摘要:

Particle simulations of the modified two‐stream instability demonstrate strong electron acceleration rather than bulk heating when the relative drift speedvdis below a critical speedVc. A very interesting nonlinear autoresonance acceleration process is observed which accelerates the electrons much above the phase speed of the linearly unstable modes. Simple criteria are presented that predict the value ofVcand the number density of the accelerated electrons.

ISSN:0031-9171    

DOI:10.1063/1.864368

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Utilizing the formalism of path integrals, the intensity moments of waves propagated through anisotropic inhomogeneous random media in the presence of a deterministic wave speed variation can be formed. In the saturated scattering regime, intensity fluctuations are expected to be Rayleigh distributed to first order. A correction to Rayleigh statistics is calculated that results in a form for the intensity moments: ⟨In⟩=n!⟨I⟩n[1+ 1/2n(n−1)&ggr;], where &ggr; depends on the fluctuation spectrum of the medium. It is shown that the correction can be approximated by an integral along the ray of a spectrally weighted local contribution. This local contribution, called the micropath focusing function, depends both on the spectrum of the medium, and on a Green’s function which is shown to be related to the phase curvature. In the straight line horizontal ray approximation, the ‘‘micropath focusing parameter’’ &ggr; can be approximated by 3/(2 ln &Fgr;) in the partially saturated regime, and by [0.83/(ln &Fgr;)3/2](1/&Lgr;&Fgr;) in the saturated regime, where &Fgr; and &Lgr; are the strength and diffraction parameters for medium fluctuations along a ray. The internal wave spectrum is chosen as an example for fluctuations in oceanic sound transmission. Theoretical results are obtained for curved rays by numerical integration, and are compared with observations from three ocean acoustics experiments.

ISSN:0031-9171    

DOI:10.1063/1.864369

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Expressions are derived for the rheological coefficients appropriate to the simple shearing, pure straining, and pure vortex motions of dilute cubic arrays of spherical particles suspended in Newtonian liquids, including simple, body‐centered, and face‐centered systems. Comparable asymptotic coefficients are also given for concentrated suspensions of almost touching spheres in simple cubic array in the lubrication‐theory limit.

ISSN:0031-9171    

DOI:10.1063/1.864370

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The nonlinear electrohydrodynamic stability of two superposed dielectric fluids is studied. The system is stressed by a normal electric field such that it allows for the presence of surface charges at the interface. The method of multiple scale perturbations is used to obtain two nonlinear Schro¨dinger equations describing the behavior of the disturbed system. The stability of the perturbed system is discussed both analytically and numerically and stability diagrams are obtained. The electric field plays a dual role in the sense that it can stabilize the system for relatively large values of wavenumbers and small values of the density ratio and vice versa. The stabilizing range of the electric field for some given density ratios is tabulated. The analogy with hydromagnetic stability is established. Expressions for the surface elevation and the cutoff wavenumber separating stable and unstable disturbances are also obtained.

ISSN:0031-9171    

DOI:10.1063/1.864371

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Nonlinear interfacial gravity waves in a two‐layer Boussinesq fluid are studied. In a previous paper, nonlinear waves on a vortex sheet separating two layers, each of constant density and velocity were considered. In the present paper the basic flow model consists of a constant vorticity upper layer bounded by a rigid surface and an irrotational lower layer of infinite depth with continuity of the unperturbed velocity at the density interface. Numerical solutions obtained from an exact formulation in terms of a complex‐valued integral equation for the shape and local vortex‐sheet strength of the wave profile are compared with results from a second‐order Stokes expansion. It is found that the wave of maximum amplitude displays different geometrical features depending on the unperturbed flow parameters. These include waves containing an S‐shaped section, waves with cusped crests and surface‐constrained waves with long flat crests. Wave integral properties calculated, including the flux of momentum and energy in the wave propagation direction, showed monotonic variation with increasing wave amplitude.

ISSN:0031-9171    

DOI:10.1063/1.864372

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Experiments on the flow development along the first diameter of a low Mach number jet are described. It is shown that the flow is governed by a local shear layer instability coupled with a feedback mechanism that determines the positions of the nonlinear amplitude saturation of the unstable modes. The spreading is a stepwise phenomenon associated with the saturation locations. In general these locations move randomly in time and space, resulting on the average in a linear spreading rate.

ISSN:0031-9171    

DOI:10.1063/1.864373

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

A comprehensive experimental examination is presented of the effect of hot‐wire length for a single‐wire probe. Specially constructed hot wires, with 2.5 &mgr;m and 5.0 &mgr;m nominal diameters and lengths from 0.3 mm to 2.8 mm, were used to measure the one‐dimensional spectrum of the streamwise fluctuating velocity componentuas well as the skewness and flatness of bothuanddu/dt. All measurements were made in air atY+=150 (corresponding toY/&dgr;=0.1) in a boundary layer growing inside a pipe. The Reynolds number was 37 000 based on the velocity at the centerline and on the boundary‐layer thickness. The longest wires exhibited some attenuation at the highest wavenumbers. This attenuation was found to roughly confirm the correction to the spectrum due to Wyngaard. A correction for the microscale &lgr;, and thus dissipation, was examined and found to be correct to a fair degree (within 8%). This correction is based on the early work of Skramstad and of Frankiel, but with the integral scale ofR11(0, 0,r) assumed to equal the &lgr; scale. The measurements of skewness and flatness did not indicate an experimentally discernible effect of the length of the hot wire.

ISSN:0031-9171    

DOI:10.1063/1.864374

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The non‐self‐similar implosion of cylindrical shock and detonation waves generated by an instantaneous energy release at a cylindrical wall is analyzed theoretically by the method of integral relations. The analysis shows that as the wave propagates towards the axis, the solutions tend to approach but never reach the self‐similar implosion limit. The rate of approach appears to be slower than expected, and the region of applicability of the self‐similar solution appears to be restricted to a very small region behind the front. This tendency is more pronounced for the detonation case. It is also demonstrated that for detonations where the initiation energy is negligible in comparison with the chemical energy, the Chapman–Jouguet detonation jump conditions apply at the front except near the axis and near the outer wall. The chemical heating in the detonation process increases the pressure and the temperature but considerably reduces the density near the front.

ISSN:0031-9171    

DOI:10.1063/1.864375

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The damping process for ultrarelativistic plane, cylindrical, and spherical waves is studied with the help of a model of the asymptotic degeneration of the shock wave in a quasi‐acoustic wave. Simple formulas are derived for the dependence of parameters of a shocked gas as functions of the distance of the shock front from the source.

ISSN:0031-9171    

DOI:10.1063/1.864376

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The quasistatic slow plasma response to an intense electromagnetic wave is shown to result in a solitary wave structure of an arbitrary amplitude.

ISSN:0031-9171    

DOI:10.1063/1.864350

出版商:AIP    

年代:1983

数据来源: AIP