摘要:

The effect of various levels of excitation on the stability and development of an axisymmetric mixing layer was studied experimentally. The flow was excited axisymmetrically by a single speaker placed at the base of the plenum chamber. Measurements of mean and phase‐averaged velocity profiles were made using an array of hot‐wire probes. The measured profiles were compared to eigenfunctions calculated from linear, viscous stability theory. It is shown that the theoretical predictions, based on measured profiles of mean velocity, compare very well with the phase‐averaged measurements, even when the local disturbance reaches levels as high as 24% of the jet speed. The cumulative effect of excitation on the mean flow is examined as a function of local Strouhal number as well as excitation level.

ISSN:0031-9171    

DOI:10.1063/1.866935

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A model calculation is presented to study turbulence at moderate Reynolds number, Re. In conformity with recent measurements, it is proposed that in the energy cascade the fractional volume occupied by eddies of various sizes depends on Re. By introducing a Re‐dependent parameter in the random beta model, it is shown that the scaling behavior of the small‐velocity fluctuations at moderate Re can be characterized by a single Re‐dependent scaling exponent &agr;0. The calculated Re dependence of &agr;0is consistent with the experimental data. Corrections to this scaling are calculated using the multifractal concept. The Reynolds number dependence of other multifractal properties in turbulent flows is also calculated.

ISSN:0031-9171    

DOI:10.1063/1.866936

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Vortex shedding and instability wave frequencies have been measured in the wakes of spheres in the Reynolds number range 500<Re<60 000. The effect of acoustic excitation was examined and an interaction between the two frequency modes was found at the lower Reynolds numbers; through this interaction, external forcing at the instability frequency could change the vortex shedding frequency. The development of the mean wake was manipulated by forcing near to the dominant shear layer instability frequency. With this forcing, the separated shear layer moved closer to the surface of the sphere and the reversed flow region of the wake was shortened. Concomitantly, the base pressure decreased and drag increased.

ISSN:0031-9171    

DOI:10.1063/1.866937

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The excitation of high‐amplitude Langmuir waves in a warm plasma is investigated in the framework of the hydrodynamic model. An integral expression is found describing the maximum possible amplitude in the limit of stationary waves. The steep increase of compression energy in the density peaks is shown to be the reason for this limitation of amplitude. If electron trapping in the potential of the Langmuir wave is taken into consideration, two consequences occur: The limit for the density peaks in the stationary wave is lowered, and the dispersion relation is modified. Because of the trapped electrons, allowed frequency bands appear below the original plasma frequency. Thus plasma waves containing trapped electrons are able to propagate in the overdense region.

ISSN:0031-9171    

DOI:10.1063/1.866938

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Theoretical and computational study for a polarization change of localized and finite amplitude Alfve´n waves propagating parallel to an applied magnetic field is presented using both reductive perturbation theory and numerical simulations. In the magnetohydrodynamic limit (&ohgr;≪&OHgr;i) where right‐ and left‐hand circularly polarized waves are degenerate, one of the transverse components of the circularly polarized Alfve´n wave is stable as it propagates, but the other component is unstable to either self‐focusing or diffraction effects. Consequently, the wave changes its polarization from circular to linear. In the high frequency regime (&ohgr;≳&OHgr;i), where two circularly polarized waves (right‐ and left‐hand circularly polarized waves) are not degenerate, two transverse components of the circularly polarized Alfve´n wave are strongly coupled to each other, and there is almost no polarization change.

ISSN:0031-9171    

DOI:10.1063/1.866939

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A new set of two‐fluid heat transport equations that is valid from collisional to weakly collisional limits is derived. Starting from gyrokinetic equations in flux coordinates, a set of moment equations describing plasma energy transport along the field lines of a space‐ and time‐dependent magnetic field is derived. No restrictions on the anisotropy of the ion distribution function or collisionality are imposed. In the highly collisional limit, these equations reduce to the classical heat conduction equation (e.g., Spitzer and Ha¨rm or Braginskii), while in the weakly collisional limit, they describe a saturated heat flux (flux limited). Numerical examples comparing these equations with conventional heat transport equations show that in the limit where the ratio of the mean free path &lgr; to the scale length of the temperature gradientLTapproaches zero, there is no significant difference between the solutions of the new and conventional heat transport equations. As &lgr;/LT→1, the conventional heat conduction equation contains a significantly larger error than (&lgr;/LT)2. The error is found to beO(&lgr;/L)2, whereLis the smallest of the scale lengths of the gradient in the magnetic field, or the macroscopic plasma parameters (e.g., velocity scale length, temperature scale length, and density scale length). The accuracy of the flux‐limited model depends significantly on the value of the flux limit parameter which, in general, is not known. The new set of equations shows that the flux‐limited parameter is a function of the magnetic field and plasma parameter profiles.

ISSN:0031-9171    

DOI:10.1063/1.866940

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The nonlinear interaction of three waves propagating in an infinite plasma with a finite spectral bandwidth is studied. A Hamiltonian formulation of the interaction is used and, with the help of a projection operator method, generalized Langevin equations are derived for the wave field amplitudes. A simple form of the evolution equations, more complete than the usual fixed‐phase equations, is obtained when the ballistic term and higher‐order corrections of the memory effects in the Langevin equations are neglected. Approximate analytical solutions are derived using a multiple time scale method. These are compared with the results of numerical integration. A number of new qualitative features related to the finite spectral bandwidth are discussed in detail.

ISSN:0031-9171    

DOI:10.1063/1.866941

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The parametric decay process of a fast wave at the ion‐cyclotron range of frequencies (ICRF) into two electromagnetic slow (or ordinary) waves is considered. It is shown that in the dipole approximation of the pump wave, the decay process is possible at around half the frequency of the pump. Plasmas of interest that are susceptible to this decay process include deuterium–hydrogen (D–H), deuterium–helium 3 (D–He3), and deuterium–tritium (D–T) plasmas. It is shown that the growth rate may approach a few percent of the pump frequency at the edge, and may thus lead to edge electron heating.

ISSN:0031-9171    

DOI:10.1063/1.866942

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The high beta regime of the lower hybrid drift instability is studied using a fully electromagnetic dispersion relation in the local approximation. The electrons are described using a kinetic model that includes the effect of finite beta orbit modifications and three different models (unmagnetized, magnetized without ∇Bdrifts, and magnetized with ∇Bdrifts) are used for the ions. It is found that when magnetized ions without ∇Bdrifts are considered, the modes remain unstable for larger values of beta and the spectrum is very different from the unmagnetized ion case. When ∇Bdrift effects are included, the results became very similar to the unmagnetized ion case and some arguments are provided to explain this behavior.

ISSN:0031-9171    

DOI:10.1063/1.866943

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Current drive by electron cyclotron waves in toroidal geometry is investigated by means of a bounce‐averaged Fokker–Planck code. The case of the extraordinary mode at a frequency lower than the electron gyrofrequency is studied in detail for a wave packet of finite width, carrying a power in the megawatt range, which is absorbed away from the plasma center. It is shown that in this case, because of the interplay of quasilinear and trapping effects (i) the current drive efficiency is strongly power dependent and (ii) wave absorption at the inboard part of the magnetic surfaces does not maximize the efficiency, in contrast to the results of linear theory.

ISSN:0031-9171    

DOI:10.1063/1.866944

出版商:AIP    

年代:1988

数据来源: AIP