摘要:

The fluctuating wall‐shear stress was measured with various types of hot‐wire and hot‐film sensors in turbulent boundary‐layer and channel flows. The rms level of the streamwise wall‐shear stress fluctuations was found to be 40% of the mean value, which was substantiated by measurements of the streamwise velocity fluctuations in the viscous sublayer. Heat transfer to the fluid via the probe substrate was found to give significant differences between the static and dynamic response for standard flush‐mounted hot‐film probes with air or oil as the flow medium, whereas measurements in water were shown to be essentially unaffected by this problem.

ISSN:0031-9171    

DOI:10.1063/1.866783

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The direct interaction approximation (DIA) is used to treat turbulence in a compressible medium with constant mean gradients. The set of coupled nonlinear integrodifferential equations is derived that is satisfied by the transverse and longitudinal energy spectral functions,QTandQL, and by the transverse and longitudinal response functions,GTandGL. Finally, expressions for the average of the product of pairs of physically relevant fluctuating quantities (velocity, temperature, density) are derived in terms ofQTandQL. The numerical solution of this set of equations will be presented in part II of this work.

ISSN:0031-9171    

DOI:10.1063/1.866784

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The thermophoretic forces acting on a Small Knudsen particle in the neighborhood of a boundary have been investigated. The applied temperature gradient is constant, but it is not normal to the wall, thereby leading to thermophoretic forces both normal to and parallel with the wall. Using the velocity distribution of the gas atoms for this problem it has been possible to obtain the variation of the thermophoretic force as a function of distance from the boundary. It is noted, that for equal temperature gradients, the force is greater in the direction normal to the wall than along it. In addition, it is observed that the velocity dependence of the mean free path has a significant effect on the force in the neighborhood of the wall. In contrast to the normal force, which is in the direction of decreasing temperature, the mass flow induced by thermal creep along the wall leads to a parallel wall force that moves the particle in the direction of increasing temperature. When these two forces are compounded they indicate that particles can move in curved paths en route to the wall surface. As a by‐product of the calculation, an exact expression for the thermal creep velocity as a function of distance from the wall for the case of constant collision cross‐section is presented.

ISSN:0031-9171    

DOI:10.1063/1.866785

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A theory is developed for compressible rotational flow that is based on the substitution principle. The theory encompasses a formula for the transformation of the vorticity under the principle. Solutions are found for four rotational flows. These are the rotational counterparts of a parallel flow, flow behind a planar oblique shock wave, Prandtl–Meyer flow, and Taylor–Maccoll flow. A surprising variation of the vorticity is found for the planar oblique shock wave and the Prandtl–Meyer flow. Irrotational flow behind a curved shock wave, where the upstream flow is rotational, is examined and shown not to be possible.

ISSN:0031-9171    

DOI:10.1063/1.866786

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The method proposed by Mott‐Smith [Phys. Rev.82, 885 (1951)] to compute the structure of shock waves according to classical kinetic theory is extended to relativistic shocks. The results for the shock thickness are in remarkably good agreement with the results obtained from Eckart’s continuum theory.

ISSN:0031-9171    

DOI:10.1063/1.866787

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The nonlinear focusing or collapse is considered for the superposed magnetic fluids in motion at uniform speeds. The magnetic field is applied along the direction of streaming. It is shown that the evolution of the amplitude is governed by a two‐dimensional Schro¨dinger equation with cubic nonlinearity. This equation gives rise to a self‐focusing singularity when the velocity difference is in the subcritical regime. It is shown that such a self‐focusing effect is direction dependent, and predominant at shorter wavelengths.

ISSN:0031-9171    

DOI:10.1063/1.867018

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The time‐dependent interaction between solid objects and the unmagnetized plasmas in which they are immersed is investigated. To this end over 1900 high‐resolution, one‐dimensional particle‐in‐cell simulations of the plasmas surrounding cylindrical and planar objects are statistically analyzed. Numerical shot noise produces an electron‐plasma‐frequency ringing in the simulations, the amplitude of which is related to the plasma temperature and to the numerical system temperature. Whenever the potential of an object is rapidly biased, the surrounding plasma rings with a large amplitude at the electron‐plasma frequency. During this ringing, a depletion layer forms around the object on ion‐acoustic time scales. Positively charged objects discharge via plasma currents in about &tgr;pe/4 and negatively charged objects discharge in about &tgr;pi. Owing to charge separations in the plasmas, for the first few ion‐plasma periods after a perturbation, the potential of an object is not directly related to the charge on it. The electron‐plasma‐frequency ringing drives large‐amplitude Langmuir waves, which energize electrons and drive cavitation in the plasma. The fluxes of electrons reaching the objects are bursty at &ohgr;pe, and the energies of the ions striking the object slowly and systematically vary with time.

ISSN:0031-9171    

DOI:10.1063/1.866788

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The two long‐range forces in nature, gravitation and electromagnetism, are generally not jointly important in determining the dynamics of physical systems. There is, however, a case in which equilibrium large‐scale electrostatic and gravitational forces can be similar in magnitude. In a two‐component plasma, in which the massive species is a charged grain with charge‐to‐mass ratio of order (G)1/2, self‐gravitation and thermalization result in strong electrical polarization since the oppositely charged low‐mass component has a scale height much greater than that of the grains. Preliminary results of a computer simulation of this behavior are presented here. Properties of steady‐state gravitational condensations in grain plasmas have been studied with a one‐dimensional particle‐in‐cell code, and an investigation of the ratio of electrostatic to gravitational energy for various values of the grain charge‐to‐mass ratio has been performed. As expected, this ratio is maximized when the grain charge‐to‐mass ratio is approximately (G)1/2.

ISSN:0031-9171    

DOI:10.1063/1.866789

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The transport coefficients for particles and heat will certainly depend upon plasma parameters. Besides making the equilibrium equations nonlinear, this introduces a multitude of new terms in the set of linearized equations, which can be used to describe the effects of perturbations to the system. A general set of such equations is obtained that includes most physical dependences of the transport coefficients. If transport is driven by gradients in density or temperature, as would be expected from most turbulence theories, significant quantitative effects result. Perturbations no longer evolve at the equilibrium transport rates, and the density and temperature perturbations can be strongly coupled. Results are presented for several specific cases.

ISSN:0031-9171    

DOI:10.1063/1.866790

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A study of some problems associated with noninductive current drive is presented. Both slow and fast waves, weak and strong power, and transient and steady‐state responses are considered. The wave‐induced deformation of the electron distribution function is expressed as a linear functional of the absorbed power. This expression is then used to study three important phenomena: saturation of the absorption, coupling with an inductive field, and wave‐induced electron cyclotron emission.

ISSN:0031-9171    

DOI:10.1063/1.866739

出版商:AIP    

年代:1988

数据来源: AIP