bluuebunny/crossref_metadata_2025_split huggingface dataset:
Measurements of third-order resonant wave interactions
Authors: Mcgoldrick L. F. , Phillips O. M. , Huang N. E. , Hodgson T. H. 
Published: March 2006

This paper presents the results of experiments on the resonant interaction of gravity waves. Two mutually-orthogonal primary wave trains are generated in a tank and their interaction products studied at various positions on the surface. Under suitable conditions, the growing resonant third-order interaction product is identified; its amplitude is shown to be a linear function of the interaction distance. The band-width of the response decreases with increasing distance, as is characteristic of the phenomenon of resonance. The ratio of the frequencies of the primary waves at resonance is very close to that predicted theoretically; the growth rate of the third component is close to, though about 20% higher than, the predicted value. Conditions far from resonance are also studied; it is found that the growing tertiary wave is absent in this case.These results offer the first unambiguous experimental demonstration of resonant wave interactions.
Fluctuating lift and drag on a long cylinder of square cross-section in a smooth and in a turbulent stream
Authors: Vickery B. J. 
Published: March 2006

The paper presents the results of measurements of fluctuating lift and drag on a long square cylinder. The measurements include the correlation of lift along the cylinder and the distribution of fluctuating pressure on a cross-section. The magnitude of the fluctuating lift was found to be considerably greater than that for a circular cross-section and the spanwise correlation much stronger.It was found that the presence of large-scale turbulence in the stream had a marked influence on both the steady and the fluctuating forces. The most significant changes were at small angles of attack (%alpha; < 10°) and included a reduction in base suction and a decrease in fluctuating lift of about 50%.
Free gravitational oscillations in rotating rectangular basins
Authors: Rao Desiraju B. 
Published: March 2006

The study of free oscillations of a homogeneous liquid under gravity in rotating rectangular basins of uniform depth is undertaken from both theoretical and experimental considerations.The theoretical study is in the framework of the quasistatic equations. It is also assumed that the curvature of the free surface can be ignored. Numerical computations for the frequencies and modal structures were carried out for several of the slowest antisymmetric and symmetric modes in a square basin and in a rectangular basin of two-to-one dimension ratio, without any restriction on the angular speed of rotation of the basin. These computations are in agreement with a numerical value obtained many years ago by Taylor, and also with several values found by Corkan & Doodson. They exhibit the typical frequency-splitting associated with certain multiplets in the zero-rotation spectrum. Further, the theoretical calculations indicate that the slopes of the curves of frequency versus speed of rotation change sign for some of the modes in rectangular geometry. Such behaviour is not present in circular basins. Negative modes are found to be ‘unstable’ in the sense of Corkan & Doodson; that is, they are transformed into positive modes for sufficiently high rotation. Calculations were also made for the slowest longitudinal oscillations in highly elongated basins to demonstrate the decreasing importance of rotation on the frequencies of these modes.Experimental work was carried out in a flat-bottomed square tank for the slowest positively and negatively propagating antisymmetric modes and the slowest positively propagating symmetric mode. Good agreement was found between theory and experiment.
Application of a magnetic mass spectrometer to ionization studies in impure shock-heated argon
Authors: Sturtevant B. 
Published: March 2006

A study of the unique role of impurities in the initial stages of ionization relaxation in shock-heated argon, using a sampling mass spectrometer to determine the ionic products of the reaction, is described. The ions are extracted from the shock tube through a small orifice in the end wall after they have diffused through the dense thermal layer adjacent to the wall from the ionizing gas behind the reflected shock wave. The ion diffusion is analysed in detail to assess the possibility that the sampling process alters the reaction products. It is shown that this is unlikely because the impurities are in dilute concentration and the reaction is studied in its initial stages. This mode of sampling is compared with others.The experiments were conducted in argon at temperature of 16,600 °K and pressure of 16 mmHg with an estimated impurity level of 300 parts per million. A surprisingly large number of different ions were detected during the initial stages of ionization. O+and H+were found in much greater amounts than any of the other products, each being about five times more abundant than A+. The results suggest that H2O is probably quite generally the most important impurity in thermal-ionization experiments, and that ionization ‘incubation’ is due to dissociation of molecular impurities (especially H2O) before ionization commences. Possible explanations of the well-known efficiency of small amounts of impurities in initiating ionization are discussed.
Intermittency measurements in the turbulent boundary layer
Authors: Fiedler H. , Head M. R. 
Published: March 2006

An improved version of Corrsin & Kistler's method has been used to measure intermittency in favourable and adverse pressure gradients, and the characteristic parameters of the intermittency have been related to the form parameterHof the mean velocity profiles.It is found that with adverse pressure gradients the centre of intermittency moves outward from the surface while the width of the intermittent zone decreases. The converse is true of favourable pressure gradients, and it seems likely that at sufficiently low values ofHthe flow over the full depth of the layer is only intermittently turbulent.A new method of intermittency measurement is presented which makes use of a photo-electric probe. Smoke is introduced into the boundary layer and illuminated by a narrow beam of parallel light normal to the surface. The photoelectric probe is focused on the illuminated region and a signal is generated when smoke passes through the focal point of the probe lens. Comparison of this signal with the output from a hot-wire at very nearly the same point shows the identity of smoke and turbulence distributions.
Computation and measurement of Hall potentials and flow-field perturbations in magnetogasdynamic flow of an axisymmetric free jet
Authors: Otis David R. 
Published: March 2006

The interaction was observed between a supersonic free jet of partially ionized argon and the magnetic field of a coil concentric with the jet. Nominal values of the parameters were: Mach number, 3; Reynolds number, 1000; magnetic Reynolds number, 0·2; magnetic interaction parameter, 1; Hall parameter, 1. The jet was strongly channelled. Axial and radial electric fields were observed in the jet with a net rise in potential across the interaction.These observations were consistent with predictions based on the single fluid, macroscopic equations, and a simple slug flow model. The current equation was solved to second order in the Hall parameter giving a closed form expression for the Hall potential which agreed with the experiments for weak fields. Flow perturbations were calculated for Mach numbers of 3.11, 6.08, and 10.05, neglecting Hall currents; the calculations are in qualitative agreement with the experiments and show Joule heating to be the main factor in perturbing the flow at high Mach numbers.
Inviscid hypersonic flow on cusped concave surfaces
Authors: Sullivan Philip A. 
Published: March 2006

An analysis of the exact equations of the inviscid flow of a perfect gas over cusped concave bodies is described. The field is examined in the limit of infinite free-stream Mach numberM∞. The slope of the shock wave in a small region adjacent to the leading edge is strongly dependent onM∞, while much further downstream the shock-wave slope is controlled primarily by the body slope. Consequently the region near the leading edge introduces into the field downstream a thin layer of gas, adjacent to the body, where the entropy is much lower than that of the gas above it. This layer is so dense that the gas velocity along it is not appreciably slowed by the pressure gradient along the body. However, it is so thin that there is little pressure change across it.The well-known self-similar solutions to the hypersonic small-disturbance equations have previously only been used to study the flow on blunted slender convex surfaces. They are known to behave singularly at the body. It is shown that there is a region on concave power-law shapes where the self-similar solutions are the correct first approximation to the exact inviscid equations in the limitM∞→ ∞; and that, further, they predict the correct first-order surface pressure.Numerical results for surface pressure from the similar solutions are presented, and comparisons are made with certain approximate theories available for more general shapes. Pressure measurements taken on a cubic surface in the Imperial College gun tunnel are presented and compared with the theoretical distributions.
The stability and transition of an axisymmetric wake
Authors: Sato Hiroshi , Okada Osami 
Published: March 2006

Experimental and theoretical investigations were made of the instability and transition of the wake behind an axisymmetric slender body at high Reynolds number. The sound from a loudspeaker was used as an artificial disturbance. The velocity fluctuations induced by the sound are selectively amplified depending on the frequency. The wavelength, the phase velocity and the amplification rate of the velocity fluctuations were measured. A linearized stability equation of the disturbance superposed on the axisymmetric wake was solved numerically for both neutral and amplified disturbances. Theoretical results on eigenvalues and eigenfunctions of the stability equation show good agreement with experimental results.Measurements on phase distributions in streamwise and azimuthal directions indicate that the line of same phase forms a helix and not a series of discrete closed loops.
An experimental investigation of the instability of an incompressible, separated shear layer
Authors: Browand F. K. 
Published: March 2006

The investigation of a separated shear layer was undertaken to clarify the non-linear mechanisms associated with instability and transition to turbulence. Such an investigation is of practical importance since profiles which resemble the separated shear layer are a common occurrence.A two-dimensional free shear layer was formed by separation of a laminar boundary layer from a rearward-facing step. The free-stream speed was approximately 16 ft./sec. Hot-wire measurements were made in the region directly downstream of the plate trailing edge. The measurements included mean velocity profiles, frequency spectra of the longitudinal fluctuation, and root-mean-square amplitude and phase distributions of various spectral components of the longitudinal fluctuation. Several measurements were designed to detect the presence of periodic spanwise structure.The most important findings were:
Significant non-linear distortion of the initial unstable wave occurred without periodic spanwise structure.Non-linear distortion was first manifest by the growth of a subharmonic oscillation, which was strongly intermittent. Numerous harmonics of the sub-harmonic oscillation were also present.Strong evidence suggests that secondary instabilities were present, which created still higher frequencies.
Wind-generated waves in thin liquid films
Authors: Craik Alex D. D. 
Published: March 2006

In the presence of an air stream, a uniform liquid film on a horizontal flat plate may be unstable to small disturbances, and waves may arise. In this paper the hydrodynamic stability of thin liquid films is examined both experimentally and theoretically.The experiments concern water films thinner than those which have been examined in the past. It is found that, when the film thickness is sufficiently small, a previously unknown type of instability occurs. The theoretical analysis explains this surprising phenomenon.Due to interaction of the mean airflow and small disturbances of the liquid-air interface, normal and tangential stress perturbations are produced at the liquid surface. It is shown that small wave-like disturbances become unstable when the joint influence of the component of normal stress in phase with the wave elevation and the component of tangential stress in phase with the wave slope is sufficient to overcome the ‘stiffness’ of the liquid surface due to gravity and surface tension. It is found that the destabilizing role of the tangential stress component is dominant for very thin films, and that instability may occur whatever the velocity of the air stream, provided the film is made sufficiently thin.
Dynamics of rotating fluids: a survey
Authors: Lighthill M. J. 
Published: March 2006

The dynamics of rotating fluids was, in the main, developed by methods special to the field, using the equationsof motion of a fluid inarotatingframe of reference. It is, nevertheless, possible to derive all the leading results from the classical principles of fluid dynamics in non-rotating frames; specifically, from the rules governing rate of change of vorticity. Although writers on the subject have adopted this approach increasingly often in recent years, the author believes that a broad survey of the field, deriving results from those classical rules concerning vorticity, has not previously been given and may be of some interest to fluid dynamicists in general.The present survey was read to the IUTAM Symposium on Rotating Fluid Systems at La Jolla, California, on 28 March 1966. It states briefly (§2) the rules governing rate of change of vorticity, and then applies them, first, to problems of steady relative motion of rotating fluids; in particular, of the atmosphere (§3), of rotating fluids in the laboratory (§4), and of the oceans (§5). Waves and wavy movements are then studied, first (§6) for systems with constant Coriolis parameter, including inertial waves, surface waves, ‘long waves’ and internal waves, and, secondly (§7)) for systems with variable Coriolis parameter, including Rossby-Haurwitz waves with and without the influence of tidal effects, as well as problems of barotropic and baroclinic instability. Vorticity principles are used as the sole theoretical tool throughout the survey.
On the instability of ventilated cavities
Authors: Woods L. C. 
Published: March 2006

It has been found that ventilated cavities extending behind hydrofoils, plates, and other two-dimensional bodies, oscillate when the air supply rate is sufficient to reduce the cavitation number to about one-fifth of its natural value. As the rate increases further, higher modes of oscillation occur in which the cavity–water interface supports several waves that are convected downstream towards the wake, which, owing to a pinching-off action replacing the usual entrainment sink, consists of a sequence of large bubbles drifting downstream. A theory of such flows that allows both for the convected velocity fluctuations in the cavity, and for the transport of bubble volume down the wake, is given in this paper. Coupled with a rather simple phenomenological relation between the pressure fluctuations within the cavity and the departure of the pinched-off rear portion of the cavity—explained in terms of the action of the re-entrant jet—this theory successfully predicts the resonance frequencies obtained in experiments by Silberman & Song.The theory also provides a solution of the more general problem of determining the fluctuations in the pressure distribution over the whole surface of the body, when it is in a prescribed unsteady motion along its axis of symmetry (the theory is confined to symmetrical bodies and flows). Thus the growth in drag due to a sudden increment in the upstream velocity can be predicted, and also the damping forces acting on the body when it is forced to oscillate at a given frequency. It is shown that in all cases the body is unstable.One important feature of the mathematical model chosen is that it completely avoids the presence of a time-dependent sink at infinity—with its associated infinite pressures—by conserving total volume of wake and cavity in just the same way as vorticity is conserved in unsteady aerofoil theory.
Numerical solutions of the steady-state vorticity equation in rectangular basins
Authors: Bye John A. T. 
Published: March 2006

The Navier-Stokes vorticity equation is solved numerically for the circulation induced in a vertical plane, by a constant stress acting on a liquid, enclosed in a basin of uniform depth and vertical sides.Solutions of the linearized vorticity equation are obtained for all Reynolds numbers (τsD2/4ρν2 where νs is the surface stress, ρ is the density, ν is the kinematic viscosity, and D is the depth of the liquid) and solutions of the complete vorticity equation for Reynolds numbers 0–400.The notable feature of the solutions is the totally different end circulations. At the upwind end the flow becomes very slack, and the vorticity equation has a boundary-layer limit, while at the downwind end a damped wave occurs and the equation has an inviscid limit.At Reynolds numbers between 400 and 600, the streamlines at the downwind end lead to a condition of hydrodynamic instability, in approximate agreement with some experimental observations by G. H. Keulegan.
The influence of Coriolis force on surface-tension-driven convection
Authors: Vidal A. , Acrivos Andreas 
Published: March 2006

The effect of uniform rotation on surface-tension-driven convection in an evaporating fluid layer is considered both theoretically and experimentally. The theoretical analysis follows the usual small-disturbance approach of perturbation theory and leads, at the neutral state, to a functional relation between the Marangoni and Taylor numbers which is then computed numerically. In addition, it is shown analytically that, in the limit of rapid rotation, the velocity and temperature fluctuations are confined to a thin Ekman layer near the surface, and that Mc = 4·42T½ and ac = 0·5T¼, where Mc and ac are, respectively, the critical Marangoni number and the critical wave number for neutral stability, and T is the Taylor number.The experimental part deals primarily with the flow pattern of a 50% solution of ethyl ether in n-heptane evaporating into still air. In this case, the convective flow is surface-tension-driven and its structure was observed using schlieren optics. In the absence of rotation, the flow shows a remarkable cellular pattern when the layer is shallow, but when the depth of the layer is increased the pattern quickly becomes highly irregular. In contrast, for T > 103, a cellular structure is always observed even for deep layers, a result which is attributable to the stabilizing effect of the Coriolis force. A further increase in T leaves the flow pattern unchanged except that the size of the cells is found to decrease as T−¼ which is in agreement with the results of the linear stability analysis.
On the viscous flow in the nose region of a symmetric blunt body in hypersonic flow
Authors: Rarity B. S. H. 
Published: March 2006

The flow field in the nose region of a blunt body in hypersonic flow is studied by considering the transport of vorticity and enthalpy. The entire region between body and shock is considered to be viscous, not necessarily thin in comparison with the nose radius of the body and to be of slowly varying density. The (given) post-shock vorticity need not be small and the density ratio ρ∞/ρs may either be small or near unity, the analysis being valid asymptotically at both limits.It is found that the vorticity equation may be uncoupled from the total enthalpy equation if μ√ρ is constant. While the equations are not expected to be necessarily restricted to the immediate vicinity of the stagnation line, only there can the solution be written down explicitly; elsewhere, numerical integration is required.
Thermal convection with shear at high Rayleigh number
Authors: Ingersoll Andrew P. 
Published: March 2006

A fluid is contained between rigid horizontal planes which move relative to each other with constant horizontal velocity. A gravitationally unstable temperature difference is maintained at the boundaries, and the heat flux and momentum flux (stress) transmitted by the fluid are measured. The Nusselt number,Nu, and the dimensionless momentum flux,Mo, are obtained for small mean rates of shear. The Rayleigh number,R, and the Prandtl number, σ, are both large in these experiments. The data are consistent with the following relations:\[ Nu \propto Mo \sigma^{\frac{1}{2}} \propto R^{\frac{1}{3}}. \]Kraichnan's mixing-length theory of turbulent thermal convection is extended to the present situation, and the above experimental dependence ofNuandMoonRand σ is obtained. The agreement between mixing-length theory and experiment provides strong support for Kraichnan's concise treatment of turbulent convection. The importance of this result is that heat and momentum fluxes may be calculated in this way for a variety of flows in geophysics.
The boundary layer in crossed-fields m.h.d.
Authors: Sears W. R. 
Published: March 2006

This study of the boundary layer of steady, incompressible, plane, crossed-fields m.h.d. flow at large Reynolds numberReand magnetic Reynolds numberRmbegins with a review of Hartmann's case, where a boundary layer occurs whose thickness is proportional to (Re Rm)−½. Following this clue, it is shown that in general the boundary layer is a ‘local Hartmann boundary layer’. Its profiles are always exponential and it is determined completely by local quantities. The skin friction and the total electric current in the layer are proportional to the square root of the magnetic Prandtl number, i.e. to (Rm/Re)½. Thus the exterior-flow problem, the solution of which precedes a boundary-layer solution, generally involves a current sheet at the fluid-solid interface.This inviscid-flow problem becomes tractable if (Rm/Re)½is small enough to permit a linearized solution. The flow field about a flat plate at zero incidence is calculated in this approximation. It is pointed out that the thin-cylinder solutions of Sears & Resler (1959), which pertain toRm/Re= 0, can immediately be extended to small, non-zero values of this parameter by linear combination with this flat-plate solution.