摘要:

A subgrid model in the large‐eddy simulation of turbulent flows is derived from the statistical viewpoint. The subgrid‐scale (SGS) Reynolds stress playing a central role in the derivation is estimated from a shear‐turbulence theory which is based on the direct‐interaction formalism incorporated with the separation of scales of mean and fluctuating fields. A resulting subgrid model is written in terms of the grid‐scale (GS) velocity, the GS pressure, and the SGS turbulent energy. The Smagorinsky model used frequently in the large‐eddy simulation is derived under the special situation that production and dissipation terms are dominant in the transport equation for the SGS turbulent energy.

ISSN:0031-9171    

DOI:10.1063/1.863940

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The buoyancy effects on the turbulence structure in unstably‐stratified shear flow generated by cooling the top of the layer in an open channel were investigated. Measurements were made of the characteristics of the turbulent fluctuations; turbulence intensities, correlation coefficients, joint probability density functions, and coherence‐phase relationships. The results show that the buoyancy‐driven motions cause substantial changes in the turbulence structure. Of the buoyancy‐driven motions, the intermittent downward motion contributes mainly to the turbulent transport of heat in the vertical direction. A parameter which characterizes the stability dependence of the turbulence structure is the local gradient Richardson number Ri, and the turbulence quantities are well correlated with Ri. A theoretical spectral equation model is applied to the stratified flow and well compared with the experimental results.

ISSN:0031-9171    

DOI:10.1063/1.863941

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Liouville’s equation for randomly forced two‐dimensional flow with Rayleigh friction is examined. An exact nonstationary solution is presented for a special form of the forcing and zero energy initial condition. This solution is such that the fluctuation‐dissipation relation is valid at all times.

ISSN:0031-9171    

DOI:10.1063/1.863942

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The problem of the motion of a single spherical particle in a rarefied gas is considered, and results for the velocity profiles and the drag forces for all rarefactions are obtained using the Bhatanagar–Gross–Krook model and diffuse reflection at the surface of the sphere. Results for the drag forces are in good agreement with previously available results of Cercignani, Pagani, and Bassanini. The velocity profile results are also in good agreement with the results of Sone and Aoki who had considered the slip limit.

ISSN:0031-9171    

DOI:10.1063/1.863943

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The propagation of internal Alfve´n‐inertio‐acoustic‐gravity waves in a rotating, stratified, inviscid, perfectly conducting nonisothermal atmosphere in the presence of a nonuniform magnetic field is investigated near the critical levels using the group velocity method based on the WKB approximation. It was found that these waves (approaching the critical level from one side) are neither transmitted nor reflected, but are completely captured in the neighborhood of the critical levels and are constrained thereafter to propagate in the horizontal directions. The same wave, if it approaches the critical level from the other side, is transmitted across the critical level without any absorption or reflection, thus establishing the valve effect, as in the case of Rudraiah and Venkatachalappa, even in the absence of the components of rotation in the plane normal to the direction in which the medium varies.

ISSN:0031-9171    

DOI:10.1063/1.863944

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Phase trapping of ’’cold’’ electrons in a finite amplitude whistler wave is possible.

ISSN:0031-9171    

DOI:10.1063/1.863945

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Steady‐state plasma turbulence and the formation of negative potential spikes and double layers in the presence of ion acoustic instabilities have been studied by means of one‐dimensional particle simulations in which the velocities of a small fraction of electrons are replaced by the initial drifting Maxwellian at a constant rate. A steady state is found where negative potential spikes appear randomly in space and time giving rise to an anomalous resistivity much greater than previously found. Comparisons of the simulation results with laboratory and space plasmas are discussed.

ISSN:0031-9171    

DOI:10.1063/1.863946

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

In a tokamak, the electron distribution deviates from a Maxwellian because magnetically untrapped electrons run away parallel to the applied electric field. A new method for calculating the dynamics of the electron distribution is presented. This method is novel because it can treat systems with comparable numbers of trapped and untrapped electrons, and an electric field comparable with (but smaller than) the Dreicer field. The electron distribution function and the plasma resistivity are presented for representative values.

ISSN:0031-9171    

DOI:10.1063/1.863947

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

A model of magnetic configurations that has both shear and variation of the field curvature along the magnetic field is constructed. The stability analysis of this system is performed by expanding the perturbation function in &fgr;0which is constant along the field and &fgr;1which varies sinusoidally along the field line. The shear term is retained in the form of differential operations in &xgr;, the coordinate parallel to the pressure gradient. After several simplifications, the equation reduces to a fourth‐order differential equation. The eigenvalue of this equation is obtained numerically. The calculated critical &bgr; plotted versus the well depth parameter,h, makes a smooth transition betweeenh<0 (maximum averageB) andh≳0 (minimum averageB). Using the same technique, the nondivergent solution to the localized shear mode (Suydam mode) is also obtained by retaining the inertia term.

ISSN:0031-9171    

DOI:10.1063/1.863948

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The numerical solution of a high‐nelectrostatic ballooning mode is obtained. Correct forms of electron and ion responses to the wave are retained. Instabilities which arise because of toroidal effects are found. Dependence on the aspect ratio, magnetic shear, wavelength, electron–ion–temperature ratio, and the electron temperature gradient is also studied. The growth rate &ggr; is large and satisfies the relation &ohgr;<&ggr;∼&ohgr;*. Depending on the parameters, the real frequency &ohgr; changes sign. The drift mode, which has been found to be stable in a slab plasma, persists in the toroidal plasma, almost always remaining stable. The density fluctuations in tokamaks, which have been observed when the microwave scattering method is used, can be qualitatively explained by the theory of the electrostatic ballooning mode.

ISSN:0031-9171    

DOI:10.1063/1.863949

出版商:AIP    

年代:1982

数据来源: AIP