摘要:

The heat flow on a cylinder immersed in an infinite rarefied gas is well defined in the free molecular limit. To first order in the inverse Knudsen number, the correction to this free molecular value diverges logarithmically, for a reason quite similar to that explaining the divergence of the virial expansion of transport coefficients of two−dimensional gases. When one accounts for the most divergent terms of an expansion of this flux in powers of the inverse Knudsen number, this logarithmic divergence does not disappear, due to hydrodynamical phenomena taking place at a distance from the cylinder that is much larger than the mean free path. This situation is somewhat similar to that encountered in studying the divergent transport coefficients of two−dimensional gases. It is discussed from the point of view of a possible experiment.

ISSN:0031-9171    

DOI:10.1063/1.861134

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

A numerical study of one stage of the transition region from laminar to turbulent flow of the boundary layer over a flat plate was performed. The theoretical development follows that of Benney and Lin in which the interaction of a two−dimensional and a restricted three−dimensional periodic disturbance was considered up to second order terms in a series expansion. Several laminar profiles were studied, all belonging to the Falkner−Skan family. The assumption of equal wave speeds for the two−dimensional and three−dimensional disturbances was relaxed, giving rise to a slow modulation of the mean secondary flow. The resulting mean secondary flow compared favorably with both the existing experimental work and the restricted analytical solution of Benney. The flow depended principally upon the disturbance spanwise wavenumber, &bgr;, whose variation resulted, in some instances, in a system of counter−rotating vortices, one on top of the other, for the mean secondary flow.

ISSN:0031-9171    

DOI:10.1063/1.861135

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

The use of a single inclined hot wire for measurments of mean flow velocity and direction in a three−dimensional turbulent flow field is presented. A constant temperature hot−wire anemometer circuit is used with and without a linearizer. The use of the linearized results were found to improve the readout ability of the instrument and also to simplify the correction for directional measurements. A study of the effect of turbulence levels on the steady calibration results has shown that if the turbulence levels and the cross correlations are known, corrections can be applied to obtain angles and velocities as reliable as those obtained from a directional pressure probe. The advantages of the use of the hot wire over a pressure probe are that the hot wire has much faster response, disturbs the flow field to a lesser degree, is easier to correct for turbulence levels, and due to its smaller dimension compared with the pressure probe is less erratic concerning mean velocity gradients. The method of measurement and correction was tested in a jet mixing with a cross wind field. The range of angles shown in the experimental graphs pertain to that of a jet mixing with a cross wind.

ISSN:0031-9171    

DOI:10.1063/1.861136

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

A one−to−one correspondence between time scales indicated by histograms of short−sampling−time autocorrelations and long−sampling−time first−moment spectral densities appears to exist. Since short−time autocorrelations are indicative of ’’bursting’’ frequencies, the implication is that ’’bursting’’ frequencies may also be obtainable from spectral data.

ISSN:0031-9171    

DOI:10.1063/1.861137

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

The behavior of thek−&egr; turbulence model at a turbulent/nonturbulent interface is examined to determine the turbulent diffusion coefficients &sgr;kand &sgr;&egr;. The analysis leads to a dilemma caused by the modeling of the generation term in thekand &egr; equations. Based on limiting values of &sgr;kand &sgr;&egr;derived from this analysis, a revised form of the low−Reynolds−number incompressiblek−&egr; turbulence model is presented which incorporates a new set of model constants, a linear form of the near wall dissipation term in thekequation, and deletion of a nonphysical term in the &egr; equation. The revised model is applied to fully developed flow in a channel and found to give better agreement with experiment than the original form of the model developed by Jones and Launder.

ISSN:0031-9171    

DOI:10.1063/1.861138

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

A generalization of the Boltzmann equation is derived from the Liouville equation for a dilute gas with macroscopic correlations. For the case of weak turbulence a solution is obtained in terms of a set of generalized hydrodynamic equations which describe the average flow.

ISSN:0031-9171    

DOI:10.1063/1.861139

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

The formal theory of geometric optics in a space− and time−varying plasma is developed assuming that there propagates a small amplitude wave, that there is a linear, causal, but, in general, nonlocal relation between current density and electric field, and that the background plasma does not change much in a locally defined wavelength or period. The description reduces to a system of ordinary differential equations along rays everywhere tangent to the group velocity, one such system of rays for each mode. These equations are formally Hamiltonian in character. They permit the introduction of an energy density such that negative energy waves will be unstable in a lossy medium, as seen by an observer moving with the group velocity, providing that the divergence of the bundles of rays is not too great.

ISSN:0031-9171    

DOI:10.1063/1.861140

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

A single fluid, two−temperature, one−dimensional magnetohydrodynamic model has been developed to describe the properties of a linear pinch plasma heated by an axially directed laser beam. The coupling of the laser light energy with the pinch dynamics is studied in detail. The laser light absorption occurs primarily through the inverse bremsstrahlung process, but the effect of enhanced absorption for sufficiently intense lasers has also been studied. Results of the calculations show a strong interaction between the heating by radial compression (pinching) and heating by the laser beam. It was found that the most important considerations in obtaining efficient heating of the plasma are the time profiles of the laser pulse and the electric current. The results of the anomalous absorption studies indicate that the increased absorption can cause the electron and ion temperatures to decouple and may actually be detrimental to the efficient laser heating of dense pinch type plasmas.

ISSN:0031-9171    

DOI:10.1063/1.861141

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

The fraction of the &agr;−particle energy absorbed in a reacting DT sphere by Rutherford interactions with ions and electrons is computed and compared to previous results obtained by neglecting interactions with ions.

ISSN:0031-9171    

DOI:10.1063/1.861142

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

Cross−field energy transport by electrostatic plasma waves has been studied theoretically and numerically for a plasma near thermal equilibrium. Energy transfer by plasma waves is significant in a high temperature plasma where collisional transport is quite small. For strong magnetic fields (&ohgr;c≳ &ohgr;p), the simulations verify the theoretical prediction that energy transport by plasma waves increases slowly with the magnetic field, and that in the limit of large field magnitudes, wave transport dominates classical collisional transport. Furthermore, in a strong magnetic field the relaxation of the temperature by wave transport may result in an anisotropic velocity distribution. For weak fields (&ohgr;p≳ &ohgr;c), the simulation shows that collisional transport is more important than wave transport, also in agreement with the theory.

ISSN:0031-9171    

DOI:10.1063/1.861143

出版商:AIP    

年代:1975

数据来源: AIP