bluuebunny/crossref_metadata_2025 huggingface dataset:
Study of waves on thin liquid films sheared by turbulent gas flows
Authors: Jurman L. A. , McCready M. J.
Published: July 2002

Waves that occur at the interface of a thin, horizontal liquid film sheared by a concurrent turbulent gas flow are investigated. Observations using liquids in the range of 13–15 cP indicate that broad-crested, steady, periodic waves appear as the gas velocity is increased above the point of neutral stability. For a sufficiently large gas Reynolds number, steady solitary waves appear. These travel at speeds significantly faster than the periodic waves. However, if the liquid flow rate is increased, solitary waves do not form. To quantitatively describe the waves, a weakly nonlinear wave equation is derived using boundary-layer approximations. The equation is valid for liquid Reynolds numbers of O (1–100) and reveals the presence of both kinematic and dynamic processes, which may (i) act together or (ii) singularly dominate the wave field. For the latter case, reduced forms of the evolution equation are derived. Linear stability analysis of the complete equation and its reduced forms is used to determine the parameter ranges where either dynamic or kinematic processes dominate. The evolution equation in its nonlinear forms should be capable of describing and predicting the amplitudes, shapes, and interactions of finite amplitude waves.
Coherent structure interactions in a two-stream plane turbulent mixing layer with impulsive acoustic excitation
Authors: Latigo Ben O.
Published: July 2002

Periodic plane acoustic waves consisting of four discrete pulses are used to trigger the Kelvin–Helmholtz (KH) instability at the origin of an initially laminar plane mixing layer. The resulting coherent large-scale structures (CLSS) which grow and interact with their neighbors are followed downstream with hot-wire probes traversed across the mixing layer to a Reynolds number Re=1.5×106. The Reynolds number here is based on downstream distance x and the velocity difference (U1−U2). The continuous hot-wire time record is conditionally sampled with respect to the tone bursts and the samples are phase averaged to reveal the CLSS footprints. Optimum phase averages are obtained using an iterative correlation technique that corrects for phase jitters due to turbulence. An enhanced study of the CLSS is therefore made possible. The ensembles reveal a vivid picture of vortex pairing between the initial eddies as well as the significant role played by the CLSS in momentum transport; hence turbulence mixing.
Subharmonic instability of compressible boundary layers
Authors: Masad J. A. , Nayfeh A. H.
Published: July 2002

The subharmonic instability of a two-dimensional compressible boundary layer over an insulated flat plate is analyzed using the Floquet model. The resulting problem is solved numerically by using both finite differences and the computer code suport. Results are presented for subsonic, transonic, and supersonic flows. For supersonic flows results for the first and second modes are presented. The effects of Mach number, spanwise wave number, amplitude of the primary wave, Reynolds number, and frequency are studied.
Nonlinear Reynolds stress models and the renormalization group
Authors: Rubinstein Robert , Barton J. Michael
Published: July 2002

The renormalization group is applied to derive a nonlinear algebraic Reynolds stress model of anisotropic turbulence in which the Reynolds stresses are quadratic functions of the mean velocity gradients. The model results from a perturbation expansion that is truncated systematically at second order with subsequent terms contributing no further information. The resulting turbulence model applies to both low and high Reynolds number flows without requiring wall functions or ad hoc modifications of the equations. All constants are derived from the renormalization group procedure; no adjustable constants arise. The model permits inequality of the Reynolds normal stresses, a necessary condition for calculating turbulence-driven secondary flows in noncircular ducts.
Computations of three-dimensional Rayleigh–Taylor instability
Authors: Tryggvason Grétar , Unverdi Salih Ozen
Published: July 2002

The fully three-dimensional deformation of an interface between two fluids resulting from a Rayleigh–Taylor instability is studied numerically in the limit of weak stratification. The Navier–Stokes equations are solved by a finite difference method, and the interface is kept sharp by front tracking. The difference between the large-amplitude stages of flows initiated by two- and three-dimensional perturbations is discussed.
Full numerical simulation of coflowing, axisymmetric jet diffusion flames
Authors: Mahalingam S. , Cantwell B. J. , Ferziger J. H.
Published: July 2002

The near field of a non-premixed flame in a low speed, coflowing axisymmetric jet is investigated numerically using full simulation. The time-dependent governing equations are solved by a second-order, explicit finite difference scheme and a single-step, finite rate model is used to represent the chemistry. Steady laminar flame results show the correct dependence of flame height on Peclet number and reaction zone thickness on Damköhler number. Forced simulations reveal a large difference in the instantaneous structure of scalar dissipation fields between nonbuoyant and buoyant cases. In the former, the scalar dissipation marks intense reaction zones, supporting the flamelet concept; however, results suggest that flamelet modeling assumptions need to be reexamined. In the latter, this correspondence breaks down, suggesting that modifications to the flamelet modeling approach are needed in buoyant turbulent diffusion flames.
A diffusion model for velocity gradients in turbulence
Authors: Girimaji S. S. , Pope S. B.
Published: July 2002

In this paper a stochastic model for velocity gradients following fluid particles in incompressible, homogeneous, and isotropic turbulence is presented and demonstrated. The model is constructed so that the velocity gradients satisfy the incompressibility and isotropy requirements exactly. It is further constrained to yield the first few moments of the velocity gradient distribution similar to those computed from full turbulence simulations (FTS) data. The performance of the model is then compared with other computations from FTS data. The model gives good agreement of one-time statistics. While the two-time statistics of strain rate are well replicated, the two-time vorticity statistics are not as good, reflecting perhaps a certain lack of embodiment of physics in the model. The performance of the model when used to compute material element deformation is qualitatively good, with the material line-element growth rate being correct to within 5% and that of surface element correct to within 20% for the lowest Reynolds number considered. The performance of the model is uniformly good for all the Reynolds numbers considered. So it is conjectured that the model can be used even in inhomogeneous, high-Reynolds-number flows, for the study of evolution of surfaces, a problem that is of interest particularly to combustion researchers.
Electroconvection at an electrically inhomogeneous permselective interface
Authors: Rubinstein I.
Published: July 2002

This paper concerns electroconvection—a nongravitational free convection in macroscopic domains of electrolyte solution. Electroconvection results from the interaction of the electric field in the system with the respective space charge within the limits of validity of local electroneutrality approximation. It follows from the dimensional analysis that the electroconvectional Péclet number, evaluated near the equilibrium in a macroscopic ionic system, characterized by a single length scale, is universally of order unity, independently of the size of the system and of the typical electrolyte concentration. An electroconvectional flow in a two-dimensional diffusion layer adjacent to a periodic electrically inhomogeneous permselective interface is calculated by an integral method to leading order in small electroconvectional Péclet number, occurring in the considered large aspect ratio case due to the presence of two separate macroscopic length scales. Explicit expressions for the flow characteristics are worked out through an asymptotic procedure valid in the vicinity of electrodiffusional limiting current.
Experiments on the inhibition of mixing in stably stratified decaying turbulence using laser Doppler anemometry and laser-induced fluorescence
Authors: Barrett T. K. , Van Atta C. W.
Published: July 2002

The decay of velocity and density fluctuations in buoyancy influenced turbulence generated by a biplanar grid, towed horizontally through a stably stratified fluid with uniform optical index of refraction was measured for three different constant density gradients. Experiments were performed in a closed rectangular test section. Fluid velocities were measured with a laser Doppler anemometer. Density was measured by adding a fluorescent dye (Rhodamine 6G) to the working fluid in such a way that the light fluoresced by the dye when excited by a laser was proportional to the local density of the solution. Simultaneous single-point measurements of the vertical velocity, the horizontal velocity in the direction of grid motion, and the density were obtained at a fixed point in the center of the test section. Statistical moments representing the state of the turbulent flow field at different periods of the decay were computed by ensemble averaging data from 100 tows with similar initial conditions. Density profiles of the quiescent fluid, before and after the grid was towed through the test section, were measured, and estimates of the total mixing accomplished by a single sweep of the grid through the fluid were calculated. The measured internal Richardson number Rii={− (1)/(2) [gρ′2/(∂ρ̄/∂z)]} ×(1/ (1)/(2) ρ̄〈w2〉) indicates that, immediately behind the grid, the turbulence was unaffected by the stable stratifications. At a time depending on the strength of the mean density gradient, the buoyancy forces acting on the turbulent eddies become as large as the inertial forces and have significant effects on the final stages of the decay of the turbulent flow field. Measurements of the convective mass flux indicate that the vertical transport of mass due to turbulence was significantly reduced by buoyancy forces. This was confirmed by integral estimates of the mixing, which indicate that the ability of the turbulent eddies to homogenize the stratified fluid decreases significantly with increasing density gradients. These results are compared with earlier measurements obtained in both stratified air and water facilities.
A three-dimensional numerical study of flow separation and reattachment on a blunt plate
Authors: Tafti D. K. , Vanka S. P.
Published: July 2002

Three-dimensional numerical simulation of the unsteady flow over a blunt plate held normal to a uniform stream has been carried out to study the physics of unsteady separation and reattachment. A finite-difference procedure is used with 32 spanwise grid cells. In comparison with previous two-dimensional results, it is observed that the inclusion of spanwise variations significantly improves the calculations and provides superior comparisons with experimental data. Mean turbulent quantities are calculated and found to be in good agreement with experiments. Instantaneous as well as statistical quantities describing the characteristic length and time scales of the large-scale structures are also presented. The calculations have also been able to capture the experimentally observed low-frequency unsteadiness of the separation bubble. In addition, a selective high-frequency shedding from the separated shear layer was found to occur with a period equal to that of the low-frequency unsteadiness.
Steep gravity–capillary waves within the internal resonance regime
Authors: Perlin Marc , Ting Chao-lung
Published: July 2002

Steep gravity–capillary waves are studied experimentally in a channel. The range of cyclic frequencies investigated is 6.94–9.80 Hz; namely, the high-frequency portion of the regime of internal resonances according to the weakly nonlinear theory (Wilton’s ripples). These wave trains are stable according to the nonlinear Schrödinger equation. The experimental wave trains are generated by large, sinusoidal oscillations of the wavemaker. A comparison is made between the measured wave fields and the (symmetric) numerical solutions of Schwartz and Vanden-Broeck [J. Fluid Mech. 95, 119 (1979)], Chen and Saffman [Stud. Appl. Math. 60, 183 (1979); 62, 95 (1980)], and Huh (Ph.D. dissertation, University of Michigan, 1991). The waves are shown to be of slightly varying asymmetry as they propagate downstream. Their symmetric parts, isolated by determining the phase which provides the smallest mean-square antisymmetric part, compare favorably with the ‘‘gravity-type’’ wave solutions determined by numerical computations. The antisymmetric part of the wave profile is always less than 30% of the peak-to-peak height of the symmetric part. As nonlinearity is increased, the amplitudes of the short-wave undulations in the trough of the primary wave increase; however, there are no significant changes in these short-wave frequencies. The lowest frequency primary-wave experiments, which generate the highest frequency short-wave undulations, exhibit more rapid viscous decay of these high-frequency waves than do the higher-frequency primary wave experiments.
Kinetic theory analysis of steady evaporating flows from a spherical condensed phase into a vacuum
Authors: Sone Yoshio , Sugimoto Hiroshi
Published: July 2002

Steady evaporating flows from a spherical condensed phase into a vacuum are considered. On the basis of the Boltzmann–Krook–Welander equation, the behavior of the gas (the velocity distribution function as well as the density, velocity, and temperature) from the sphere to downstream infinity is analyzed numerically in detail for the whole range of the Knudsen number. The discontinuity of the velocity distribution function in the gas, a typical behavior of the gas around a convex body, is analyzed accurately. It extends, with appreciable size, to downstream infinity not only for intermediate and large Knudsen numbers but also even for rather small Knudsen numbers. The flow is highly in nonequilibrium over the whole field except for very small Knudsen numbers. The velocity and temperature at downstream infinity take finite values, determined by the Knudsen number, except the temperature at zero Knudsen number. An analytic solution for small Knudsen numbers, especially the temperature at infinity (frozen temperature) is obtained with the aid of the asymptotic theory and the hypersonic expansion.
Oblique and head-on collisions of solitary waves in Marangoni–Bénard convection
Authors: Linde Hartmut , Chu Xiaolin , Velarde Manuel G.
Published: July 2002

In Marangoni–Bénard convection with mass transfer from gas phase to liquid phase, it is found that solitary waves, excited and sustained by Marangoni stresses, are very stable during interactions with others, keeping their original shapes and celerities after interactions, thus experiencing at most a phase shift. Both positive and negative phase shifts have been observed for different types of oblique interaction. A ‘‘critical’’ angle, at which zero phase shift occurs as a result of the interaction, has been defined to delineate regions of negative and positive phase shifts.
A note on turbulent energy dissipation in the viscous wall region
Authors: Bradshaw Peter , Perot J. Blair
Published: July 2002

From simulation data it is shown that the difference between the true rate of dissipation of turbulent kinetic energy and the ‘‘isotropic dissipation’’ used by Reynolds-averaged modelers is less than 2% in a conventional viscous wall region, and negligible elsewhere. The difference is a contribution to viscous diffusion which is a small fraction of the total.
Görtler vortices and heat transfer: A weakly nonlinear analysis
Authors: Smith Sonya T. , Haj-Hariri Hossein
Published: July 2002

A weakly nonlinear analysis of the Görtler vortex problem is performed. It is demonstrated that the system of streamwise vortices enhances the heat transfer rate at the wall. The inherently nonlinear physical mechanism responsible for this enhancement is discussed both mathematically, as well as heuristically; the latter using kinematic arguments. Remarks are made on the relation of the weakly nonlinear results to their fully nonlinear counterparts, as well as on the inadequacy of linear analyses.
On the two-way interaction between homogeneous turbulence and dispersed solid particles. I: Turbulence modification
Authors: Elghobashi S. , Truesdell G. C.
Published: July 2002

The modification of decaying homogeneous turbulence due to its interaction with dispersed small solid particles (d/η<1), at a volumetric loading ratio φv≤5×10−4, is studied using direct numerical simulation. The results show that the particles increase the fluid turbulence energy at high wave numbers. This increase of energy is accompanied by an increase of the viscous dissipation rate, and, hence, an increase in the rate of energy transfer T(k) from the large-scale motion. Thus, depending on the conditions at particle injection, the fluid turbulence kinetic energy may increase initially. But, in the absence of external sources (shear or buoyancy), the turbulence energy eventually decays faster than in the particle-free turbulence. In gravitational environment, particles transfer their momentum to the small-scale motion but in an anisotropic manner. The pressure-strain correlation acts to remove this anisotropy by transferring energy from the direction of gravity to the other two directions, but at the same wave number, i.e., to the small-scale motion in directions normal to gravity. This input of energy in the two directions with lowest energy content causes a reverse cascade. This reverse cascade tends to build up the energy level at lower wave numbers, thus reducing the decay rate of energy as compared to that of either the particle-free turbulence or the zero-gravity particle-laden flow.
Pulsation of stimulated Raman scattering in a laser plasma
Authors: Sakagami H. , Mima K. , Nishihara K.
Published: July 2002

Stimulated Raman scattering near the quarter critical density saturates by two different mechanisms. The first is the high-energy electrons generated by Landau damping of plasma waves and the other is plasma density profile modification by the ponderomotive force of plasma waves. Simulations were performed with a one-and-a-half dimensional relativistic electromagnetic particle code to investigate those mechanisms and the pulsation dynamics of the scattered wave found. Finally, simulation results are compared with a simple theoretical model.
Nonadiabatic behavior of the magnetic moment of a charged particle in a dipole magnetic field
Authors: Murakami Sadayoshi , Sato Tetsuya , Hasegawa Akira
Published: July 2002

Nonadiabatic behavior of the magnetic moment of a charged particle in a dipole magnetic field is studied both numerically and analytically in the presence of a low-frequency electrostatic wave with an azimuthal component. The adiabaticity can be broken as a result of two types of resonance, i.e., the bounce-E×B drift resonance and the wave-drift resonance.
Nonlinear Raman scattering behavior with Langmuir and sound waves coupling in a homogeneous plasma
Authors: Bonnaud Guy , Pesme Denis , Pellat René
Published: July 2002

By means of wave-coupling simulations, the typical nonlinear evolution of stimulated Raman scattering (SRS) is investigated in a homogeneous sub-quarter-critical plasma for present-day low laser irradiances and kilo-electron-volt electron temperatures. The decrease of the Langmuir energy observed after the SRS growth is found to be basically the result of the electrostatic decay instability (EDI) onset, which generates a high-amplitude ion-acoustic wave. The resulting strong modulation of the plasma density causes a conversion process that transforms the initial one-wave-vector Langmuir wave driven by SRS into a Bloch wave and induces SRS detuning and larger damping. The conditions involved herein have allowed isolation of these processes from the modulational instability; in addition, the Langmuir collapse is found not to occur owing to the high electron temperature.
Backward and forward stimulated Raman scattering from thin foil targets with a 0.26 μm laser
Authors: Labaune C. , Baldis H. A. , Fabre E. , Briand F. , Villeneuve D. M. , Estabrook K.
Published: July 2002

Time-resolved spectra of backward and forward stimulated Raman scattering were measured from the interaction of 0.26 μm laser radiation with foil targets. The plastic foil targets were designed to go underdense before the peak of the 450 psec, 2×1015 W/cm2 laser pulse. The Raman spectra were quite narrow at any one time (Δω/ω∼0.05) and shifted to shorter wavelengths during the pulse. The backward Raman spectra can be well predicted by assuming that the Raman occurs in a disk-shaped region on the top of the parabolic density profile, using well accepted threshold estimates.
Initial results from investigation of three-dimensional magnetic reconnection in a laboratory plasma
Authors: Yamada M. , Perkins F. W. , MacAulay A. K. , Ono Y. , Katsurai M.
Published: July 2002

A comprehensive laboratory experiment has been proposed to investigate the fundamental 3-D physics of magnetic reconnection regions and their associated hydromagnetic flows. Two toroidal plasma rings, with equal or opposite magnetic helicity, are formed and then brought together, contacting along a toroidally symmetric line. This research addresses three important questions: (1) How does magnetic helicity affect reconnection? (2) Will three-dimensional processes arise spontaneously and modify the usual Sweet–Parker or Petschek picture of two-dimensional reconnection where the global configuration is that of an axisymmetric X-point line? (3) How does the reconnection rate respond to global forcing? In a preliminary experiment carried out at the University of Tokyo [Phys. Rev. Lett. 65, 721 (1990)], the direction of the toroidal field plays an important role in the merging process. It is found that plasmas of opposite helicity merge appreciably faster than those of similar helicity. It is also found that the reconnection rate is proportional to the external force suggesting that magnetic reconnection, in the present experiment, is a forced phenomenon. A comparison of the present experimental data with 2-D computer simulation results is made.
Collisionless two-fluid theory of toroidal ηi stability
Authors: Mondt J. P. , Weiland J.
Published: July 2002

A collisionless two-fluid theory based on a 14-moment generalization of the ‘‘double-adiabatic’’ equations is developed to lowest order in the Larmor radius parameter, and applied to derive the toroidal ηi stability boundary for all values of the ratio of the density gradient scale length divided by the field curvature length. The present model is an improvement over existing two-fluid models in view of the collisionless nature of the ηi instability, while retaining the advantage over kinetic theory of the practicability of mode-coupling simulations. The linear stability boundary, linear growth rate, and real frequency agree fairly accurately with drift-kinetic theory.
Ion-temperature-gradient modes in noncircular tokamak geometry
Authors: Hua D. D. , Xu X. Q. , Fowler T. K.
Published: July 2002

The work of Xu and Rosenbluth [Phys. Fluids B 3, 627 (1991)] on ion-temperature-gradient modes in toroidal geometry is extended to noncircular cross sections to examine the scaling with the elongation κ. The growth rate, wave numbers, and stability thresholds are calculated for a flat density profile characteristic of H-mode operation, and a mixing-length estimate of the ion thermal diffusivity χi is calculated. It is found that in the plasma core, growth rates decrease as the central elongation κ increases, due to finite-Larmor-radius effects. The calculated χi is approximately of the form χi=DTi3/2[(LTi)c−LTi] where D and (LTi)c depend on τ=(Te/Ti) and geometry, and LTi=‖Ti/∇Ti‖. Preliminary evidence indicates fair agreement with experimental data near the plasma edge, where LTi≪ (LTi)c, but the fit is poorer in the core where LTi∼(LTi)c and the theoretical χi is very sensitive to the accuracy in calculating (LTi)c.
Thermal magnetic fluctuations of whistlers in a Maxwellian plasma
Authors: Golubyatnikov G. , Stenzel R. L.
Published: July 2002

Thermal fluctuations have been measured with a magnetic loop antenna inside a large afterglow plasma in the regime of whistler waves (f≲fce≂30 MHz≪fpe≂3000 MHz; Ar, 2×10−4 Torr, 1 m diam×2.5 m length). The magnetic fluctuations B̃(ω) exhibit a 1/f-like spectrum for whistlers (f<fce), no resonant enhancement at the electron cyclotron frequency fce, and a flat spectrum in the evanescent regime (fce<f≪fpe). Thus the observed fluctuations are neither described by blackbody radiation laws (B̃∝ω) nor by cyclotron emission (lines at nfce), but resemble the decaying Alfvénic fluctuation spectrum calculated by Cable and Tajima [Phys. Rev. A 46, 3413 (1992)].
Ultrahigh intensity laser–plasma interaction: A Lagrangian approach*
Authors: Rax J.-M. , Fisch N. J.
Published: July 2002

A Lagrangian analysis simplifies considerably the description of the ultrahigh intensity regime of laser–plasma interaction, and facilitates the identification of several new and important effects. First, the vacuum figure ‘‘8’’ orbit is shown to be unstable, with respect to a stochastic instability leading to collisionless heating. Second, in the generation of plasma wakes using ultrahigh intensity laser pulses, it can be shown that, for long-duration laser pulses, the plasma wake is insignificant, but, through proper phasing of a set of short-duration pulses, a dramatic amplification of the wake amplitude occurs. Third, in the generation of third-harmonic waves using ultrahigh intensity, long-duration laser pulses, it can be seen that a mismatch in the phase velocity limits severely the power conversion, but a conversion efficiency free from saturation might indeed be possible by employing either a density-modulated plasma or an ionized buffer gas.
Toroidal effects on drift wave turbulence
Authors: LeBrun M. J. , Tajima T. , Gray M. G. , Furnish G. , Horton W.
Published: July 2002

The universal drift instability and other drift instabilities driven by density and temperature gradients in a toroidal system are investigated in both linear and nonlinear regimes via particle simulation. Runs in toroidal and cylindrical geometry show dramatic differences in plasma behavior, primarily due to the toroidicity-induced coupling of rational surfaces through the poloidal mode number m. In the toroidal system studied, the eigenmodes are seen to possess (i) an elongated, nearly global radial extent, (ii) a higher growth rate than in the corresponding cylindrical system, (iii) an eigenfrequency nearly constant with radius, and (iv) a global temperature relaxation and enhancement of thermal heat conduction. Most importantly, the measured χi shows an increase with radius and an absolute value on the order of that observed in experiment. On the basis of the present observations, it is argued that the increase in χi with radius observed in experiment is caused by the global nature of heat convection in the presence of toroidicity-induced mode coupling.
Argon gas puff implosion experiments and two-dimensional modeling
Authors: Deeney C. , LePell P. D. , Cochran F. L. , Coulter M. C. , Whitney K. G. , Davis J.
Published: July 2002

The K-shell x ray yields from argon gas puff Z pinches are observed to increase from 3.5±1.0 kJ to 13±1.0 kJ when inwardly tilted nozzles are used on a 6 TW, 4 MA generator. This increase is associated with the elimination of the zipper effect and the achievement of higher density plasmas, as confirmed by x-ray diagnostics. Two-dimensional (2-D) magnetohydrodynamic modeling of the gas puff implosions indicates that the collapsing shell reaches a higher density and smaller diameter when the axial zipper is eliminated. The calculations show that axial mass flow in the zippering cases result in nonhollow collapses which limit the final pinch radii and densities. The calculations also indicate that narrower nozzle exits contribute to increasing the assembled plasma density.
Cyclotron harmonic lines in the thermal magnetic fluctuation spectrum of spiraling electrons in plasmas
Authors: Stenzel R. L. , Golubyatnikov G.
Published: July 2002

Radio frequency (rf) magnetic fluctuations B̃ have been measured with loop antennas in a large pulsed discharge plasma column (ne≲1012 cm−3, kTe≲3 eV, B0≂20 G, Ar, 2×10−4 Torr, 1 m diam×2.5 m length). A 1/f-like noise spectrum is observed in the whistler wave regime (ωce1/2ωci1/2<ω<ωce) both in the Maxwellian afterglow plasma and in the active discharge which contains energetic (45 eV) electrons. Discrete emission lines at the electron cyclotron frequency and its harmonics are found only in the presence of spiraling energetic electrons. These are naturally present in the active discharge but have also been injected as a controlled oblique electron beam into the Maxwellian afterglow plasma. In the latter case up to 15 cyclotron harmonic lines with weak amplitude decay B̃z(ω) are generated in the beam flux tube. From two-point correlation measurements it is shown that the line spectrum is due to ballistic beam modes rather than plasma eigenmodes driven unstable by the beam. The lines evolve from broadband thermal current fluctuations of the beam through a filtering effect. Those fluctuations which rotate synchronously with the ordered cyclotron motion (ω=nωc) constructively interfere (k∥=0) and produce coherent solenoidal rf fields, while others interfere destructively. Axial and azimuthal phase velocity measurements for rf-modulated beams clearly demonstrate the filtering effect. In the present parameter regime (ωp≫ωc) the fluctuations are evanescent and localized near the electron flux tube (rc≳c/ωp). In low density plasmas the fluctuations may couple to propagating electromagnetic waves and be observable externally as in earlier observations by Landauer or Ikegami.
The wall-shear-stress contribution to boundary-layer noise
Authors: Hariri H. Haj , Akylas T. R.
Published: July 2002

The effect of viscosity on the wall-pressure spectrum of a slightly compressible, turbulent boundary layer is studied. Apart the resolving the singularities of the inviscid spectrum, the viscous shear-stress dipole sources also partake in sound generation. The dominant viscous contribution to the low-wavenumber part of the spectrum is evaluated in terms of the flow Mach and Reynolds numbers. It is concluded that, in typical applications, the viscous contribution is small compared with the inviscid one.
Analytical solutions of the discrete Boltzmann equation for the Rayleigh flow problem for gas mixtures
Authors: Longo Eugenia , Monaco Roberto
Published: July 2002

In this paper the Rayleigh flow problem for multicomponent gas mixtures is studied by means of a discrete velocity model of the Boltzmann equation. The analysis carried out leads to analytical solutions for the number densities, which are the pertinent quantities capable of describing the space-time evolution of the system at a molecular level. The macroscopic behavior of the mixture is also deduced, and a comparison is made between our results and those of other authors.
Stability of an accelerated shear layer
Authors: Mjolsness R. C. , Ruppel H. M.
Published: July 2002

A fluid shear layer with free boundary conditions is subject to a Kelvin–Helmholtz-like instability. When the shear layer is accelerated by a difference in applied pressures it is also subject to a Rayleigh–Taylor-like instability. The combined action of these instabilities leads to at most one unstable mode at each wavelength, whose behavior depends in detail on fluid parameters, the fluid acceleration and the perturbation wavelength. Typically, at longest wavelengths the instability is essentially of Rayleigh–Taylor form; its behavior resembles the Kelvin–Helmholtz-like mode at shorter wavelengths, near the thickness of the shear layer, cutting off when the Kelvin–Helmholtz-like mode does. At still shorter wavelengths, the shear layer is subject to a Rayleigh–Taylor-like instability. Careful control of fluid parameters could place the most unstable wavelength for Rayleigh–Taylor instability, calculated from viscous theory, in the range of wavelengths where the accelerated shear layer has no unstable mode. However, this may be difficult to achieve in practice. If this can be realized, the most unstable growth rate may be reduced by about an order of magnitude by the presence of shear.
Hamiltonian formulation of the baroclinic quasigeostrophic fluid equations
Authors: Holm Darryl D.
Published: July 2002

A Hamiltonian formulation using a noncanonical Poisson bracket is presented for the nonlinear dynamics of baroclinic quasigeostrophic fluid flow. Nonlinear integral invariants for the system are found to be in the kernel of the noncanonical Poisson bracket. A novel feature of this formulation is that dynamical boundary conditions are generated from the constrained energy Hamiltonian, via the noncanonical Poisson bracket.
The convective nature of instability in plane Poiseuille flow
Authors: Deissler Robert J.
Published: July 2002

By numerical solution of the Orr–Sommerfeld equation for complex frequency and complex wavenumber for a wide range of Reynolds numbers R and by asymptotic analysis for large R, it is shown that there is no absolute instability in a two-dimensional plane Poiseuille flow for any R and that the flow is convectively unstable for Rc <R<∞, where Rc is the critical Reynolds number.
Hamiltonian structure for two-dimensional hydrodynamics with nonlinear dispersion
Authors: Holm Darryl D.
Published: July 2002

A Hamiltonian formulation using a noncanonical Poisson bracket is presented for a recently proposed model (Dokl. Akad. Nauk SSSR 293, 818 (1985) [Sov. Phys. Dokl. 32, 262 (1987)]) of two-dimensional shallow-water hydrodynamics with nonlinear dispersion. Nonlinear integral invariants for this model are found to be in the kernel of the noncanonical Poisson bracket. A generalized Kelvin theorem is also given for the model.
The Rayleigh–Taylor instability in ablatively accelerated targets with 1, 1/2 , and 1/4 μm laser light
Authors: Emery Mark H. , Dahlburg Jill P. , Gardner John H.
Published: July 2002

The results of a series of detailed numerical simulations of the Rayleigh–Taylor instability in laser ablatively accelerated targets are presented for a fairly wide range of initial conditions. It is shown that the Rayleigh–Taylor growth rate in an ablative environment is a strong function of the laser wavelength. For perturbation wavelengths about three times the in-flight target thickness, the ratios of the numerical growth rates to the classical growth rates are of the order of 1/1.5, 1/2.5, and 1/3.5 for 1, 1/2 , and 1/4 μm laser light, respectively. The numerical results are in good agreement with the theoretical model presented here based on the ablative convection of vorticity away from the unstable ablation front. These results provide strong evidence for the viability of high-aspect-ratio shells in direct-drive laser fusion.
Double diffusion in a vertical fluid layer: Onset of the convective regime
Authors: Gobin D. , Bennacer R.
Published: July 2002

This paper considers the onset of double diffusive natural convection in a vertical layer of a binary fluid submitted to horizontal thermal and compositional gradients. The analysis deals with the particular situation where the resulting buoyancy forces (the Grashof numbers corresponding to the thermal and solutal effects) are opposing and of equal intensity. The stability analysis for the infinite layer shows that the purely diffusive (motionless) solution prevails at moderate Grashof numbers, and an analytical expression of the critical Rayleigh number as a function of the Lewis number is obtained. These results are then compared to the critical values which are determined from numerical simulations in an enclosure. Numerical calculations in the transient regime are used to give an interpretation of the stability of the steady state diffusive regime when the buoyancy forces are below the critical value.
Traveling wave instability in sustained double-diffusive convection
Authors: Predtechensky A. A. , McCormick W. D. , Swift J. B. , Rossberg A. G. , Swinney Harry L.
Published: July 2002

Experiments on buoyancy-driven double-diffusive convection sustained by imposed vertical concentration gradients (one stabilizing, the other destabilizing) have been conducted in a thin (Hele–Shaw) isothermal rectangular cell. Novel gel-filled membranes were used to sustain the concentrations at the boundaries. When the destabilizing solute diffuses more rapidly than the stabilizing one, the primary instability leads to traveling waves with a high reflection coefficient at the ends of the cell. The measured critical Rayleigh numbers and frequencies are in reasonable accord with a stability analysis that includes corrections for the finite thickness of the cell and cross-diffusion effects. The weakly nonlinear waves that appear at onset do not stabilize, even very close to the transition, but continue to evolve, eventually becoming a packet of large amplitude plumes. The packet travels back and forth along the cell in a nearly periodic manner. This behavior and the absence of measurable hysteresis are consistent with the present weakly nonlinear analysis which predicts tricritical scaling (∼ε1/4 rather than the usual ε1/2) all along the instability boundary. However, the range of this scaling in ε was found to be less than 0.005, which is inaccessible in the present experiments.
Near-limit behavior of high-Lewis number premixed flames in tubes at normal and low gravity
Authors: Pearlman Howard G. , Ronney Paul D.
Published: July 2002

The high-Lewis number (Le) diffusive-thermal instability in premixed gas combustion is studied experimentally in tubes using lean C4H10–O2–He mixtures (Le≊3.0). Two self-excited instabilities are observed for flames propagating in a tube open at the ignition end and closed at the other end: (1) a combined spiral wave and radial pulsation and (2) a pure radial pulsation. These modes are observed at both earth gravity (1 g) and microgravity (μg) and therefore are believed to be a direct manifestation of the diffusive-thermal instability rather than a buoyancy-induced instability. Moreover, the frequencies of the observed modes are lower than the fundamental acoustic frequencies of the tubes which renders unlikely the possibility that they are acoustically-induced instabilities. A critical fuel concentration for the onset of the combined spiral wave and radial pulsation, as well as the pure radial pulsation is found. The transition between spiral waves and radial pulsations is also observed. These observations may be analogous to the propagating chemical wave and rotating spiral wave solutions found in reaction–diffusion systems other than premixed gas combustion.
Phenomenology for the decay of energy-containing eddies in homogeneous MHD turbulence
Authors: Hossain Murshed , Gray Perry C. , Pontius Duane H. , Matthaeus William H. , Oughton Sean
Published: July 2002

We evaluate a number of simple, one-point phenomenological models for the decay of energy-containing eddies in magnetohydrodynamic (MHD) and hydrodynamic turbulence. The MHD models include effects of cross helicity and Alfvénic couplings associated with a constant mean magnetic field, based on physical effects well-described in the literature. The analytic structure of three separate MHD models is discussed. The single hydrodynamic model and several MHD models are compared against results from spectral-method simulations. The hydrodynamic model phenomenology has been previously verified against experiments in wind tunnels, and certain experimentally determined parameters in the model are satisfactorily reproduced by the present simulation. This agreement supports the suitability of our numerical calculations for examining MHD turbulence, where practical difficulties make it more difficult to study physical examples. When the triple-decorrelation time and effects of spectral anisotropy are properly taken into account, particular MHD models give decay rates that remain correct to within a factor of 2 for several energy-halving times. A simple model of this type is likely to be useful in a number of applications in space physics, astrophysics, and laboratory plasma physics where the approximate effects of turbulence need to be included.
On the unsteady separated flow past a semi-infinite plate: Exact solution of the Brown and Michael model, scaling, and universality
Authors: Cortelezzi L.
Published: July 2002

Two-dimensional unsteady separated flow past a semi-infinite plate is considered. The rolling up of the separated shear layer is modeled by a point vortex, whose time-dependent circulation is predicted by an unsteady Kutta condition. The equation of motion for the starting vortex is derived and solved in closed form for any free-stream condition. A time-dependent scaling that captures the universality of the flow is proposed.
A non-slip boundary condition for lattice Boltzmann simulations
Authors: Inamuro Takaji , Yoshino Masato , Ogino Fumimaru
Published: July 2002

A non-slip boundary condition at a wall for the lattice Boltzmann method is presented. In the present method unknown distribution functions at the wall are assumed to be an equilibrium distribution function with a counter slip velocity which is determined so that fluid velocity at the wall is equal to the wall velocity. Poiseuille flow and Couette flow are calculated with the nine-velocity model to demonstrate the accuracy of the present boundary condition.
Three-dimensional instabilities of film flows
Authors: Liu Jun , Schneider J. B. , Gollub J. P.
Published: July 2002

Two-dimensional (2-D) interfacial waves on flowing films are unstable with respect to both two- and three-dimensional instabilities. In this paper, several distinct three-dimensional instabilities that occur in different regions of the parameter space defined by the Reynolds number R and the frequency f of forced two-dimensional waves are discussed in detail. (a) A synchronous 3-D instability, in which spanwise deformations of adjacent wave fronts have the same transverse phase, appears over a wide range of frequency. These transverse modulations occur mainly along the troughs of the primary waves and eventually develop into sharp and nearly isolated depressions. The instability involves many higher harmonics of the fundamental 2-D waves. (b) A 3-D surbharmonic instability occurs for frequencies close to the neutral curve fc(R). In this case, the transverse modulations are out of phase for successive wave fronts, and herringbone patterns result. It is shown that this weakly nonlinear instability is due to the resonant excitation of a triad of waves consisting of the fundamental two-dimensional wave and two oblique waves. The evolution of wavy films after the onset of either of these 3-D instabilities is complex. However, sufficiently far downstream, large-amplitude solitary waves absorb the smaller waves and become dominant.
Attractors of finite-sized particles: An application to enhanced separation
Authors: Omurtag A. C. , Dutta P. , Chevray R.
Published: July 2002

It is shown in the context of the periodically driven eccentric annular system that small rigid spherical particles advected in a time-periodic bounded flow can be represented by a dissipative dynamical system with simple or strange attractors. The behavior of such particles differs significantly from the patterns in the fluid flow driving the dynamical system. This may have implications for transport, mixing, and separation in multiphase flows as well as in predicting the dispersion of trace substances in natural fluid media. Numerical results suggest that the sensitivity of the limit sets to parameter values may be used to separate species of particles differing slightly in the value of some physical property.
Chaotic properties of internal wave triad interactions
Authors: Kim W. , West Bruce J.
Published: July 2002

We discuss the stochasticities of two-triad interactions that occur in two-degree-of-freedom autonomous Hamiltonian systems. The system we study is a two-triad test-wave system consisting of a single internal wave mode (test-wave) interacting with a spectrum of ambient internal wave modes; the ambient modes, however, do not interact among themselves except through a three-wave interaction which includes the test-wave. The present study concerns the effect of nonlinearities on the ocean internal wave field. Our numerical results using the physical parameters appropriate for the deep ocean confirm that the test-wave system is non-integrable. Moreover, there exists a certain separatrix net that fills the phase space and is covered by a thin stochastic layer for a two-triad pure resonant interaction. The stochastic web implies the existence of diffusion of the Arnold type for the minimal dimension of a non-integrable autonomous system. For the non-resonant case, the stochastic layer is formed where the separatrix from KAM theory is disrupted. However, the stochasticity does not increase monotonically with increasing energy.
Visualization of the fluid flow field around a laser generated oscillating bubble
Authors: Schiffers W. P. , Emmony D. C. , Shaw S. J.
Published: July 2002

We report on a new method of visualizing qualitatively a fluid flow field around an oscillating cavitation bubble. The near infrared radiation of a Q-switched Nd:YAG laser is focused in deionized water and in a solution of copper sulphate for contrast enhancement to generate a cavitation bubble in free space and in front of a solid boundary. Using high-speed photography and an accurately positioned schlieren knife edge we are able to visualize the heated path of the laser beam and the different stages of the oscillating bubble with high spatial resolution. For the case of a bubble in free space the marked laser path indicates radial fluid flow only. For a bubble in front of a solid boundary the marked laser path clearly shows the motion of the fluid through the bubble during the collapse process. The marked paths are similar to numerically calculated streamline plots.
Numerical investigation of the effects of large particles on wall-turbulence
Authors: Pan Y. , Banerjee S.
Published: July 2002

Particle-laden turbulent flows, at average volume fraction less than 4×10−4, in open channels are numerically simulated by using a pseudospectral method. The motion of particles, that are large compared with the dissipative length scale, is coupled to the fluid motion by a method that generates a “virtual” no-slip boundary on the particle surface by imposition of an external force field on the grid-points enclosed by the particle. Cases for both moving and stationary particles, lying on the wall, are simulated. The investigations focus on particle-turbulence interaction. It is found that particles increase turbulence intensities and Reynolds stress. By examining higher order turbulence statistics and doing a quadrant analysis of the Reynolds stress, it is found that the ejection-sweep cycle is affected—primarily through suppression of sweeps by the smaller particles and enhancement of sweep activity by the larger particles. An assessment of the impact of these findings on scalar transfer is made, as enhancement of wall heat/mass transfer rates is a motivation of the overall work on this subject. In the cases considered, comparison of the calculations with an existing experiment was possible, and shows good agreement. At present, due to limitations in available computational resources, this method cannot be used when the particle diameter is smaller than the smallest turbulence scale (e.g. the Kolmogorov length scale) and the volume fraction is of the same order as studied in this paper, i.e. between 10−3 and 10−4.