摘要:

Collapse times of minute gas bubbles in both water and a dilute polymer solution were measured precisely by means of the streak schlieren method. It was found that when the bubble becomes smaller in size, the collapse time in a polymer solution is shortened in comparsion with it in water due to the relaxation effect.

ISSN:0031-9171    

DOI:10.1063/1.864661

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

An experimental study of the characteristics of the turbulent field upstream of a flat plate placed normal to the mean flow was performed when the integral scale of turbulence is small with respect to the characteristic dimension of the obstacle. Measurements of the turbulence intensitiesu′1andu2, the spectrumE11( f˜) taken at different distances from the plate, and the pressure spectrum &thgr;pp( f˜) measured at the stagnation point are given in the framework of a comparison with Hunt’s theory.

ISSN:0031-9171    

DOI:10.1063/1.864662

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

Flow visualization experiments are conducted to study the phenomena of separation and reattachment behind a segmented step in subsonic turbulent flow. The results show that the effect of castellations on the step face is to produce strong three‐dimensional disturbances in the initial recirculation region. This is followed by a complex reattachment process and a transition region in which the three‐dimensional disturbances are attenuated, the flow eventually resembling a two‐dimensional flow in the relaxation far downstream.

ISSN:0031-9171    

DOI:10.1063/1.864663

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

The problem of the stability of the universal mode in a sheared magnetic field is considered using the method of matched asymptotic expansions, which, among the analytical approaches, can perhaps provide the clearest physical insight. An eigenvalue equation is derived predicting marginal stability in the limit in which previous papers using the same technique find instability. The discrepancy is explained by showing that the analysis used therein is applicable only in the stability regime.

ISSN:0031-9171    

DOI:10.1063/1.864664

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

Computational simulations indicate that proper programming of the toroidal and poloidal magnetic fields in a reversed field pinch (RFP) can drive a steady‐state current. This current drive technique, calledF‐&THgr; pumping, assumes that the RFP plasma remains in a quasirelaxed state and that the field programming is not deleterious to pinch performace. Field modulation experiments on the Los Alamos ZT‐40M experiment have shown that these assumptions are reasonable, and henceF‐&THgr; pumping is a plausible candidate for RFP current drive.

ISSN:0031-9171    

DOI:10.1063/1.864665

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

Spatially resolved soft x‐ray measurements on the ZT‐40M reversed‐field pinch using a surface barrier diode array are reported. In plasma discharges with x‐ray sawteeth present, the sawteeth have anm=0,n=0 character. In normal discharges, without large‐amplitude sawteeth, a new ly observed internalm=1 oscillation is described. This quasicontinuousm=1 mode may be a manifestation of a saturated magnetohydrodynamic tearing mode.

ISSN:0031-9171    

DOI:10.1063/1.864666

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

Hybrid simulations, which include kinetic ions, demonstrate that them=2 rotational instability in the field‐reversed configuration can be suppressed by the application of an external quadrupole magnetic field without stopping the plasma rotation.

ISSN:0031-9171    

DOI:10.1063/1.864667

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

A flat eddy is defined by the condition &egr;=&dgr;/l≪1, wherel=(l−21+l−23)−1/2, andl1,l3, and &dgr; are the streamwise, spanwise, and vertical scales of the eddy, respectively. An approximate solution of the initial value problem for the evolution of a flat eddy in an inviscid flow is obtained using a Lagrangian formulation together with an expansion of the Euler equations in a Taylor series in time. The effects of horizontal pressure gradients on the eddy evolution are found to be small for small times such that &egr;U0t/l≪1, whereU0is a measure of the initial horizontal velocity. Nonuniformity of the spanwise stretching rate, together with the shearing of the eddy by the mean flow, is found to lead to the formation of internal shear layers, also when such layers were absent initially. The separate contributions to the streamwise fluctuation velocity due to redistribution of background momentum and direct driving by the pressure gradient are compared; the latter is found to be significantly smaller than the former and plays very little role in the shear layer formation process.

ISSN:0031-9171    

DOI:10.1063/1.864668

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

It is shown that resonant triads exist for Kelvin–Helmholtz waves. The loci of various triad modes in wavenumber space are discussed in detail. In most cases of interest one of the three triad modes lies very close to the Kelvin–Helmholtz cutoff wavenumber and hence direct resonance occurs. Evolution equations are first derived in wavenumber space and then transformed into physical space. Interactions among various triad modes occur on a much faster time scale and the amplitudes of various modes are much larger than the amplitudes for a regular triad. The stability of a plane wavetrain subject to perturbations from resonant triad modes is examined.

ISSN:0031-9171    

DOI:10.1063/1.864653

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

The wall‐pressure field in a turbulent channel flow has been obtained from a numerical solution of the Navier–Stokes equations. The root mean square pressure normalized with respect to the wall shear stress, as well as the convection velocity of the field, are in good agreement with experiment. Certain limitations of the calculation, associated with its finite spatial resolution, are discussed.

ISSN:0031-9171    

DOI:10.1063/1.864654

出版商:AIP    

年代:1984

数据来源: AIP