摘要:

A theoretical analysis of the stability of a stratified, two‐dimensional Kelvin–Helmholtz billow against three‐dimensional perturbations is presented. This predicts the three‐dimensional spectrum to be dominated by a shear aligned convective instability, which is localized in the region surrounding the billow core. The results of a direct numerical simulation of the evolution of the three‐dimensional stratified mixing layer fully verify the dominance of this convective mode in the mixing transition. The origin of the streamwise streaks of vorticity, which precede turbulent collapse in a stratified shear layer, is thereby explained. ©1994 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868370

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The spatiotemporal evolution of a turbulent vortex ring with an initial Reynolds number of 7500 is experimentally investigated using the technique of digital particle image velocimetry. The flow is initially characterized by the laminar/turbulent transition via azimuthal bending instabilities. After transition, the shedding of vorticity from peripheral regions of the ring is found to be responsible for the formation of a wake region. This shedding process results in the staircase‐like decay of the circulation and propagation speed of the vortex ring. ©1994 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868371

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

This paper uses an acoustic technique to determine the concentration profile developing during the sedimentation of noncolloidal bidisperse suspensions of glass beads in a Newtonian fluid. Various bead diameter ratios have been used and a wide range of relative concentrations is covered. From the shock front velocities and the concentrations in different zones, the sedimentation velocities of small and large particles in a homogeneous suspension of respective concentrationscs0andcl0have been determined. The semidilute regime (c0=ss0+cl0<20%) has many similarities with the dilute regime, where large particles provide the dominant hydrodynamic hindrance to settling. In the concentrated regime (c0≳35%), a mutual hindrance leads to a velocity reduction of large particles and to an enhancement of small ones, as compared to a monodisperse suspension. The data clearly demonstrate that size segregation in the concentrated regime disappears at a critical concentration, which for the size ratio 1.68 is equal toc0=45%. ©1994 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868372

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A recursive relation for the unit isotropic tensor of an arbitrary even order is derived from a rotational Brownian motion of the unit vector. An expression for the eighth‐order velocity gradient correlation function is derived from the eighth‐order isotropic tensors, after preserving the solenoidal property of the velocity field. This correlation function is compared with measurements in a turbulent wake. ©1994 American Institute of Physics.  

ISSN:1070-6631    

DOI:10.1063/1.868373

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The stability of time modulated Taylor–Couette flow with co‐ and counter‐rotating cylinders and modulated inner cylinder velocity is investigated. The mean velocities of the inner and outer cylinders are chosen to be in constant relative distance to the stability border of nonmodulated Taylor–Couette flow and the inner cylinder velocity is periodically modulated. The critical modulation amplitude, which leads to linear instability of the modulated laminar flow, is calculated with numerically integrated Floquet theory, as well as with perturbation theory. The critical modulation amplitude as a function of the outer cylinder Reynolds number has a rich structure for counter‐rotating cylinders, which is open to experimental test. The critical modulation amplitude decreases monotonically with increasing rotation frequency for corotating cylinders. ©1994 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868374

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The analytical solution of Stokes’ equations between two concentric, confocal ellipses is derived here. This bounded flow, similar in certain respects to the journal bearing flow, was imagined in order to investigate two‐dimensional mixing and Lagrangian chaos in a bounded flow with two symmetry axis. The derived streamfunction is in the form of a Fourier cosine series and, when the eccentricity ratio of the inner ellipse is not very low, the solution converges very rapidly. When the ellipses turn in opposite directions, there are cases where two saddle points are connected by two different streamlines, a necessary and sufficient condition for structural instability according to Peixoto’s theorem. This flow geometry could be particularly effective for mixing of viscous fluids since the number of low period hyperbolic and elliptical points during time periodic boundary motion is greater than for the eccentric rotating cylinder system. The Poincare´ sections obtained with a discontinuous velocity protocol suggest that the size of regions of poor mixing can be reduced by increasing the inner ellipse motion per period. For this geometry, the Poincare´ sections indicate that counter‐rotation yields a more chaotic long term behavior than co‐rotation. ©1994 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868375

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The occurrence of reverse flow in a channel when a bluff body is kept at the entry is already known. In the earlier investigations, attention was focused on the generation of the reverse flow with bluff bodies, such as flat plate and other geometries, having the same width as the channel. The separation of the shear layers from the obstruction at the front end and the interaction of the shear layers at the rear end are mainly responsible for the reverse flow. To gain further insight into the phenomenon, the effects of the width of the obstruction at the front and that of placing another at the rear end in tandem with the front one are examined in this study. It is observed that the reverse flow occurs even when the width of the flat plate (b) is less than the channel width (w); the lower limit beingb/w=0.6. At thisb/wthe reverse flow velocity is small, but it increases progressively withb/wuntil a maximum of about 30% of the forward velocity is attained forb/w≥2.0. However, reverse flow as high as 0.6 times the free‐stream velocity is obtained when another plate is kept close to the rear end in addition to the front plate. Further increase in the reverse flow to 0.83 times the free‐stream velocity has been achieved by replacing the flat plate model at the rear with a semicircular scoop. ©1994 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868376

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The direct simulation Monte Carlo (DSMC) scheme is used to study the gas flow under a read/write head positioned nanometers above a moving disk drive platter (the slider bearing problem). In most cases, impressive agreement is found between the particle‐based simulation and numerical solutions of the continuum hydrodynamic Reynolds equation which has been corrected for slip. However, at very high platter speeds the gas is far from equilibrium, and the load capacity for the slider bearing cannot be accurately computed from the hydrodynamic pressure. ©1994 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868377

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Liquid film flow in a fracture between two porous bodies is mainly driven by pressure. The pressure drop across such a small distance could be significant. The flow of a liquid film is governed by the pressure drop across the fracture space, and therefore, understanding of liquid film flow in a single liquid bridge along a solid wall between two porous bodies is needed. The shape of the liquid bridge between the porous blocks is an unknown. The first step is to determine the shape of the free surface. Due to the nature of the problem, a boundary integral technique is found to provide the solution to the whole problem. Solutions are reported for a range of capillary numbers observed in cracked porous media. Pressure drop is correlated using a dimensionless capillary number group. Using analogy from the Darcy flow in porous media, a correlation for the equivalent Darcy permeability is developed. ©1994 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868378

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Laboratory experiments have been performed for a double diffusive system in which opposing vertical gradients of temperature and salinity are heated from one side. Details of the internal structure of the intrusions that form along the heated endwall are discussed. Fingering motions and convective overturns are prominent characteristics of the internal intrusion structure, particularly when the rate of lateral heating is high relative to the strength of the ambient vertical density gradient. Analysis of the overturning scale indicates that the RMS size of the overturns is typically 10%–30% of the total layer thickness. Comparison of the flow inside the convective layers with that which develops in a long box heated and cooled at opposite sides based on the analysis of Jeevaraj and Imberger [J. Fluid Mech.222, 565 (1991)] shows poor agreement when molecular values of the diffusivity and viscosity are used in the theory. However, moderate increases in the diffusivity values (to account for increased vertical mixing) give good agreement between the experiments and theory. Building on the results of Schladowetal. [J. Fluid Mech.236, 127 (1992)], further evidence of the ability of the intrusions to continue propagating following removal of the endwall heating is presented. Modification of the flow field ahead of the intrusion fronts can create conditions for which the stability ratio,R&rgr;, drops below a critical value, resulting in continued propagation of the intrusions. ©1994 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868379

出版商:AIP    

年代:1994

数据来源: AIP