摘要:

An aspect of the stability of boundary‐layer profiles containing a point of zero shear away from the wall is examined. Analysis of a viscous critical layer at the point of zero shear shows that the classical unsteady boundary‐layer equation contains a short‐wavelength unstable solution. The presence of this instability has consequences for the numerical solution of the classical boundary‐layer equation, and may be connected with the initiation process of Van Dommelen and Shen’s nonlinear unsteady separation singularity.

ISSN:0031-9171    

DOI:10.1063/1.865105

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A new low‐frequency cutoff for them=−1 fast wave in a cold cylindrical plasma in the ion‐cyclotron frequency range is reported for the completely‐filled plasma waveguide. The presence of a vacuum gap between the plasma and the conducting wall enables the wave to propagate to arbitrarily small frequencies. For &ohgr;∼&ohgr;ci, this mode is a surface wave and undergoes a resonance at the ion‐cyclotron frequency.

ISSN:0031-9171    

DOI:10.1063/1.865106

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

By utilizing a laser‐produced plasma technique, the possibility of realizing a field‐reversed configuration (FRC) with a nearly spherical plasma cross section is examined experimentally on the basis of a double‐cusp bias magnetic field. Plasma measurements by magnetic probes and x‐ray pinhole camera confirm the formation of a nearly spherical FRC.

ISSN:0031-9171    

DOI:10.1063/1.865107

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Experiments were performed to determine the stability characteristics of impulsive spin‐down to rest in a circular cylinder of moderate aspect ratio. The characteristics measured include the onset time, onset wavelength, and time history of the wavelength for the Taylor–Go¨rtler instability, as well as the global stability limit of the apparatus. Results compare favorably with earlier theoretical, numerical, and experimental investigations.

ISSN:0031-9171    

DOI:10.1063/1.865108

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Experiments in which a thick layer of fluid was heated from below at a constant rate were conducted at flux Rayleigh numbers in the range of 109to 1012. The resulting convection was investigated by statistical analysis of the temperature signals from a pair of thermistor probes at different horizontal spacings. Power spectra showed that the temperature variance was distributed over a broad band of periods but that the centroid of this band, the characteristic period, depended very nearly on the − (1)/(2) power of the heat flux. Measurements of the coherency between the probe signals at varying distances were interpreted as defining a characteristic horizontal length scale that depended approximately upon the − (1)/(4) power of the heat flux. These results tend to substantiate the boundary layer instability model of convection at very high Rayleigh numbers.

ISSN:0031-9171    

DOI:10.1063/1.865109

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A geometric interpretation for the operation of probes that use multiple sensors is presented. This interpretation provides a method for visualizing how the individual sensor reponse and the geometrical arrangement of the sensors are related to the measurements made with a given probe. The interpretation also provides a simple explanation for the occurrence of multiple solutions of the nonlinear equations for a probe. It is shown that a measured set of three cooling velocities, using a three‐sensor hot‐wire probe, can be produced by as many as eight different velocity vectors, only one of which is the correct velocity vector.

ISSN:0031-9171    

DOI:10.1063/1.865110

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The relation between turbulent diffusivities for transferable quantities and the Lagrangian displacement of fluid particles in a flow field with a mean velocity distribution is derived on the assumption of a gradient transport. The turbulent diffusivities in a plane jet and in two interacting plane parallel jets are calculated by using Lagrangian trajectories that have been computed based on a simple model. The results indicate that the mean velocity distribution affects the turbulent transport, and that neglect of this factor can give a fivefold overestimation of the diffusivity in interacting jets.

ISSN:0031-9171    

DOI:10.1063/1.865111

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A two‐color laser Doppler anemometry technique has been used to measure velocity statistics and Reynolds stress in a turbulent boundary layer over a strongly heated wall (Tw=1100 K) with premixed ethylene–air combustion. Measurements in the isothermal and nonreacting heated wall turbulent boundary layers were also made for comparison. Conditional velocity covariance −∼(uv) are deduced using the quadrant and ‘‘hole’’ analysis methods. The conditional data obtained in the isothermal layer are in exellent agreement with those deduced by others based on hot‐wire data. The results obtained in the heated and reacting layers indicate that the decrease in mean Reynolds stress can be attributed to a reduction in the −∼(uv) contributions associated with a burst of low momentum fluid form the wall. The reduction is much more significant in the reacting layer than in the heated layer. Comparison with previous density shows that the changes in conditional −∼(uv) contributions occur mostly within the constant density sublayer of combustion products adjacent to the wall. In the region where combustion reaction takes place, no significant change in the turbulence intensities or Reynolds stress is found.

ISSN:0031-9171    

DOI:10.1063/1.865112

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The interaction of a supersonic plane or axially symmetric jet with a background gas of equal or different species is investigated in the transition regime between continuum and free‐molecule flow by solving a set of Boltzmann equations with simplified collision terms. In the solution, the discrete ordinate method is applied and extended to axially symmetric flow by introducing an approximation to the curvature terms. The rate of convergence of the solution is increased by using the deviations of the distribution functions from the equilibrium solutions as dependent variables. The results show that three flow regions characterized by the ratio of cross to self‐collisions can be identified in the jet structure. The penetration of the jet by background molecules is studied with regard to isotope separation. The influence of various assumptions for the boundary conditions on the solution is discussed. Density profiles computed for axially symmetric flow are compared with recent experimental data.

ISSN:0031-9171    

DOI:10.1063/1.865409

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

We consider the problem of the thermal excitation of vortex waves on quantized vortex lines and rings. We discover that this problem, which has been largely neglected in the past, apparently presents a ‘‘free energy catastrophe’’ above about 1.85 K; that is, if one naively proceeds to compute the thermally induced vortex waves on rings or lines, the entropy of the waves becomes so large that the free energy of vortices is driven negative, and superfluidity could not exist. Various mechanisms for eliminating this problem are considered; it appears to be necessary to consider the relevant system to be the vortex plus the bath in order to avoid double counting of states. We comment on the relevance of this calculation to the problem of thermally induced nucleation of vortices.

ISSN:0031-9171    

DOI:10.1063/1.865113

出版商:AIP    

年代:1985

数据来源: AIP