摘要:

The diffusion of a marked fluid particle is considered in a random incompressible, isotropic velocity field, stationary in space and time. The integrodifferential equations proposed earlier by Bourret and Saffman, for the time evolution of the probability density of marked fluid particles, are introduced and solutions are found for a Gaussian Lagrangian covariance function. An improved equation is derived by including the effects of higher‐order moments. Solutions are found to be in good agreement with independent results based on fluid particle diffusion in a multivariate‐normal field.

ISSN:0031-9171    

DOI:10.1063/1.1694747

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A novel pressure perturbation was applied in the irrotational region outside of a turbulent boundary layer to simulate the natural pressure disturbance created by a free‐stream flowing over the turbulent bulges at the interface. Conditional sampling was used to reveal the perturbation and its effect on the boundary layer. The natural mean bursting frequency scaled with the outer variables in agreement with the theoretical and experimental results reported previously in the literature. The mean frequency of the bursting phenomenon in the wall region during the disturbance was independent of the pressure perturbation frequency.

ISSN:0031-9171    

DOI:10.1063/1.1694748

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The properties of doubly diffusive convection of large amplitude are investigated using the upper bounding method. The specific problem treated is the thermohaline instability in a horizontal layer of fluid containing temperature and solute concentration increasing in the direction opposite to that of gravity. For large values of the solute and thermal Rayleigh numbers,Rand Ra, a boundary‐layer analysis is used to calculate an upper bound on the solute flux. This upper bound is found to bef*(R/Ra)Ra11/8, where the functionf*is a monotonically increasing function ofR/Ra. Thus, for large values of Ra, the solute flux may be very large even whenR/Rais only slightly in excess of unity.

ISSN:0031-9171    

DOI:10.1063/1.1694749

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The motion of a viscous fluid contained between two eccentric rotating spheres is obtained as a perturbation solution valid through first‐order terms in the Reynolds number and second‐order terms in a parameter describing the eccentricity.

ISSN:0031-9171    

DOI:10.1063/1.1694750

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Detailed numerical results and comparison with wind tunnel experiments are presented. These include the nonlinear unstable region of the wake behind adiabatic and heated flat plates, the far wake of cooled and adiabatic wedges, and the bow‐shock generated outer wake of an adiabatic circular cylinder. In general, it is found that disturbance energy production is primarily contributed by the Reynolds shear stress integral. However, the development of mean flow kinetic and thermal energy fluxes is primarily contributed by their respective streamwise diffusion fluxes due to the disturbance density fluctuation. The peaking of the averaged disturbance kinetic energy and the extent of mean flow decay is, in terms of the appropriately scaled streamwise distance, hastened by either (i) an increase of the Reynolds number for the same body geometry and surface thermal conditions; (ii) surface cooling for the same body geometry and Reynolds number; or (iii) an increase in the body length for the same wedge angle, Reynolds number, and surface thermal conditions.

ISSN:0031-9171    

DOI:10.1063/1.1694751

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The energy theory is applied to investigate the stability of a general viscoelastic simple fluid heated from below. The linear theory predicts overstability for this configuration and the present energy results provide a sufficient condition for stability which coincides with the linear limit for the Newtonian case.

ISSN:0031-9171    

DOI:10.1063/1.1694752

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The general case of transport in axisymmetric fully ionized quasineutral plasmas is formulated for systems immersed in a strong magnetic field and close to local kinetic equilibrium. The particle density and temperature for each species, which depend on position only via the magnetic streamfunction, obey a closed system of transport equations involving coefficients which can be optimally computed via minimax variational principles. The electric field is eliminated from the transport equations, apart from the electrostatic potential which is obtained from a rate equation, as is the toroidal magnetic field. The magnetic streamfunction is determined by an elliptic boundary value problem. The theory goes over to previous work in the limit where the frequency associated with the guiding center motion of a representative particle in the absence of collisions is much greater than the collision frequency, and vice versa.

ISSN:0031-9171    

DOI:10.1063/1.1694753

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Magnetic mirror compression and electron cyclotron heating have been investigated in a high field, simple mirror, compression device (midplane field rises from 200 G to 50 kG in 0.5 msec, mirror ratio = 1.4), using a contact ionization lithium source(n ≈ 108−1010 cm−3, Te = Ti ≈ 0.2 ev)to produce the initial plasma. Experimental results are: (1) For magnetic mirror compression, the plasma is macroscopically unstable, and, althoughTi ≈ 10 eV, confinement is destroyed with∼200 &mgr;sec, for all ranges of initial plasma density and neutral pressure. A set of rate equations for electron and ion density and temperature and ambipolar potential predicts classical confinement times of the order of milliseconds. (2) For short pulse, electron cyclotron heating (250 kW, 0.25&mgr;sec, 8.95 GHz pulse) followed by magnetic compression,Te ≈ 45 keV, Ti ≈ 10 eVand the plasma is stably confined, provided the neutral gas pressure exceeds∼2 × 10−5Torr. The observed stability is in rough agreement with the Guest‐Beasley criterion for stabilization of flute modes by flow of cold plasma to the conducting chamber walls. The confinement time is in agreement with previous rate equation calculations of hot electron plasma confinement in magnetic mirrors.

ISSN:0031-9171    

DOI:10.1063/1.1694754

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The interaction of a streaming plasma with a magnetic dipole field is studied in the framework of a linearized, two‐fluid continuum description. For the case of low‐density flows, in which the pressure gradients are small in comparison to magnetic and electric forces, it is possible to extend the solution beyond the first‐order one, which gives the distribution of induced current in the azimuthal direction, and calculate space‐charge density and plasma potential distributions, along with the Hall current components in a restricted spatial region. The quantities of second‐order scale directly with the appropriate Cowling number, whereas the azimuthal current density increases with the square root of this ratio. It is also shown that the Hall current densities and potential variation increase rapidly with increasing plasma density, while the normalized space charge density scales with the square of the plasma velocity. Moreover, the theory predicts a significant decrease in plasma potential in the vicinity of the dipole; a result which tends to agree with recent experimental indications.

ISSN:0031-9171    

DOI:10.1063/1.1694755

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The interaction of a movingElayer with the resistor structures that line the walls of the Livermore astron apparatus are studied theoretically and experimentally. It is shown that for a weak interaction no change in mean layer radius results from the energy loss in the resistors. Using the approximation of a long thin layer in axisymmetric geometry, curves are given for calculating the energy dissipated in the resistors per meter traveled or, equivalently, the axial force on the moving layer. Results of experiments are reported showing transport of the incoming layer in the astron tank and trapping in the magnetic mirrors. Measurements of the damping ofE‐layer axial motion in several resistor geometrics show typical damping times of 0.5&mgr;sec. Comparison of experiment and theory show agreement within about a factor of 2.

ISSN:0031-9171    

DOI:10.1063/1.1694756

出版商:AIP    

年代:1974

数据来源: AIP