摘要:

Both the theory of the absorption process in the ion‐cyclotron range of frequencies and some of the experiments which slow the promise and problems with radio frequency plasma heating in this range are discussed. It is shown that mode conversion is invariably involved in the process and so an extensive review of mode conversion theory, expecially as it applies to problems with back‐to‐back cutoff‐resonance pairs, is included. This includes a discussion of the tunneling equation with and without absorption effects and with and without energy conservation. The general theory is applied to various ion‐cyclotron harmonics, the two‐ion hybrid resonance, and to a case where a wave converts to a Bernstein mode at the plasma edge. The results are given analytically for a variety of cases without absorption, and empirical formulas are given for the second and third harmonics of the ion‐cyclotron frequency, which include effects of absorption. Various problem areas in the theory are also discussed with some of the limitations caused by the approximations involved. A number of experiments are also discussed which show effective heating, and some show the features of the mode conversion process, indicating that the general processes of absorption are reasonably well understood. Areas where further work is necessary, both in fundamental theory and in comparing theory with experiment, are also discussed.

ISSN:0031-9171    

DOI:10.1063/1.865224

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Theoretical and experimental results show that it is possible to obtain flow separation or reverse flow for low Reynolds number shear flow past a wavy wall. The region of reverse flow depends on the amplitude of the wall waves as well as the gap width. A comparison between the fourth‐order perturbation results and experiment is given.

ISSN:0031-9171    

DOI:10.1063/1.865225

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The nature of the viscous boundary‐layer flow induced near a wall, caused by a pair of counter‐rotating vortices above the wall, is investigated. Solutions for the unsteady flow that develops near the wall are obtained numerically for a variety of cases. The vortices are taken to be of equal strength with cores located at equal distances from the wall; depending upon the assumed sense of rotation, the vortices either move toward the wall or recede from it, creating a region of either inflow or outflow near the wall, respectively. The calculated results show that the adverse pressure gradient near the wall induced by the vortex motion gives rise to explosively growing regions of recirculating flow near the wall. The possible relevance of these results to Go¨rtler vortex instability and turbulent flow near walls is discussed.

ISSN:0031-9171    

DOI:10.1063/1.865226

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

In a recent paper, Renardy and Joseph [Phys. Fluids28, 788 (1985)] studied the Be´nard problem for two layers of different fluids lying on top of each other and bounded by walls. Their study shows that, in contrast to the Be´nard problem for one fluid, the onset of instability can be oscillatory. The number of parameters involved in the problem is large, and there is as yet no comprehensive picture of when the instability is oscillatory and when it is not. The study of limiting cases, accessible by perturbation methods, may be helpful in this respect. In this paper, an analysis is given for the case when the properties of the two fluids are nearly equal and the fluids are allowed to slip at the boundaries.

ISSN:0031-9171    

DOI:10.1063/1.865227

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

In this paper the stability of the interface separating fluids of widely differing viscosities has been examined. It is shown that a viscous–inviscid (V–I) model offers a consistent zeroth‐order approximation to the stability problem. The zeroth‐order solution is obtained by neglecting the smallest‐order effect, viz., viscosity on the less viscous side of the interface. In this sense, the V–I model significantly differs from the Kelvin–Helmholtz (K–H) approach where both the viscosities are dropped in a single step. A closed form solution for the stability criterion governing the V–I model has been obtained, and a novel instability mechanism is described. It is shown that the V–I model is also a consistent zeroth‐order approximation for the Rayleigh–Taylor problem of a viscous–viscous, nonflowing interface when the viscosity ratio tends to zero. For the interface separating two viscous, nonflowing, incompressible fluids, exact solutions for the velocities, pressures, and interface displacement for a disturbance of a given wavelength have been provided for the stable (lighter fluid on top) wave motion. By discussing the roles played by the dynamic and kinematic viscosities, it is made clear why neither the V–I nor the K–H model should apply to the air–water interface. The results of the V–I model compare well with experimental observations. The V–I model serves as an excellent basis for comparison in detailed numerical studies of the viscous–viscous interface.

ISSN:0031-9171    

DOI:10.1063/1.865228

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The effect of rigid vertical boundaries on the onset of convective instability is calculated for the salt finger regime of double‐diffusive convection. The unperturbed state is a quiescent fluid with constant vertical gradients of temperature and solute, which are stabilizing and destabilizing, respectively. The horizontal boundaries are taken to be stress‐free and perfectly conducting. The lateral boundaries are perfectly insulating for solute. Changing from thermally insulating to thermally conducting sidewalls results in a strong destabilization of the flow for large thermal Rayleigh numbers even in the limit that the separation between the sidewalls approaches infinity. Further, for thermally conducting sidewalls, a decrease in the separation of the sidewalls may destabilize the system.

ISSN:0031-9171    

DOI:10.1063/1.865229

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Recent measurements by Sreenivasen [FrontiersinFluidMechanics(Springer, New York, 1985), pp. 41–67] have shown the presence of temporal frequencies (other than the shedding frequency) in the vortex wake generated by a circular cylinder. It is shown here that these modes of oscillation can be described in terms of the classical analysis of the von Karman vortex trail. A comparison of theory with experiment identifies the individually excited modes.

ISSN:0031-9171    

DOI:10.1063/1.865230

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The effect of viscosity on the wall‐pressure spectrum of a slightly compressible, turbulent boundary layer is studied. Apart the resolving the singularities of the inviscid spectrum, the viscous shear‐stress dipole sources also partake in sound generation. The dominant viscous contribution to the low‐wavenumber part of the spectrum is evaluated in terms of the flow Mach and Reynolds numbers. It is concluded that, in typical applications, the viscous contribution is small compared with the inviscid one.

ISSN:0031-9171    

DOI:10.1063/1.865231

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

In this paper the Rayleigh flow problem for multicomponent gas mixtures is studied by means of a discrete velocity model of the Boltzmann equation. The analysis carried out leads to analytical solutions for the number densities, which are the pertinent quantities capable of describing the space‐time evolution of the system at a molecular level. The macroscopic behavior of the mixture is also deduced, and a comparison is made between our results and those of other authors.

ISSN:0031-9171    

DOI:10.1063/1.865414

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The Euler equations for a reacting polytropic gas with model losses, applied to unsupported steady detonation waves, lead to fast and slow detonation speeds for a given loss. The reaction rate is taken to have the Arrhenius form. The differential equations for the Mach number squaredmand the reaction progress variable &lgr; are integrated numerically using a fourth‐order Runge–Kutta–Fehlberg method. The calculations are presented as plots of trajectories in the &lgr;‐log (m) plane. The main parameters that are varied are the activation temperature, the heat of reaction, the order of the reaction, and the polytropic exponent &ggr;. The onset of detonation failure, and a continuum of solutions that appears in some parameter ranges, are explored in detail. The results are summarized in plots of the detonation Mach number squaredm0versus the logarithm of the loss parameter.

ISSN:0031-9171    

DOI:10.1063/1.865232

出版商:AIP    

年代:1985

数据来源: AIP