摘要:

Various studies of thermal turbulence in thin shear flows are re‐examined in order to evaluate the ratio of the time scale of the scalar‐fluctuation field to that of the velocity‐fluctuation field. In all cases considered it is found that the ratio is nearly uniform with a value close to 0.5.

ISSN:0031-9171    

DOI:10.1063/1.862228

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The response of a nonlinear dumbbell with varying frictional coefficient in a pseudo‐turbulent flow field is examined. It is shown that in such flows, the dumbbells will be able to maintain their highly dissipative extended configurations even though the mean flow is weak.

ISSN:0031-9171    

DOI:10.1063/1.862229

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

A one‐dimensional electron fluid model is investigated using the mathematical methods of modern fluid turbulence theory. Nondissipative equilibrium canonical distributions are determined in a phase space whose coordinates are the real and imaginary parts of the Fourier coefficients for the field variables. Spectral densities are calculated, yielding a wavenumber electric field energy spectrum proportional tok−2for large wavenumbers. The equations of motion are numerically integrated and the resulting spectra are found to compare well with the theoretical predictions.

ISSN:0031-9171    

DOI:10.1063/1.862230

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

A predominantly numerical study, using the direct simulation Monte Carlo method, of the flow from outgassing surfaces with plane, cylindrical, or spherical geometry is presented. The numerical results for the plane flow case are compared with existing theoretical and experimental results, and also with a new formulation of the jump conditions across the Knudsen layer between the surface and the continuum outflow. This outflow is sonic for the free expansion case, but subsonic in the presence of back‐pressure effects. The self‐scattered return flux is 19% of the outgassed flux in the sonic outflow case and increases at lower Mach numbers. Thermal accomodation and reflection of this return flux produces an outgassing cooling effect. The cylindrical and spherical geometries introduce an additional Knudsen number defined by the ratio of the mean free path in the effusing gas to the radius. The plane flow results apply when this Knudsen number is less than 0.001 and, when it is of order unity or above, the return flux is inversely proportional to it.

ISSN:0031-9171    

DOI:10.1063/1.862231

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Using a simplified equation of state, an analytic solution for the temperature, pressure, velocity, and acceleration of two semi‐infinite liquids initially at two distinct temperatures is obtained by ignoring the effects of the generated pressure on the temperature distribution. It is found that the pressure wave generated in the cooler medium by the temperature discontinuity develops into a shock after passing through the thermal boundary layer. The cooler material is initially accelerated into the hotter material and thus if the cooler material is lighter, the interface is Raleigh–Taylor unstable. The pressures and velocities at the interface relax with a characteristic time &tgr;=&agr;/c2, where &agr; is the thermal diffusivity andcis the speed of sound. For the particular case of liquid Na in contact with liquid UO2specific results are given. The linearized approximation is compared with exact numerical integration of the full hydrodynamic equations and reasonable agreement is found.

ISSN:0031-9171    

DOI:10.1063/1.862232

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The kinetic equation for particles interacting with turbulent fluctuations is derived by a new nonlinear technique which successfully corrects the difficulties associated with quasi‐linear theory. In this new method the effects of the fluctuations are evaluated along particle orbits which themselves include the effects of a statistically averaged subset of the possible configurations of the turbulence. The new method is illustrated by calculating the pitch angle diffusion coefficientD&mgr;&mgr;for particles interacting with ’’slab model’’ magnetic turbulence, i.e., magnetic fluctuations linearly polarized transverse to a mean magnetic field ⟨B⟩. Results are compared with those of quasi‐linear theory and also with those of Monte Carlo calculations reported in a companion paper. The major effect of the nonlinear treatment in this illustration is the determination ofD&mgr;&mgr;in the vicinity of 90° pitch angles where quasi‐linear theory breaks down. The spatial diffusion coefficient &kgr;∥parallel to ⟨B⟩ is evaluated usingD&mgr;&mgr;as calculated by our technique. It is argued that the partially averaged field method is not limited to small amplitude fluctuating fields and hence is not a perturbation theory.

ISSN:0031-9171    

DOI:10.1063/1.862233

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Monte Carlo simulation experiments have been performed in order to study the velocity diffusion of charged particles in a static turbulent magnetic field. By following orbits of particles moving in a large ensemble of random magnetic field realizations with suitably chosen statistical properties, a pitch‐angle diffusion coefficient is derived. Results are presented for a variety of particle rigidities and rms random field strengths and compared with the predictions of standard quasi‐linear theory and the nonlinear partially averaged field theory.

ISSN:0031-9171    

DOI:10.1063/1.862234

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

A Korteweg–de Vries equation appropriate to radially symmetric, focusing waves is attacked by assuming a large amplitude, small width solution. The results are compared with recent numerical work.

ISSN:0031-9171    

DOI:10.1063/1.862235

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Asymptotic solutions of nonlinear wave propagation in media with gentle, but otherwise arbitrary density inhomogeneities, are discussed. In the adiabatic approximation, a nonlinear Schro¨dinger equation is derived for media with linear, but time‐dependent density gradients. The resulting extra term can be eliminated by going to an accelerated reference frame, which makes this modified nonlinear Schro¨dinger equation completely integrable by the inverse scattering method. Multi‐solitons are therefore still the asymptotic states of waves propagating in media with arbitrary inhomogeneities.

ISSN:0031-9171    

DOI:10.1063/1.862236

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Bernstein, Greene, and Kruskal showed how, given the electrostatic potential as a function of position, the entire distribution function for ions, and the distribution function for untrapped electrons in a one‐dimensional, two‐species Vlasov plasma, one could determine the distribution function for the trapped electrons as the solution of an Abel integral equation. This result is generalized by replacing the statements about the distribution functions with the specification of the velocity space distribution functions of both species at a single position in the plasma. Limiting cases are shown to correspond to the earlier treatment, and the applicability of the theory to experiment is discussed.

ISSN:0031-9171    

DOI:10.1063/1.862237

出版商:AIP    

年代:1978

数据来源: AIP