摘要:

A stochastic model based on a linearized theory is developed. It is assumed that the large scale structure of the turbulence is created randomly at the jet orifice and that its subsequent convection downstream may be described in terms of quantities predicted by a linearized inviscid stability theory for slowly diverging flows. The process of the generation of turbulence is modeled by the imposition of a stochastic initial condition for the corresponding stability problem. The application of this condition secures the homogeneity of mean values of the turbulent quantities with respect to the angular coordinate; the contribution of each spiral mode is then proportional to a certain spectral function, the latter being an unknown function within the framework of the present study. Since experiments have shown that the turbulence of jets is far from being adequately represented by white noise conditions, the spectral functions must be determined by fitting to experimental data. It is shown that the model is capable of predicting the experimental trends for circumferential and radial coherence functions fairly well. However, complications arise with respect to the axial coherence functions and an attempt is made to alleviate the difficulties by either a cutoff assumption or by a process of continuation of the inviscid quantities.

ISSN:0031-9171    

DOI:10.1063/1.863364

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The Ievlev multipoint probability density function closure proposal is applied at the two‐point level to the mass balance equation for a single stochastically distributed reactant undergoing molecular diffusion and nonlinear reaction. The solution is shown to satisfy the major realizability properties of a probability density function and to be unique. In the case of a linear reaction, it produces the exact solution for an initially multivariate normal distribution. By short‐time numerical integration of a second‐order reaction with diffusion, the solution is shown to represent molecular mixing more satisfactorily than do existing one‐point theories.

ISSN:0031-9171    

DOI:10.1063/1.863365

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A nonlinear analysis of the Rayleigh–Taylor stability of two thermally conducting fluids is presented. Mass transfer across the interface due to the evaporation and condensation process is explicitly taken into account. The method of multiple scale expansion is employed for the analysis. The stability criterion is expressible in terms of various competing parameters representing the equilibrium heat flux, latent heat of evaporation, vapor pressure, gravity, surface tension, densities of the fluids, and thermal properties of the fluids. It is found that when the heat flux is strong enough and when the heat transfer mechanism is efficient enough, the system can be stabilized in the regimes where it would be linearly unstable. The study has also shed light on the validity of the quasi‐equilibrium approximation employed in a previous nonlinear study of the stability problem. The implication for the phenomenon of boiling heat transfer is also discussed.

ISSN:0031-9171    

DOI:10.1063/1.863366

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A mixture of heat‐conducting viscous inert fluids is given the structure of material with hidden variables. As a consequence, a model for sediment transport is elaborated within the approximation of sediments as a dilute suspension. Such a model turns out to be hyperbolic and allows for exceptional transverse waves.

ISSN:0031-9171    

DOI:10.1063/1.863367

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

An elongational flow of dilute polymer solution in a falling jet was studied. Measurements were made of the stress and velocity distribution along the falling jet by means of a photographic technique. The results show that the relaxation time in dilute drag reducing fluids is much longer than those predicted in a simple shear flow, and suggest the possibility of using a very high value of the relaxation time in a wall turbulence model. Also, the increase in Trouton ratios determined for a simple elongational flow is possibly as high as 27 000.

ISSN:0031-9171    

DOI:10.1063/1.863368

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

In certain materials, shock waves of sufficient intensity spontaneously emit transverse rarefaction waves and are therefore unstable. A criterion is derived by calculating the relative amplitudes of reflected acoustic waves incident on the shock from behind. This criterion is found to delimit the conditions under which both acoustic amplification and acoustic emission can occur; it agrees with earlier results by Kontorovich.

ISSN:0031-9171    

DOI:10.1063/1.863369

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Explicit solutions of the equations describing the space–time interaction of three random phase wave packets are presented. The solutions describe the collision and decay of previously known shock‐like solutions. Stability of the solutions to a perturbation in initial conditions is also analyzed. For those solutions which are unstable, a closed form solution of the nonlinear evolution of the interaction is given.

ISSN:0031-9171    

DOI:10.1063/1.863370

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Sufficient conditions are derived for the stability of cylindrically symmetric plasma equilibria with steady flow when the magnetic field is unidirectional and the flow is tangential to the field. The stability criteria involve only the flow, sound, and Alfve´n speeds.

ISSN:0031-9171    

DOI:10.1063/1.863371

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A previous two‐dimensional electrostatic calculation for the spatial structure of drift and trapped‐electron modes is extended to include finite‐&bgr; effects. Specifically, the parallel perturbed vector potential, and the parallel Ampere’s law are added to the calculation. Illustrative results are presented. The effect of increasing &bgr; can be either stabilizing or destabilizing, depending on the parameter values chosen. For present tokamak &bgr; values, the finite‐&bgr; effects are small.

ISSN:0031-9171    

DOI:10.1063/1.863372

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

In the limit of weak collisions, a narrow boundary layer forms at the trapped/untrapped boundary in velocity space. When the electron collision frequency &ngr;eis small compared with the wave frequency &ohgr; times the inverse aspect ratio &egr;, a realistic collision operator must be employed to evaluate the response of the boundary layer electrons. The parallel current carried by the barely circulating boundary layer electrons results in tearing mode modifications which are evaluated from a slab model in the banana regime (&ngr;e<&egr;&ohgr;) by using a Lorentz model collision operator.

ISSN:0031-9171    

DOI:10.1063/1.863352

出版商:AIP    

年代:1981

数据来源: AIP