摘要:

Random‐walk models have long been used to calculate the dispersion of passive contaminants in turbulence. When applied to nonstationary and inhomogeneous turbulence, the model coefficients are functions of the Eulerian turbulence statistics. More recently the same random‐walk models have been used as turbulence closures in the evolution equation for the Eulerian joint probability density function (pdf) of velocity. There are, therefore, consistency conditions relating the coefficients specified in a random‐walk model of dispersion and the Eulerian pdf calculated using the same random‐walk model. It is shown that even if these conditions are not satisfied, the dispersion model does not violate the second law of thermodynamics: all that is required to avoid a second‐law violation is that the mean pressure gradient be properly incorporated. It is also shown that for homogeneous turbulence the consistency conditions are satisfied by a linear Gaussian model; and that for inhomogeneous turbulence they are satisfied by a nonlinear Gaussian model.

ISSN:0031-9171    

DOI:10.1063/1.866127

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Discrete tones generated from the interaction of high‐speed jets with solid boundaries are investigated experimentally. Measurement of far‐field pressure waves and Schlieren flow visualization were conducted. Emphasis is placed upon the generation mechanism of a discrete tone that is generated when a circular cylinder is placed in a jet at a high Reynolds number. This discrete tone was observed to be generated for jet exit Mach numbers greater than 0.6 and distances from the nozzle lip to the cylinder less than about eight times the nozzle diameter. By the Schlieren flow visualization, it is confirmed that this discrete tone is radiated from the jet by a strong feedback resonance. When an eddy (a ring vortex) interacts with the cylinder, an acoustic pulse is emitted with a strong directional peak in the upstream direction. The acoustic pulse propagates outside the jet and, upon reaching the nozzle lip, forces the generation of a new eddy. As the new eddy approaches the cylinder it regains the strength of the original eddy by extracting energy from the potential core of the jet and so maintains the cycle.

ISSN:0031-9171    

DOI:10.1063/1.866128

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A conditional sampling technique was designed and employed to determine the ensemble‐averaged flow structures in the developing region of a symmetric wake (r=1.0) and an unsymmetric wake (r=0.5). It was found that at a velocity ratio of 0.5 the large scale structure, which was observed from a reference frame traveling with its phase speed, is initially confined in the high‐speed stream, elongated in the stream direction, and is shaped elliptically. As the flow convects downstream it grows in the cross‐stream direction, as well as tilting toward the wake center, and then evolves into a nearly circular shape. Results for a velocity ratio of 1.0, corresponding to the case of a 2‐D wake flow, show that two rows of counter‐rotating vortices develop in two streams originating in the linear instability region. Similar to the case forr=0.5, these vortices grow, as well as tilt, when the flow convects them downstream. However, the growth rate of these flow structures is slower than that of the case forr=0.5.

ISSN:0031-9171    

DOI:10.1063/1.866129

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Yakhot and Orszag have recently developed a theory of turbulence [J. Sci. Comput.1, 3 (1986)] based on dynamic renormalization‐group (RNG) techniques. They predict parameters of the Kolmogorov inertial range and then successfully use eddy‐viscosity formulas from the inertial‐range theory in computations of shear flows. In the present paper a critical analysis of the Yakhot–Orszag theory is offered based on comparison with a simple perturbative model. The latter appears to parallel much of the physical and operational content of the lowest order of the Yakhot–Orszag theory, without using RNG methods. The essence is as follows: (1) the dynamics of modes in the inertial and dissipation ranges are assumed to be dominated by interactions more‐or‐less local in wavenumber that are modeled by a white‐noise force acting against an effective viscosity; and (2) the effective viscosity is estimated by extrapolation from the small contributions of interactions very nonlocal in wavenumber (distant interactions). In common with the Yakhot–Orszag theory, the only explicit contacts with the Navier–Stokes equation are overall energy conservation by nonlinear terms and the interaction coefficients of highly nonlocal wave vector triads.

ISSN:0031-9171    

DOI:10.1063/1.866130

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The relativistic extensions of the stability properties of shock waves in classical gasdynamics are derived and studied in detail. In particular, the shock front instability resulting from the spontaneous emission of acoustic waves is thoroughly investigated.

ISSN:0031-9171    

DOI:10.1063/1.866131

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Symmetry properties are presented for a multidimensional dispersion functional. If the system of linearized Vlasov‐field equations is ‘‘completely Hamiltonian,’’ the dispersion operator satisfies a certain formal self‐adjointness property as a function of omega. For appropriate boundary conditions this implies a relation between an eigenfunction and its dual. If the equilibrium admits ‘‘conjugate orbits’’ for a completely Hamiltonian system and if the ‘‘conjugate‐orbit parity condition’’ is satisfied, then the kinetic part of the dispersion matrix is symmetric. For this case and for appropriate boundary conditions the entire dispersion matrix for the multispecies Vlasov or Vlasov‐fluid models is symmetric. It then follows that the complex conjugate of the dual eigenfunction is proportional to the eigenfunction itself. The analytic continuation of the dispersion functional of the linearized Vlasov‐field equations into the lower half of the frequency plane is derived.

ISSN:0031-9171    

DOI:10.1063/1.866132

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Marfes are toroidally symmetric bands of high density radiating plasma that form at the edge of tokamak plasmas. The marfe results from a process of radiative condensation: A local increase in the plasma density increases the radiation rate and lowers the temperature, allowing the density to rise further to maintain pressure balance. It is demonstrated that the marfe onsets when the plasma density exceeds a critical threshold that is just below the density limit, in agreement with observations. This threshold results from a balance between condensation and cross‐field thermal flux from the central hot plasma. Finally, it is noted that radiative condensation is also the driving mechanism of solar prominences and other astrophysical objects.

ISSN:0031-9171    

DOI:10.1063/1.866133

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The stability of a sheared configuration with high beta is studied by investigating the normal modes of the linearized magnetohydrodynamic equations. First, a single ordinary differential equation is obtained that governs the behavior of modes that are localized around rational surfaces. In addition, the equation that describes the properties of ballooning modes is derived.

ISSN:0031-9171    

DOI:10.1063/1.866134

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A gyroaveraged Lagrangian is used to describe the motion of a single electron undergoing anomalous Doppler resonance with a wave that has both electrostatic and electromagnetic components, propagating at arbitrary inclination to the magnetic field. It is shown that the flows of parallel and perpendicular energy are oppositely directed, and have magnitudes in the ratio 1+&ohgr;/(&OHgr;/&ggr;):1. The change in perpendicular kinetic energy, or equivalently Larmor radius, is related very simply to theE×Bdrift occurring at Landau resonance. The classical single‐particle description of the anomalous Doppler resonance underlies the existing classical collective descriptions, and is the classical counterpart to the well‐known quantum single‐particle description.

ISSN:0031-9171    

DOI:10.1063/1.866135

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Trapped‐particle effects on electron–cyclotron resonance heating (ECRH) and lower‐hybrid current drive are studied using the adjoint formalism. A square‐well model for the collision operator is used to obtain an analytic form for the Green’s function that is shown to be quite accurate. In constructing the current, the wave‐heated distribution function is allowed to have an effective temperature along the velocity‐space resonance curve that is different from the bulk temperature of the scattering electrons. These calculations are used to illustrate the deleterious effect of heating on the outside of a flux surface. Relativity and strong heating are each shown to increase current‐drive efficiency for heating on the inside of the flux surface withY≡harmonic number×cyclotron frequency/wave frequency less than 1, and decrease or reverse the driven current in the opposite limits. A significant limit on ECRH current drive in a mildly relativistic plasma, resulting from an upper bound on the minimum resonant energy, is demonstrated.

ISSN:0031-9171    

DOI:10.1063/1.866136

出版商:AIP    

年代:1987

数据来源: AIP