摘要:

Recent experiments by Wolf have demonstrated the possibility of dynamically stabilizing the Rayleigh‐Taylor instability of the interface between two heavy fluids. This problem is considered and it is shown quite generally that both viscosity and interfacial tension are necessary to stabilize the short wavelength perturbations. As an example the experimental results obtained with a liquid‐air interface are reproduced.

ISSN:0031-9171    

DOI:10.1063/1.1693294

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The structure of turbulent line vortices is examined. A general argument is constructed to show that the vortex must develop an overshoot of circulation if it entrains fluid at a rate greater than that due to molecular diffusion. A weak hypothesis on the distribution of Reynolds stress leads to the logarithmic profile of Hoffman and Joubert and an estimate of the maximum Reynolds stress. The results of a turbulence model due to Saffman are presented and shown to be poor.

ISSN:0031-9171    

DOI:10.1063/1.1693295

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

A linear stability analysis was performed of a horizontal layer of the reactive fluid mixture(A2, A)which is undergoing both a simple surface catalyzed dissociation and plane Couette flow. When only transverse disturbances were considered, it was found that the instabilities for this system are similar to those discussed previously. By including the longitudinal disturbances, it was found that the shear has a stabilizing influence on the overstableS>0instability.

ISSN:0031-9171    

DOI:10.1063/1.1693296

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Two apparently disparate lines of inquiry in kinetic theory are shown to be equivalent: (1) Brownian motion as treated by the (stochastic) Langevin equation and Fokker‐Planck equation; and (2) Boltzmann's equation. The method is to derive the kinetic equation for Brownian motionfromthe Boltzmann equation for a two‐component neutral gas by a simultaneous expansion in the density and mass ratios.

ISSN:0031-9171    

DOI:10.1063/1.1693297

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

An extension of the usual treatment of Brownian motion is given. The motion of a body in a fluid is considered to be described by a quite general form of retarding force. To describe the expected fluctuations, correlation functions are obtained from a fluctuation‐dissipation theorem. Under very modest conditions on the retarding force the “equipartition theorem” is obtained. Frequently, such correlation functions are obtained by postulating certain fluctuating forces. Here, no such assumption is made. However, it is possible to exhibit those fluctuating forces which would give our correlation functions. Some examples are discussed. Finally, the results for a realistic retarding force are obtained.

ISSN:0031-9171    

DOI:10.1063/1.1693298

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The dynamic behavior of magnetically polarizable fluids (ferrofluids) is considered in two instances differing in the dependence of the fluid magnetization on temperature. When the magnetization can be regarded as temperature independent, the influence of the magnetic field on the force distribution acting on a submerged body is demonstrated, the flow pattern of the associated ordinary fluid dynamics problem remaining unaffected. In the other instance, when the temperature dependence of the magnetization must be considered, the question of thermal stability of the ferrofluid is considered and the appropriate magnetic Rayleigh number and its influence are determined. Furthermore, it is shown that small disturbances in a ferrofluid are quickly damped so that vorticity generated by the thermomagnetic interaction for such disturbances does not propagate.

ISSN:0031-9171    

DOI:10.1063/1.1693299

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

An electrically driven shock wave is launched axially down a tube filled withTe = 1−2‐eVplasma, carrying an axial current and associated azimuthal magnetic field (250 G). The number density is between1016and1017 cm−3and the shock Mach numbers range from 1.2 to 6.5. Flux compression of 10‐25% at the front is observed, but this amount is very much smaller than ideal theory predicts and its duration is briefer. The flux increase is closely followed by diamagnetic discharge plasma producing an abrupt flux decrease. Mixing with this plasma and boundary‐layer flow to the walls of the shock tube account for the small and brief field compression observed. Rough calculations support this picture and include an effective fractionq1of the flow not undergoing compression. Thisq1is found to be as high as 50%.

ISSN:0031-9171    

DOI:10.1063/1.1693300

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The propagation of a shock wave, originating in a stellar interior, is considered when it approaches the surface of the star and assumes a self‐similar character, “forgetting” its initial conditions. The flow behind the shock is assumed to be spatially isothermal rather than adiabatic to simulate the conditions of large radiative transfer near the stellar surface. The adiabatic and isothermal flows behind such a shock are compared. The exact shock‐propagation laws, obtained by solving the equations in similarity variables, for different values of the parameter &dgr; in the undisturbed density law,&rgr;0∞&dgr;, and&ggr;, the ratio of specific heats, are compared with the approximate values calculated by Whitham's characteristic rule and the two show a generally good agreement.

ISSN:0031-9171    

DOI:10.1063/1.1693301

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

A numerical model is presented which describes the evolution with time of a short segment of a spark channel in air and its associated acoustic wave. The model assumes a straight, cylindrical conducting column in which local thermodynamic equilibrium exists at every point. The electrical energy input to the column is determined by a prescribed electrical current waveform, coupled with a computation of the plasma conductivity. The evolution with time of the conducting column and its surrounding flow field is then found by numerical integration of the equations of gas dynamics. The model employs a realistic equation of state for air at high temperatures, and incorporates kinetic and radiative energy transport processes. It is shown that a satisfactory description of the properties of a spark channel cannot be achieved when radiative transport processes are neglected. The model agrees well with experimental measurements of spark channel radii, temperatures, pressures, and electron densities, and predicts the resultant shock wave strengths closely. The voltage gradients along the spark channel predicted by the model, and the total energy input to the channel, are not as uniformly in agreement with experiment. Possible reasons for these discrepancies are discussed.

ISSN:0031-9171    

DOI:10.1063/1.1693302

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The numerical model of a spark discharge in air, described in the preceding paper [Phys. Fluids14, 2111 (1971)], is used to compute the properties of the discharge channel of a lightning return stroke and its acoustic wave. Initial conditions and electric current waveforms were varied over a wide range, to correspond with the observed variability of natural lightning events. Computed temperatures, pressures, and electron densities in the discharge channel agree well with available observational data. The model predicts temperatures near the upper limit of the range of spectroscopically determined temperatures. Discharge channel radii agree well with similarity solutions for the growth of a spark channel. The energy dissipated per unit length of the discharge channel and the strength of the pressure wave lie well below most literature estimates. The computed energy inputs fall between 10 and 60 J/cm, and depend primarily on the maximum values of the discharge current.

ISSN:0031-9171    

DOI:10.1063/1.1693303

出版商:AIP    

年代:1971

数据来源: AIP