摘要:

The viscous buckling phenomenon studied experimentally by Suleiman and Munson is discussed theoretically. In the linear theory used, no true instability is found, but the steepness of a wavy disturbance may grow in time when the dimensionless shear stress exceeds (3/2)(2)1/2. Maximal kinematic contraction horizontally determines a critical angle equal to 35.27° between the wave crests of the preferred disturbance and the channel walls.

ISSN:0031-9171    

DOI:10.1063/1.864895

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

The stability of the Taylor vortex state in a circular Couette geometry is investigated by measuring its transition to a wavy vortex state. Seven different radius ratios, ranging from 0.7767 to 0.7972, are used to map out the stability boundaries in the Reynolds number–radius ratio plane. The results verify several important features predicted by Jones.

ISSN:0031-9171    

DOI:10.1063/1.864896

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

It is shown that density profile modifications near a plasma surface can survive at moving localized spots because of the radiation pressure of leaking wave field fluctuations. The properties of these luminous surface cavitons are studied.

ISSN:0031-9171    

DOI:10.1063/1.864897

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

Harmonic generation and parametric decay are examined in a toroidal ACT‐1 plasma using electrostatic plate antennas. The harmonic generation, which is consistent with sheath rectification, is sufficiently strong that the nonlinearly generated harmonic modes themselves decay parametrically. Resonant and nonresonant parametric decay of the second harmonic are observed and compared with uniform pump theory. Resonant decay of lower‐hybrid waves into lower‐hybrid waves and slow ion‐cyclotron waves is seen for the first time. Surprisingly, the decay processes are nonlinearly saturated, indicating absolute instability.

ISSN:0031-9171    

DOI:10.1063/1.864898

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

A high‐power ordinary wave launched from the low field side is observed to undergo nonlinear mode conversion process and is partially transformed into a short‐wavelength electrostatic upper‐hybrid wave at the upper‐hybrid layer. The nonlinear conversion process can be explained as due to a periodic modulation of upper‐hybrid frequency at twice its value, while the frequency modulation comes from the combination of electron mass oscillation and oscillating density modulation induced by the ordinary wave.

ISSN:0031-9171    

DOI:10.1063/1.864899

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

By axially tapering the current density at each end of an axial pinch, a scheme is developed for injecting electrons across magnetic field lines into a modified betatron accelerator. This scheme produces only a minimal perturbation to the circulating electron beam. The axial pinch provides beam equilibrium in the transverse field and the tapering reduces the nonadiabatic growth of the perpendicular particle velocity from 0.5cto 0.05cfor the parameters of the Naval Research Laboratory modified betatron.

ISSN:0031-9171    

DOI:10.1063/1.864907

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

The flow that results when a potential vortex rotates normal to a stationary horizontal disc is considered. This takes the form of an inward flowing boundary layer on the disc which effuses into, and ultimately forms, the core of the vortex. The pressure on the disc falls sharply toward the vortex axis and, over the boundary layer region, is essentially that of the outer flow. The radius at which the base pressure field first deviates appreciably from that of the interior is defined asr1, the radius of the effusing core. This radius and the pressure coefficient at the disc center,C0(0), are determined from radial pressure measurements on the disc over the range of Reynolds numbers, Re, from 3.0×103to 3.0×104, where Re is based on the radius and the velocity at the disc edge. Three phases are observed as Re is varied. In the first the flow is laminar,r1∝Re−1/2andC0(0)+1∝Re. The radiusr1continues to decrease andC0(0) to increase with Re in the second phase (1.0×104<Re<1.55×104), but faster than in the first, while in the thirdr1grows andC0(0) falls. The behavior in the latter phases is thought to reflect a transition to turbulence, but several puzzling features are present and further experiments are necessary.

ISSN:0031-9171    

DOI:10.1063/1.864900

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

In order to explore the relationship between the large‐ and small‐scale motions in turbulent shear flows, a number of flows have been studied based on short‐time correlation measurements. The shear flows investigated are boundary layers, plane and axisymmetric mixing layers, plane wakes and the far fields of plane and circular jets. The coupling between the scales has been obtained by correlating the low‐frequency component of theu‐velocity signal with a signal that is similar to the envelope of the high‐frequency part of the velocity signal. The coupling is found to be significant in all flows. Phase reversal across the shear region is found to occur in the boundary layers and the mixing layers only. This is interpreted to mean that in boundary layers and mixing layers, the streamwise extent of the large structure over which the small‐scale activity reaches a peak moves at one transverse end of the large structure over which its ensemble averagedufluctuation is positive to another where it is negative. This phase shift (&bartil;180°) of the location of the peak activity of the small scales with respect to the large structure takes place gradually resulting in a midlayer where the phase difference is about 90°. On the other hand, in the far field of the jets and wakes, all across the layer, the peak activity of the small scales always remains confined to that half of the streamwise extent of the large structure where its ensemble averagedufluctuation is positive; in the remaining streamwise half of the large structure, the small scales remain dormant.

ISSN:0031-9171    

DOI:10.1063/1.864901

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

The variational approach to the closure problem of turbulence theory, reported in an earlier paper, is applied to the study of the universal equilibrium range of turbulence. Two integral equations are derived for two unknown functions, taking account of viscous dissipation. The equation‐error method of parameter estimation of control theory is used to obtain the approximate solution of the two integral equations. The resulting energy spectrum for the universal equilibrium range isE(k)=&egr;2/3k−5/3F(k/kd) withF(x)=1.19(1+5.3x2/3) ×exp(−5.4x4/3). Here &egr; is the energy dissipation rate andkdis the Kolmogorov wavenumber. The corresponding one‐dimensional energy spectrum and dissipation spectrum are calculated and are in agreement with the experimental results. SinceF(x) is not a monotonically decreasing function, but has a maximum nearx=0.1, the usual experimental values of the Kolmogorov constant will be greater than its theoretical valueF(0)&bartil;1.2 and will depend upon the range ofk/kd.

ISSN:0031-9171    

DOI:10.1063/1.864902

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

The ion‐acoustic decay instability is investigated for a finite‐length plasma with density somewhat below the cutoff density of the electromagnetic driver (n∼0.7nc). For this regime, the heating in a very long system can overpopulate the electron tail and cause linear saturation of the low phase velocity electron plasma waves. For a short system, the instability is nonlinearly saturated at larger amplitude by ion trapping. Absorption can be significantly increased by the large‐amplitude ion waves. These results compare favorably with microwave experiments.

ISSN:0031-9171    

DOI:10.1063/1.864877

出版商:AIP    

年代:1984

数据来源: AIP