bluuebunny/crossref_metadata_2025_split huggingface dataset:
Observations on distorted turbulent wakes
Authors: Reynolds A. J. 
Published: March 2006

The question considered here is whether turbulent wakes can undergo a simple self-preserving development when convected through a duct of varying crosssection. A flow is described as self-preserving if the distributions of its mean-value properties at successive sections have the same basic shape, differing only in magnitude and lateral extent. In such flows the scales of the mean velocity variation and turbulence have been found to be proportional.The wakes studied are concentrated near a plane of symmetry through a duct of changing rectangular section but constant area; the portion of the wake near the other plane of symmetry is considered. Measurements in the flows behind three circular cylinders reveal a lack of universality of the scales associated with self-preserving solutions of the momentum equation. The wakes of the smaller cylinders adopt the predicted form, but that of the largest does not. As it moves through the channel, this wake is compressed less rapidly by the imposed lateral strain than is predicted.Interest is then concentrated on the equation governing the kinetic energy of the turbulence and in particular on the relative size of its two production terms, that representing the action of the mean wake shear and that representing the effect of the distortion applied by the duct. It is found that self-preserving wakes can form only in a few kinds of distorting convecting flow. In the experimental duct, which has exponentially varying walls, one of these is set up. Attention is concentrated on this case. A stability analysis (based on the criterion that a stable wake is one in which the total turbulence production decreases as a result of more rapid wake expansion) suggests that, when production by wake shear is greater than that by distortion, a stable self-preserving development is possible in which the turbulence scale and the velocity defect scale remain proportional. But in the alternative, distortion-dominated, case the velocity defect decreases continually relatively to the turbulence.The experimental results are in accord with these predictions. The transition from shear-dominated to distortion-dominated wakes appears to take place when the production terms are about equal. The failure of the one experimental wake to adopt a simple self-preserving form can be attributed to the relatively slight organization of its turbulence when distortion is begun.
The ‘starting plume’ in neutral surroundings
Authors: Turner J. S. 
Published: March 2006

The advancing front of a buoyant plume which is being established in uniform surroundings has some properties in common with the plume, while in other respects it behaves more like a ‘thermal’ released from rest. The solutions for these two cases cannot be matched directly, since the dependence of velocity on height is different. It is shown here that a similarity solution, which is consistent with the equations describing both parts of the flow, can be obtained once it is recognized that the velocity of the front may be less than that of the steady plume. The cap moves with a constant fraction of the plume velocity at the same level, and the total buoyancy in the cap is increasing, so a modification of the simple relations for thermals is required.This prediction is verified experimentally, and numerical values for the ratio of the velocities and the rate of increase of the cap radius with height determined. The extreme front of plume cap advances at about 0·6 times the mean velocity on the axis of a steady plume, and it spreads at just over half the angle of a thermal. This implies a smaller rate of entrainment and therefore a smaller rate of dilution per unit height compared with a thermal, especially since about half of the fluid mixed into the cap comes from the plume below. The model allows one to estimate the time necessary for convection from a known steady source on the ground to lead to the formation of a cloud.
A periodic boundary-layer flow in magnetohydrodynamics
Authors: Turcotte D. L. , Lyons J. M. 
Published: March 2006

It is the purpose of this paper to solve a boundary-value problem posed by induction electromagnetic pumps and generators. Solutions are obtained by an expansion technique and a momentum method for the laminar, incompressible flow problem. For large values of the interaction parameter (μ2σH20λ/ρμe viscous effects are shown to be restricted to periodic boundary layers. In regions of high-field strength a local Hartmann solution is valid. Where the applied field is weak an inertial boundary layer is present which thickens in the upstream direction.A logical explanation of this phenomenon is given. The condition that a boundary-layer type flow exist is obtained and is shown to be in general satisfied. The results show that inviscid theory may be used to calculate the overall performance of electromagnetic pumps and generators while the boundary-layer theory developed here may be used to obtain the wall shear stress.
On Oseen's approximation
Authors: Chester W. 
Published: March 2006

An investigation is made into the validity of the Oseen equations, for incom-pressible, viscous flow past a body, as an approximation to the Navier-Stokes equations. It is shown that, when the body is such that a reversal of the uniform flow at infinity merely reverses any component of the force on the body without changing its absolute magnitude, that component can be determined correctly to the first order in the Reynolds number, though the detailed velocity field is not correct to this order. Moreover, this force can be deduced simply from a knowledge of the force on the body according to Stokes's approximation.The analysis is also generalized to include the magneto-hydrodynamic effects when the fluid is conducting and the flow takes place in the presence of a magnetic field.
Hydromagnetic waves in a cylindrical plasma: an experiment
Authors: Jephcott D. F. , Stocker P. M. 
Published: March 2006

In a previous paper (Woods 1962) a theory is presented which leads to a general dispersion relation for hydromagnetic waves in a dissipative plasma contained in a cylindrical tube. In the present paper experimental observations are compared with the predictions of this theory.An experiment is described in which torsional hydromagnetic waves are excited in a gas discharge. Measurements of the wave velocity and damping are compared with solutions of the dispersion equation which are computed from measured values of the plasma parameters. The results are consistent with the theory, and good numerical agreement is obtained by assuming a loss of particles to the tube walls. There is evidence of a cut-off in wave propagation in the region of ion cyclotron resonance.
The Preston tube as a means of measuring skin friction
Authors: Head M. R. , Rechenberg I. 
Published: March 2006

Preston's method of measuring skin friction, which makes use of a Pitot tube resting on the surface, depends upon the assumption of a region of flow similarity, adjacent to the wall, common to fully developed turbulent pipe flow and the turbulent boundary layer. Experiments performed elsewhere have cast considerable doubt on the validity of this assumption, and the present investigation was undertaken to establish whether or not it is justified.Experiments were carried out in a short length of large-diameter pipe which could either form part of a very much longer pipe, giving fully developed turbulent pipe flow, or could be preceded by a conventional contraction and screens, giving a developing turbulent boundary layer.Final results showed that for a given skin friction the Pitot tube reading was the same for both boundary layer and pipe flows, thus vindicating Preston's method and confirming the existence of a universal region of wall similarity. Initial experimental difficulties were found to be due to unexpectedly large circumferential variations in skin friction in the growing boundary layer.
The movement of single large bubbles in closed vertical tubes
Authors: Goldsmith H. L. , Mason S. G. 
Published: March 2006

The flow and deformation of single large air and liquid bubbles suspended in wetting liquids when moving under gravity in closed vertical tubes have been studied. The flow patterns in both phases and thickness of the liquid film surrounding the bubbles and the shape of the bubble ends were determined by direct observation through a microscope.Quantitative agreement with a theory relating bubble velocity to film thickness in viscous flow, and to the tube radius in inertial flow was obtained. In viscous flow the deformation at a given interfacial tension was independent of bubble length and of the viscosity of the suspending liquid.In viscous flow the end of the sedimenting bubbles were spheroidal, with the leading ends prolate and the trailing ends generally oblate. The axis ratio of the leading ends increased with increasing interfacial tension. Wave disturbances occurred at the trailing ends of the bubbles. In inertial flow the leading ends of rising air bubbles were hemispherical.
Behaviour of macroscopic rigid spheres in Poiseuille flow Part 1. Determination of local concentration by statistical analysis of particle passages through crossed light beams
Authors: Segré G. , Silberberg A. 
Published: March 2006

An apparatus is described for determining particle passages through any selected point on a tube cross-section. The method depends on the simultaneous blocking out of two mutually perpendicular light beams by a particle passing through their common region. The coincidence is registered and counted electronically. At higher particle concentrations coincidences are also registered arising from a pair-wise occupation of the light beams by two particles. An analysis is presented showing that these pair coincidences can be allowed for exactly in terms of experimentally measurable quantities.The reliability and reproducibility of the method is discussed and illustrated by examples from sphere suspensions in Poiseuille flow.
Behaviour of macroscopic rigid spheres in Poiseuille flow Part 2. Experimental results and interpretation
Authors: Segré G. , Silberberg A. 
Published: March 2006

It is shown that a rigid sphere transported along in Poiseuille flow through a tube is subject to radial forces which tend to carry it to a certain equilibrium position at about 0.6 tube radii from the axis, irrespective of the radial position at which the sphere first entered the tube. It is further shown that the trajectories of the particles are portions of one master trajectory and that the origin of the forces causing the radial displacements is in the inertia of the moving fluid. An analysis of the parameters determining the behaviour is presented and a phenomenological description valid at low Reynolds numbers is arrived at in terms of appropriate reduced variables. These phenomena have already been described in a preliminary note (Segré & Silberberg 1961). The present more complete analysis confirms the conclusions, but it appears that the dependence of the effects on the particle radius go with the third and not the fourth power as was then reported.It is also shown that the description of the phenomena becomes more complicated at tube Reynolds numbers above about 30.
Measurements of the fluctuating pressure at the wall beneath a thick turbulent boundary layer
Authors: Willmarth W. W. , Wooldridge C. E. 
Published: March 2006

Measurements of the turbulent pressure field at the wall beneath a thick (5-inch) turbulent boundary layer produced by natural transition on a smooth surface are reported. The data include the mean-square pressure, parallel to the stream, and spatial correlation of the pressure transverse to the stream.The root-mean-square wall pressure was 2.19 times the wall shear stress. The power spectra of the pressure were found to scale with the free-stream speed and the boundary-layer displacement thickness. A few tests with a rough surface showed that the increase in root-mean-square wall pressure was greater than the increase in wall shear stress.The space-time correlation measurements parallel to the stream direction exhibit maxima at certain time delays corresponding to the convection of pressure-producing eddies at speeds varying from 0.56 to 0.83 times the stream speed. The lower convection speeds are measured when the spatial separation of the pressure transducers is small, or when only the pressure fluctuations at high frequencies are correlated. Higher convection speeds are observed when the spatial separation of the pressure transducers is large, or when only low frequencies are correlated. The result that low-frequency pressure fluctuations have the highest convection speed is in agreement with the measurements of Corcos (1959, 1962) in a fully turbulent tube flow. Analysis of these measurements also shows that both large- and small-scale pressure-producing eddies decay after travelling a distance proportional to their scale. More precisely, a pressure-producing eddy of large or small wavelength λ decays and vanishes after travelling a distance of approximately 6λ.The transverse spatial correlation of the wall-pressure fluctuations was measured and compared with the longitudinal scale. Both the transverse and the longitudinal scale of the pressure fluctuations were of the order of the boundary-layer displacement thickness. The transverse and longitudinal scales of both large- and small-scale wall-pressure fluctuations were also measured and were also found to be approximately the same.
The motion of rigid particles in a shear flow at low Reynolds number
Authors: Bretherton F. P. 
Published: March 2006

According to Jeffery (1923) the axis of an isolated rigid neutrally buoyant ellipsoid of revolution in a uniform simple shear at low Reynolds number moves in one of a family of closed periodic orbits, the centre of the particle moving with the velocity of the undisturbed fluid at that point. The present work is a theoretical investigation of how far the orbit of a particle of more general shape in a non-uniform shear in the presence of rigid boundaries may be expected to be qualitatively similar. Inertial and non-Newtonian effects are entirely neglected.The orientation of the axis of almost any body of revolution is a periodic function of time in any unidirectional flow, and also in a Couette viscometer. This is also true if there is a gravitational force on the particle in the direction of the streamlines. There is no lateral drift. On the other hand, certain extreme shapes, including some bodies of revolution, will assume one of two orientations and migrate to the bounding surfaces or to the centre of the flow. In any constant slightly three-dimensional uniform shear any body of revolution will ultimately assume a preferred orientation.
The growth of Taylor vortices in flow between rotating cylinders
Authors: Davey A. 
Published: March 2006

In flow between concentric rotating circular cylinders, it was shown by Taylor (1923) that instability may occur in the form of toroidal vortices spaced regularly along the axis. When the vortex motion occurs additional torque is required to keep the cylinders in motion at given speeds. Stuart (1958) used an energy-balance method, in the case when the annular gap is small compared with the radius, to estimate the additional torque and the associated finite amplitude attained by the vortices. He included the effect of distortion of the mean motion, but ignored the generation of harmonics of the fundamental mode and the distortion of the velocity associated with the fundamental mode. It is now known that these are not valid mathematical approximations and a rigorous perturbation expansion is developed here to remedy the deficiency. The analysis is valid for any gap width and any angular speeds of the containing cylinders, but requires the amplification rate of the disturbance to be small.Numerical results using a digital computer are obtained for the shape and amplitude of the vortices in three cases: (i) when the outer cylinder has twice the radius of the inner one and is kept at rest, (ii) when the gap is small and the cylinders rotate with nearly the same speeds, and (iii) when the gap is small and the outer cylinder is kept at rest. The equilibrium amplitude obtained in the last case is substantially the same as that found by Stuart.The results for cases (i) and (iii) give close agreement with the experimental values obtained by Taylor (1936) and Donnelly (1958) for the torque required to keep the inner cylinder rotating with constant speed while the outer one is at rest, for a certain range of speeds. In the small-gap problem it is shown that the equilibrium amplitude is almost proportional to 1 − m, where m is the ratio of the angular speeds of the outer and inner cylinders.
Large-amplitude compression waves in an adiabatic two-fluid model of a collision-free plasma
Authors: Morton K. W. 
Published: March 2006

The development of large amplitude compression waves in a collision-free plasma is studied by considering the motion of a plane piston into a uniform stationary plasma containing a magnetic field parallel to the plane of the piston. The adiabatic two-fluid equations are solved by finite-difference methods and the form of the waves after a long time is compared with the possible steady-state solutions.A generalized discontinuous solution of the steady-state equations is found for sufficiently high Mach numbers. At the highest Mach numbers this leads to a constant state at the piston; while at lower speeds a wave train results whose amplitude increases as the speed decreases. In each of these cases the numerical solutions of the time-dependent equations converge rapidly to the steady-state solutions. At still lower speeds, where the solitary-wave solution exists, the situation is less clear.
Experimental investigation of the penetration of a high-velocity gas jet through a liquid surface
Authors: Banks Robert B. , Chandrasekhara D. V. 
Published: March 2006

This investigation is concerned with the phenomenon of a gas jet impinging on and penetrating into a liquid. The study has been restricted to the cases of circular and plane jets penetrating a liquid at right angles. Both ‘free streamline’ and turbulent jets were considered. The phenomenon was analysed from two viewpoints. The first, a stagnation-pressure analysis, related the depth of the surface depression or cavity to the stagnation pressure based on the centre-line velocity of the jet in the neighbourhood of the surface. The second, a displaced-liquid analysis, related the weight of the liquid displaced from the cavity to the momentum of the jet.Numerous experiments were conducted in which the cavity depth, diameter or width, and peripheral lip height were measured. The role of surface tension in affecting the cavity depth was considered and the phenomenon of drop formation was examined. Some attention was given to the case of a plane jet impinging on a moving liquid. It was found that the experimental data fit into the framework of these analyses quite consistently.
Experiments with a self-propelled body submerged in a fluid with a vertical density gradient
Authors: Schooley Allen H. , Stewart R. W. 
Published: March 2006

It is shown that the turbulent wake of a self-propelled body moving in a fluid with a vertical density gradient is considerably different than of the same body moving in a fluid having no density gradient. In the uniform density case, the turbulent mixed fluid behind the body expands into an irregular conical shape. In the case of a density gradient, the initial expansion of the mixed fluid is quickly followed by a collapse in the vertical direction which is accompanied by a further spreading in the horizontal direction. This phenomenon is caused by the force of gravity. The volume of fluid behind the self-propelled body has a more or less constant density due to mixing. Thus, it is forced to seek its own density level in the undisturbed fluid.The collapsing vertical wake is shown to be an efficent generator of internal waves, many of high order. These manifest themselves in surface movements.The assumption that the internal waves are damped only by viscosity, not by turbulence, leads to results in general accord with the observations.
Heat transfer across rough surfaces
Authors: Owen P. R. , Thomson W. R. 
Published: March 2006

It is argued that the heat transfer between a roughened surface and a stream of incompressible fluid flowing over it is dependent on both the viscosity and thermal conductivity of the fluid even when the roughness is large enough for viscosity to have ceased to affect the skin friction.Concentrating on closely spaced roughness, sufficiently large for the skin friction to be independent of Reynolds number, a simple model is constructed of the flow near the surface. It consists of horseshoe eddies which wrap themselves round the individual excrescences and trail unsteadily downstream; the eddies are imagined to scour the surface and thereby to transport heat between the surface and the more vigorous flow in the neighbourhood of the roughness crests. Taken in conjunction with Reynolds analogy between temperature and velocity distributions in the fluid away from the surface, the model leads to an expression for the rate of heat transfer which contains a function of the roughness Reynolds number and the Prandtl number of the fluid whose detailed form is found by appeal to the limited experimental data available. An order-of-magnitude argument suggests that the functional form established empirically is consistent with the assumed model of the flow close to the surface.The object of the work is to establish a basis for the analysis of experimental data and for their extrapolation with respect to Reynolds number and Prandtl number.
The mean velocity profile in three-dimensional turbulent oundary layers
Authors: Hornung H. G. , Joubert P. N. 
Published: March 2006

The mean velocity distribution in a low-speed three-dimensional turbulent boundary-layer flow was investigated experimentally. The experiments were performed on a large-scale model which consisted of a flat plate on which secondary flow was generated by the pressure field introduced by a circular cylinder standing on the plate. The Reynolds number based on distance from the leading edge of the plate was about 6 x 106.It was found that the wall-wake model of Coles does not apply for flow of this kind and the model breaks down in the case of conically divergent flow with rising pressure, for example, in the results of Kehl (1943). The triangular model for the yawed turbulent boundary layer proposed by Johnston (1960) was confirmed with good correlation. However, the value ofyuτ/vwhich occurs at the vertex of the triangle was found to range up to 150 whereas Johnston gives the highest value as about 16 and hence assumes that the peak lies within the viscous sublayer. Much of his analysis is based on this assumption.The dimensionless velocity-defect profile was found to lie in a fairly narrow band when plotted againsty/δ for a wide variation of other parameters including the pressure gradient. The law of the wall was found to apply in the same form as for two-dimensional flow but for a more limited range ofy.
An experimental investigation of a two-layer inviscid shock cap due to blunt-body nose injection
Authors: Baron Judson R. , Alzner Edgar 
Published: March 2006

Blunt-body solutions for suspersonic flow usually concern closed body surfaces. This paper reports on an experimental investigation of a two-layer shock cap and indicates the existence of a predictable contact surface separating the layers. The inner layer was generated by injecting air through a contoured axisymmetric channel on a blunt body so as to simulate a hemispherical contact surface in a Mach number 4.8 flow.Results show the existence of the contact surface and the influence of a range of mass-injection rates upon the displacement of the bow shock and contact surface from the body.