摘要:

A linear secondary instability mechanism is presented that leads to the occurrence of subharmonic three‐dimensional disturbances in wall‐bounded shear flows. The instability originates from the periodic redistribution of vorticity in the shear flow by small but finite‐amplitude Tollmien–Schlichting waves. Low threshold amplitudes and other characteristics of this instability are consistent with experiments and may elucidate various obscure observations.

ISSN:0031-9171    

DOI:10.1063/1.864226

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Turbulence is measured by resonant fluorescence of sodium atoms seeded into a supersonic nitrogen jet. By varying the frequency of the pumping laser, the mean square velocity, temperature, and pressure fluctuations (as well as their correlations) may be determined.

ISSN:0031-9171    

DOI:10.1063/1.864227

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Under what conditions turbulent motions produced by two different mechanisms can coexist and have energy spectra separated by a gap, as often observed in astro‐ and geophysical flows, are investigated. The dynamical consistency of such a gap is illustrated using a two‐point statistical closures and some experiments are suggested.

ISSN:0031-9171    

DOI:10.1063/1.864228

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The eigenmode structure of drift waves in a straight stellarator using the ballooning mode formalism is investigated. The electrons are assumed to be adiabatic and the ions constitute a cold, magnetized fluid. The ‘‘effective potential’’ has an overall parabolic envelope but is modulated strongly by helical ripples alongB. Two classes of solutions are found: those that are strongly localized in local helical wells, and those that are weakly localized and have broad spatial extent. The weakly localized modes decay spatially due to the existence of Mathieu resonances between the periods of the eigenfunction and the modulation of the ‘‘effective potential.’’

ISSN:0031-9171    

DOI:10.1063/1.864229

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The forces on a small rigid sphere in a nonuniform flow are considered from first prinicples in order to resolve the errors in Tchen’s equation and the subsequent modified versions that have since appeared. Forces from the undisturbed flow and the disturbance flow created by the presence of the sphere are treated separately. Proper account is taken of the effect of spatial variations of the undisturbed flow on both forces. In particular the appropriate Faxen correction for unsteady Stokes flow is derived and included as part of the consistent approximation for the equation of motion.

ISSN:0031-9171    

DOI:10.1063/1.864230

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

By specializing the class of unsteady extensional flows considered by Slattery to motions which are periodic perturbations about a rest state and using the proper representation for Noll’s simple fluid undergoing such flows, explicit formulas for the induced stresses in terms of measurable material properties were achieved. It is found that oscillations of a sufficiently large amplitude will generate steady stresses in the fluid due to its nonlinear viscoelastic properties implying that orientation effects might be found in such flows.

ISSN:0031-9171    

DOI:10.1063/1.864231

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The onset of steady natural convection in a rotating cylindrical volume of fluid completely bounded by rigid surfaces is examined for moderate Taylor numbers (Ta≤2×106) and aspect ratios (A≤2). The critical Rayleigh number for three dimensional disturbances is found to be lower than that for the radially unbounded problem by up to a factor of six. The thermal boundary condition on the lateral walls is shown to have a greater effect here than in the nonrotating case.

ISSN:0031-9171    

DOI:10.1063/1.864238

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The behavior of nonlinear progressive waves at the interface between two inviscid fluids in the presence of an upper free boundary is studied as a model of waves on the thermocline. A set of relationships between the integral properties of bounded waves in a general two‐fluid model is first developed and the Stokes expansion to third order is derived. The exact free boundary problem for the wave profile is then formulated within the Boussinesq approximation as a nonlinear integral equation, which is solved numerically using two different numerical methods. For finite velocity difference across the two‐fluid interface bifurcation of solutions into upper and lower branch wave profiles with quite different properties is obtained. Numerically calculated wave shapes and integral properties show good agreement with third‐order Stokes expansion predictions in the weakly nonlinear regime for waves which are not too long. Very long waves were found to exhibit distinct solitary wave‐like features.

ISSN:0031-9171    

DOI:10.1063/1.864239

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Large amplitude disturbances in a boundary layer shear flow are considered in an inviscid and long‐wave theory. Initially weak horizontal convergences are concentrated and amplified in time, thereby increasing the maximum normal velocity (v) until it becomes comparable with the horizontal velocity. The effect is first demonstrated in a two‐dimensional model having piecewise uniform vorticity. The leading edge of the compact disturbance propagates downstream more rapidly than the trailing edge, but no ‘‘quiet’’ zone appears in the center. Insteadv→∞ and a tendency for wavebreaking occurs. The evolving largevpattern is consistent with observations of the laminar spike just prior to its breakdown. The long‐wave theory is generalized to three‐dimensional motions, and the effect of an initial spanwise divergence is such as to rationalize the initial vorticity assumed in the two‐dimensional model.

ISSN:0031-9171    

DOI:10.1063/1.864240

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Experiments with disks having diameters from 2.0 to 3.1 cm falling freely in air at speeds of 1.7–3.3 m sec−1were conducted with a vertically driven, also falling, high‐speed camera in order to show that the gyroscopic‐like motion, gyration, is the most general type of rotational motion. From analysis of the films, position and orientation were measured throughout the fall and translatory and angular motions were determined, together with the forces and torques which caused the motion. The existence of gyrational motions was confirmed together with straight oscillatory and rotational fall behavior. Typical nutation/precession frequencies were in the range of 6–11 Hz while typical angular frequencies about the minor axis were 1.8–3.0 Hz. The likelihood of gyration increased with particle density. The observed gyration characteristics of disks were similar to predictions for oblate spheroids. Further, it is postulated that air influences can cause rotation about a horizontal diameter to become unstable and to consequently evolve into gyration. This gyrational motion may represent a very general mode of free fall for other shapes in a similar range of Reynolds numbers from 2000–5000.

ISSN:0031-9171    

DOI:10.1063/1.864241

出版商:AIP    

年代:1983

数据来源: AIP