摘要:

Heat transport in a porous medium is governed by molecular diffusion and hydrodynamic dispersion. The latter factor, represented by a second‐order tensor which depends on the fluid velocity, is taken into account for the first time in the problem of thermal convective currents in a layer heated from below. The linearized and the energy analysis yield the same results and value of the critical Rayleigh number as those obtained with neglect of dispersion. Finite amplitude convective currents are influenced by dispersion, which has a stabilizing effect. Dispersion may have an influence for relatively coarse materials or at high values of Rayleigh number.

ISSN:0031-9171    

DOI:10.1063/1.861234

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

Vertical oscillations, together with a uniform rotation about a vertical axis, are found to have a stabilizing influence on a stratified fluid with density increasing exponentially in a direction (vertically up) opposite to that of gravitational acceleration.

ISSN:0031-9171    

DOI:10.1063/1.861235

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

An ensemble of instantaneous temperature profiles across a two‐dimensional heated jet was obtained by shooting a fine platinum resistance thermometer across the jet at speeds much higher than the local velocity of the jet. Each recording of the ensemble was shifted to have a common center, and ensemble averages were obtained counting only the profiles for which the spatial location in question was inside the jet. This conditional averaging produced mean and mean‐square fluctuations which are quite flat across the jet, indicating that the fluid inside the jet is well‐mixed. Visual inspection of instantaneous records showed no cold spots in the jet. Ensemble averages were also measured by aligning the edge or beginning of the instantaneous profiles. This led to a sharper rise in the mean temperature profile and an overshoot in the mean‐square temperature fluctuations. This indicates that mixing across the turbulent‐nonturbulent interface is stronger than in the main body of the jet.

ISSN:0031-9171    

DOI:10.1063/1.861236

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

A heated two‐dimensional turbulent jet flow has been studied experimentally using conditional sampling techniques. The aim of the investigation was to establish the details of the spread of the temperature and velocity fields. Results confirmed previous observations showing that the conventionally measured mean temperature profile was wider than the velocity. However, when considered in terms of the turbulent fluid only, the spread of thermal and of momentum fields was found to be coincident.

ISSN:0031-9171    

DOI:10.1063/1.861237

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

The necessary conditions for stability of a shock, that the shock travel with supersonic velocity with respect to the medium ahead and with subsonic velocity with respect to the medium behind, are shown to be implied by the second law of thermodynamics for very general classes of viscous, heat‐conducting fluids; the Weyl conditions are not invoked. The results are shown to be also compatible with the Le Chatelier–Braun principle. They further imply that under certain conditions it is not thermodynamically permissible to assume the existence of a shock transition layer in which entropy production is due to heat conduction alone.

ISSN:0031-9171    

DOI:10.1063/1.861238

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

The stagnation region of a shock layer in front of a blunt body re‐entering the atmosphere at high altitude and high velocity is considered. At these flight conditions the ionization is out of equilibrium in a considerable fraction of the shock layer thickness. The nonequilibrium phenomenon modifies the distribution of all the thermodynamic, radiative, and transport properties of the gas. To quantitatively evaluate these modifications and their influence upon the radiative heat flux, the governing equations for nonequilibrium conditions are obtained and numerically solved for real air. A simple method of evaluating nonequilibrium absorption and emission coefficients is also proposed. The numerical results show that, while nonequilibrium slightly affects the body heating load, important variations occur in the shock layer structure and the radiative flux emergent at the shock front is considerably reduced with respect to complete equilibrium conditions. This fact must be taken into account in all studies dealing with precursor effects.

ISSN:0031-9171    

DOI:10.1063/1.861239

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

The processes occurring in an electroconductive fluid in a magnetic field and subjected to an electric current flowing through it are discussed. In a fluid whose electrical conductivity rises with temperature, a phenomenon similar to free convection occurs when the entropy drops along the direction of the force j × B. The joint action of this phenomenon and the overheating instability which also stems from the temperature dependence of the electrical conductivity and the effect of electromagnetic braking is considered. In this case, the criterion of stability is obtained. Instability in the form of free convection also appears due to a Hall current jHflowing along the electrodes of a magnetohydrodynamic channel when a high temperature gradient occurs along the direction of the force jH× B. The instability arises near the cathode and the appearance of one is determined by the value of a dimensionless parameter identical to the Rayleigh number Ra = (dln&sgr;/dT) g˜l4‖∇T‖ /&ngr;&khgr; where g˜ =jHB/&rgr; plays the role of the gravitational acceleration anddln&sgr;/dTplays the role of the expansion coefficient.

ISSN:0031-9171    

DOI:10.1063/1.861240

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

A nonlinear analysis is given of the oscillations of the magnetohydrodynamic flow instability caused by the dependence of the conductivity on the temperature. The effect of the flow parameters is studied.

ISSN:0031-9171    

DOI:10.1063/1.861241

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

The analogy between the free surface flow of a conducting liquid and the compressible flow of a conducting gas was studied. It was shown that the analogy exists if the Joule heating is negligible and if the conductivity of the gas is proportional to the temperature. Tests were carried out with mercury to simulate the oscillating flow of a plasma in shock‐wave magnetohydrodynamic generators. A single‐ and a twin‐Hartmann–Sprenger tube configuration were used to produce the oscillating flow. The experiments performed so far showed that as much as 21% of the energy of the jet driving the Hartmann–Sprenger tubes may be converted into electricity.

ISSN:0031-9171    

DOI:10.1063/1.861242

出版商:AIP    

年代:1975

数据来源: AIP

摘要:

Analytic theory for two‐dimensional turbulent equilibria for the inviscid Navier–Stokes equation (or the electrostatic guiding center plasma) is tested numerically. A good fit is demonstrated for the approach to a predicted energy per Fourier mode obtained from the two‐temperature canonical ensemble of Kraichnan: ⟨‖u(k) ‖2⟩ = (&agr;+&bgr;k2)−1, where k is the wavenumber and &agr; and &bgr; are reciprocal energy and enstrophy temperatures. Negative as well as positive temperature regimes are explored. Fluctuations about the mean energy per mode also compare well with theory. In the regime &agr;<0, &bgr;≳0, with the minimum value of &agr;+&bgr;k2near zero, contour plots of the stream function reveal macroscopic vortex structures similar to those seen previously in discrete vortex simulations by Joyce and Montgomery. Kraichnan’s assertion that thermodynamic limits exist for the negative temperature states is questioned. Eulerian direct interaction equations, which can be used to follow the approach to inviscid equilibrium, are derived.

ISSN:0031-9171    

DOI:10.1063/1.861243

出版商:AIP    

年代:1975

数据来源: AIP