bluuebunny/crossref_metadata_2025 huggingface dataset:
Extensional flow of a suspension of fibers in a dilute polymer solution
Authors: Harlen O. G. , Koch Donald L.
Published: July 2002

The extensional properties of a suspension of slender fibers in a dilute polymer solution at high Deborah numbers are derived by modeling the polymer solution as an anisotropic viscous fluid. The fiber stress is found to exceed its value in a Newtonian fluid with the same shear viscosity by at most a factor of log r (where r is the aspect ratio of the fiber). There is, however, a change to the form of the velocity disturbance caused by a fiber resulting in a decrease in the orientational dispersion due to fiber–fiber interactions.
A renormalization group scaling analysis for compressible two-phase flow
Authors: Chen Yupin , Deng Yuefan , Glimm James , Li Gang , Zhang Qiang , Sharp David H.
Published: July 2002

Computational solutions to the Rayleigh–Taylor fluid mixing problem, as modeled by the two-fluid two-dimensional Euler equations, are presented. Data from these solutions are analyzed from the point of view of Reynolds averaged equations, using scaling laws derived from a renormalization group analysis. The computations, carried out with the front tracking method on an Intel iPSC/860, are highly resolved and statistical convergence of ensemble averages is achieved. The computations are consistent with the experimentally observed growth rates for nearly incompressible flows. The dynamics of the interior portion of the mixing zone is simplified by the use of scaling variables. The size of the mixing zone suggests fixed-point behavior. The profile of statistical quantities within the mixing zone exhibit self-similarity under fixed-point scaling to a limited degree. The effect of compressibility is also examined. It is found that, for even moderate compressibility, the growth rates fail to satisfy universal scaling, and moreover, increase significantly with increasing compressibility. The growth rates predicted from a renormalization group fixed-point model are in a reasonable agreement with the results of the exact numerical simulations, even for flows outside of the incompressible limit.
Discrete shedding modes in the von Kármán vortex street
Authors: König Michael , Noack Bernd R. , Eckelmann Helmut
Published: July 2002

In the laminar vortex street behind a circular cylinder, only few distinct shedding modes for a large range of aspect ratios and various end conditions are observed. The nth mode is characterized by a Strouhal number Stn and a shedding angle, θn, both of which vary continuously with the Reynolds number. The experimentally observed discontinuities in the Strouhal–Reynolds number relationship are transitions between these shedding modes.
Modification of magnetohydrodynamic activity by lower hybrid current drive
Authors: Romanelli F. , Cardinali A. , Bartiromo R.
Published: July 2002

The effect of magnetic islands on the propagation of lower hybrid waves is investigated analytically for a model slab equilibrium with a single helical perturbation corresponding to an m=1 or m=2 island. From the variation of k∥ inside the island, the parallel current is estimated with the same pitch as the original perturbation. The effect of such a current on the nonlinear evolution of an m=2 island corresponds to a destabilization of the mode if the rf driven current is in the same direction as the global parallel current. A comparison with numerical solutions of the problem using a toroidal ray trajectory code is also presented.
Suppression of sidebands by diffraction in a free-electron laser
Authors: Antonsen T. M. , Laval G.
Published: July 2002

The stability of sidebands in a free-electron laser is determined in the regime in which the three-dimensional structure of the modes is important. It is found that the growth rate of the sidebands in the presence of a fundamental mode that is weakly guided by the beam is reduced. The indications are that for some parameters the spatial growth rate of the sidebands can be made less than that of the fundamental.
Structure of electron-cyclotron ordinary mode damping
Authors: Fidone I. , Giruzzi G. , Granata G. , Krivenski V. , Meyer R. L. , Bornatici M. , Mazzucato E.
Published: July 2002

The ordinary mode damping near the electron-cyclotron frequency is investigated using the relativistic dispersion relation. It is shown that the Doppler splitting of the absorption spectrum claimed by Arunasalam et al. [Phys. Rev. A 36, 3909 (1987); 37, 2063 (1988)] is an erroneous result obtained from an oversimplified dispersion relation and assuming an infinite speed of light in the resonance condition.
Transport and stability of long-pulse relativistic electron beams in UV laser-induced ion channels
Authors: Lucey R. F. , Gilgenbach R. M. , Miller J. D. , Tucker J. E. , Bosch R. A.
Published: July 2002

Results are presented for the first experiments in which long-pulse (0.4–1 μsec), relativistic (0.8 MV) electron beams have been transported in the ion focused regime (IFR) in ion channels formed in low pressure diethylaniline gas by means of KrF excimer laser-induced ionization. These experiments demonstrate that the most efficient (50%–80%) and longest pulse (0.6 μsec) e-beam transport is obtained with laser-induced channels over a very narrow gas pressure range (0.3–1.7 mTorr). Higher than optimal pressures cause excess e-beam-induced ionization and instability of the electron beam. At lower pressures, the laser-induced ion channel density is insufficient for initial e-beam guidance. Transverse oscillations of the electron beam have been measured at a frequency close to that predicted for the ion hose instability. The growth length and wavelength of the transverse oscillations are comparable to the betatron wavelength, further suggesting that these oscillations result from the ion hose instability.
Particle-in-cell simulations of stochastic electron acceleration
Authors: Akimoto K. , Karimabadi H.
Published: July 2002

The results of a series of particle-in-cell simulations of stochastic wave–particle interaction are presented. The threshold for stochasticity was confirmed. The simulations demonstrate that in a strong magnetic field plasma waves with quiver velocities much less than the speed of light but above a certain threshold can stochastically accelerate electrons to energies far greater than 1 MeV. Moreover, self-consistency effects drive return currents and produce energetic runaway electrons that violate an invariant of motion.
Sideband instabilities in free electron lasers
Authors: Rosenbluth Marshall N. , Vernon Wong H. , Moore B. N.
Published: July 2002

The linear stability of sideband modes for a one-dimensional free electron laser is investigated in detail. The dependence on wiggler taper, slippage between optical pulse and electrons, and trapped electron distribution functions are included in the analysis. Nyquist plots are used to delineate the parameter space in which sideband instabilities occur and approximate analytic expressions for the linear growth rate are derived. In special cases a complete analytic solution is given. Essentially all equilibria are unstable to sideband growth. The linear growth rates agree well with numerical simulations.
Free-electron lasers: Spontaneous emission of electrons with chaotic orbits
Authors: Spindler G. , Renz G.
Published: July 2002

A single-particle computer model is used to calculate the electron trajectories in combined helical wiggler and guide fields. The analysis considers the effects of a realizable wiggler field. Special attention is focused on the stability of the electron orbits near magnetoresonane. Upon calculating Poincaré surface-of-section plots and Lyapunov exponents, it is shown that the electron dynamics is chaotic. Analyzing the spectra of spontaneous emission, it is found that regular and chaotic orbits might be discriminated by optical measurements: off resonance, the spectra consist of well-defined lines. On the other hand, near magnetoresonance, chaotic orbits are revealed by noisy spectra concentrated at low frequencies, and a sudden breakdown of the integral radiant energy.
Ion-temperature-gradient-driven turbulence and transport in a sheared magnetic field
Authors: Dimits A. M. , Drake J. F. , Guzdar P. N. , Hassam A. B.
Published: July 2002

An analytical and numerical investigation has been completed of the nonlinear growth and saturation of long-wavelength ion-temperature-gradient-driven turbulence in a sheared magnetic field for the case of an isolated rational surface. The radial correlation length of the turbulence is found to be of order ρiLs/ Lt, with ρi the ion Larmor radius, Ls the magnetic shear length, and LT the temperature scale length. The scaling of the resulting anomalous cross-field diffusivity is χ⊥∝g(ηi)(ρ2i/LTLy) (Ls/LT)2ρivti, where g(ηi) is obtained from the numerical results. In the poloidal direction, the spectrum collapses to the longest wavelength, Ly, available. Explicit results for the function g(ηi) are presented for values of ηi ranging from just above marginal stability ηic to ηi=∞. Although the quasilinear temperature profile is held fixed, the transport rates are very small as a result of local flattening of the temperature profile near the rational surface. The implications of these results for understanding anomalous transport in tokamaks are discussed.
Drift stabilization of tearing modes and the L–H transition
Authors: Strauss H. R.
Published: July 2002

The L to H-mode transition is accompanied by a drop in the magnetic signal of a mode with low toroidal mode number, usually n=1. Quasilinear drift stabilization can cause a saturated tearing mode to collapse, under the conditions at the edge of a tokamak plasma. The criterion for stabilization gives a relation of temperature to magnetic field T∝Bα, with 6/7≥α≥2/3. The collapse of the island could trigger the H mode by reducing the stochastic magnetic field layer around the magnetic separatrix. This would improve edge confinement, helping to form a transport barrier. Improvement of electron edge confinement could make the electric field more negative, producing the rotation seen in experiments.
Self-similar model for tamped ablation driven by thermal radiation
Authors: Sanz J. , Piriz A. R. , Tomasel F. G.
Published: July 2002

A self-similar model for the ablation driven by thermal radiation is developed for the case in which the ablated flow cannot freely expand and it is optically thick to the impinging radiation. The results show that the motion is governed by a dimensionless parameter, which determines the importance of the radiation heat conduction relative to the convective energy flux. The model can be applied to the situation arising in ion beam fusion targets, where a cold pusher is ablated by blackbody radiation emitted by the beam absorber layer that surrounds it and tamps the ablated flow. The scaling laws for the magnitudes of the flow are found and the main differences with free flow ablation are discussed.
Formation of a velocity shear layer in confined plasmas
Authors: Tsui H. Y. W.
Published: July 2002

It is shown, by allowing for finite momentum loss, that an E×B velocity shear layer forms naturally from the radial potential profile created at the boundary or scrape-off region of a confined plasma where magnetic field lines come into contact with material surfaces. The loss of perpendicular momentum governs the structure of the velocity shear in the scrape-off layer via the generation of a radial current that modifies the potential profile. Increase in velocity shear is coupled to improvement in momentum confinement.
The mode structure of high-n resistive ballooning modes
Authors: Connor J. W. , Hastie R. J. , Wilson H. R.
Published: July 2002

The higher-order corrections (in an n−1/2 expansion) to resistive ballooning theory are analyzed in order to gain information about the radial structure of the Δ′-driven modes. This higher-order theory also predicts that the ballooning phase angle θ0 (which is undetermined in the leading-order theory) must be chosen so as to maximize the value of Δ′. The importance of applying this maximization is illustrated by an analytical calculation of Δ′ as a function of θ0 for the s-α model in the limit of small α. It is demonstrated that for this case, one should choose θ0=90° and that the resulting value of Δ′ can be very different from that obtained by setting θ0=0, as is frequently imposed.
Exponential growth of an unstable l=1 diocotron mode for a hollow electron column in a warm-fluid model
Authors: Rasband S. N. , Spencer Ross L. , Vanfleet Richard R.
Published: July 2002

Numerical investigations of a warm-fluid model with an isothermal equation of state for the perpendicular dynamics of an axisymmetric, magnetically confined pure electron plasma predict an exponentially unstable, l=1, diocotron mode for hollow density profiles. The unstable mode can be identified with a stable, nonsmooth mode that exists in cold drift models but which is destabilized by finite temperature effects. The unstable mode has many properties similar to the experimental results reported by Driscoll [Phys. Rev. Lett. 64, 645 (1990)].
Kinetic simulation of a plasma collision experiment
Authors: Larroche Olivier
Published: July 2002

The ionic Fokker–Planck code which was written for describing plasma shock wave fronts [M. Casanova et al. Phys. Rev. Lett. 67, 2143 (1991)] is applied to model the collision of two plasmas in plane geometry. Improvements brought to the code for that purpose are described. The initial phase of the experiment during which the plasmas interpenetrate is accounted for by a simple fluid model, which yields qualitative insight into the phenomena at play as well as an initial condition to start the kinetic simulation. The kinetic results obtained in the stagnation and thermalization phases are discussed with respect to a specific laser-produced plasma collision experiment, as well as to existing fluid and kinetic (‘‘particle-in-cell’’) simulations.
Propagations and collisions of drift wave vortices in a cylindrical plasma
Authors: Yabuki K. , Ueno K. , Kono M.
Published: July 2002

Drift wave vortices in a cylindrical plasma is studied based on the modulated point vortex model for the Hasegawa–Mima equation. Various types of interactions are revealed such as exchange scattering, trapping, and detrapping, which sometimes bring about a long-distance radial excursion, resulting in an enhancement of transport.
The small tight aspect ratio tokamak experiment
Authors: Colchin R. J. , Carolan P. G. , Duck R. , Edlington T. , Erents S. K. , Ferreira J. , Fielding S. J. , Gibson K. , Goodall D. H. J. , Gryaznevich M. , Hender T. C. , Hugill J. , Jenkins I. , Li J. , Manhood S. J. , Parham B. J. , Robinson D. C. , Singleton M. , Sykes A. , Todd T. N. , Turner M. F. , Valovic M. , Walsh M. , Wilson H. R.
Published: July 2002

Low-aspect-ratio tokamaks offer both the economic advantage of smaller size and a number of physics advantages which are not available at conventional aspect ratio. The Small Tight Aspect Ratio Tokamak (START) [Fusion Technology 1990, edited by B. E. Keen, M. Huguet, and R. Hemsworth (North-Holland, Amsterdam, 1991), Vol. 1, p. 353] was conceived as a first substantial test of tokamak plasma behavior at low aspect ratio. It has achieved plasma currents up to 200 kA, peak densities of ∼2×1020 m−3 and central electron temperatures of ∼500 eV at an aspect ratio of 1.3–1.5. Central beta values of ∼13% have been measured and the volume-averaged beta 〈β〉 can approach the Troyon limit. Plasmas are naturally elongated (κ≲2.0) and are vertically stable without feedback control. Major disruptions have not been observed at low aspect ratios (A≤2.0).
Saturation of drift instabilities by electron dynamics
Authors: Smith Ralph A. , Krommes John A. , Lee W. W.
Published: July 2002

An analytical explanation is provided for phenomena observed in gyrokinetic particle simulations used to study the nonlinear evolution of the universal instability. The nonlinear E×B advection of nearly resonant electrons is related to the deactivation of the instability mechanism, leading to oscillations of the perturbation amplitude around a nonzero saturated level. An equivalent description is given in terms of nonresonant mode coupling. A self-consistent evolution equation for the amplitude is obtained for the case where this mechanism predominates, and is successfully compared to the gyrokinetic simulations.
Finite Larmor radius and wall effects on the M=1 ballooning mode at arbitrary beta in axisymmetric tandem mirrors
Authors: Kaiser T. B. , Pearlstein L. Donald
Published: July 2002

The equation for the m=1 ballooning mode is derived; the dominant finite Larmor radius terms force the mode to be rigid. A dispersion relation with the wall near the plasma is obtained for arbitrary β, and it is also shown that with the wall at infinity only the vacuum curvature contributes to all orders in β.
Container geometry and the transition to unsteady Bénard convection in porous media
Authors: Steen Paul H.
Published: July 2002

For convection in three-dimensional boxes of fluid-saturated porous media, an analog of the Rayleigh–Bénard problem at high Prandtl number, it is shown that cascades to time-dependent motion cannot occur within the weakly nonlinear regime for boxes up to moderate size. The convective nonlinearity forces the phase space that describes the interaction of finite-amplitude convection patterns to be globally attracting to large disturbances. In addition, the fastest growing patterns inhibit others in a way that precludes transitions to time-periodic motions. The analysis exploits a closed-form dependence on geometry for the infinite set of coefficients describing the nonlinear interactions in a representation of the governing equations by eigenfunction expansion.
Interfacial stability in a two-layer Bénard problem
Authors: Renardy Yuriko
Published: July 2002

A linear stability analysis of the Bénard problem for two layers of different fluids lying on top of each other and bounded by free surfaces is considered. The fluids are assumed to be similar and perturbation methods are used to calculate the interfacial eigenvalue in closed form. The case of the Rayleigh number and wavenumber of the disturbance being close to the first criticality of the one-fluid Bénard problem has been investigated in a previous paper [Phys. Fluids 28, 2699 (1985)], and was found to exhibit both overstability and convective instability. In this paper, the Rayleigh number is assumed to be less than that of the first criticality of the one-fluid problem, and in this situation, overstability does not occur. An unexpected result is that by an appropriate choice of parameters, it is possible to find linearly stable arrangements with the more dense fluid on top.
Vaporization of irradiated droplets
Authors: Armstrong R. L. , O’Rourke P. J. , Zardecki A.
Published: July 2002

The vaporization of a spherically symmetric liquid droplet subject to a high-intensity laser flux is investigated on the basis of a hydrodynamic description of the system composed of the vapor and ambient gas. In the limit of the convective vaporization, the boundary conditions at the fluid–gas interface are formulated by using the notion of a Knudsen layer in which translational equilibrium is established. This leads to approximate jump conditions at the interface. For homogeneous energy deposition, the hydrodynamic equations are solved numerically with the aid of the CON1D computer code (‘‘CON1D: A computer program for calculating spherically symmetric droplet combustion,’’ Los Alamos National Laboratory Report No. LA-10269-MS, December, 1984), based on the implict continuous–fluid Eulerian (ICE) [J. Comput. Phys. 8, 197 (1971)] and arbitrary Lagrangian–Eulerian (ALE) [J. Comput. Phys. 14, 1227 (1974)] numerical mehtods. The solutions exhibit the existence of two shock waves propagating in opposite directions with respect to the contact discontinuity surface that separates the ambient gas and vapor.
Turbulent passive scalar field of a small Prandtl number
Authors: Qian J.
Published: July 2002

The statistical-mechanics theory of the passive scalar field convected by turbulence, developed in an earlier paper [Phys. Fluids 28, 1299 (1985)], is extended to the case of a small molecular Prandtl number. The set of governing integral equations is solved by the equation-error method. The resultant scalar-variance spectrum for the inertial range is F(k)∼x−5/3/[1+1.21x1.67(1+0.353x2.32)], where x is the wavenumber scaled by Corrsin’s dissipation wavenumber. This result reduces to the − (5)/(3) law in the inertial-convective range. It also approximately reduces to the − (17)/(3) law in the inertial-diffusive range, but the proportionality constant differs from Batchelor’s by a factor of 3.6.
Relaxation to the steady state in neutral-beam-injected mirrors
Authors: Mirnov V. V. , Meiss J. D. , Tennyson J. L.
Published: July 2002

The nonlinear stability of ion velocity distributions in simple beam-injected mirror machines is investigated using a one-dimensional kinetic equation. The steady states and their linear response functions are calculated, and the nonlinear behavior is studied both analytically and numerically. It is shown that when charge exchange effects are negligible, oscillatory instabilities found are suppressed. When unstable steady states are present, the eventual equilibrium of the system is shown to depend not only on the initial density, but on the initial velocity distribution as well.
Collisional effects on coherent nonlinear wave–particle interactions at cyclotron harmonics
Authors: Carter M. D. , Callen J. D. , Batchelor D. B. , Goldfinger R. C.
Published: July 2002

When particle orbits are trapped very near a cyclotron harmonic resonance, the quasilinear concept of weakly perturbed, uncorrelated passages through resonance breaks down, and nonlinear effects become important. In numerical as well as analytic studies, it is demonstrated that relativistic detuning of the resonance can be important for electrons even at low initial energies (∼20 eV) and that coupling to perturbed parallel motion can lead to strong interactions for values of the turning point where the wave frequency differs from a harmonic multiple of the bounce-averaged gyrofrequency by an integral multiple of the bounce frequency. The resultant motion is described by large periodic energy excursions for which small-angle Coulomb collisions or other randomization processes are required to realize net heating. Analytic formulas are derived describing the energy excursion behavior and heating in a mildly relativistic limit. Also, a Monte Carlo numerical model of the collisional effects on the orbits has been employed to study electron heating at the second-harmonic cyclotron resonance and to test the analytic results. In certain regimes of collisionality, a strong enhancement over quasilinear heating has been found.
Trapping of gun-injected plasma by a tokamak
Authors: Leonard A. W. , Dexter R. N. , Sprott J. C.
Published: July 2002

It has been seen that a plasma produced by a Marshall gun can be injected into and trapped by a tokamak plasma. This trapping of a gun-injected plasma is explained in terms of a depolarization current mechanism. A model is developed that describes the slowing of a plasma beam crossing into the magnetic field of a tokamak. The slowing down time is shown to go as τs∝T3/2eL2/nbα20, where nb and Te are the density and temperature of the plasma beam and α0/L is the pitch of the field lines per unit length in the direction in which the beam is traveling. Experimental tests of this model are consistent with the scaling predictions.
Plasma boundary layer in the presence of fast primary electrons
Authors: Schott L.
Published: July 2002

The fluid equations are solved for a three-component plasma in contact with a plane object whose surface is negatively biased or electrically floating. The plasma is assumed to consist of cold ions, fast monoenergetic primary electrons with isotropic velocities, and secondary electrons with a Maxwellian distribution. The rate of ionization is assumed to be proportional to the density of the primary electrons. For a certain range of plasma parameters multiple double layers are formed whose positions are quasiperiodic.
Cross-linking of two vortex rings
Authors: Oshima Yuko , Izutsu Naoki
Published: July 2002

The mechanism for cross-linking two vortex rings has been investigated experimentally. Phase-locked velocity measurements using x-type hot wires were carried out point by point over the entire flow field, and time-dependent vorticity fields were educed. It is found that a new pair of vortex tubes is created, which connects the interacting vortex rings and grows gradually, while the vorticity intensity of the main rings decreases. This is considered to be a type of bridging phenomenon of the vortex tubes, as proposed by Kida and Takaoka [Phys. Fluids 30, 2911 (1987)] in their numerical simulation.
Numerical simulation of the reversed field pinch
Authors: Kirby P.
Published: July 2002

In this paper a series of numerical simulations of field reversal in the reversed field pinch is in the simulations described using an incompressible magnetohydrodynamic (MHD) model and a reference set of plasma conditions. Field reversal and maintenance are observed, but require values of the pinch parameter θ larger than in experiment. This discrepancy is shown to arise largely from the unrealistic resistivity profile in the reference conditions and may not be fundamental. Qualitative agreement with experiment is demonstrated in several areas. The view that field reversal occurs because of a simple MHD dynamo is therefore given support.
Convective instabilities induced by exothermic reactions occurring in a porous medium
Authors: Subramanian Sudhakar , Balakotaiah Vemuri
Published: July 2002

A pseudohomogeneous model is developed and used to analyze the onset of reaction-driven convection in an open rectangular box containing a porous medium. For exothermic reactions, the one-dimensional conduction state exhibits multiplicity as well as oscillatory behavior. Singularity theory is used to classify the different possible bifurcation diagrams of conduction states, along with their stabilities. Linear instability theory is used to determine the stability of the conduction states to convective perturbations. The dependence of both simple zero and Hopf bifurcation neutral stability curves on various problem parameters is presented. It is shown that the Lewis number, Le (ratio of thermal to mass diffusivity), has a pronounced effect on the stability boundaries. Increasing the value of Le, shifts the stationary stability boundary toward higher Rayleigh numbers (Ra). It is also shown that in the region of multiple conduction solutions, there exists a critical value of Lewis number, Le1 (resp., Le2) below which the entire ignited (resp., extinguished) conduction branch is stable to convective perturbations for 0<Ra<Rac, where Rac is the critical Rayleigh number at the extinction (resp., ignition) point. Moreover, depending on the value of Le, a branch of the neutral stability curve for simple bifurcation is found for negative values of Ra. The results indicate that oscillatory instability is more likely to occur for the case of liquids, while for gases only stationary instability is possible for a practical range of parameter values. Numerically computed global bifurcation diagrams of conductive and convective solutions along with flow patterns, isotherms, and concentration profiles are also presented.
Chaotic phenomena in the interaction of vortex rings
Authors: Konstantinov M.
Published: July 2002

The problem of interaction of several coaxial vortex rings in an inviscid fluid is investigated numerically. It is assumed that the core shape of the vortex rings remain circular. At the initial time the rings are located at the same distance ρ0 from the center of the system. This distance is a control parameter of the problem. The cases of interaction of three, four, and five vortex rings are studied. It is shown, that in spite of the nonintegrability of the problem, there are certain domains of values ρ0 where the motion of the vortex rings is quasiperiodical. The transition from one such domain to another occurs always through a domain of chaotic interaction. The results of the interaction of the vortex rings are compared with the interaction of their plane analogies–vortex pairs.
Particle deposition from a turbulent flow. I. A steady-state model for high inertia particles
Authors: Swailes David C. , Reeks Michael W.
Published: July 2002

The steady-state transport and deposition of ‘‘high inertia’’ particles in turbulent duct flow is studied. A relatively simple yet fundamental PDF (probability density function) equation (kinetic equation) is proposed as a model for such systems together with models describing perfectly and partially adsorbing boundaries. Using the idea of particle mean free path it is shown how this equation may be reduced to a universal form. A numerical method of solution based on spectral collocation is presented, and the effectiveness of this is shown to be the result of the substantial reduction in complexity that it affords. Results from this approach are shown to be in very good agreement with those obtained from a random walk simulation, in contrast to those obtained from simple gradient-diffusion and gradient-diffusion/free-flight models. Evaluation of the separate terms in the momentum and energy equations derived from the kinetic equation illustrates how gradient diffusion breaks down close to the depositing wall. Finally, results of particle deposition as predicted by the kinetic equation are compared with those from experiment. Again there is extremely good agreement, despite the relatively simple form of the basic model.
Centrifugal instability of pulsed flow
Authors: Aouidef A. , Normand C. , Stegner A. , Wesfreid J. E.
Published: July 2002

The stability of a pulsed flow in a Taylor–Couette geometry with both cylinders rotating at the same angular velocity Ω(t)=Ω0 cos (ωt) is investigated. The first experimental evidence showing that the flow is less unstable in the limit of low and high frequency while destabilization is maximum for an intermediate frequency ω0 is reported. A detailed analysis of the restabilization at frequencies just above ω0 reveals a behavior not accounted for by previous theoretical analysis. Thus, the linear stability analysis is reconsidered by using a different implementation of the Floquet theory and a satisfactory agreement with the present experimental results is found.
Coriolis instability of pulsed flow
Authors: Aouidef A. , Normand C. , Wesfreid J. E.
Published: July 2002

The linear stability of a time-periodic flow is considered. The fluid motion is taking place in a Hele–Shaw cell made of two vertical rectangular parallel plates separated by a gap of small extent compared to the dimensions of the plates. The flow is generated by oscillating the cell about its vertical symmetry axis. Our stability analysis was motivated by the experimental results reported some years ago by Bolton and Maurer [Bull. Am. Phys. Soc. 32, 2097 (1987)] who observed the onset of longitudinal rolls in this configuration. The inviscid stability criterion for steady flow subjected to Coriolis force is applied at different times to assess the instability mechanism in the two opposite regimes of respectively low and high frequency of oscillation. For moderate values of the frequency, implementation of Floquet theory is used to find the critical values of the instability parameters. Finally a connection is established between the present results and those we obtained recently for a pulsed flow in a Taylor–Couette geometry.
Electroconvection in a layer and in a loop
Authors: Rubinstein I. , Zaltzman T. , Zaltzman B.
Published: July 2002

This paper concerns electroconvectional stability of a conduction state in an electrolyte layer flanked by cation-permselective walls (electrodialysis membranes, electrodes) under constant current conditions. It is shown through a numerical finite difference solution of the linear stability problem that above a certain current threshold the basic conduction state becomes electroconvectionally unstable. Marginal stability curves in the current/wave number plane are calculated and the dependence of the critical threshold characteristics on the system’s parameters (ionic diffusivities ratio, electroconvectional Péclet number) studied. Electroconvectional instability is shown to occur for an arbitrary ionic diffusivities ratio. A one-dimensional model of electroconvection in a loop is developed and the respective problem solved explicitly for a steady state. It is shown that above a certain current threshold, the quiescent conduction in the loop bifurcates into a pair of electroconvectional steady-state circulations.
Transient, nonaxisymmetric modes in the instability of unsteady circular Couette flow. Laboratory and numerical experiments
Authors: Neitzel G. P. , Kirkconnell C. S. , Little L. J.
Published: July 2002

Laboratory and numerical experiments were conducted to quantitatively determine the modal structure of transient, nonaxisymmetric modes observed during the instability of an impulsively initiated circular-Couette flow. The instability develops initially as an axisymmetric, Görtler-vortex state and persists ultimately as a steady, axisymmetric Taylor-vortex state of different wavelength. The transition between these two states results from the instability of the Görtler mode combined with the underlying developing swirl flow and is dominated by nonaxisymmetric modes. The laboratory experiments employed flow visualization coupled with digital video and image-processing techniques; numerical experiments were performed using the spectral-element code, nekton.
Thermal and electrohydrodynamic plumes. A comparative study
Authors: Vázquez P. A. , Pérez A. T. , Castellanos A.
Published: July 2002

In this paper we deal with self-similar thermal and electrohydrodynamic (EHD) plumes. The former arises from hot lines or points, whereas the latter arises when sharp metallic contours submerged in nonconducting liquids support high electrostatic potential, resulting in charge injection. Although the motive force is buoyancy in one case and Coulomb force in the other, it is shown that the solution for EHD plumes is the same as for thermal plumes in the limit of large Prandtl numbers. We present the analysis of axisymmetric plumes for large values of Prandtl number, and this analysis is subsequently applied to EHD plumes. The validity of the approximations for EHD plumes is discussed in the light of experimental data.
Simulation of hypersonic flows using a detailed nitric oxide formation model
Authors: Bose Deepak , Candler Graham V.
Published: July 2002

In this paper the extensive quasiclassical trajectory (QCT) study recently concluded [J. Chem. Phys. 104, 2825 (1996)] is used to model the kinetics of the primary NO formation reaction, N2+O→NO+N, in hypersonic nonequilibrium flows. The QCT data are used to obtain expressions for the thermal rate constant, reactant energy removal, and product energy disposal rates of this reaction. The QCT results are coupled with the continuum conservation flow equations, and these equations are used to simulate the Bow-Shock UltraViolet2 (BSUV2) flow at altitudes between 75 to 87.5 km. It is found that the use of the Macheret and Rich [Chem. Phys. 174, 25 (1993)] vibration–dissociation coupling model along with the QCT rates gives improvements in the NO concentration predictions at altitudes between 80 and 85 km. Also, it is found that the vibrational and rotational temperatures of NO are much higher than that of the N2 and O2 in the gas, in accordance with the BSUV2 measurements. The amount of NO produced in the flow fields at 87.5 km and above is found to be strongly dependent on the free-stream density of atomic oxygen.
Görtler-crossflow vortices
Authors: Le Cunff Cédric , Zebib Abdelfattah
Published: July 2002

Crossflow influence on steady Görtler vortices is studied in a pressure-driven boundary layer developing along a curved wall. Linear theory shows that crossflow is destabilizing in the long wavelength range. In the non-linear simulations, co-rotating vortices are transformed into counter-rotating vortices for accelerated boundary layers. The streamwise location, where counter-rotating vortices appear, depends on the amplitude of the pressure gradient. Linear studies indicate that a secondary instability may develop on the steady vortex profile.
On nonlinear transient free-surface flows over a bottom obstruction
Authors: Zhu Songping , Zhang Yinglong
Published: July 2002

A fully nonlinear integral-equation model based on the potential theory is used to study transient free-surface flow induced by a bump moving along a flat bottom. Comparison is made between the large-time solution of the transient model and the fully nonlinear steady-state solution in Zhang and Zhu [J. Eng. Math. 30, 487 (1996)] for both subcritical and supercritical flows, and asymptotic agreement between the two is observed. For the transcritical flow, some features associated with the resonant flow motion previously found by approximate theories are confirmed. A diagram of the parameter space is presented showing the regions of different solution categories including wave breaking. Through this diagram, a discrepancy between two previous steady-state models can now be clearly explained. Interestingly, the dividing line separating those solutions with or without wave breaking levels out asymptotically for very large Froude numbers, indicating that there is a limiting dimension of the bump, beyond which physically meaningful steady-state solutions do not exist, although they may well be obtained mathematically.
Magnetic damping of buoyant convection during semiconductor crystal growth in microgravity. Steady transverse residual acceleration
Authors: Ma Nancy , Homan K. O. , Walker John S.
Published: July 2002

In this paper we treat the steady, three-dimensional buoyant convection in a circular cylinder with an axisymmetric temperature, with a uniform axial magnetic field, and with a steady transverse acceleration or gravitational force. The magnetic field is sufficiently strong that inertial effects and convective heat transfer are negligible for the typical residual accelerations on an earth-orbiting vehicle. Since the governing equations are linear, the solution for any steady acceleration or gravitational vector is given by the superposition of the solutions for an axial acceleration and for a transverse acceleration. The solution for an axial acceleration is given by a rescaling of the well-known solution for a terrestrial vertical Bridgman crystal-growth process, and in this paper we present the solution for a transverse acceleration. With an axial magnetic field, the melt motion consists of two circulations in opposite directions around two axial lines on opposite sides of the diameter plane which is parallel to the transverse acceleration vector a: both circulations involve flows in the direction opposite to a near the diameter plane and flows in the same direction as a near the cylinder wall. The small electrical conductivity of the crystal is important because the crystal provides a path for a major part of the electric current responsible for the magnetic damping.
Added mass of a disc accelerating within a pipe
Authors: Sherwood J. D. , Stone H. A.
Published: July 2002

The flow of inviscid fluid around a disc in a pipe is computed, and the results are used to determine the added mass of the accelerating disc in the frame in which the mixture velocity is zero. The added mass of an array of discs spaced at regular intervals along the pipe is then computed, and is related to the pressure gradient along the pipe. Some flow profiles are also presented. The results show that the added mass per particle increases as the pipe diameter is reduced relative to the particle size. The added mass per particle decreases as the number density of particles increases, but the added mass per unit length of the pipe nevertheless increases. Thus an increase of either the particle size or number density leads to a tighter coupling between the liquid and the particles; this result should hold for other particle shapes and configurations. Results are also presented for the drift, i.e., the displacement of fluid particles caused by the motion of an isolated disc along the axis of the pipe. If the diameter of the pipe is sufficiently small, the added mass of the disc is modified from that in unbounded fluid, and the background drift at the walls of the pipe can no longer be estimated from the added mass of the disc.
On the collision rate of small particles in isotropic turbulence. I.  Zero-inertia case
Authors: Wang Lian-Ping , Wexler Anthony S. , Zhou Yong
Published: July 2002

Numerical experiments have been performed to study the geometric collision rate of finite-size particles with zero inertia (i.e., fluid elements) in isotropic turbulence. The turbulent flow was generated by the pseudospectral method. We argue that the formulation of Saffman and Turner [J. Fluid Mech. 1, 16 (1956)] for the average collision kernel is correct only under the assumptions that the particles are kept in the system after collision and allowed to overlap in space. This was confirmed, for the first time, by numerical experiments to within a numerical uncertainty as small as 1%. Finite corrections to the Saffman and Turner result must be made if one applies the theory to actual coagulation process where particles are not allowed to overlap before collision and particles are removed from a given size group after collision. This is due to the fact that Saffman and Turner assumed a uniform, time-independent concentration field in their formulation of the average collision kernel, while in the actual modeling of population evolution the particle number concentration changes in time and may be locally nonuniform as a result of a biased removal process due to spatially nonuniform coagulation rates. However, the quantitative level of the deviations from the Saffman and Turner result remain to be explained. Numerical experiments in simple shear flow were also conducted to elaborate our findings.
A geometric explanation of the effects of mild streamline curvature on the turbulence anisotropy
Authors: Holloway A. G. L. , Tavoularis S.
Published: July 2002

A new explanation of the well documented effects of mild streamline curvature on the anisotropy of sheared turbulence has been developed. Its main underlying assumption is that the mean streamline curvature has no direct effect on the production and dynamics of each individual turbulent eddy, which is produced with the structure of purely sheared turbulence, and is subsequently convected downstream while retaining its initial anisotropy relative to fixed inertial coordinates. The local Reynolds stress anisotropy accumulates the contributions of all surviving eddies produced upstream, which, because the mean shear keeps changing direction, have different anisotropies, when viewed in terms of the local curvilinear coordinates; thus, the local anisotropy is influenced by the history of flow curvature only indirectly. A model developed to demonstrate the validity of the hypothesis requires only the specification of the turbulence anisotropies in a fully developed, rectilinear, reference flow (e.g., a rectilinear uniformly sheared flow), the geometrical features of the flow under study, and a dimensionless mean eddy lifetime. It predicts accurately the observed asymptotic turbulence structure of uniformly sheared flow subjected to prolonged, constant curvature and the exponential adjustment of this structure to stepwise changes in flow curvature. Predictions of the shear stress anisotropy in a curved mixing layer are also in good agreement with published data. In all these cases, the present model makes better predictions than two popular Reynolds stress models and a rapid distortion model.
Effect of acoustic radiation on the stability of spherical bubble oscillations
Authors: Gumerov Nail A.
Published: July 2002

A recent analysis of the stability of spherical bubble oscillations shows that the high order shape modes are parametrically unstable with respect to small but finite perturbations [Z. C. Feng and L. G. Leal, J. Fluid Mech. 266, 209 (1994)]. Using a heuristic approach it is shown here that the acoustic radiation due to the liquid compressibility plays an important role in stabilization of the high frequency modes and overall stability of the bubble spherical shape.
Subharmonic resonance in a laminar wall jet
Authors: Tumin Anatoli
Published: July 2002

Receptivity and stability of a two-dimensional laminar wall jet is considered. The wall jet and the disturbance source examined in a direct numerical simulation by S. Wernz and H. F. Fasel (AIAA Paper 96-0079) are chosen for the analysis. The disturbances are introduced by blowing and suction through a slot in the wall. The disturbances of two frequencies (28 and 56 Hz) are considered. Because two eigenmodes may be unstable in the wall-jet flow, both of them are taken into account for each frequency. Therefore, the linear receptivity problem is solved for two pairs of eigenmodes, and their development and interaction downstream from the source are analyzed. The analysis allows one to explain the behavior of the fundamental and subharmonic disturbances observed in the numerical simulation.
Asymptotic scaling of a decaying turbulent compressible vortex
Authors: Grasso F. , Pirozzoli S.
Published: July 2002

In the present work a theoretical analysis of the asymptotic decay of an isolated, compressible turbulent vortex is presented. The governing equations are expanded in powers of the vortex Mach number squared, and the asymptotic scaling both of the O(1) and the O(M2) variables is theoretically analyzed in the framework of an isotropic eddy viscosity two-equation turbulence model. Numerical simulations are presented to confirm the analytically predicted power-law decay, and to assess the influence of turbulence and of the initial thermodynamic state on the evolution both of a finite- and zero-circulation vortex. Regardless of the initial state, the simulations show that isolated turbulent compressible vortices decay according to a two-stage mechanism. For a finite-circulation vortex, the latter is found to be controlled primarily by the decay of vorticity and heat conduction, while for a zero-circulation vortex, the two-stage mechanism is associated with the reversal of the radial motion within the vortex core.
Core instabilities and “bridging” in the first pairing of plane mixing layers
Authors: Estevadeordal Jordi , Kleis Stanley J.
Published: July 2002

Core instabilities and “bridging” in the first pairing of transitional plane mixing layers are identified experimentally through phase-locked measurements. Spanwise-uniform acoustic excitation is employed to stabilize the two-dimensional (2D) instability modes. The three-dimensional (3D) instability modes undergo amplification during pairing of the spanwise rollers. The most amplified 3D instability modes are associated with upstream perturbations generated by small imperfections in the splitter-plate surface that produce a kink [e.g., as in the graph of tanh(z)] and a bend [e.g., as in the graph of sech(z)] in the rollers. These perturbations produce core dynamics, bridging phenomena, and “lambda” structures.