification
Authors: Balsa T. F.
Published: April 2006

Lighthill, in his elegant and classic theory of jet noise, showed that the far-field acoustic pressure of noise generated by turbulence is proportional to the integral over the jet volume of the second time derivative of the Lighthill stress tensor, the integrand being evaluated at a retarded time. The purpose of this paper is to generalize the above results to include the effects of mean flow (velocity and temperature) surrounding the source of sound. It is shown quite generally that the integrand is now a certain functional of the Lighthill stress tensor evaluated at a retarded time. More important, however, at low and high frequencies this functional assumes an extremely simple form, so that the acoustic field can once more be given by integrals of the time derivatives of the Lighthill tensor. Both the self- and the shear-noise contributions to the pressure are evaluated.
A double-scale investigation of the asymptotic structure of rolled-up vortex sheets
Authors: Guiraud J. P. , Zeytounian R. Kh.
Published: April 2006

A double scale technique is used to determine the asymptotic behaviour of a rolled-up vortex sheet. The technique relies on a process of averaging out the saw-tooth-like behaviour of the flow variables, which generates a continuous solution having the structure of a vortex filament. The fine-scale behaviour of the flow is described and includes concentrated vorticity on the sheet. Application to the conical vortex sheet allows the solution of Mangler & Weber (1967) to be rederived. A further application, to Kaden's problem, is worked out and the results are in complete agreement with Moore's asymptotic formulae for the shape of the spiral.
Rotating elliptic cylinders in a viscous fluid at rest or in a parallel stream
Authors: Lugt Hans J. , Ohring Samuel
Published: April 2006

Numerical solutions are presented for laminar incompressible fluid flow past a rotating thin elliptic cylinder either in a medium at rest at infinity or in a parallel stream. The transient period from the abrupt start of the body to some later time (at which the flow may be steady or periodic) is studied by means of streamlines and equi-vorticity lines and by means of drag, lift and moment coefficients. For purely rotating cylinders oscillatory behaviour from a certain Reynolds number on is observed and explained. Rotating bodies in a parallel stream are studied for two cases: (i) when the vortex developing at the retreating edge of the thin ellipse is in front of the edge and (ii) when it is behind the edge.
The calculation of flow fields by panel methods: a report on Euromech 75
Authors: Körner H. , Hirschel E. H.
Published: April 2006

Self-diffusion coefficients were determined experimentally for lateral dispersion of spherical and disk-like particles in linear shear flow of a slurry at very low Reynolds number. Using a concentric-cylinder Couette apparatus, recurrent observations were made of the lateral position of a particular radioactively labelled particle. The self-diffusion coefficient D was calculated by means of random-walk theory, using the ergodic hypothesis. Owing to great experimental difficulties, the calculated values of D are not of high accuracy, but are correct to within a factor of two. In the range 0 < ϕ < 0·2, D/a2ω increases from zero linearly with ϕ up to D/a2ω ≈ 0.02 (where ϕ = volumetric concentration of particles, a = particle radius, ω = mean shear rate of suspending fluid). In the range 0.2 < ϕ < 0.5, the trend of D/a2ω is not clear because of experimental scatter, but in this range D/a2ω ≈ 0·025 to within a factor of two. Within the experimental accuracy, spheres and disks have the same value of D/a2ω.
A moving fluid interface. Part 2. The removal of the force singularity by a slip flow
Authors: Hocking L. M.
Published: April 2006

If the no-slip condition is used to determine the flow produced when a fluid interface moves along a solid boundary, a non-integrable stress is obtained. In part 1 of this study (Hocking 1976), it was argued that, when allowance was made for the presence of irregularities on the solid boundary, an effective slip coefficient could be found, which might remove the difficulty.

This paper examines the effect of a slip coefficient on the flow in the neighbourhood of the contact line. Particular cases which are solved in detail are liquid–gas interfaces at an arbitrary angle, and normal contact of fluids of arbitrary viscosity. The contribution of the vicinity of the contact line to the force on the boundary is obtained.

The inner region, near the contact line, must be matched with an outer flow, in which the no-slip condition can be applied, in order to obtain the total value of the force on the boundary. This force is determined for the flow of two fluids between parallel plates and in a pipe, with a plane interface. The enhanced resistance produced by the presence of the interface is calculated, and it is shown to be equivalent to an increase in the length of the column of fluid by a small multiple of the pipe radius.
Experimental determination of the main features of the viscous flow in the wake of a circular cylinder in uniform translation. Part 1. Steady flow
Authors: Coutanceau Madeleine , Bouard Roger
Published: April 2006

A visualization method is used to obtain the main features of the hydrodynamic field for flow past a circular cylinder moving at a uniform speed in a direction perpendicular to its generating lines in a tank filled with a viscous liquid. Photographs are presented to show the particular fineness of the experimental technique. More especially, the closed wake and the velocity distribution behind the obstacle are investigated; the changes in the geometrical parameters describing the eddies with Reynolds number (5 < Re < 40) and with the ratio λ between the diameters of the cylinder and tank are given. A comparison with existing numerical and experimental results is presented and some remarks are made about the calculation techniques proposed up to the present. The limits of the Reynolds-number range for which the twin vortices exist and adhere stably to the cylinder are determined.
Spectral energy transfer in high Reynolds number turbulence
Authors: Helland K. N. , Atta C. W. Van , Stegen G. R.
Published: April 2006

The spectral energy transfer of turbulent velocity fields has been examined over a wide range of Reynolds numbers by experimental and empirical methods. Measurements in a high Reynolds number grid flow were used to calculate the energy transfer by the direct Fourier-transform method of Yeh & Van Atta. Measurements in a free jet were used to calculate energy transfer for a still higher Reynolds number. An empirical energy spectrum was used in conjunction with a local self-preservation approximation to estimate the energy transfer at Reynolds numbers beyond presently achievable experimental conditions.

Second-order spectra of the grid measurements are in excellent agreement with local isotropy down to low wavenumbers. For the first time, one-dimensional third-order spectra were used to test for local isotropy, and modest agreement with the theoretical conditions was observed over the range of wavenumbers which appear isotropic according to second-order criteria. Three-dimensional forms of the measured spectra were calculated, and the directly measured energy transfer was compared with the indirectly measured transfer using a local self-preservation model for energy decay. The good agreement between the direct and indirect measurements of energy transfer provides additional support for both the assumption of local isotropy and the assumption of self-preservation in high Reynolds number grid turbulence.

An empirical spectrum was constructed from analytical spectral forms of von Kármán and Pao and used to extrapolate energy transfer measurements at lower Reynolds number to Rλ = 105 with the assumption of local self preservation. The transfer spectrum at this Reynolds number has no wavenumber region of zero net spectral transfer despite three decades of $k^{-\frac{5}{3}}$. behaviour in the empirical energy spectrum. A criterion for the inertial subrange suggested by Lumley applied to the empirical transfer spectrum is in good agreement with the $k^{-\frac{5}{3}}$ range of the empirical energy spectrum.
The stability of a rotating flow in the presence of an axial and a toroidal magnetic field
Authors: Sung Chao-Ho
Published: April 2006

Making a normal-mode assumption, we shall investigate stability with respect to non-axisymmetric perturbations of an inhomogeneous incompressible fluid rotating between two perfectly conducting, infinite, coaxial cylinders in the presence of an axial and a toroidal magnetic field. We shall establish sufficient conditions for stability, discuss the westward-drift nature of unstable modes and estimate upper bounds on the azimuthal phase speeds and growth rates. There will also be a discussion of the validity of the sufficient conditions for stability when the normal-mode assumption is not made, so that the stability is based on an initial-value problem.
Directional slope and curvature distributions of wind waves
Authors: Wu Jin
Published: April 2006

The slope and curvature distributions of wind waves along two principal axes (upwind-downwind and cross-wind) have been measured in a laboratory tank under various wind velocities. In both directions, the slope distributions are very closely Gaussian, and the components of the mean-square water-surface slope vary loga,rithmically with the friction velocity of the wind. As the windvelocityincreases, the ratio of the upwind-downwind and cross-wind components increases and lies between 0.5 and 0.6 at high wind velocities in the gravity-governed regime of wind-wave interaction. The radius of .water-surface curvature, along either direction of measurement, is generally found to be greater at a steeper viewing angle from the normal to the mean water surface. The average radius of curvature of the disturbed surface varies inversely with the friction velocity of the wind. The ratio of the upwind-downwind and cross-wind components of the average radius of curvature is unity at all wind velocities, indicating that the wind-disturbed water surface is isotropic on the smallest scale. Other results show that both the slope and the curvature distributions are asymmetric along the upwind-downwind direction, either because of the presence of parasitic capillaries or because of the occurrence of wave breaking. The results also indicate that even the high frequency portion of the spectrum is saturated locally but the spectrum is not universal, and that the long waves suppress the growth of the nearly saturated ripples.
Wave breakdown in stratified shear flows
Authors: Landahl M. T. , Criminale W. O.
Published: April 2006

The wave-mechanical condition (Landahl 1972) for breakdown of an unsteady laminar flow into strong small-scale secondary instabilities is applied to some simple stratified inviscid shear flows. The cases considered have one or two discrete density interfaces and simple discontinuous or continuous velocity profiles. A primary wavelike disturbance to such a flow produces a perturbation velocity that is discontinuous at a density interface. The resulting instantaneous system, defined as the sum of the mean flow and the primary oscillation, develops a local secondary shear-flow instability that has a group velocity equal to the arithmetic mean of the instantaneous velocities on the two sides of the interface. According to the breakdown criterion, the disturbed flow will become critical whenever this velocity reaches a value equal to the phase velocity of the primary wave. The calculations show that for a single density interface breakdown may occur for low to moderate wave amplitudes in a fairly narrow range of Richardson numbers on the stable side of the stability boundary. On the other hand, in the unstable regime maximum wave slopes of order unity may be reached before breakdown occurs; this conclusion is in qualitative agreement with experiments. When the system includes two density interfaces, it is found that there exists a range of high Richardson numbers far into the stable regime for which breakdown may take place even for very small and zero wave interface deflexions.
On Green's functions for small disturbances of plane Couette flow
Authors: Marcus Philip S. , Press William H.
Published: April 2006

The linearized stability of plane Couette flow is investigated here, without using the Orr–Sommerfeld equation. Rather, an unusual symmetry of the problem is exploited to obtain a complete set of modes for perturbations of the unbounded (no walls) flow. An explicit Green's function is constructed from these modes. The unbounded flow is shown to be rigorously stable. The bounded case (with walls) is investigated by using a ‘method of images’ with the unbounded Green's function; the stability problem in this form reduces to an algebraic characteristic equation (not a differential-equation eigenvalue problem), involving transcendental functions defined by integral representations.
The near field in the mixing of a round jet with a cross-stream
Authors: Moussa Z. M. , Trischka John W. , Eskinazi S.
Published: April 2006

In the mixing of a jet with a cross-stream, it is found that in the near field, defined as the region of the flow from the jet exit to a distance of a few diameters downstream of this exit, a considerable amount of dynamical adjustment takes place. This near-field region characterizes the subsequent behaviour and development of the jet, its wake and the cross-stream in the vicinity of this mixing region. The rapid evolution of the flow gives rise to a pair of bound vortices attached to the lee side of the jet boundary, to fast development of the turbulent and mean vorticity, to a vortex-shedding system, and to the largest rates of entrainment of cross-stream flow into the jet. Furthermore, it is found that the geometrical configuration of the boundaries at the jet exit plays an important role in the mixing and development processes.

An intrinsic method is proposed for the delineation of the flow boundaries between the jet and the cross-stream. Calculations of mass, momentum and vorticity fluxes have been made. The vorticity flux gives evidence of the rapid stretching and tilting of the vorticity vector field in the near-field region.
Secondary flow due to Alfvén waves
Authors: Shercliff J. A.
Published: April 2006

Large (gigajoule) amounts of energy can in principle be stored as kinetic energy in liquid metal circulating round a torus and can be extracted at the gigawatt level by Alfvén waves propagating along an imposed axial field. A major limitation on the energy that may be so stored is the disruption of these primary Alfvén waves by secondary flows in meridional planes, associated with out-of-balance centrifugal forces ahead of and behind the waves and non-uniform magnetic pressures at the wave fronts. Vorticity, created at the wave, itself propagates in secondary Alfvén waves.

This paper gives a linearized treatment of these secondary motions and the associated perturbations of the imposed axial field and compares the resulting disruption of the primary wave mode with crude estimates made in an earlier paper. The main case treated is the discharge of the stored energy into a matched resistor by an Alfvén step wave but the secondary consequences of standing primary waves are also explored. The nature of the solutions depends on the electromagnetic characteristics of the walls normal to the imposed field. The problem is mathematically interesting because it involves the joint solving of elliptic and hyperbolic equations that are coupled by the boundary conditions at these walls.
On solutions of the compressible laminar boundary-layer equations and their behaviour near separation
Authors: Davies T. , Walker G.
Published: April 2006

A numerical solution of the two-dimensional compressible laminar boundary-layer equations up to the point of separation is presented. For a particular mainstream velocity distribution it is necessary to specify the surface temperature (or the heat flux across the surface), the suction velocity, the free-stream Mach number and the viscosity-temperature relationship for a solution to be generated. The effect upon the position of separation of a hot or cold wall and of varying the free-stream Mach number is given special emphasis. The variations of the skin friction, heat transfer and various boundary-layer thicknesses for compressible flow past a circular cylinder and for flow with a linearly retarded mainstream were found. The behaviour of the solutions close to separation is investigated. Known functions which model the skin friction and heat transfer are introduced and are used to match the numerical solutions with the Buckmaster (1970) expansions.
High frequency limiting virtual-mass coefficients of heaving half-immersed spheres
Authors: Davis A. M. J.
Published: April 2006

High frequency surface waves are generated by the forced heaving of either two half-immersed spheres in infinite water or by a half-immersed sphere in a hemispherical lake. The virtual-mass coefficients can be found, to leading order, in terms of wave-free limit potentials.
The collapse of a non-hemispherical bubble attached to a solid wall
Authors: Shima A. , Nakajima K.
Published: April 2006

The problem of the collapse of a vapour/gas bubble attached to a solid wall and initially perturbed from a hemispherical shape is solved numerically by the variational method, in which the bubble's viscosity and compressibility in liquid are neglected. The effects of surface tension on the collapsing bubble are taken into account. The rebounding processes of a non-hemispherical gas bubble are simulated: the gas inside the bubble undergoes an adiabatic process. The results of numerical calculations are given for two initial shapes: one is close to a prolate spheroid, the other is close to an oblate spheroid. The governing equations for the motion of a bubble can be written in matrix form, which is simpler than that derived from perturbation theory. This analysis using the variational method may be applied to more complicated problems.
Cavitation noise and inception as influenced by boundary-layer development on a hydrofoil
Authors: Blake William K. , Wolpert M. J. , Geib F. E.
Published: April 2006

This paper describes measurements of noise from two-phase flow over hydrofoils. The experiments were performed in a variable-pressure water tunnel which was acoustically calibrated so that sound power levels could be deduced from the sound measurements. It is partially reverberant in the frequency range of interest.

Cavitation was generated on a hydrofoil in the presence of either a separated laminar boundary layer or a fully turbulent attached boundary layer. The turbulent boundary layer was formed downstream of a trip which was positioned near the leading edge. High-speed photographs show the patterns of cavitation which were obtained in each case. The noise is shown to depend on the type of cavitation produced; and for each type, the dependence on speed and cavitation index has been determined. Dimensionless spectral densities of the sound are shown for each type of flow.
The hydraulics of rotating-channel flow
Authors: Gill A. E.
Published: April 2006

Flow of a homogeneous inviscid fluid down a rotating channel of slowly varying crosssection is considered, with particular reference to conditions under which the flow is ‘hydraulically controlled’. This problem is a member of a general class of problems of which gas flow through a nozzle and flow over a broad-crested weir are examples (Binnie 1949). A general discussion of such problems gives the means for determining the position of the control section (which is generally flow dependent) and shows that at this position there always exist long-wave disturbances with zero phase speed (i.e. disturbances are always ‘critical’ at the control section). The general theory is applied to the rotating-channel problem for the case of uniform potential vorticity. For this problem, three parameters are needed to specify the upstream flow, and the control theory gives a relationship between these parameters which depends on the geometry of the channel.
Cavitation in lubrication. Part 1. On boundary conditions and cavity—fluid interfaces
Authors: Savage M. D.
Published: April 2006

The flow of viscous lubricant in narrow gaps is considered for those geometries in which cavitation arises. A detailed review is presented of those boundary conditions which have been proposed for terminating the lubrication regime (i.e. those valid where the cavity forms). Finally it is shown that a uniform cavity-fluid interface remains stable to small disturbances provided that
\[
\frac{d}{dx}\left(P+\frac{T}{r}\right) < 0,
\]
in which T and r represent the surface tension of the fluid and the radius of curvature of the interface respectively whilst dP/dx is the gradient of fluid pressure immediately upstream of the interface.
Cavitation in lubrication. Part 2. Analysis of wavy interfaces
Authors: Savage M. D.
Published: April 2006

The steady and unifrom flow of a viscous fluid past a unifrom cavity in a gemoetry with small, yet arbitrary, film thickness is considered. A mathematical model for describing steady perturbations to such a flow is presented in which the perturbation to the cavity-fluid interface is represented by a small amplitude harmonic wave of wavenumber n. A linearized perturbation analysis then permits the formulation of a boundary-value problem involving the homogeneous Reynolds equation, the solution to which determines both n and the perturbed pressure field.

Numerical and approximate analytic solutions are determined for the cylinderplane geometry in which fluid flows between a rotating cylinder and a Perspex block. Whilst these compare well with experimental data over the whole range
\[
0.1 < \eta U/T < 3,
\]
closest agreement between theory and experiment is attained for small values of both ηU/T and n.
Relationship between tumour-node-metastasis stage of laryngeal squamous cell carcinoma and duration of symptoms
Authors: Özbilen S , Aydil U , Dinç A E , Kizil Y , Köybaşioğlu A
Published: April 2006

Objective: To evaluate the relationship between disease stage and duration of symptoms for squamous cell carcinoma of the larynx.

Method: We evaluated retrospectively data from 102 laryngeal squamous cell carcinoma patients seen in the otorhinolaryngology department of the Gazi University School of Medicine between December 1996 and June 2004. Inclusion criteria were a histologic diagnosis of laryngeal squamous cell carcinoma, no previous medical, surgical or radiological treatment for carcinoma, and the absence of any other primary tumours or distant metastatic disease.

Results: There was no relationship between duration of symptoms and stage of the disease, both for glottic and supraglottic tumours. When glottic and supraglottic tumours were compared, no significant difference in symptom duration could be found.

Conclusion: There is no correlation between symptom duration and the stage of the disease for squamous cell carcinoma of the larynx. Therefore, symptom duration does not reflect the stage of the tumour.
Natural and anthropogenic forcing on the dynamics of virioplankton in the Yangtze river estuary
Authors: Jiao Nianzhi , Zhao Yanlin , Luo Tingwei , Wang Xiulin
Published: April 2006

Seasonal investigation of virus dynamics by flow cytometry was conducted in the Yangtze river estuarine area in April, August, November 2002 and February 2003, and a supplemental investigation in the inner estuary and downstream of the river was conducted in October 2005. The majority of the total viral abundance was bacteriophage and only 5.4% of the total was algal virus. Total viral abundance varied with season and location, ranging from 6.75×105–1.68×107 particles/ml, and the virus:bacterium ratio (VBR) ranged from 1.52 to 72.02 with a mean of 8.7. In the present study, viral abundance peaked in both the summer and the winter, unlike the typical seasonal pattern reported in the literature, in which viral abundance peaks in the summer when bacterial hosts are also at their most abundant. However, the driving forces for the two peaks reported here were totally different, the summer viral abundance peak coupled with the development of bacterial hosts which were controlled largely by temperature year-round and by trophic state occasionally, while the winter one seemed to be multi-factor controlled. The host-phage interaction was no longer predominant in control of the winter viral abundance as bacterial abundance was lowest in this season. The winter low temperature would help maintain a high viral abundance as high temperatures might increase viral inactivation and viral decay; the VBR peak values actually occurred in the winter. More importantly, the high virus-containing freshwater discharge in winter due to a higher proportion of anthropogenic sewage relative to low natural flooding in winter run-off, turned out to be the first factor contributing to the high winter viral abundance and VBR values. In addition, the variation of intrusion of warm and relatively oligotrophic water from oceanic currents played a role alternating the distribution patterns of temperature, salinity and trophic conditions and consequently the distribution patterns of virus and bacteria seasonally and spatially. Dynamics of virus in the Yangtze river estuarine area is thus characterized by distinct seasonal and spatial variations due to natural forcing and by pronounced alternation of the regular patterns due to anthropogenic impacts.
The music is the message II: Adémar de Chabannes' music for the apostolic Office of Saint Martial
Authors: GRIER JAMES
Published: April 2006

When in 1028–9 Adémar de Chabannes created his apostolic liturgy for Saint Martial, patron saint of the abbey that bears his name in Limoges, he preserved most items in the Divine Office from the existing liturgy for the saint that venerated him as a confessor-bishop. For the last two responsories of the third nocturn of Matins, however, Adémar provided new compositions that depart from the prevailing style of this genre: the penultimate responsory, Gloriosus est, is entirely newly composed, while the final responsory, O sancte dei apostole, consists of a new introduction that is prefixed to the existing final responsory of Matins, O Marcialis princeps. The newly composed material contains music that is much more dramatic in conception in comparison with the typical undulating style of the genre, and with this music, Adémar makes a strong statement about Martial's new status as an apostle.
MINIMALLY INVASIVE MANAGEMENT OF TWIN REVERSED ARTERIAL PERFUSION SEQUENCE (TRAP)
Authors: SEBIRE NJ , WONG AE , SEPULVEDA W
Published: April 2006

Acardiac anomaly, twin reversed arterial perfusion (TRAP) sequence or chorioangopagus parasiticus are synonymous terms referring to a rare complication unique to monochorionic multiple pregnancies in which there is apparent lack of a well-formed cardiac structure in one fetus (the acardiac twin), which is abnormally perfused by a structurally normal co-twin (the pump twin) through a single superficial artery-to-artery placental anastomosis. The condition therefore results in arterial blood flowing in a retrograde fashion from the pump twin towards the affected fetus, and this underlying pathophysiology indicates the preferred use of the clinical term TRAP-sequence. Pathophysiologically, the acardiac fetus acts as a parasite that can only survive in-utero as it is haemodynamically dependent upon the pump twin. The principal perinatal problems being associated with the TRAP sequence are pump-twin congestive heart failure, polyhydramnios and severe preterm delivery, although intrauterine death of the pump twin has been reported even in the absence of such features.