摘要:

It is shown that the equivalent blowing velocity of a three‐dimensional boundary layer can be either positive or negative near separation depending on the geometry of the flow. The situation is illustrated by a computation of the boundary layer in the planes of symmetry of a thin ellipsoid moving in the direction of its major axis. This result is of importance in estimating the effect of the boundary layer on the external flow and is distinct from the two‐dimensional problem where the blowing velocity always appears to be positive near separation.

ISSN:0031-9171    

DOI:10.1063/1.864267

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

An experimental study of turbulent, two‐layer stratified entrainment in an annular, surface stress driven geometry is described. Density and velocity probes were used to determine interfacial locations and flow patterns. Annular geometry effects were examined using three channel widths. The entire flow system was constructed of clear plastic to facilitate observations. The measurements and observations of these flows over a wide range of Richardson number indicate a strong secondary circulation, radial interfacial tilt, and a dominant dependence of flow parameters on the annular radius. It is concluded that the entrainment results in annuli such as the one used in this study may not be directly applicable to one‐dimensional entrainment processes.

ISSN:0031-9171    

DOI:10.1063/1.864268

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The relative energy fluxFrelof capillary‐gravity waves on a moving stream is calculated for various wavelengths, as a function of wave amplitude. In some cases, abrupt changes in the sign ofFrelare found to occur for a small increase in wave amplitude. The way in which the phenomena might be observed experimentally is indicated.

ISSN:0031-9171    

DOI:10.1063/1.864269

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The two‐dimensional steady‐state reaction‐zone structure of a homogeneous and a heterogeneous explosive is studied. To do this theoretical results obtained from the Euler equations of compressible flow are combined with experimental data on steady‐state detonation shock‐wave speed and shape as a function of the explosive charge size. The theoretical results, constrained by the experiments, define an inverse problem for the chemical heat‐release function in the reaction zone which follows the shock wave; this problem is solved. The heterogeneous explosive is made from the homogeneous one by adding small quantities of other materials. Because of this, the two explosives were closely related in many respects. In spite of this, quite large differences in the detonation characteristics are observed between the two explosives, both in the wave speed as a function of charge size and in the shape of shock‐wave loci near the explosive edge. It is found that a single forward rate exponentially dependent on the inverse of the local shock pressure can ‘‘explain’’ the homogeneous case observations, but that a rate with much less state dependence near the explosive edge is necessary to understand the observations for the heterogeneous material. Near the failure diameter of both materials, prominent structures are found on the experimental shock‐wave loci. These structures would seem to be incompatible with a steady flow and call into question whether two‐dimensional steady‐state detonations occur at all in condensed explosives near their failure diameter.

ISSN:0031-9171    

DOI:10.1063/1.864270

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Statistics of the streamwise derivative of temperature fluctuations on the centerline and at one streamwise station in the self‐preserving region of a turbulent plane jet have been obtained by two techniques. In one, the derivative is approximated by the difference between the signals from two cold wires separated in the streamwise direction. In the other, the streamwise derivative is inferred from the temporal derivative and different convection velocities. Instantaneous values, root‐mean‐square values, skewness, and flatness factors are in reasonable agreement for a sufficiently large range of separations regardless of whether the mean velocity (Taylor’s hypothesis) or the instantaneous velocity are used as the convection velocity. Corrections for turbulence intensity, that are often applied to Taylor’s hypothesis, do not seem warranted. The difference between spectra of the temperature difference and temporal derivative spectra cannot be resolved by identifying the convection velocity with a frequency‐dependent phase velocity of the temperature fluctuation.

ISSN:0031-9171    

DOI:10.1063/1.864271

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

A simple empirical model is developed for the isotropic turbulent kinetic energy spectrum. The model can define the spectral distribution as a function of Reynolds number at all wavenumbers. It is compared with spectral data from different types of flows over the 13 ≤Re&lgr;≤7000 range.

ISSN:0031-9171    

DOI:10.1063/1.864272

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The asymptotic self‐similar form of cylindrically or spherically imploding shock waves is extracted by numerically solving non‐self‐similar problems. The shock wave is generated by a contracting piston with finite initial velocity. For the initial shock motion, a perturbation method is used to determine the starting condition for the numerical calculation. Propagation of the shock wave and flow field properties are obtained and the transition of the non‐self‐similar motion of the shock wave into the self‐similar one is presented. Good agreement between self‐similar exponents determined from the variation of the shock strength and those calculated by Guderley is obtained.

ISSN:0031-9171    

DOI:10.1063/1.864273

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Particle diffusion transverse to a strong uniform magnetic field is studied within a three‐dimensional electrostatic guiding‐center model in which particle streaming along the magnetic field lines is admitted as well as transverseE×Bdrifting. Only the randomizing influence of thermal fluctuations excited by particle discreteness is considered. The electric field autocorrelation coefficient is obtained in the form of a modified fluctuation‐dissipation theorem. It is shown that because of the competition between the parallel and perpendicular decorrelation mechanisms, the divergence of the diffusion coefficient in the previous finite‐volume theories disappears. The diffusion coefficient is Bohm like (proportional to 1/B) for magnetic fields which are not too strong, and goes over to a classical variation (proportional to 1/B2) in the limitB→∞.

ISSN:0031-9171    

DOI:10.1063/1.864282

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The Rostoker equation has been numerically compared with the Fokker–Planck equation for a plasma with axially symmetric velocity distributions by computing electron slowing‐down rates and velocity‐space diffusion rates due to collisions with background electrons. The results in a case of &OHgr;e∼&ohgr;peagree well with the corresponding quantities obtained from the Fokker–Planck operator. In the case of a strongly magnetized plasma (&OHgr;e≫&ohgr;pe), the results indicate corrections to the Fokker–Planck coefficients which are different from the one suggested by a previous work.

ISSN:0031-9171    

DOI:10.1063/1.864283

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Collisional transport of a high‐temperature plasma across the confining field of a bumpy torus magnetic confinement system depends sensitively upon the functional form of the radial guiding center drift, and thus upon details of the confinement geometry. A general and relatively explicit formula for the radial drift is derived, using the large aspect ratio results of a previous equilibrium study. Allowance is made for (i) arbitrary toroidal variation of the confining field; (ii) field distortion due to plasma currents; (iii) noncircular deformation of the toroidal field coils. However, the analysis pertains only to the plasma core, and not to the high‐beta annuli (electron rings) which are usually present in experiments. The question of bumpy torus omnigenity—whether any bumpy torus field configuration is consistent with a vanishing, or nearly vanishing, radial drift— is also investigated. It is found that omnigenity cannot occur, at least in the vicinity of the magnetic axis.

ISSN:0031-9171    

DOI:10.1063/1.864284

出版商:AIP    

年代:1983

数据来源: AIP