摘要:

The effect of flow oscillations on the axial diffusion of a solute in a pipe is analyzed theoretically by a perturbation method for small oscillation Reynolds numbers. The specific case of an initial step disribution in concentration is solved to second order. Numerical results of diffusion enhancement are given for several values of the parameter involved and are found to be conveniently summarized in terms of an equivalent diffusion parameter.

ISSN:0031-9171    

DOI:10.1063/1.863450

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Two approximate solutions, which become exact for Prandtl numbers 1 and 2, are given for laminar round plumes for arbitrary Prandtl number &sgr;. The first of these gives a very accurate solution for laminar found plumes in air, for which &sgr; = 0.73, and the second provides a good solution for laminar round plumes in water, for which &sgr; = 6.7. Two approximate solutions, which become exact for turbulent Prandtl numbers 1.1 and 2, respectively, are given for turbulent round plumes. The one which becomes exact at &sgr; = 1.1 is used to give a highly accurate solution for &sgr; = 1, which, for an eddy‐viscosity coefficient &lgr; equal to 0.0156, provides a remarkably good agreement with the experimental data of Beuther, Capp, and George, Jr.

ISSN:0031-9171    

DOI:10.1063/1.863451

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The results of an experimental investigation of a turbulent boundary layer with zero‐pressure gradient directed toward extending the data base at low Reynolds numbers are presented. The data obtained are concerned primarily with mean‐velocity distributions, skin‐friction coefficients, and distributions of intensity of the longitudinal‐component of the turbulent‐velocity fluctuations for Reynolds numbers based on momentum thickness as low as 465. The validity, at low Reynolds numbers, of the semi‐empirical laws characterizing the inner and outer regions of the boundary layer is examined.

ISSN:0031-9171    

DOI:10.1063/1.863452

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A numerical scheme is presented for the computation of time and space periodic standing waves of finite amplitude in water of arbitrary uniform depth. The dynamic and kinematic boundary conditions are used in their exact nonlinear form. The numerical procedure involves series truncation. Accurate solutions are presented for various values of the amplitude and of the depth. It is found that for some values of the depth, the frequency is not a monotonic function of the amplitude.

ISSN:0031-9171    

DOI:10.1063/1.863453

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Numerical solutions are given for surface waves generated by underwater explosions. The shallow water approximation is used for the governing equations. An axisymmetric initial impulse is applied to the surface of the water. The initial impulse distribution is taken from the results obtained by Falade and Holt. Due to the unavailability of the initial displacement distribution, the latter is assumed to be zero, but the method can work for the case when both initial impulse and displacement are presented. Glimm’s scheme is used to obtain the solution for the shallow water equations. The scheme was developed to solve one‐dimensional gas dynamic problems. With slight modification, the scheme can be used for the shallow water equations. It is tested successfully on the dam break problems. The calculations confirm the existence of the upper critical depth effect, namely, that for near surface explosions, the maximum amplitude surface wave is that generated by an explosion just below the surface.

ISSN:0031-9171    

DOI:10.1063/1.863454

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A strong external dc magnetic field introduces a basic anisotropy into incompressible magnetohydrodynamic turbulence. The modifications that this is likely to produce in the properties of the turbulence are explored for the high Reynolds number case. The conclusion is reached that the turbulent spectrum splits into two parts: an essentially two‐dimensional spectrum with both the velocity field and magnetic fluctuations perpendicular to the dc magnetic field, and a generally weaker and more nearly isotropic spectrum of Alfve´n waves. A minimal characterization of the spectral density tensors is given. Similarities to measurements from the Culham–Harwell Zeta pinch device and the University of California, Los Angeles Macrotor tokamak are remarked upon, as are certain implications for the Belcher and Davis measurements of magnetohydrodynamic turbulence in the solar wind.

ISSN:0031-9171    

DOI:10.1063/1.863455

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Strong Langmuir turbulence acts to generate localized spiky fields and ion cavities. It is shown that these cavities generate potential barriers that can trap a significant fraction of the electrons. For example, a 20% cavity traps 50% of the electrons. This can seriously change the direct current properties of a collisionless plasma and lead to the appearance of anomalous dc resistivity.

ISSN:0031-9171    

DOI:10.1063/1.863456

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

During the implosion of weakly ionized plasmas in a theta pinch, monoenergetic electron beams in the direction of the magnetic field lines are observed, the energy of the beam corresponding to the ionization energy in each case. The distribution function has been measured by incoherent Thomson scattering.

ISSN:0031-9171    

DOI:10.1063/1.863457

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Electron heat flux in a fully ionized gas between two parallel boundaries is studied. The moment method is employed to analyze the relation between the electron heat flux and the plasma collisionality parameterL/&lgr;e, both with and without electron‐electron collisions (&lgr;eis the electron mean‐free‐path). The electron heat flux is determined as a function ofL/&lgr;eover the entire range from free flow of collisional regimes. These calculations employ various combinations of trial distribution function and collision operator. Comparisons are made between the various models and with the known result in the collisional limit. Discrepancies between the present model and the known collisional behavior are discussed, and an approximate heat flux expression is presented.

ISSN:0031-9171    

DOI:10.1063/1.863444

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A method is developed for evaluating transport coefficients in asymmetric geometries using the Monte Carlo method. The method is applied to the stellarator.

ISSN:0031-9171    

DOI:10.1063/1.863445

出版商:AIP    

年代:1981

数据来源: AIP