摘要:

The problem of transition between regular and Mach reflection of planar shock waves over straight wedges in steady flows was numerically studied by the DSMC method. It is shown that the transition from regular to Mach reflection takes place, in accordance with the detachment criterion, while the opposite transition occurs at smaller angles. The hysteresis effect was observed at increasing and decreasing shock wave angles. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868591

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

This Letter examines how intrinsic three‐dimensionality influences the flow structure of unsteady shear flows, and models its effects by considering spanwise averages of vorticity fluctuations. In particular, uniform flow past a half‐cylinder is considered: the two‐dimensional wake is very different from its three‐dimensional counterpart and the evolution equation for the spanwise spatially averaged vorticity is analyzed. It is shown that the vector flux which transports spanwise vorticity due to spatial vorticity fluctuations is of considerable magnitude (large in comparison with molecular diffusion) and is far from co‐linear with the gradient of the spanwise averaged vorticity. Modeling this flux using classical eddy viscosity ideas would therefore seem inappropriate. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868592

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Experiments in high Reynolds number turbulence call into question the experimental verification of Kolmogorov’s refined similarity hypothesis. Previous experimental support for the hypothesis is shown to be distorted by the severe simplifications used in estimating the turbulence energy dissipation, which has invariably been based only on the streamwise gradient of the streamwise velocity component. However, when the dissipation is estimated by the streamwise gradient of a transverse velocity component the dependence between ‖&Dgr;ur‖ and (r&egr;r*)1/3all but disappears. These results suggest that previous experimental support for the hypothesis may have been premature. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868593

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Instantaneous velocity fields in thex‐yplane of a zero pressure gradient turbulent boundary layer are measured using particle image velocimetry. It is found that there exist random, time‐varying zones in theu‐&ngr; fields in which the streamwise momentum is remarkably uniform. The largest dimension of a typical zone is proportional to the boundary layer thickness. The zone closest to the wall contains viscous‐inertial inclined structures similar to those found in low Reynolds number wall turbulence. A second zone is located above the wall zone in a region that coincides roughly with the logarithmic layer. The wake region of the boundary layer contains a complicated, time‐varying pattern of several nearly‐constant‐momentum zones. The zones are separated from each other and from the free stream by thin viscous shear layers that contain concentrations of spanwise vorticity. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868594

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The electrophoretic velocity of a charged disk of zero thickness is computed in the limit of small surface potentials, but with arbitrary double layer thickness. The disk represents an idealized clay particle, and has uniform surface charge over its flat surface, together with a uniform line charge around its edge. The contributions of these two charges to the electrophoretic velocity are considered separately. Asymptotic results are obtained for thin and thick double layers, and intermediate results are obtained by numerical integration. The singularities in both the electrical and hydrodynamic fields at the edge of the particle enhance the importance of the edge charge when the double layer thickness &kgr;−1is small compared to the disk radiusa. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868595

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The possible existence of equilibrium salt fingering in a deep fingering domain, equivalent to large Rayleigh number fingering convection, is investigated with a numerical model of a continuously heat and salt stratified fluid system. The growth of fingering heat and salt fluxes in this system is shown to be limited by the instability of fingering cells, and an equilibrium state is achieved when the instability has increased the energy dissipation sufficiently to balance the buoyancy forcing generated by double diffusion. The equilibrium fluxes are shown to vary proportional to the mean verticalTandSgradients, in contrast to the inverse relationship that holds for convection across a finite height, or low Rayleigh number, fingering zone. The structure of equilibrium convection is shown to be disorganized and turbulent‐like, characterized by incoherently rising and sinking blobs of anomalous density fluid. A model of equilibrium fingering based on the blob structure is presented to interpret the convective processes. Estimates of the finger blob scale and the heat‐to‐salt flux ratio are also made from the model and are shown to be consistent with those determined from direct numerical simulations. Finally, a critical Rayleigh number is suggested and estimated to delineate the equilibrium fingering regime from the low Rayleigh number fingering regime. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868596

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

As has recently been reported by Tsitverblit and Kit [Phys. Fluids A5, 1062 (1993)], a vertical rectangular enclosure containing stably stratified brine and differentially heated from its side walls is characterized by complex steady bifurcation phenomena. In the present work, the structure of steady solutions in the enclosure has been studied in detail for several values of the salinity Rayleigh number, RaS, fixed near the commencement of the double‐diffusive region. It was found that when the thermal Rayleigh number, RaT, is either very small or sufficiently large, the steady solution is unique while in an intermediate region of this parameter, there exists a great variety of the multiple steady flows, being the result of nondegenerate hysteresis points and isolas of asymmetric solutions forming as RaSis increased. In particular, at the maximal value of RaSconsidered there have been observed symmetric and asymmetric one‐, two‐, three‐, four‐, and five‐cell flows. Despite the multiplicity of the flow patterns, a critical interval of the buoyancy ratio has been distinguished, above and below which the generic characteristics of the steady solutions were found to resemble the respective features of the ‘‘successive’’ and ‘‘simultaneous’’ regimes of layer formation whose existence was established in previous studies. Although the set of the steady solutions has been found to contain no linearly stable multicell flows, the perturbation was so long retained in the close proximity of the unstable steady solutions that such flows could be easily observable in the experiment. In spite of the appreciably different range of the Rayleigh numbers, the physically meaningful parameters suggested in previous studies were found to be represented in the present results. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868776

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A detailed experimental study has been made of the formation and motion of vortex rings generated when drops of liquid are allowed to come into contact at zero velocity with a quiescent flat surface of the same liquid. In this case the vortex motion is driven principally by the drop surface energy. It is shown that in the case considered, the phenomenon depends on two dimensionless parameters alone, the reciprocal of a Bond number B−1and a global Reynolds number Re where the velocity scale is based on the surface energy. Using a video camera, measurements were made, over the whole trajectory, of the ring velocity and diameter for a number of drop sizes and liquids. Vortex rings generated by small drops reach higher peak velocities but decelerate more rapidly than ones generated by large drops. The latter, however, may become turbulent before relaminarizing during deceleration. Although the most interesting finding of the investigation was the discovery of oscillations in the ring translational velocity and diameter during deceleration, the identification of the appropriate scaling laws and a suggestion regarding the early stages of vorticity generation should also be of interest. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868597

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The phenomenological derivation of a phase field model of capillarity that accounts for the structure of an interfacial layer formed by two immiscible incompressible liquids is addressed. A rheological expression for the reversible component of capillary stresses is obtained in terms of the free energy of a binary fluid, which depends on the absolute temperature, composition, and gradient of composition. This model can be applied to those flows that involve change of topology of a capillary interface, such as coalescence and breakup of drops. As an illustration, an equilibrium of a binary fluid with either a flat or spherical interfacial layer is analyzed, and a thermocapillary flow in an infinite gap is considered. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868598

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Several techniques have been proposed for determining two‐ or three‐dimensional velocity fields from measurements of one passive scalar. It is shown that measurements of one scalar and knowledge of the equation governing its transport determine a velocity field, only up to an additive vector field locally perpendicular to the gradient of the scalar field but otherwise arbitrary. Three previously proposed procedures for selecting a unique velocity field from among the uncountable infinity consistent with the scalar transport data and equation are then discussed, and it is shown that a recent ‘‘iterative inversion’’ procedure for ‘‘solution’’ of a singular linear equation system (obtained using only measurements of one scalar and the equation governing its transport) cannot converge as claimed. A method for determining the correctn‐dimensional (n=2 or 3) divergence‐free velocity field from measurements ofn−1 passive or reactive scalars is then developed. Finally, it is shown how the velocity field in ann‐dimensional compressible flow can be determined from measurements of density andn−1 passive or reactive scalars. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868599

出版商:AIP    

年代:1995

数据来源: AIP