摘要:

To study the nonlinear physics of incompressible turbulent flow, the unaveraged Navier–Stokes equations are solved numerically. Initial three‐dimensional cosine velocity fluctuations and periodic boundary conditions are used. No mean gradients are present. The three components of the mean‐square velocity fluctuations are equal for the initial conditions chosen. The resulting solution shows characteristics of turbulence, such as the nonlinear excitation of small‐scale fluctuations. For the higher Reynolds numbers the initially nonrandom flow develops into an apparently random turbulence. Thus, randomness or turbulence can apparently arise as a consequence of the structure of the Navier–Stokes equations.

ISSN:0031-9171    

DOI:10.1063/1.863580

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The statistics of the time evolution of a nonlinearly coupled system of first‐order equations representing the Euler equations is studied. The probability distribution of functions is nearly Gaussian, while that of their time derivatives has exponential tails and moments of order 4, 6, and 8 that approach those of the exponential distributions.

ISSN:0031-9171    

DOI:10.1063/1.863581

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A closure representation is developed for the velocity/pressure‐gradient correlation arising in the Reynolds stress equation through the interaction of the turbulent velocity field with mean shear flow. The development has the advantage of putting the closure on a rational footing. The resulting expression is used in a simple example where nearly steady, homogeneous turbulence is present in a rectilinear mean shear flow. Reynolds stresses and various aspects of the behavior of the velocity/pressure‐gradient correlation seem to be predicted correctly.

ISSN:0031-9171    

DOI:10.1063/1.863582

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Benjamin and Davis and Acrivos have established the existence of an algebraic solitary wave in deep fluids. This theory is here extended to second order in amplitude. The second‐order correction to the wave amplitude, the second‐order correction to the wave phase speed, and the first‐order correction to the wavelength are all determined for solitary waves propagating in a stratified shear flow. Two special cases are discussed in detail.

ISSN:0031-9171    

DOI:10.1063/1.863583

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The evolution of packets of water waves described by the long‐wave limit of the Davey–Stewartson equations is considered. The stationary one‐dimensional kink solutions are analyzed with respect to transverse perturbations. They are shown to be stable when surface tension can be neglected. To demonstrate this, the two‐dimensional inverse scattering transform found by Anker and Freeman is applied. The present stability analysis supplements previous investigations in the long‐wavelength limit of water waves where finite depth effects have been ignored.

ISSN:0031-9171    

DOI:10.1063/1.863584

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The presence of convective cells in a purely poloidal field Levitated Octupole has been associated with diffusion that scales asDv∝(T*/n)1/2, independent ofB, whereT* is an ’’effective temperature,’’T*∝T. The electric field spectrum of the convective cells can be used to estimate the magnitude ofT* andDv. The results are in reasonable agreement with previous measurements of cross‐field transport, and agree qualitatively with theortical models of vortex diffusion.

ISSN:0031-9171    

DOI:10.1063/1.863585

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

When the amplitude of a coherent lower hybrid wave exceeds a certain stochasticity threshold, the ion motion becomes ergodic. The problem of determining the resulting diffusion coefficient is considered. For large amplitude waves it is found that the diffusion coefficient oscillates with decreasing amplitude about the quasi‐linear value as the wave amplitude is increased. Furthermore, the diffusion coefficient is shown to be subject to narrow resonances at particular values of the wave amplitude.

ISSN:0031-9171    

DOI:10.1063/1.863586

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A theory of collisionless electrostatic drift wave turbulence in a circular cylinder with shear is presented. A renormalized electron drift kinetic equation which is consistent with conservation of energy is derived. For low levels of turbulence, a perturbative solution indicates that the turbulence has a stabilizing effect and that total mode spectrum energy decreases. For strongly turbulent regimes, results quantitatively but not qualitatively different from those of Hirshman and Molvig are found. In particular, additional stabilizing terms lead to a lower saturation amplitude, but the basic picture of turbulent destabilization competing against linear and nonlinear shear damping persists.

ISSN:0031-9171    

DOI:10.1063/1.863587

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A physical description is given of the wave‐particle resonance which can occur when particles undergoing a ∇ Bdrift are in phase with a wave propagating across the magnetic field (i.e., &ohgr; ∼ k⋅V∇ B, where V∇ Bis the ∇ Bdrift velocity). Specifically, the physics of the energy exchange mechanism is discussed and a general equation for the wave damping/growth rate is derived based upon physical arguments. The theory is applied to the lower‐hybrid‐drift instability and collisionless trapped electron instability.

ISSN:0031-9171    

DOI:10.1063/1.863588

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A variational theory for Ohm’s law, valid for arbitrary collision frequency, is presented for a slab geometry using a pitch angle scattering collision operator. Shear and temperature gradient effects are included but toroidicity is neglected. The variational conductivity is used to examine the effects of shear and electron temperature gradients on tearing mode instabilities. It is shown that shear has a strong stabilizing effect on the so‐called ’’collisional’’ and ’’semicollisional’’ electron temperature gradient tearing mode instabilities. In addition, the stabilizing effect of shear on the classical resistive and inertial tearing modes is discussed.

ISSN:0031-9171    

DOI:10.1063/1.863576

出版商:AIP    

年代:1981

数据来源: AIP