摘要:

Values of the amplification rate and torque for Taylor‐vortex flows are given for radii ratios of 0.95 and 0.5 for several Taylor numbers greater than critical and for a number of different axial wavenumbers. The relevance of the results to experimental observations is discussed.

ISSN:0031-9171    

DOI:10.1063/1.861852

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Laminar source flow between co‐rotating disks can often be adequately described by boundary layer equations. However, when the disk spacing and rotation speed are large (small Ekman number) and the source strength is small (small Rossby number), recirculation occurs near the edges of the disks and the boundary layer equations cannot correctly describe the flow there. The flow is studied in the vicinity of the disk edges for swirl Reynolds number of order 106using approximate equations that are valid in a small region far from the rotation center. Numerical solutions are obtained by finite differences for three values of the Ekman numberEand several values of the Rossby number &egr;. The Ekman numbers chosen give flows between the disks characterized by linearized theory, boundary layer theory, and rotating flow theory with Ekman layers.

ISSN:0031-9171    

DOI:10.1063/1.861853

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Results showing the three‐dimensional vortex shedding structure when a sphere is towed at a constant velocity through a stratified fluid are presented. It is found that for small Richardson numbers (weak stratification) and Reynolds numbers in the range from 4×103to 2×104the vortex is shed three‐dimensionally. However, stratification quickly and effectively inhibits the vertical motion and the initially turbulent wake collapses and reveals the vertically oriented portion of the vortex structure, reminiscent of a two‐dimensional vortex street behind a circular cylinder when viewed from above. The structure is, however, distinctly three‐dimensional. It is also found that the estimated vortex shedding frequency is in reasonable agreement with previously published results for a sphere in a homogeneous fluid. It is suggested that a weak stratification is an excellent means for revealing the vortex structure of a three‐dimensional body in a homogeneous fluid, and that the vortex tube in the wake of a sphere in a homogeneous fluid has a closed‐end double helical structure. Two branches of the double helix are continuously unwinding in an opposite sense from the formation region. Moreover, the present double helical model satisfies Thompson’s circulation theorem in contrast to previously proposed helical models.

ISSN:0031-9171    

DOI:10.1063/1.861854

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Hot‐wire anemometry and digital techniques are used to develop time series for the streamwise and transverse velocity components and for the mass fraction of helium in the low‐speed turbulent boundary layer of air involving the slot injection of helium. Profiles are developed at several streamwise stations. In the present contribution the data are presented principally in terms of conventional, unconditioned averages. It is found that the mean velocity and concentration profiles develop in the downstream direction in the expected fashion but that the intensities of the velocity, concentration, and density fluctuations increase sharply downstream of the slot and peak in the region where the mixing region from the splitter plate impinges on the wall. Close to the wall where the helium concentration is relatively high counterfluxes of helium are found. The data at one streamwise station are reanalyzed in terms of Favre‐averaging so that the two means of averaging can be compared in a flow involving significant density fluctuations.

ISSN:0031-9171    

DOI:10.1063/1.861855

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

A plane wall jet over a convex surface with constant radius is studied experimentally. Classical and intermittent properties of the flow are analyzed. The distribution of the Reynolds shear stress is examined and the contributions of the two centers of turbulent production are emphasized, especially in the region where the mean velocity is a maximum. All the terms included in the turbulent kinetic energy budget and in the Reynolds stress equation are measured or evaluated. Two interfaces are considered: the first one located in the edge region of the jet, the second one in the inner part located in the vicinity of the ordinate where the velocity is a maximum. Also, two intermittency factors are determined. Accordingly, conditional mean velocities can be measured. Results indicate that the wall region seems strongly dependent on the jet region.

ISSN:0031-9171    

DOI:10.1063/1.861856

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Application of the photon correlation technique to the measurement of sinusoidally perturbed fluid flows is described. An expression is derived for the shape of the count correlation function in terms of mean flow velocity and perturbation amplitude. Measurements have been made both in the wake behind a circular cylinder and with a mechanical flow simulator. These show that perturbation amplitudes can be measured by direct visual observation of the correlation displays. Maximum entropy transformation and curve fitting techniques are also described which produce results of higher accuracy.

ISSN:0031-9171    

DOI:10.1063/1.861857

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The momentum transferred from the ions to the neutrals in the positive column of a discharge is derived from the kinetic equation of the ions. A weakly ionized plasma, charge exchange collisions between the atomic ions and the neutrals at constant collision frequency, and the thermal energy of the neutrals small relatively to the field energy of the ions are assumed. For the collision dominated case the expression for the force on the neutrals is that obtained by Leiby and Oskam for low field energy, multiplied by a factor of 2. The calculated pressure gradient in the neutral gas is compared with the experimental results of Bergman and Chanin for helium on the high pressure side of the maximum.

ISSN:0031-9171    

DOI:10.1063/1.861858

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

A theory is developed to determine the parameters for simulation of the hypervelocity impact of small particles on a surface by focusing a high power pulsed laser upon that surface. It is proposed that the early time history of hypervelocity impact effects can be simulated by requiring that the laser induced surface pressure equal the surface impact pressure, equating the laser pulse time to a particle/surface interaction time, and setting the focal spot diameter equal to the particle diameter. Calculations of impact pressure versus projectile velocity are performed using an idealized one dimensional model for shock wave propagation. A simplified theory for the interaction of a highly focused pulsed laser beam with a surface is developed to obtain the laser induced surface pressure. Results of a sample calculation for water droplets impacting a graphite surface between 3 and 6 km/sec indicate that laser intensities of 5×1010–5×1011W/cm2are required. The corresponding laser energies are strongly particle size and shape dependent and range from 0.01 J with a 5 nsec pulse time for 20 &mgr; radius particles to 2 J with a 20 nsec pulse time for 80 &mgr; radius particles. A qualitative comparison of data for high power laser produced craters and hypervelocity impact produced craters is also presented.

ISSN:0031-9171    

DOI:10.1063/1.861859

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The results of a numerical modeling of the Los Alamos implosion heating experiment, based on self‐consistent inclusion of turbulent effects is presented and the simulation results along with the experimental measurements are analyzed. The computer simulation shows very strikingly the mechanisms involved in the experimentally observed plasma motion. The structure of the density profile is explained and the implosion and diffusion of the magnetic field seen in the experiment are reproduced in the simulation.

ISSN:0031-9171    

DOI:10.1063/1.861860

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

A stationary, spherical flow model gives the form of laser‐driven ablation fronts and scaling laws for the dependence of implosion parameters on laser wavelength, pusher atomic number, and other input quantities.

ISSN:0031-9171    

DOI:10.1063/1.861861

出版商:AIP    

年代:1977

数据来源: AIP