摘要:

The quantitative interpretation of the recent experiments on turbulent diffusivity in high‐Reynolds‐number Couette–Taylor flow by Tam and Swinney [Phys. Rev. A36, 1374 (1987)], is presented.

ISSN:0031-9171    

DOI:10.1063/1.866805

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A new heat flux formula valid in strong inhomogeneous laser‐produced plasmas has been derived from the Fokker–Planck equation. The nonlocal treatment of Lucianietal. [Phys. Rev. Lett.51, 1664 (1983); Phys. Fluids28, 835 (1985)] is improved by taking into account the electric potential effect. A simple and reliable phenomenological formula is proposed, which is in good agreement with numerical Fokker–Planck computations.

ISSN:0031-9171    

DOI:10.1063/1.866806

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

It is demonstrated that radio‐frequency (rf) waves applied below the ion cyclotron frequency (&ohgr;/&OHgr;i≊0.75) can stabilize a mirror plasma against the interchange instability. A set of phased antennas in the Phaedrus‐B tandem mirror central cell [Phys. Rev. Lett.59, 206 (1987)] is used to select the rf azimuthal mode number (m). When them=−1 mode is selected, the ponderomotive force from the left‐hand polarized wave fields is sufficient to stabilize the plasma. Measurements of the rf mode number and of the wave polarization indicate strong excitation ofm=−1 modes with much of the wave having a left‐hand polarization.

ISSN:0031-9171    

DOI:10.1063/1.866807

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

An explicit expression for the sedimentation velocity at low particle Reynolds number in a concentrated suspension is derived and evaluated for two different approximations to the hydrodynamic interactions: a strict pairwise additive approximation and a far‐field, or Rotne–Prager, approximation. It is shown that the simple Rotne–Prager approximation gives a very accurate prediction for the sedimentation velocity of random suspensions from the dilute limit all the way up to close packing. The pairwise additive approximation, however, fails completely, predicting an aphysical negative sedimentation velocity above a volume fraction &fgr;≊0.23. The explanation for these different behaviors is shown to be linked to the ‘‘effective medium’’ behavior of the suspensions. It is shown analytically and by Stokesian dynamics simulation that a suspension of neutrally buoyant particles may be modeled as a homogeneous fluid with an effective viscosity, but a sedimenting suspension cannot. As a result, the Rotne–Prager approximation actually captures the correct features of the many‐body interactions in sedimentation. An analytical expression for the sedimentation rate, which is in good agreement with experiment, is obtained using the Percus–Yevick hard‐sphere distribution function.

ISSN:0031-9171    

DOI:10.1063/1.866808

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Averaged equations describing the evolution of the orientation distribution in a dilute suspension of axisymmetric ‘‘tracer’’ particles flowing through a fixed bed of spheres or fibers are derived. The orientations of the freely suspended ‘‘tracer’’ particles change because of the stochastically fluctuating velocity field induced by the randomly distributed fixed‐bed particles. In the limit of small fixed‐bed volume fraction the orientation distribution satisfies a local orientational advective‐diffusion equation with an effective rotary diffusivity and ‘‘drift’’ velocity, which are functions of the orientation of the particle relative to the direction of the average flow. Solutions of this equation show that the tracer particles tend to align with their ‘‘thin side’’ toward the direction of the bulk flow and that the degree of this alignment depends on the aspect ratio of the particle.

ISSN:0031-9171    

DOI:10.1063/1.866809

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Optical and photographic surveys of microbubble boundary layers are presented. The results show that the outer edge of the bubble cloud diffuses away from the wall as the bubbles are swept downstream. The plate‐on‐bottom orientation contains a bubble‐free region near the wall that cannot be discerned for the plate‐on‐top configuration. Skin friction measurements made when the bubble‐free region extends toy+=200 show there is no longer anyCfreduction present suggesting bubbles are not effective when they are outside the near‐wall region of the boundary layer. Bubble sizes, which increase with airflow and distance from the injection section and decrease with free‐stream velocity, were measured to be between 150 and 1100 &mgr;m.

ISSN:0031-9171    

DOI:10.1063/1.866810

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A model of the interaction of small, well‐separated bubbles of one fluid propagating through another ‘‘resident’’ fluid in a Hele–Shaw cell is introduced and studied. In the model each bubble acts on the others by setting up a velocity field of the dipole type. A system of ordinary differential equations is developed for the bubble positions. The system is solved completely for the two‐bubble problem. The three‐bubble problem is addressed by numerical simulations. A set of self‐similar motions are also found analytically. The dynamics of rows of bubbles is investigated analytically and via numerical simulations. Although clearly an extreme idealization, the model appears to shed considerable light on what to expect in laboratory experiments.

ISSN:0031-9171    

DOI:10.1063/1.866811

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Numerical experiments were carried out on the pattern formation of bioconvection that is observed in cultures of motile aquatic microorganisms. New features were revealed on how the bioconvective system evolves and how the number of falling fingers is selected at each stage of evolution. At the onset of convection, the relevant dynamical regime is that of the Rayleigh–Taylor instability. Then, readjustment of the wavenumber occurs as the adjacent convection cells combine with each other. The trajectory in (total kinetic energy, total potential energy) space shows that evolution of the system proceeds in the direction of intensifying the downward advection of microorganisms and reducing the total potential energy of the system. Finally the system reaches a stationary state, where the aspect ratio of the convection cells resembles that of Be´nard–Rayleigh convection and is optimum for the efficient downward advection of microorganisms. Furthermore, it is demonstrated that trajectories of the two cases deviate from this major evolution. In the case where the diffusion time of the system is large, the system shows remarkable oscillation and repeats the Rayleigh–Taylor instability intermittently. In the case where the viscous effect is large, the system ceases to evolve before reaching the optimum mode.

ISSN:0031-9171    

DOI:10.1063/1.866812

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The primary instabilities and bicritical curves for flow between counter‐rotating cylinders have been computed numerically from the Navier–Stokes equations assuming axial periodicity. The computations provide values of the Reynolds numbers, wavenumbers, and wave speeds at the primary transition from Couette flow for radius ratios from 0.40–0.98. Particular attention has been focused on the bicritical curves that separate (as the magnitude of counter‐rotation is increased) the transitions from Couette flow to flows with different azimuthal wavenumbersmandm+1. This lays the foundation for further analysis of nonlinear mode interactions and pattern formation occurring along the bicritical curves and serves as a benchmark for experimental studies. Preliminary experimental measurements of transition Reynolds numbers and wave speeds presented here agree well with the computations from the mathematical model.

ISSN:0031-9171    

DOI:10.1063/1.866813

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Stability features are studied experimentally for the unstable three dimensional boundary layer flow on a swept‐back flat plate. A pressure gradient on the flat plate is induced by a displacement body. Infinite sweep conditions are approximated by means of contoured endplates. For the measurements, hot‐wire and surface hot‐film anemometry as well as flow visualization techniques are used. In addition to stationary waves, traveling waves are also traced. The cross‐flow Reynolds numbers for the first appearance of either instability mode are of approximately the same magnitude. Wavelength and the direction of stationary vortices, as well as the frequencies of the most amplified traveling waves, are measured for different Reynolds numbers. The data obtained by the measurements are compared with the results of linear stability theory. The location of the final transition on the swept flat plate has proved to be fairly well predicted by the empirical transition criterion of Coustols (The`se de Docteur Inge´nieur, Ecole Nationale Supe´rieur de l’Ae´ronautique et de l’Espace, Toulouse, 1983).

ISSN:0031-9171    

DOI:10.1063/1.866814

出版商:AIP    

年代:1988

数据来源: AIP