摘要:

This Letter considers the evolution of perturbations at an interface between two fluids subjected to an oblique shock. The normal component of the shock generates the Richtmyer–Meshkov (RM) instability, and the parallel component generates the Kelvin–Helmholtz (KH) instability. If a constant normal acceleration is also present it induces the Rayleigh–Taylor (RT) instability or, depending on the sign ofgA(g=acceleration,A=Atwood number), it acts to stabilize the KH and RM instabilities. Treating the shock as an instantaneous acceleration, analytic formulas are derived for the evolution of the perturbations. This Letter illustrates with an application to inertial‐confinement‐fusion capsules.

ISSN:1070-6631    

DOI:10.1063/1.868198

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A new method for computing the hydrodynamic stability eigenvalue problem in the presence of wall suction by using the eigenvalue and eigenfunctions of the no‐suction problem is presented. The perturbation approach is computationally efficient in comparison with the direct approach; it is potentially applicable in various ways to hydrodynamic stability, particularly in laminar flow control.

ISSN:1070-6631    

DOI:10.1063/1.868199

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Simulations of oblique transition in the spatial domain are presented, covering the complete transition process into the turbulent regime. It is conjectured that the three stages identified here and elsewhere are universal for oblique transition in all shear flows: first a nonlinear generation of a streamwise vortex by the oblique waves, second a transient growth of streaks from the vortex by the lift‐up effect, and third a breakdown of the streaks due to secondary instability.

ISSN:1070-6631    

DOI:10.1063/1.868200

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Countergradient flow is explained in terms of a nonlocal flux‐gradient relationship, obtained through solution of the widely‐used BGK or ‘‘relaxation time’’ model kinetic equation in one dimension for a layered source in an atmosphere with stationary turbulence. It is also pointed out that the classical formula derived by Taylor [Proc. London Math. Soc. Ser. A20, 196 (1921)] for dispersion from a point source is fully recoverable from BGK theory.

ISSN:1070-6631    

DOI:10.1063/1.868201

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

This paper discusses the properties of a semidilute suspension of disks, for whichnl3≫1 and &fgr;≪1, wherenis the number of disks per unit volume,lis their large dimension, and &fgr; is their volume fraction. The effective conductivity of a dispersion of aligned, highly conducting disks is shown to beO[k(nl3)2], wherekis the conductivity of the matrix. The extensional viscosity is shown to beO[&mgr;(nl3)2], where &mgr; is the viscosity of the fluid. In addition, similar scaling results are shown to hold for the case of a semidilute suspension of aligned, two‐dimensional slabs which are of infinite extent in the direction perpendicular to their plane of cross section. Specifically, fornl2≫1, the effective conductivity and the extensional viscosity are shown to beO[k(nl2)2] andO[&mgr;(nl2)2] respectively, wherenis now the number of slabs per unit area andlis the width of the slab. Planar extensional flow simulations of a periodic array of aligned slabs confirm the quadratic scaling for stress in the semidilute regime. The simulations also show the crossover from a linear dependence of the stress on particle concentration in the dilute regime to the quadratic, semidilute scaling.

ISSN:1070-6631    

DOI:10.1063/1.868202

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Image Scanning Ellipsometry, a technique to measure the two‐dimensional thickness profile of a nonuniform, thin, liquid film, from several nanometers up to tens of microns, in the steady and transient states, was developed and tested. The ability of this full‐field imaging technique to map every point on the surface simultaneously was demonstrated by measuring the thickness profiles of very thin, draining, liquid films in the interfacial, transition, hydrodynamic, and capillary regions. Depending on the relative size of the intermolecular, gravitational, and capillary forces, four flow regions were identified. Using a simple model for the transient film thickness profiles of a completely wetting, draining film of FC‐70, the experimental results were successfully analyzed in the interfacial, transition, and hydrodynamic regions. A diffusion coefficient for the junction line between the interfacial and transition regions was theoretically and experimentally evaluated.  

ISSN:1070-6631    

DOI:10.1063/1.868203

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Some measurements have been obtained for the axial velocity of the fully developed laminar flow in a circular straight pipe with radiusa, which is rotating with constant angular speed &OHgr; about an axis perpendicular to its own axis. A diode laser LDA system was mounted together with a circulating pipe flow system on a rotating table for the experiment. According to previous analyses and calculations, there exist four types of axial velocity distributions that result from the various effects of the secondary flow on the main stream via the convection and Coriolis effect for different values ofR( =wm’a/&ngr;) andR&OHgr;(=&OHgr;a2/&ngr;), wherewm’is the mean axial velocity and &ngr; is the kinematic viscosity of the fluid. The present study provides experimental validation for the previous theoretical and numerical analyses. Experiments have also been carried out for studying the asymptotic nature of the slow flow in a rapidly rotating pipe (R&OHgr;≫1 andR&OHgr;≫R) and the rapid flow in a slowly rotating pipe (RR&OHgr;≫1 andR≫R&OHgr;).

ISSN:1070-6631    

DOI:10.1063/1.868204

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The evolution of localized three‐dimensional disturbance in two‐ and three‐dimensional laminar boundary layers is examined. The linearized Navier–Stokes equations for three‐dimensional disturbances in a three‐dimensional parallel shear flow are solved numerically using Fourier transform Chebyshev collocation techniques. Modal analysis shows that substantial short‐term energy growth can be obtained even when all instability waves are damped. This transient growth can increase the initial disturbance energy by two or three orders of magnitude, at which stage nonlinear interactions might lead to a breakdown to turbulent flow, bypassing the traditional Tollmien–Schlichting instability mechanism. The dependence of the transient growth on wave number, Reynolds number, sweep angle and Hartree parameter is determined and a method for predicting the maximum transient growth is proposed and found to be reasonably accurate over a wide parameter range. Localized disturbances are also examined and it is found that the bypass growth mechanism can enhance the formation of cross‐flow vortices in a three‐dimensional flow. Some implications are discussed, particularly with respect to the observed effects of roughness on transition location.

ISSN:1070-6631    

DOI:10.1063/1.868205

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Direct numerical simulations of the transition process from laminar to chaotic flow in converging–diverging channels are presented. The chaotic flow regime is reached after a sequence of successive supercritical Hopf bifurcations to periodic, quasiperiodic, and chaotic self‐sustained flow regimes. The numerical experiments reveal three distinct bifurcations as the Reynolds number is increased, each adding a new fundamental frequency to the velocity spectrum. In addition, frequency‐locked periodic solutions with independent but synchronized periodic functions are obtained. A scenario similar to the Ruelle–Takens–Newhouse scenario of the onset of chaos is verified in this forced convective open system flow. The results are illustrated for different Reynolds numbers using time‐velocity histories, Fourier power spectra, and phase space trajectories. The global structure of the self‐sustained oscillatory flow for a periodic regime is also discussed.

ISSN:1070-6631    

DOI:10.1063/1.868206

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Two noninvasive measurement techniques are described for investigating transitional flow in a rigid straight tube. Photochromic trace recordings, representing fluid displacement profiles, were made within the core and at the leading and trailing edges of turbulent slugs generated by a disturbance at the tube inlet. Consistent photochromic trace recordings were made possible through the discovery that Doppler ultrasound can be used to detect the passage of turbulent slugs, thereby allowing precise timing of the photochromic traces. Upstream and downstream Doppler probes enabled the leading and trailing edge velocities of slugs to be measured and also enabled the laminar/turbulent transition regions to be investigated in detail. Trailing edge interface velocities were in excellent agreement with existing data, while leading edge velocities agreed less well. Turbulence generation at the trailing edge appeared to be associated with a localized vortical structure which originated in the near wall region and then grew toward the tube centerline. The generation of this vortex is postulated to be induced by a shear layer instability. Mean velocity profiles obtained from within the core of turbulent slugs were similar to those obtained for fully developed turbulent flow.

ISSN:1070-6631    

DOI:10.1063/1.868207

出版商:AIP    

年代:1994

数据来源: AIP