摘要:

A liquid spreading over a dry solid surface with a contact angle of 180° has always been regarded as a special case. It is commonly thought that this represents a unique situation in which a singularity does not arise in an analysis of the dynamics of the liquid when the usual hydrodynamic assumptions are made, i.e., when the liquid is Newtonian and incompressible and obeys the no‐slip boundary condition at the solid. In fact, there are strong indications that it is not a special case.

ISSN:0031-9171    

DOI:10.1063/1.864591

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

A simple stress model has been developed to explain the observations of reduced drag when small gas bubbles are introduced into a turbulent boundary layer. The drag reduction is caused by a combination of density reduction and turbulence modification. The maximum reduction is obtained when the gas volume fraction approaches the bubble packing limit; the medium viscosity also increases markedly in this limit and becomes the important factor in restricting further reduction in drag. The derived analytical expression represents experimental data well.

ISSN:0031-9171    

DOI:10.1063/1.864592

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

A stability criterion for the tilt mode of a spheromak‐ion ring hybrid configuration has been developed for the case where the ring current is small compared to the spheromak azimuthal current. It is shown that the stability is related to the distortion of the spheromak separatrix.

ISSN:0031-9171    

DOI:10.1063/1.864593

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

Rayleigh–Be´nard convection in a laterally unbounded classical fluid layer with low Prandtl numberP(ratio of kinematic viscosity to thermal diffusivity) is reexamined. An amplitude expansion with only a few normal modes yields lateral oscillations of the convective rolls, which are therefore only weakly nonlinear. For free boundary conditions, additional modes (absent for rigid boundaries) lead to long wavelength (‘‘hydrodynamic’’) oscillations, with explicit nonlinear distortions in the velocity and temperature fields. For oscillations with rigid boundaries, the finite critical wavenumbers are approximately independent ofPfor smallP, and the calculated Rayleigh number, frequency, and wavenumber at onset agree well with observations in air. Discrepancies with experiments in dilute superfluid3He–4He systems with small aspect ratios (ratio of horizontal to vertical dimensions) suggest that lateral boundaries or two‐fluid effects play an important role in these systems.

ISSN:0031-9171    

DOI:10.1063/1.864594

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

A theoretical study is made of the free‐surface flow induced by a traveling oscillatory pressure distribution acting at the surface of water of finite depth. Depending on the relative values of the water depth and the frequency and speed of the applied pressure, the solution of the linearized water‐wave problem may be singular, owing to a resonance phenomenon. Through an asymptotic analysis, it is shown that, close to resonant conditions, the finite‐amplitude response is bounded, and it is governed by a forced nonlinear Schro¨dinger equation. It is deduced that, under certain circumstances, the generated wave disturbance may not reach a steady state; in particular, for deep water, a steady state is never attained.

ISSN:0031-9171    

DOI:10.1063/1.864595

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

The linear stability problem for the compressible Ekman boundary layer common to rotating fluids is formulated and the stability properties determined numerically. Three classes of unstable waves are identified (called class A, B, and C), their properties are described. The class C waves have only recently been reported in the literature and are present only in compressible Ekman boundary layers. Most of the calculations presented here are for uranium hexafluoride gas; however, critical Reynolds numbers are also computed for air and ammonia gas. Compressibility is generally found to decrease the critical Reynolds number for each class of wave. A comparison of results for the three different gases shows the stability to be largely unaffected by changes in the gas properties. Maximum growth rate calculations for each wave show the class A and B waves to be the dominant instabilities. OFF

ISSN:0031-9171    

DOI:10.1063/1.864596

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

A method for approximately analyzing the feedback between downstream and upstream edges in incompressible shear flow is described. The shear flow is modeled by a vortex sheet. Equations for resonance eigenvalues are derived. After the reduction of growth rate by finite shear layer thickness is allowed for, agreement is found between calculated resonances and those that have been observed experimentally.

ISSN:0031-9171    

DOI:10.1063/1.864597

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

Kolmogorov’s third hypothesis, as extended by Gurvich and Yaglom, is found to be obeyed by a diffusive scalar for a class of homogeneous, two‐dimensional mixing models. The mixing models all involve the advection of fluid by discrete vortices distributed in a square region with periodic boundary conditions. By computer simulation, it is found that the squared gradient of a diffusive scalar so advected is lognormally distributed, obeys the predicted scaling when a spatial smoothing is applied, and exhibits a power‐law range in the spatial autocorrelation. In addition, it is found that the scaling property cuts off at the Batchelor length, as predicted by Gibson. Since the mixing models employed do not incorporate the dynamical features of high‐Reynolds‐number turbulence, these results suggest that scalar lognormality and associated scaling behavior may be more robust or persistent than the scaling laws of the flow field.

ISSN:0031-9171    

DOI:10.1063/1.864598

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

The structure of one‐dimensional rotational discontinuities in a collision‐free plasma is analyzed. The plasma consists of cold streaming ions, isothermal streaming electrons, and electrostatically trapped warm electrons or ions. Allowances for large deviations from charge neutrality are made. By properly adjusting the trapped particle population, the rotational discontinuity can acquire a width comparable to or greater than the ion inertial length. Its structure then consists of an approximately charge neutral region of helical magnetic field sandwiched between a reversible electrostatic shock pair in which large deviations from charge neutrality occur. The discontinuity has left‐hand (ion) polarization for positive potentials, &fgr;>0, and right‐hand (electron) polarization for &fgr;<0. The magnetic field magnitude is constant throughout the discontinuity and the plasma state is identical on its two sides.

ISSN:0031-9171    

DOI:10.1063/1.864586

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

The propagation and absorption of electromagnetic waves in a relativistic Maxwellian plasma are investigated by solving the uniform plasma dispersion relation. Both the Hermitian and the anti‐Hermitian parts of the plasma conductivity tensor &sgr; are calculated relativistically. The Bessel functions occurring in &sgr; are not expanded, and many cyclotron harmonic terms are included at high temperatures. The dispersion relation is solved numerically for perpendicular propagation,k∥=0, where the relativistic effects are maximum and are not masked by Doppler broadening, which has been more thoroughly investigated. It is found that relativistic broadening has a substantial effect on wave dispersion, shifting the extraordinary mode right‐hand cutoff and the upper‐hybrid resonance to higher magnetic field with increasing temperature. Above a critical temperature, the cutoff disappears entirely. There is a broad range of temperatures, 20 keV≤Te≤500 keV, for which the wavenumberk⊥differs significantly from both the cold‐plasma value and the vacuum value. This has important implications for ray tracing in relativistic plasmas. Wave damping rates are calculated and compared to results from a previous formulation using the Poynting theorem, in which only the Hermitian part of &sgr; is calculated relativistically.

ISSN:0031-9171    

DOI:10.1063/1.864587

出版商:AIP    

年代:1984

数据来源: AIP