摘要:

The inviscid flow produced by the initial conditions proposed by Taylor and Green illustrates some basic features of the turbulent flow process, namely, the build‐up of the mean square vorticity and the decay of the integral scale. The rate of deformation and the rate of vortex stretching along one direction shows some differences. The deformations occur mostly in lenticular regions. The evolution process of the flow development is discussed.

ISSN:0031-9171    

DOI:10.1063/1.862317

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The drag force on a sphere moving slowly through an incompressible fluid with viscosity depending on distance from the sphere as &eegr;=&eegr;∞[1+(R0/r)s] is calculated. Boundary conditions at the sphere appropriate to a rigid sphere (no‐slip) and to a bubble are considered. The general results are given, as well as specializations to (i) large spheres, (ii) small spheres, and (iii) the cases=4, which originally motivated the calculation.

ISSN:0031-9171    

DOI:10.1063/1.862326

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The gradient of an advected scalar in turbulent flow is treated in a coordinate system moving with a fluid particle. This Lagrangian coordinate system is chosen to be rotating such that the gradient vector lies along the 1‐axis at all times; the final results are independent of this particular coordinate system. In the absence of diffusion, the magnitude of the gradient at the position of the fluid particle is shown to be a lognormal random process. The action of diffusion is to reduce the probability density for the largest values of the gradient.

ISSN:0031-9171    

DOI:10.1063/1.862327

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

It is pointed out that numerical and simulational calculations of the Lagrangian decay time &tgr;Lare easily explained by straightforward analytical approximations. This result demonstrates the usefulness of these approximations for problems of turbulent transport. A calculation is then made of the turbulent diffusion coefficientDin terms of an arbitrary energy spectrum, for homogeneous isotropic turbulence. This relation is compared with previous results.

ISSN:0031-9171    

DOI:10.1063/1.862328

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

One of the well‐known, fundamental features of some free turbulent shear flows, such as the wake downstream of a cylinder, is similarity of the profiles of unconditioned quantities of interest in fluid mechanics such as the mean and higher moments of the temperature. Recent work has indicated that profiles of zone‐averaged, conditioned statistics are also similar. Results are presented which are based on conditioned sampling and show that similarity within the turbulent fluid exists in much greater detail than has heretofore been thought; in particular, the statistical properties of structures having nominally the same characteristic lengths are similar. Additionally, it is shown that these turbulent structures consist of two interface regions having relatively large gradients and a core region with relatively smaller gradients of the temperature moments. These latter gradients indicate preferential entrainment on the downstream edges of the turbulent structures.

ISSN:0031-9171    

DOI:10.1063/1.862329

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The effect of high molecular weight drag‐reducing polymers on flow‐induced cavitation was studied. Cavitation was generated on the surface of a disk rotating in a test chamber. Cavitation inception was determined as a function of flow velocity, temperature, and pressure. Polymeric additives used included polyethylene oxide and polyacrylamide at various concentrations. The results showed that the inception of flow‐induced cavitation was suppressed by the high molecular weight polymers. This suppression was represented by a substantial reduction in the values of the cavitation inception parameter in polymer solutions. Polymeric additives were also found to have a strong effect in reducing the overall size of fully developed cavities. These effects were briefly discussed in terms of polymer viscoelasticity.

ISSN:0031-9171    

DOI:10.1063/1.862330

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

A free boundary problem concerning equilibrium plasma configurations is investigated. It is shown that under some conditions a plasma is only one‐component, i.e., can have only one simply connected component of connectivity. There are cases where a plasma must be two‐component, i.e., have two components of connectivity (each component is simply connected). The conditions under which a plasma can be both one‐component and two‐component are also described. A complete description is given of all (symmetric) plasma configurations for a magnetic field flux &kgr; in a shell with a special cross section &Ggr;&agr;. It is shown that the manifold of all such configurations is homeomorphic to the two‐parameter manifold of the real zeros of some third‐order polynomial with real coefficients dependent on &agr; and &kgr;.

ISSN:0031-9171    

DOI:10.1063/1.862331

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

It is shown that laser fields in plasmas, in addition to the usual ponderomotive force in the center‐of‐mass momentum equation, give rise to significant modifications of the material stresses and additional terms in the Ohm’s law and equation for the heat flow vector. These are evaluated explicitly for the case of a single geometric optics field assuming that the slowly varying part of the electron distribution function is collision dominated, that the equation for the high frequency part can be linearized, and that the effective charge numberZ≫1. The new terms are all roughly proportional to the energy density in the high‐frequency field. They are thus comparable in magnitude with conventional ponderomotive forces.

ISSN:0031-9171    

DOI:10.1063/1.862332

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The effect of a constant, external magnetic field on the bremsstrahlung spectrum of a Maxwellian plasma is investigated. The collisional damping, assumed small, is obtained and by means of Kirchoff’s law, an expression for the bremsstrahlung emission coefficient is written. It is found that the emission is greatly enhanced for certain frequencies and directions.

ISSN:0031-9171    

DOI:10.1063/1.862333

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Transport of a nonneutral electron plasma across a magnetic field is caused by electron scattering from ambient neutral atoms. A theoretical model of such transport is presented, assuming the plasma is quiescent and the scattering is elastic scattering from infinite mass scattering centers of constant momentum transfer cross section. This model is motivated by recent experiments. A reduced transport equation is obtained by expanding the Boltzmann equation for the electron distribution in inverse powers of the magnetic field. The equation together with Poisson’s equation for the radial electric field, which must exist in a nonneutral column, determine the evolution of the system. When these two equations are properly scaled, they contain only a single parameter: the ratio of initial Debye length to initial column radius. For cases where this parameter is either large or small, analytical solutions, or at least partial solutions, are obtained. For intermediate values of the parameter, numerical solutions are obtained.

ISSN:0031-9171    

DOI:10.1063/1.862334

出版商:AIP    

年代:1978

数据来源: AIP