摘要:

The deformation of a free surface caused by the roll up of a vortex sheet below the surface is studied. The large amplitude motion depends on both the strength and depth of the vortex sheet. A distinction is made between three different scenarios of the free‐surface motion: a breaking wave, entrainment of air, and the generation of relatively short surface waves.

ISSN:0031-9171    

DOI:10.1063/1.866713

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Numerical calculations of vortex breakdown generated within a closed circular cylinder by rotation of one of the end walls have been performed. The solutions are used to compute steady streak‐line patterns for dye introduced both symmetrically and asymmetrically about the symmetry axis upstream of the breakdown bubble. Beginning from such a steady state, the end‐wall angular speed is impulsively increased and the asymmetric streak‐line pattern is observed during a portion of the subsequent period of unsteady flow.

ISSN:0031-9171    

DOI:10.1063/1.866714

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Using a counterstreaming electron beam to support a nonlinear wakefield for particle acceleration is investigated theoretically. This scheme has the following advantages over the plasma wakefield accelerator (PWFA): (1) easier beam loading; (2) higher transformer ratio; and (3) easier beam‐pulse shaping. Additionally, the achievable accelerating gradient (GeV/m) is as large as that which can be obtained with a PWFA. A numerical comparison between this scheme and the PWFA is given.

ISSN:0031-9171    

DOI:10.1063/1.866715

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The dispersion of a passive tracer resulting from flow through heterogeneous porous media is studied. When the correlation length for the permeability fluctuations is finite, a normal diffusive process, with the mean‐square displacement of the tracer growing linearly with time, is obtained at long times. However, it is shown that, when the correlation length diverges,anomalousdiffusion occurs in which the mean‐square displacement growsfasterthan linearly with time. The space–time evolution of the tracer’s concentration is calculated and shown to be universal—uniquely related to the covariance of the permeability field—in the anomalous regime.

ISSN:0031-9171    

DOI:10.1063/1.866716

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Joukovskii’s model [J. Russian Physico. Chem. Soc.22, 19 (1891)] for a two‐dimensional bubble rising in an unbounded fluid is considered. This model approximates the wake behind the bubble by a region bounded by vertical walls. By using an inverse method, Joukovskii obtained an exact solution characterized by a Froude numberF=U/(gL)1/2= (2&pgr;)−1/2. HereUis the velocity at which the bubble rises,gis the acceleration of gravity, andLis the distance between the vertical walls. It is shown numerically that Joukovskii’s solution is not unique. When surface tension is neglected there is a solution for each value of 0<F<Fc, whereFc∼0.66. In addition there are solutions with a cusp at the apex of the bubble forF>Fc, and an isolated solution with a 120° at the apex forF=Fc. When surface tension is taken into account there is a discrete set of solutions. Each of these solutions corresponds to a different value ofF. As the surface tension tends to zero, all these solutions approach a unique solution. The numerical results indicate that this limiting solution is Joukovskii’s exact solution.

ISSN:0031-9171    

DOI:10.1063/1.866717

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Steady, inviscid, incompressible, two‐dimensional flows with vortex patches bounded by vortex sheets (Batchelor flows) are calculated numerically. Two particular cases are considered: the vortex on a plane wall (Sadovskii vortex) and the vortex in a right‐angled corner. Nonlinear integral equations are derived for the shape of the bounding vortex sheet which are solved numerically. Two different formulations are employed to check the results. Previous results by Sadovskii [Appl. Math. Mech.35, 773 (1971)] and Chernyshenko (Royal Aircraft Establishment library translations Report No. 2133, 1983) for specific values of the parameters are confirmed. Only symmetrical solutions are found to exist.

ISSN:0031-9171    

DOI:10.1063/1.866718

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The nonlinear evolution of periodic disturbances on vortex trails is considered. In addition to following small initial perturbations, large amplitude initial disturbances of the vortex trails are also studied. It is shown that the equations support a rich variety of essentially nonlinear solutions including unbounded and quasisteady ones. These solutions are found to correspond to various modes of vortex clustering in the physical plane. At the close of the paper, comparisons of these results with recent numerical and experimental findings on the wakes behind stationary cylinders, and also transversely oscillating bluff objects, are made.

ISSN:0031-9171    

DOI:10.1063/1.866719

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The linear parallel and incompressible stability of a family of bluff‐body wake profiles is studied at Reynolds numbers close to the onset of Ka´rma´n vortex shedding. The family of mean flow profiles allows for the variation of the wake depth as well as for a variable ratio of wake width to mixing layer thickness. The absolute or convective nature of the sinuous instability is determined as a function of the profile parameters and Reynolds number. A comparison of this survey with experimental data shows that in bluff‐body near wakes a region of local absolute instability begins to form at a Reynolds number of approximately one‐half the critical value for Ka´rma´n vortex shedding. Hence, at the onset of the global response (Ka´rma´n vortex shedding), a substantial region of local absolute instability already exists in the wake. This confirms the qualitative model prediction of Chomaz, Huerre, and Redekopp [submitted to Phys. Rev. Lett.] and also shows that the prediction of vortex shedding frequencies, when based on local stability properties alone, is somewhat arbitrary evenatthe critical Reynolds number.

ISSN:0031-9171    

DOI:10.1063/1.866720

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The results of a series of detailed numerical simulations of the Rayleigh–Taylor instability in laser ablatively accelerated targets are presented for a fairly wide range of initial conditions. It is shown that the Rayleigh–Taylor growth rate in an ablative environment is a strong function of the laser wavelength. For perturbation wavelengths about three times the in‐flight target thickness, the ratios of the numerical growth rates to the classical growth rates are of the order of 1/1.5, 1/2.5, and 1/3.5 for 1, 1/2 , and 1/4 &mgr;m laser light, respectively. The numerical results are in good agreement with the theoretical model presented here based on the ablative convection of vorticity away from the unstable ablation front. These results provide strong evidence for the viability of high‐aspect‐ratio shells in direct‐drive laser fusion.

ISSN:0031-9171    

DOI:10.1063/1.866782

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Results of an experimental and numerical study of a turbulent free jet of air from a sharp‐edged square slot are presented. The jet was treated as elliptic in the numerical study that predicted mean streamwise velocity, mean static pressure, and turbulence kinetic energy distributions. The experimental results include the mean velocity, the turbulent normal and shearing stresses obtained with hot‐wire anemometry, and the mean static pressure acquired with a pitot‐static tube in conjunction with a pressure transducer. The experimental results reveal the existence of pronounced mean streamwise velocity off‐center peaks in the very near field, where positive mean static pressures were also found. Furthermore, the square jet is found to spread significantly faster in the near flow field than a jet from a round slot that has the same exit area as the square slot under identical test conditions. The general agreement between experimental results and numerical predictions is relatively good.

ISSN:0031-9171    

DOI:10.1063/1.867007

出版商:AIP    

年代:1988

数据来源: AIP