摘要:

The effect of counter‐rotating streamwise steady vortices on three‐dimensional Tollmien–Schlichting (TS) waves in a growing boundary layer on a concavely curved surface has been studied by using the method of multiple scales. The nonlinear evolution of the amplitudes with streamwise position has been obtained analytically. It is shown that the TS waves are amplified by the presence of streamwise vortices. The effect of combination resonance between TS waves and Goertler vortices is much more pronounced than the effect of parametric resonance between the TS waves and the periodic basic flow. The amplification rates increase with Goertler number and spanwise wavenumber.

ISSN:0031-9171    

DOI:10.1063/1.866298

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

An analytical study of the developed steady flow in saturated packed beds is presented with emphasis on the influence of the microstructure on the (ensemble) average momentum equation. The fluctuating interstitial velocity field is modeled by a spatial Ornstein–Uhlenbeck process and the functional form of the inertial (Forchheimer) drag term is correctly deduced. For Couette flow in porous media of constant porosity, the velocity profile is uniform except near the solid boundaries where boundary layers appear with thickness comparable to the bead diameter. It is proved that the random macroscopic variations of the porosity in statistically homogeneous packed beds enhance the average flow rate when the Forchheimer law holds.

ISSN:0031-9171    

DOI:10.1063/1.866299

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

It is shown that gas mixtures can be separated at relatively high differential flow rates by an enhanced diffusion technique involving the oscillation of gases within a capillary bundle in the presence of axial concentration gradients. Experimental data for the diffusional separation of both He–CO2and CO2–SF6mixtures into an O2carrier are presented.

ISSN:0031-9171    

DOI:10.1063/1.866300

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A modeled transport equation for the joint probability density function (pdf) of the velocities and a scalar has been solved numerically for four self‐similar turbulent free shear flows: the plane mixing layer, the plane wake, the plane jet, and the axisymmetric jet. In the pdf equation, convection is treated exactly but the effects of viscosity, molecular diffusion, and the fluctuating pressure gradient have to be modeled. Five different models are evaluated; four of these are based on the Langevin equation for the fluid particle velocities, and the fifth is a particle pair interaction model. In each case, a stochastic mixing model represents the effects of molecular diffusion, and conditional modeling is included to account for intermittency. The resulting modeled pdf equation is solved by a Monte Carlo method. Calculated spreading rates and profiles of the mean velocity and Reynolds stresses show generally good agreement with experimental data for three of the five velocity models. However, discrepancies between the calculated and measured intermittency factor profiles, in particular for the plane mixing layer, reveal weaknesses in the conditional modeling.

ISSN:0031-9171    

DOI:10.1063/1.866301

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The linear stability of a plane relativistic shock has been investigated. Restrictions are found on the equation of state in order for the shock to be stable. A comparison is made with the results obtained in the framework of (nonlinear) corrugation stability.

ISSN:0031-9171    

DOI:10.1063/1.866302

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A consistent hydrodynamic model is established to describe the evolution of thin disk galaxies. A previous result showing that the heat flow vanishes in the limit of small epicyclic motion is generalized and simplified, providing a closure relation for the hierarchy of fluid equations. Its systematic use at first allows the obtainment of a new invariant of the fluid motion, governing the evolution of the anisotropic pressure (stress) tensor. This might represent a step toward explicit adiabatic laws relating the pressure and the density of the stellar gas. The same closure relation is then used to derive a powerful variational principle, describing linearized perturbations of the system, which takes the pressure anisotropy fully into account. This principle is independent of the usual WKB (‘‘tightly wound’’) approximation, which is easily recovered in the asymptotic study of spiral waves and spiral modes.

ISSN:0031-9171    

DOI:10.1063/1.866509

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Electron motion in a single electrostatic wave in a sheared magnetic field is shown to become stochastic in the presence of a second wave at an amplitude well below that obtained from the overlapping pendulum resonance approximation. The enhanced stochasticity occurs for low parallel velocity electrons for which the parallel trapping motion fromeE∥/minteracts strongly with theE×Btrapping motion due to the presence of magnetic shear. The single‐wave particle motion is given by a two‐parameter family of one‐degree‐of‐freedom Hamiltonians which bifurcate from a pendulum phase space to a topology with three chains of elliptic and hyperbolic fixed points separated in radius about the mode‐rational surface. In the presence of a perturbing wave with a different helicity, electrons in the small parallel velocity regime become stochastic at an amplitude scaling as the fourth root of the wave potential. The results obtained for stochastic motion apply directly to the problem of the diffusion of electrons in drift waves.

ISSN:0031-9171    

DOI:10.1063/1.866520

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A generalized, deterministic Fokker–Planck equation with Eulerian coefficients is derived from the Vlasov equation without making use of the Markovian approximation. Such an equation is equivalent to a simpler (no time derivative) propagation equation for the average distribution  f¯ , and identical at orderE2with a previously derived non‐Markovian quasilinear equation. By expressing the perturbed distribution in terms of  f¯ , propagation of  f¯  in terms of all of its past history is obtained, with space and time correlations of the orbit perturbations.

ISSN:0031-9171    

DOI:10.1063/1.866303

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A boundary‐layer solution to a high magnetic Reynolds numberRperiodic dynamo model shows that: (1) flux expulsion forces the magnetic field into flux sheets; (2) the principal contribution to the &agr; effect arises from regions of flow stagnation along a flux sheet; and (3) the &agr; effect scales asR−1/2. Arguments for these effects persisting in turbulent dynamos are given.

ISSN:0031-9171    

DOI:10.1063/1.866304

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The effects of electron Rutherford scattering on the Weibel instability are investigated using the implicit plasma simulation codevenus[J. Comput. Phys.46, 271 (1982);63, 434 (1986)]. Collisions decrease the Weibel growth rate below the collisionless value, in agreement initially with simple linear theory. Likewise, collisions decrease the saturation level, in agreement with a magnetic trapping mechanism. When the collision rate exceeds the collisionless Weibel growth rate, the instability is suppressed altogether. The results provide benchmarks for analysis of proposed short‐wavelength laser‐fusion schemes.

ISSN:0031-9171    

DOI:10.1063/1.866305

出版商:AIP    

年代:1987

数据来源: AIP