bluuebunny/crossref_metadata_2025 huggingface dataset:
Large- and small-scale stirring of vorticity and passive scalar in a 3-D temporal mixing layer
Authors: Lesieur Marcel , Comte Pierre
Published: July 2002

We investigate, with the aid of a three-dimensional direct-numerical simulation at high resolution, the origin and topology of the longitudinal vortex filaments which appear in the temporally growing mixing layer. The basic velocity field is a hyperbolic-tangent profile U tanh(2y/δi), with a Reynolds number of Uδi/ν =100. The calculation uses pseudospectral methods, and is carried out at a resolution of 1283 grid points in a cubic box of size L containing four fundamental most-amplified wavelengths (L=4λa). The initial velocity field is the basic velocity, upon which is superposed a three-dimensional Gaussian perturbation of wide spectrum, peaking at ka=1/2πλa, with kinetic energy equal to 10−4U2, modulated by a Gaussian exp[−(y/δi)2] in the transverse direction. A passive-scalar transport equation is solved as well, with the same initial profile as the basic velocity profile. Isosurfaces of the passive scalar and three vorticity components are visualized, permitting the 3-D vortex structure of the flows to be revealed.

Because of the initial spanwise decorrelation of the perturbation phase, the fundamental spanwise vortices that appear have strong spanwise oscillations which are not in phase, and hence cannot be interpreted in terms of the translative instability investigated by Pierrehumbert and Widnall.1 Pairings between the primary vortices exhibit the same spanwise decorrelation, and reconnections of the billows occur. Visualizations also show the generation of thin longitudinal vortices from the regions of reconnection. The vortex lines at the same moment indicate that the thin longitudinal vortices result from an intense longitudinal stretching of the spanwise vortex line when it is severely twisted in the reconnection region. Reconnected Kelvin–Helmholtz billows give rise to a three-dimensional Λ-shaped structure of the passive scalar. This behavior is in agreement with the theory developed by Lesieur et al., where it was proposed that the large-scale three-dimensionality of the temporal mixing layer perturbed randomly was governed by two-dimensional turbulence unpredictability mechanisms. It might explain the experimental findings of Breidenthal2 and Bernal and Roshko3 concerning a spatially-growing, unforced mixing layer. Notice also that the thin longitudinal filaments forming in our calculation do not seem to be explained by the mechanism proposed by Lasheras and Choi,4 where they originate from the straining by the big rollers of a spanwise vortex filament perturbed about the stagnation line.
Computation of mixing and chemical reaction in stirred vessels
Authors: Ziman H. , Middleton J. C.
Published: July 2002

Chemical reactions carried out in stirred vessels in industrial practice are commonly complex in that they have several steps which, for example, may complete for a reactant and form different products from it. Usually, one of these products is desired whereas the other is a waste product giving rise to extra costs of recycling or disposal. In some cases, the rate and pattern of mixing in the vessel influences the reaction selectivity to the desired product. One example of this occurs when one of the competing reactions is fast (its rate being mixing-controlled), while the other is slower. In such cases, it is found that the only successful predictive design or scale-up method available is to model the mixing process in detail and superpose the reaction kinetics, using computational fluid dynamics. This paper presents a suitable computational procedure, which first calculates the three-dimensional flow field in a stirred vessel, then solves the equations describing the mixing and chemical reactions that will occur at each place and time in that calculated flow field. Special attention is given to formulating the concentration transport equations in a manner that is efficient and simple to apply to various reaction schemes. The approach adopted uses the concepts of mixture fraction and reaction extent, and enables the program to handle almost any reaction scheme without any additional programming. Macro- and micromixing regimes are included. In the latter case, consideration is given to the interaction of turbulence and the local reaction rate, using a simple model based on a local mixing time scale. Example calculations are presented that show the ability of the method to calculate mixing times and reaction yields. Experiments with a competitive reaction scheme are described and their results compared with computed predictions, in a regime in which the traditional rules of thumb for scale-up are unsuccessful.
Instabilities of roll-like patterns for degenerate marginal curves
Authors: Proctor Michael R. E.
Published: July 2002

The Eckhaus, cross-roll, and zigzag instabilities of convective patterns in the form of rolls are examined in the case that the minimum in the marginal stability curve is degenerate (flat). The effects of asymmetry in the growth rate curves are also considered. Examples are given for the quartic degeneracy.
Numerical simulations of multifrequency instability-wave growth and suppression in the Blasius boundary layer
Authors: Pal A. , Bower W. W. , Meyer G. H.
Published: July 2002

A mathematical model based on the Orr–Sommerfeld equation is developed to describe the growth and suppression of multifrequency, two-dimensional instability waves in the Blasius boundary layer over a flat place through localized perturbations at the surface caused by time-varying suction/blowing. It is shown for harmonic (single-frequency) perturbations that the instability wave can be decomposed into two components: an idealized Tollmien–Schlichting wave and a second perturbation that approximately cancels the first component upstream of the surface disturbance and becomes small downstream. Because the first component alone fully expresses the instability of the flow, the need to perform numerical Fourier transformation over the wave number is eliminated, permitting easy extension of the analysis to the more general case of arbitrary waveform of the perturbation. Numerical results are presented for examples of instability-wave generation and suppression in the boundary layer.
Causality and the stability of parallel flow
Authors: Chimonas George
Published: July 2002

The linearized theory of the stability of parallel flow is formulated in terms of causality theory. In this method the properties of the Fourier spectrum—the disturbances with real frequency ω—provide the stability criteria for a system. In particular, if the Fourier spectrum provides a complete cause-and-effect formulation, the flow is assuredly stable. This single restriction leads to the critical-level theorems for parallel flows in both stably and unstably stratified fluids, Howard’s semicircle theorem, the Miles–Howard Richardson-number criterion, an extended semicircle theorem for unstable flows, and several other results concerning instabilities in fluids. The causal approach supersedes group-velocity concepts, and resolves the problems of defining upward- and downward-propagating signals in a rapidly varying medium. It also associates a causal behavior to scattering from a discontinuity or an interface. Causality is examined by subdividing the continuous fluid into a set of homogeneous intervals within which propagation and interfacial scattering can be explicitly computed. As the intervals are reduced toward infinitesimal size the solution for the continuous fluid is obtained.
Dissipation and contact-line motion
Authors: Haley Patrick J. , Miksis Michael J.
Published: July 2002

The forced motion of a droplet trapped in a cone including contact-line motion is determined. In addition, the viscous and contact-line dissipation associated with the motion is determined. It is found that in certain limits, the contact-line dissipation can dominate the viscous dissipation.
Averaged-equation and diagrammatic approximations to the average concentration of a tracer dispersed by a Gaussian random velocity field
Authors: Koch Donald L. , Shaqfeh Eric S. G.
Published: July 2002

Approximate methods for predicting the spread of a passive tracer due to a Gaussian random velocity field are examined. Previous methods such as the direct-interaction approximation [J. Fluid Mech. 11, 257 (1961)] and Phythian’s short-time expansion [J. Fluid Mech. 67, 145 (1975)] make predictions for the second moment of the concentration that are qualitatively consistent with numerical simulations [Phys. Fluids 13, 22 (1970)]. However, it is shown that the higher moments obtained from these approximations are incorrect. The derivations of the direct-interaction approximation using a diagrammatic expansion and the method of averaged equations are reviewed [Phys. Fluids A 1, 47 (1989)]. A higher-order (two-body) approximation is developed which makes predictions for the fourth moment that are qualitatively correct in both the short- and long-time limits.
Gallery of Fluid Motion
Authors: Reed Helen L.
Published: July 2002

This article displays winning photographs from the ninth Annual Fluid Mechanics Photo Contest held at the November 1991 meeting of the American Physical Society, Division of Fluid Dynamics, Scottsdale, Arizona.
Stokes flow through a two-dimensional filter
Authors: Wang C. Y.
Published: July 2002

A simple eigenfunction expansion and matching method is used to solve the slow viscous flow through a wall with two-dimensional periodic gaps. The normalized pressure drop is determined as a function of the geometric parameters.
Calculation of interfacial flows and surfactant redistribution as a gas/liquid interface moves between two parallel plates
Authors: Wassmuth Fred , Laidlaw W. G. , Coombe D. A.
Published: July 2002

The hydrodynamic equations governing the evolution of the interface of a gas/liquid ‘‘bubble’’ as it moves through the space between two plates are solved using a finite difference algorithm defined on a moving deforming surface grid. Particular attention is paid to the redistribution of surfactant as the bubble deforms on squeezing through a constriction. The surfactant redistribution, the surface tangential flows, and the surface normal velocity depend on factors such as bulk viscosity, the bulk/surface surfactant exchange rate, the surface tension dependence on surfactant concentration, and the bulk velocity pattern. Zones of surfactant depletion just behind the bubble front and an accumulation near the bubble front are observed; there are also depletion and accumulation zones associated with the constriction. A variety of other effects are investigated and a detailed analysis permits the generation of schematic representations of the interplay of the coupled bulk and surface hydrodynamic equations for the system.
A study of multiscalar mixing
Authors: Girimaji Sharath S.
Published: July 2002

A model for passive mixing of multiple scalars in homogeneous turbulence is developed. This model is based on the time-evolving mapping closure methodology. Calculations from the model are compared against heat conduction simulations (HCS) data. The agreement is excellent at the early stages of mixing and at the later stages it is fair.
Measurement of lower-hybrid-driven current profile by Abel inversion of electron-cyclotron wave transmission spectra
Authors: Fidone I. , Giruzzi G. , Caron X. , Meyer R. L.
Published: July 2002

A method for measuring the radial profile of the lower-hybrid-driven current in a low-density tokamak plasma using electron-cyclotron wave attenuation is discussed. This diagnostic scheme is reminiscent of the transmission interferometry approach, commonly used in tokamaks to measure the plasma density, but now the wave amplitude instead of the phase is measured. Wave attenuation of the ordinary mode at ωp ≪ ω < ωc along vertical chords is measured; at these frequencies, the waves are absorbed by the superthermal tail sustained by lower-hybrid waves and the local wave absorption coefficient is proportional to the noninductive current density. The radial profile of this current is obtained from Abel inversion. An application to the Tore Supra tokamak is presented.
Reply to ‘‘Comment on ‘Modification of stimulated Brillouin, saturated Raman scattering and strong Langmuir turbulence by nonlocal heat transport’ ’’
Authors: Rose H. A. , DuBois D. F.
Published: July 2002

The effects of nonlocal heat transport on stimulated Brillouin scattering are discussed, and so is the applicability of nonlocal transport on the effects. (AIP)
General theory of Onsager symmetries for perturbations of equilibrium and nonequilibrium steady states
Authors: Krommes John A. , Hu Genze
Published: July 2002

The theory of Onsager symmetry is reconsidered from the point of view of its application to nonequilibrium, possibly turbulent steady states. A dynamical formalism based on correlation and response functions is used; understanding of its relationship to more conventional approaches based on entropy production enables one to resolve various confusions about the proper use of the theory, even near thermal equilibrium. Previous claims that ‘‘kinematic’’ flows must be excluded from considerations of Onsager symmetry are refuted by showing that suitably defined reversible and irreversible parts of the Onsager matrix separately obey the appropriate symmetry; fluctuating hydrodynamics serves as an example. It is shown that Onsager symmetries are preserved under arbitrary covariant changes of variables; the Weinhold metric is used as a fundamental tensor. Covariance is used to render moot the controversy over the proper choice of fluxes and forces in neoclassical plasma transport theory. The fundamental distinction between the fully contravariant Onsager matrix Lij and its mixed representation Lij is emphasized and used to explain why some previous workers have failed to find Onsager symmetry around turbulent steady states. The generalized Onsager theorem of Dufty and Rubí [Phys. Rev. A 36, 222 (1987)] is reviewed. An explicitly soluble Langevin problem is shown to violate Onsager’s original symmetry but to obey the generalized theorem. The physical content of the generalized Onsager symmetry is discussed from the point of view of Nosé–Hoover dynamics. A set of extended Graham–Haken potential conditions are derived for Fokker–Planck models and shown to be consistent with the generalized Onsager relations. Finally, for quite general, possibly turbulent steady states it is argued that realizable Markovian statistical closures with underlying Langevin representations must also obey the generalized theorem. In the special case in which all state variables have even parity and there are no external parameters that change sign under time reversal, the steady-state energy balance fully determines the Onsager matrix, which is guaranteed to be symmetric.
Particle simulation of non-neutral plasma behavior
Authors: Ramachandran H. , Morales G. J. , Decyk V. K.
Published: July 2002

It is demonstrated that particle-simulation techniques can explore a broad range of dynamical behavior exhibited by non-neutral plasmas. New features isolated by this approach include collective relaxation through generation of core and halo populations; self-organization without radial transport as the plasma is cooled; spontaneous generation of solitonlike structures upon axial reflection from the external confining potential.
Nonlinear waves in two-fluid hydrodynamics
Authors: Gedalin M.
Published: July 2002

Two assumptions, one-dimensionality and quasineutrality, in the framework of the two-fluid hydrodynamics for hot plasmas, allow a close set of three equations (for inverse density and two components of the transverse magnetic field) to be obtained. These equations describe nonlinear waves in a wide range of wave vectors (up to the inverse electron inertial length) and frequencies (up to the low-hybrid frequency or in some cases electron gyrofrequency). The obtained set of equations is valid for arbitrary plasma temperatures. Linear dispersion relations are easily recovered from the obtained nonlinear equations. Nonlinear wave equations for different modes, which include new terms due to finite pressure, are derived using methods of the reductive perturbation theory. Stationary solutions are analyzed by the pseudopotential method. Conditions for the existence of solutions with homogeneous asymptotics are found.
Excitation of electrostatic ion cyclotron waves by a power-modulated transversely excited atmosphere (TEA) CO2 laser
Authors: Sasaki K. , Fujii T. , Takahashi K. , Nagatsu M. , Tsukishima T.
Published: July 2002

Electrostatic plasma waves are excited in a laboratory plasma by means of the ponderomotive force of a high-power, power-modulated transversely excited atmosphere (TEA) CO2 laser beam [W. W. Duley, CO2 Lasers (Academic, New York, 1976), Chap. 2.5]. The strengths of the magnetic field, for which the plasma waves are excited selectively, and the group velocity of the excited plasma waves are consistent with the dispersion relation of the electrostatic ion cyclotron wave. This wave-excitation experiment gives proof to the principles of the driven scattering plasma diagnostics which has been proposed as a method for the ion temperature measurement of high-temperature plasmas.
A new method to calculate the electron collision frequency in a cold plasma
Authors: Sharma S. K.
Published: July 2002

In this Brief Communication it is proposed that one can calculate the electron collision frequency in a cold plasma using a phase velocity versus frequency plot of an adiabatic ion-acoustic wave (IAW).
A novel Lagrangian formulation for self-consistent fluid mechanics problems
Authors: Lewis H. Ralph
Published: July 2002

A novel Lagrangian formulation is proposed for self-consistent fluid mechanics problems, like magnetohydrodynamics, in which the force field is to be determined along with the velocity field. A key feature of the formulation is that the momentum balance and continuity equations are exactly satisfied locally, even in numerical applications. This is accomplished by choosing a Lagrangian representation for the velocity field and using the momentum balance equation to cmpute the force field pointwise in terms of the Lagrangian mapping function. The formulation is illustrated for one-dimensional electrohydrodynamics, in which case the mass continuity and momentum balance equations are coupled to the Poisson equation. The formulation may be suitable for describing phenomena like the formation of convective cells, boundary layers, or discontinuities in a natural way.
Spectroscopic investigation of a reversed field pinch operated without limiters
Authors: Weber P. G.
Published: July 2002

Spectroscopic measurements on the ZT-40M reversed field pinch [submitted to Fusion Tech.] discharge are reported. Specifically, the impurity content, Zeff, recycling behavior, ion temperature, and propagation of magnetohydrodynamic modes in the edge are addressed. The importance of the plasma–wall interaction in ZT-40M, operated without limiters, is demonstrated.
Statistics of surface flow structures on decaying grid turbulence
Authors: Loewen S. , Ahlborn B. , Filuk A. B.
Published: July 2002

The surface flow pattern of decaying turbulence generated by a vertical bar grid in a towing tank has been studied. The velocities and positions of about 2×104 Al aluminum filing tracers following the surface motion have been measured and statistically analyzed. From ten photo series, each of 10 mesh width span and 40 mesh widths long, 2000 surface eddies were identified and studied. The flow field is decomposed into ‘‘closed’’ rotating flow structures (the surface eddies) and ‘‘open’’ flow structures (rivers) of predominantly translational motion. There are also relatively stagnant areas that grow in number and size with time. Immediately behind the grid the average energy densities of the surface eddies ee and the rivers er are about equal but ee decays at a greater rate than er. The eddy size distribution gradually shifts to larger eddies and an increasing fraction of the fluctuating kinetic energy is transferred into the rivers. In the collision-dominated equilibrium region, the eddy and the river energy distributions can be described by Boltzmann-type functions. These distributions may then be characterized by ‘‘temperature’’ θe and θr for eddies and rivers. These temperatures decay steadily throughout the observation period, but remain approximately equal to each other. The relationship between the surface flow dynamics and the subsurface and bulk turbulent grid-flow is examined.
Stability of Hele–Shaw flows: The wetting-layer effect
Authors: Schwartz Leonard
Published: July 2002

The classical viscous instability calculation of Chouke et al. [Trans. AIME 216, 188 (1959)], further clarified by Saffman and Taylor [Proc. R. Soc. London, Ser. A 245, 312 (1958)] in the context of flow in a Hele–Shaw cell, is reconsidered and the importance of the wetting condition and the associated interfacial pressure jump is established. Important qualitative changes appear in the resulting dispersion relation when this effect is included. The wavelength of maximum growth is increased and the growth rate is reduced, however, the critical wavelength for instability remains unchanged. Comparison of the theoretical results with some recent experiments appears to provide support for the new effect.
Power dependence of electron cyclotron wave damping in tokamak plasmas
Authors: Krivenski V. , Fidone I. , Giruzzi G. , Meyer R. L. , Ziebell L. F.
Published: July 2002

The power dependence of the wave damping in a tokamak plasma for arbitrary direction of propagation, mode of polarization, and wave frequency is investigated. Using a 2-D Fokker–Planck code, it is shown that the wave damping increases or decreases with the rf power depending on the velocity range of the absorbing electrons.
Drift modes with differential rotation and passing electrons
Authors: Liu J. , Horton W. , Sedlak J. E.
Published: July 2002

Differential rotation characteristic of the tandem mirror central cell is shown to have a significant stabilizing effect on the rotation and unfavorable curvature driven drift modes. Stability boundaries and parametric dependence of the growth rate and frequency of the m=1 and m=2 modes are given as a function of the passing electron density fraction, wall-to-plasma radius, and rotation speed.
The subgrid-scale modeling of compressible turbulence
Authors: Speziale C. G. , Erlebacher G. , Zang T. A. , Hussaini M. Y.
Published: July 2002

A subgrid-scale model recently derived by Yoshizawa [Phys. Fluids 29, 2152 (1986)] for use in the large-eddy simulation of compressible turbulent flows is examined from a fundamental theoretical and computational standpoint. It is demonstrated that this model, which is only applicable to compressible turbulent flows in the limit of small density fluctuations, correlates somewhat poorly with the results of direct numerical simulations of compressible isotropic turbulence at low Mach numbers. An alternative model, based on Favre-filtered fields, is suggested which appears to reduce these limitations.
Kink instabilities with short wavelengths along an elliptic magnetic stagnation line in anisotropic plasmas
Authors: Spies Günther Otto
Published: July 2002

Pressure anisotropy cannot stabilize the kinks with short axial wavelengths that, according to ideal magnetohydrodynamics, are unstable at an elliptic magnetic stagnation line. This result is derived by applying the guiding center theory to the simple but generic case of circularly cylindrical Z-pinch equilibria with anisotropic Maxwellian distribution functions of ions and electrons.
Relative velocity fluctuations in turbulent flows at moderate Reynolds number. II. Model calculation
Authors: Tong P. , Goldburg W. I.
Published: July 2002

A model calculation is presented to study turbulence at moderate Reynolds number, Re. In conformity with recent measurements, it is proposed that in the energy cascade the fractional volume occupied by eddies of various sizes depends on Re. By introducing a Re-dependent parameter in the random beta model, it is shown that the scaling behavior of the small-velocity fluctuations at moderate Re can be characterized by a single Re-dependent scaling exponent α0. The calculated Re dependence of α0 is consistent with the experimental data. Corrections to this scaling are calculated using the multifractal concept. The Reynolds number dependence of other multifractal properties in turbulent flows is also calculated.
The evolution of a plane mixing layer with spanwise nonuniform forcing
Authors: Collis S. Scott , Lele Sanjiva K. , Moser Robert D. , Rogers Michael M.
Published: July 2002

The experimental studies of Nygaard and Glezer (AIAA Paper No. AIAA-91-0625, 1991) show that vortical structures shaped like a ‘‘chain-link fence’’ form in a plane mixing layer subjected to spanwise nonuniform phase excitation. Using direct numerical simulation of temporally evolving mixing layers, initial conditions have been developed that result in flows with vortex structures similar to those observed in the experiments. The simplest initial condition that leads to the experimentally observed structures is composed of a pair of equal-strength oblique disturbances and this initial condition is a low-order Fourier approximation to the square-wave excitation used in the experiments. This simple disturbance suggests an explanation for the short wavelength cutoff observed in the experiments and allows comparison with previous stability analyses. The simulated flow fields show that the chain-link-fence vortex structures are different from the usual rib/roller mixing layer structures. Furthermore, detailed examination of the overlap region between adjacent structures shows that ‘‘local pairing’’ does not occur in the chain-link-fence flow due to strong self-induction effects. By increasing the spanwise wavelength, self-induction effects can be delayed and regions of local pairing are observed. However, the local pairings do not lead to a single amalgamated vortex at late times as is observed in two-dimensional pairings. Finally, it is shown that the chain-link-fence flow produces increased mixing compared to standard rib/roller flows.
A closed probability density function evolution equation for stochastic diffusion
Authors: Ros Javier , Valiño Luis
Published: July 2002

A closed probability density function (PDF) evolution equation is obtained for a homogeneous scalar field undergoing stochastic diffusion. The closure assumes that the scalar field is made up of an ensemble of one-dimensional periodic waves.
Scalar measurements in compressible axisymmetric mixing layers
Authors: Clemens N. T. , Paul P. H.
Published: July 2002

An experimental study was conducted of mixing in supersonic axisymmetric shear layers using planar laser-induced fluorescence of seeded nitric oxide. Passive scalar images were obtained at convective Mach numbers (Mc) of 0.35, 0.82, and 1.3. Values of the fraction of mixed fluid (δm/δ1) were obtained at Mc=0.35 and 0.82, using a technique that employs fluorescence quenching to provide a direct measurement of fluid unmixedness. From the unmixedness measurement, an accurate value of the fraction of mixed fluid can be derived, even when the smallest mixing scales are not resolved. The visualizations reveal that at low Mc, the turbulent structures appear primarily as rollers, although the degree of axisymmetry of the rollers could not be determined. At high Mc, the structures were more irregular, and often appeared as the jagged, irregular structures that have been previously documented in planar layers. Concentration fluctuation statistics show that fluctuations are smaller at Mc=0.82 than at Mc=0.35, particularly on the high-speed edge of the layer, but those at Mc=1.3 are comparable to those at Mc=0.35. Using the quenching method, the fraction of mixed fluid, δm/δ 1, was found to be 0.45 and 0.48 at Mc=0.35 and 0.82, respectively. If the effect of the Reynolds number difference is taken into account, then the measured trend is even stronger. One can conclude then that the moderately compressible axisymmetric mixing layer has a slightly greater mixing efficiency than its incompressible counterpart.
Unsteady separation from the leading edge of a thin airfoil
Authors: Degani A. T. , Li Q. , Walker J. D. A.
Published: July 2002

At high Reynolds numbers, the process leading to dynamic stall on airfoils initiates in the leading-edge region. For thin airfoils, the local motion near rounded leading edges can be represented as flow past a parabola and when the mainstream flow is at an angle of attack to the airfoil, a portion of the boundary layer will be exposed to an adverse pressure gradient. Once the angle of attack exceeds a certain critical value, it is demonstrated that unsteady boundary-layer separation will occur in the leading-edge region in the form of an abrupt focused boundary-layer eruption. This process is believed to initiate the formation of the dynamic stall vortex. For impulsively-started incompressible flow past a parabola, a generic behavior is found to occur over a range of angles of attack, and a limit solution corresponding to relatively large angles is found. The separation in the leading-edge region develops in a zone of relatively limited streamwise extent over a wide range of angles of attack. This suggests that localized control measures (such as suction) may possibly be effective at inhibiting separation.
Two-point velocity and vorticity correlations for axisymmetric turbulence
Authors: Zhu Y. , Antonia R. A. , Kim J.
Published: July 2002

Direct numerical simulation data for two-point velocity and vorticity correlations at small separations near the centerline of a fully developed turbulent channel flow are more closely approximated by axisymmetry than isotropy.
Viscous damping and instabilities in stratified liquid film flowing down a slightly inclined plane
Authors: Kliakhandler Igor L. , Sivashinsky Gregory I.
Published: July 2002

A reduced nonlinear model for density stratified viscous film flowing down a slightly inclined wall is derived and explored. Under buoyancy stable stratification the system exhibits various long-wavelength instabilities of noninertial, purely kinetic origin. Unlike many existing models for the film interface evolution in the present study regularization of the pertinent long-scale dynamics is provided directly by the film viscosity rather than surface tension.
Conserved quantities in Stokes flow with free surfaces
Authors: Cummings L. J. , Howison S. D. , King J. R.
Published: July 2002

We derive an infinite set of invariants for two-dimensional Stokes flow with a free surface, driven by a point sink, in the case that surface tension effects are negligible. The complex variable methods used are closely related to those used for certain Hele–Shaw flows, which likewise have infinitely many conserved quantities.
Further experimental support for the Kolmogorov refined similarity hypothesis
Authors: Praskovsky Alexander , Praskovskaya Eleanor , Horst Thomas
Published: July 2002

The validity of the Kolmogorov refined similarity hypothesis (RSH) is tested experimentally in the mixing layer and in the return channel of a large wind tunnel, at Rλ≈2.0×103 and 3.2×103, respectively. The energy dissipation rate is estimated from the longitudinal gradient of both the longitudinal ∂u/∂x and transverse ∂w/∂x velocity components, and the results are in good agreement with RSH in both cases.
On longitudinal and lateral moment hierarchy in turbulence
Authors: Boratav Oluş N.
Published: July 2002

A quantity of interest, β (degree of intermittency between scale hierarchy), which appears in a recent intermittency model by She and Lévêque [Phys. Rev. Lett. 72, 336 (1994)] is computed from turbulent simulations with Taylor-Reynolds number in the order of 100. It is found that the lateral and longitudinal β values are unequal. For most of the moment hierarchy order p, lateral βdeviates more from K-41 (i.e., smaller) than the longitudinal. β is also found to be p dependent.
Direct numerical simulations of the Batchelor trailing vortex by a spectral method
Authors: Abid M. , Brachet M. E.
Published: July 2002

The nonlinear vorticity dynamics of the Batchelor trailing vortex is presented. Direct numerical simulations of the three-dimensional, incompressible, Navier-Stokes equations by a spectral method are used. It is shown that the initial vortex core is subjected to three transformations: a twisting phase, a lateral expansion of its cross section and formation of a spiral structure. These transformations are accompanied by a gradual deceleration of axial velocity. Mean kinetic energy is then transferred to radial velocity. When the transfer is maximum a secondary instability is initiated leading to the spiral structure. These transformations are in agreement with vortex breakdown observed in recent experiments.
Theory of attached and lifted diffusion flames
Authors: Wichman Indrek S. , Ramadan Bassem
Published: July 2002

Diffusion flame (DF) attachment and liftoff are examined, leading to (1) explanations of the origins of previous, successful empirical correlations; (2) the discovery of multiple lifting regimes. The latter includes a very slow flow regime, a slow-to-moderate flow regime, and a moderate-to-fast flow regime. Formulas for liftoff height (l̂g) and characteristic flame tip breadth (l̂r) are developed from a combination of the differential and integral form of the conservation equations. These formulas are compared with numerical solutions of the same equations.
Propagation of medium energy electrons (10–20 keV) in carbon dioxide
Authors: Farley David R. , Cattolica Robert J. , Clapp Leonard H.
Published: July 2002

Propagation of medium energy electrons (10–20 keV) in carbon dioxide is studied for application to using the Electron Beam Fluorescence (EBF) technique with CO2 as a rarefied gas dynamic diagnostic tool. A collimated (1 mm nominal diameter) electron beam at energies of 10, 15, and 20 keV was injected into a vacuum chamber containing ambient CO2 maintained at pressures of 50, 100, 200, 400, 600, and 800 mTorr. Through deconvolution of recorded two-dimensional images of the emitted fluorescence, the growth of the root-mean-square (rms) radius of the beam was obtained as a function of axial distance. An empirical form of the differential scattering cross section which includes the contributions of both elastic and inelastic collisions between fast electrons and CO2 molecules is used in single scattering, multiple scattering, and scattering envelope models developed for describing the electron distribution along the electron beam. Using these models, the rms radius of the electron beam is calculated and compared with experimental results obtained from imaging of the electron beam fluorescence. It is found that the multiple scattering and envelope models significantly overestimate experimental rms radius data. The single scattering model underestimates radius data at target thickness higher than ∼2 and 4 Torr cm for the 10 and 15 keV beams, respectively. The single scattering model matches well with the 20 keV beam data.
Radial displacement of a fluid annulus in a rotating Hele–Shaw cell
Authors: Carrillo Lluı́s , Soriano Jordi , Ortı́n Jordi
Published: July 2002

The radial displacement of a fluid annulus in a rotating circular Hele–Shaw cell has been investigated experimentally. It has been found that the flow depends sensitively on the wetting conditions at the outer interface. Displacements in a prewet cell are well described by Darcy’s law in a wide range of experimental parameters, with little influence of capillary effects. In a dry cell, however, a more careful analysis of the interface motion is required; the interplay between a gradual loss of fluid at the inner interface, and the dependence of capillary forces at the outer interface on interfacial velocity and dynamic contact angle, result in a constant velocity for the interfaces. The experimental results in this case correlate in the form of an empirical scaling relation between the capillary number Ca and a dimensionless group, related to the ratio of centrifugal to capillary forces, which spans about three orders of magnitude in both quantities. Finally, the relative thickness of the coating film left by the inner interface, αi, is obtained as a function of Ca.
Receptivity of three-dimensional boundary-layers to localized wall roughness and suction
Authors: Bertolotti F. P.
Published: July 2002

The receptivity of three-dimensional, incompressible, boundary-layer flows to localized roughness or suction is studied theoretically and numerically. Three-dimensional boundary-layer flows have a strong “nonparallel” component due to the curvature of the potential-flow streamlines. Our theoretical model extends the Fourier-transform methodology to nonparallel flows using a Taylor expansion of the laminar mean-flow at the location of the roughness. Both the near-field and the far-field solutions are contained in the model. Additionally, the use of MacLaurin expansions in the complex plane leads quickly to the receptivity factors of eigenmodes. The theoretical results are validated with solutions to the linearized Navier–Stokes equations efficiently obtained by replacing the actual wall forcing with a smoother, but equivalent, forcing. We find that the far-field response is proportional to both H̃(αe) and dH̃(αe)/dα, where H̃(α) is the Fourier transform of the forcing distribution, and αe is the eigenmode’s wave number. The receptivity coefficient for H̃(αe) decreases when nonparallelism is included in our models, while that for dH̃(αe)/dα increases.
Space and time behavior of backscattering instabilities in the modified decay regime
Authors: Mounaix Philippe , Pesme Denis
Published: July 2002

The space and time behavior of parametric backscattering instabilities is computed analytically in the so-called modified decay regime. The plasma is assumed to be homogeneous and of finite length. The propagation of the pump wave and its finite pulse duration both are taken into account, its depletion is ignored. The parametric growth is solved in terms of fluctuating initial and boundary conditions corresponding to thermal noise at equilibrium. Fluctuating source terms, representing spontaneous emission of waves, are accordingly retained in the coupled mode equations. The initial stage of the instability is investigated in detail; the time from which the time asymptotic concept of absolute or convective instability applies is computed. Approximate expressions for the fluctuations of the waves, that are uniformly valid for any gain factor and any time, are derived.
Nonlinear electrostatic turbulence
Authors: Garbet X. , Laurent L. , Samain A.
Published: July 2002

A variational method expressing the self-consistency of an electrostatic turbulence in a confined plasma is presented. It properly takes into account all types of resonances if the Kolmogorov broadening due the diffusion is small compared to the spectral width of the turbulent electric potential. The method is applied to study the possibility of a new type of drift turbulence in tokamaks where the various modes are strongly localized by nonlinear effects around their resonant surface. The passing electrons play a major role in driving such a turbulence, a result compatible with the observed increase of transport at the edge.
Comments on finite Larmor radius models for ion cyclotron range of frequencies heating in tokamaks
Authors: Phillips C. K. , Wilson J. R. , Hosea J. C. , Majeski R. , Smithe D. N.
Published: July 2002

The accuracy of standard finite Larmor radius (FLR) models for wave propagation in the ion cyclotron range of frequencies (ICRF) is compared against full hot plasma models. For multiple ion species plasmas, the FLR model is shown to predict the presence of a spurious second harmonic ion–ion type resonance between the second harmonic cyclotron layers of two ion species. It is shown explicitly here that the spurious resonance is an artifact of the FLR models and that no absorption occurs in the plasma as a result of this ‘‘resonance.’’
Current diffusion and toroidal electric field response to a non-Ohmic current drive
Authors: Litwin C.
Published: July 2002

Resistive current diffusion and inductive electric field evolution in response to an auxiliary current drive are analyzed for conditions typical for tokamak operation. Special emphasis is placed on a localized current drive. Relaxation time scales for a broad range of conductivity profiles are calculated, using the Wentzel–Kramer–Brillouin (WKB) approximation, and compared with numerical solutions. Validity of the effective circuit model is discussed.
Theory for a direct magnetic-field measurement of microturbulence enhanced electron collisionality
Authors: Mayo R. M. , Gerwin R. A. , Black D. C.
Published: July 2002

A theoretical framework is established for the description of anchored field-line distortion by a flowing resistive magnetofluid. The Hall effect is included and significantly influences field-line deformation in two dimensions when the electron magnetization parameter, ωce/νe≳1. Moreover, the formalism described herein provides for a direct and local magnetic-field determination of the effective electron collisionality, which may then be gauged against microturbulence descriptions of enhanced transport.
Comment on ‘‘Investigations of diffusional effects in applied-B ion diodes’’ [Phys. Fluids B 5, 4408 (1993)]
Authors: Krall Nicholas A. , Rosenthal Stephen E.
Published: July 2002

Turbulent diffusion modeling for the applied-B diode is considered. It is proposed that the simulation can be improved by considering the instability structure. (AIP)
Response to ‘‘Comment on ‘Investigations of diffusional effects in applied-B ion diodes’ ’’ [Phys. Fluids B 5, 4408 (1993)]
Authors: Schuldt R.
Published: July 2002

The turbulent diffusion simulation for the applied-B diode is discussed. For modeling and instability in this device, the dispersion relation and the field profiles of the instability were used. (AIP)
Dynamics of second-order phase transitions in resistive pressure-gradient-driven turbulence
Authors: Carreras B. A. , Lynch V. E. , Garcia L. , Diamond P. H.
Published: July 2002

The phase transition model for the transition from a low-confinement to a high-confinement regime is based on general properties of the coupling between turbulence and averaged sheared flows. To extract these generic properties several turbulence models have been investigated. In particular, the structure of the turbulent Reynolds stress tensor is crucial to the phase transition model. For a broad range in parameters of the resistive pressure-gradient-driven turbulence, we find a strong competition in the Reynolds stress tensor between the dynamo instability term and the turbulent viscosity. This result is contrary to expectations based on simplified local models. This competition allows the transition to take place only near marginal stability. The particle flux is calculated, and its reduction at the transition is caused not only by fluctuation suppression but also by the change in the relative phase of the pressure and potential fluctuations. The results obtained with this turbulence model are presented here, with special emphasis on the structure of the Reynolds stress and the existence of limit cycle solutions, which may be related to the observed phenomenon of dithering.
Magnetic helicity in magnetohydrodynamic turbulence with a mean magnetic field
Authors: Stribling Troy , Matthaeus William H. , Oughton Sean
Published: July 2002

A computational investigation of magnetic helicity of the fluctuating magnetic field Hm in ideal and freely decaying three-dimensional (3-D) magnetohydrodynamics (MHD) in the presence of a uniform mean magnetic field is performed. It is shown that for ideal 3-D MHD Hm, which is a rugged invariant in the absence of a mean magnetic field [Frisch et al., J. Fluid Mech. 77, 796 (1975)], decays from its initial value and proceeds to oscillate about zero. The decay of Hm is shown to result from the presence of a new ‘‘generalized’’ helicity invariant, which includes contributions from the uniform magnetic field. The loss of invariance of Hm will diminish the effects of inverse transfer of Hm on freely decaying turbulence. This is demonstrated in a discussion of the selective decay relaxation process.
Inhomogeneous cyclotron emission source for the five branch mode conversion problem
Authors: Ng C. S. , Shvets V. F. , Swanson D. G.
Published: July 2002

Recent results on determining the spatial profile of the emission source distribution from a cyclotron mode conversion layer are extended to the five branch wave coupling case which occurs when the X-mode is coupled to the O-mode. A variational problem for an inhomogeneous source is formulated and solved. Computations demonstrate that the basic features, including a highly localized emitter, are generally similar to those of the three branch mode conversion problem.
Optimization of multistage harmonic gyrodevices
Authors: Nusinovich G. S. , Levush B. , Dumbrajs O.
Published: July 2002

In low voltage gyrodevices operating at high cyclotron harmonics the electrons are weakly coupled to the synchronous harmonic of an electromagnetic field. To realize efficient operation of a device it is necessary to compensate for this weak coupling by increasing the resonator Q-factor. When the required Q-factor is on the order of the Ohmic Q-factor, the optimization of gyrotron parameters should be modified by taking into account Ohmic losses of the microwave power. In the present paper this modification has been done and the optimization of two- and three-cavity gyrodevices operating at cyclotron harmonics has been considered.
Anomalous pinch effect and energy exchange in tokamaks
Authors: Isichenko M. B. , Gruzinov A. V. , Diamond P. H. , Yushmanov P. N.
Published: July 2002

It is shown that, under very generic assumptions of the nature of plasma turbulence in an inhomogeneous magnetic field, the anomalous particle and energy fluxes, in addition to usual diffusive terms, involve purely convective terms not directly associated with density or temperature gradients. The anomalous convective transport results from the conservation, on turbulent time scale, of adiabatic invariants of particle motion in the inhomogeneous magnetic field and the Liouville-theorem constraint on the microscopic dynamics, thus furnishing the mechanism of the anomalous pinch effect in tokamaks. This theory also predicts an electron-turbulence energy exchange, which can be interpreted as a turbulent enhancement of the electron-ion energy exchange. Collisions introduce important modifications to the turbulent mechanisms of the pinch effect. It is argued that the nondiffusive effects are intrinsic to tokamak transport and should be included in power-balance analyses.
Rotation of weakly collisional plasmas in tokamaks, operated with Alfvén waves
Authors: Tsypin V. S. , Elfimov A. G. , de Azevedo C. A. , de Assis A. S.
Published: July 2002

The effect of the kinetic Alfvén waves on weakly collisional plasma rotation in tokamaks has been studied for the plateau and banana regimes. The quasistationary rotation velocities and radial electric field have been found. The estimation of these quantities for the Phaedrus-T tokamak [S. Wukitch et al., Phys. Rev. Lett. 77, 294 (1996)] and for the Joint European Torus (JET) [A. Fasoli et al., Nucl. Fusion, 36, 258 (1996)] has been presented. It is shown that the kinetic Alfvén waves, which are needed for current drive, change weakly the quasistationary rotation velocities and radial electric field, as found from the experimental data of these tokamaks. In conditions with increased rf power, the plasma rotation and radial electric field can essentially grow up.