摘要:

Excess pressure drops are measured for the flow through very small orifices whose diameter ranges from the order of 1 mm to 10 &mgr;m using water, silicon oils, and solutions of glycerin in water: For larger orifices it is almost the same as, but for smaller orifices several times higher than, that of the numerical analysis of Newtonian fluid. Water gives the highest among the liquids used. Velocities are also measured along the centerline of a small orifice and found inconsistent with the result of the numerical analysis. A different mechanism may be dominant in the flow through very small orifices. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869170

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Relationships between diverse thermodynamic quantities appropriate to weakly compressible turbulence are derived. It is shown that for turbulence of a finite turbulent Mach number there is a finite effect of compressibility. A methodology for generating initial conditions, consistent with finite Mach number, for the fluctuating pressure, density, and dilatational velocity is given. Use of these initial conditions gives rise to a smooth development of the flow, in contrast to cases in which these fields are specified arbitrarily or set to zero. Comparisons of the effect of different types of initial conditions are made using direct numerical simulation of decaying isotropic turbulence. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869152

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The onset of surface‐tension‐driven convection in a horizontal liquid layer cooled suddenly from above is analyzed by using linear stability theory. To obtain the critical condition to mark the onset of instability in the form of regular cellular motion the time domain is divided into two. With small time the basic temperature profile is strongly nonlinear and therefore the propagation theory we have developed is employed. Based on the propagation theory a new set of stability equations are derived and their scale analysis is discussed. It is found that the fluid layer is more stable with decreasing the Biot number and the Prandtl number. With large time the frozen‐time model is applied. In this time domain the stability criteria are independent of the Prandtl number. By connecting the predictions from the above two models the overall stability criteria are constructed. The interesting role of the Biot number on the critical condition is discussed in detail. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869171

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A comparison is made of the ‘‘apparent viscosity’’ (as defined by Stokes law) between two different cases of a test sphere moving slowly through an unbounded, otherwise quiescent, globally homogeneous, dilute suspension of identical, neutrally buoyant spherical particles dispersed in an incompressible Newtonian liquid. In case I the force on the test sphere is maintained constant for all time (and the torque‐free sphere allowed to rotate) — corresponding to the so‐called ‘‘falling ball’’ case — and its instantaneous velocity allowed to vary with proximity to each suspended sphere encountered during its trajectory; in case II the non‐rotating test sphere is towed with a uniform (instantaneous) velocity through the suspension and the force experienced by it allowed to vary with proximity to each suspended sphere. Allowing for two‐body hydrodynamic interactions between the ball and a suspended particle, the ensemble‐average velocity of the test sphere is calculated in case I and ensemble‐average force in case II, and Stokes law used to calculate the apparent viscosity of the suspension from the ensemble‐averaged, linear force/velocity ratio obtained. In each case the ‘‘apparent suspension viscosity’’ coefficient attains, as expected, the limiting, continuum, Einstein value of 2.5 when the test sphere is much larger than the freely suspended particle. However, in the case of disparate relative sizes, the apparent viscosity is found to be significantly larger in case II than in case I. The difference arises from the locally inhomogeneous nature of the suspension and points up a fundamental non‐continuum aspect of suspension behavior above and beyond the expected test/suspended‐sphere size ratio ‘‘Knudsen’’ non‐continuum effect. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869158

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A reduced nonlinear model for density stratified viscous film flowing down a slightly inclined wall is derived and explored. Under buoyancy stable stratification the system exhibits various long‐wavelength instabilities of noninertial, purely kinetic origin. Unlike many existing models for the film interface evolution in the present study regularization of the pertinent long‐scale dynamics is provided directly by the film viscosity rather than surface tension. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869165

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The focus of this work is analysis of mixing in a rotating cylinder—a prototype system for mixing of granular materials—with the objective of understanding and highlighting the role of flow on the dynamics of the process. The analysis is restricted to low speeds of rotation, when the free surface of the granular solids is nearly flat, and when particles are identical so that segregation is unimportant. The flow is divided into two regions: arapidflowregionof the cascading layer at the free surface, and afixedbedof particles rotating at the angular speed of the cylinder. A continuum model, in which averages are taken across the layer, is used to analyze the flow in the layer. Good agreement is obtained between the predictions of the flow model for the layer thickness profile and experimental results obtained by digital image analysis. The dynamics of the mixing process are studied by advecting tracer particles by the flow and allowing for particlediffusionin the cascading layer. The mixing model predictions for distribution of tracer particles and mixing rates are compared qualitatively and quantitatively to experimental data. Optimal operating conditions, at which mixing rates are maximum, are determined. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869172

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The behavior of bubbles with radii of 0.5–0.7 mm rising through water in the presence of a solid boundary were observed using a high‐speed video camera. Fluid inertia and surface tension cause a bubble to bounce several times from a horizontal wall before viscosity dissipates the energy. An energy balance involving the kinetic energy of the fluid motion, the surface energy of the air–water interface, and the gravitational potential energy aids in the interpretation of the dynamics of the collision. We also observed the motion of a bubble rising under an oblique wall with an angle of 10°–85° to the horizontal. When the angle was less than about 55° corresponding to We<0.4, the bubble slid steadily along the wall. At steeper angles the bubble was observed to bounce repeatedly from the inclined wall without any apparent loss of amplitude. It was also determined that the critical Weber number of coalescence of a bubble rising toward a stationary bubble is 1.6. At Weber numbers below this critical value, the two bubbles coalesce on impact while bubbles bounce at higher Weber numbers. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869168

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Three‐dimensional interactions between an elliptic vortex ring and two no‐slip parallel walls are visualized in a numerical simulation and an experiment. The vortex ring induces a surface vorticity layer on the wall which reconnects with the vortex ring. During the interaction core‐area‐varying axial waves are generated and carry the surface layer away from the wall. The vortex ring then becomes two tornado‐like structures with strong upward helical flows near the wall surface, which provides entrainment of the surface fluid into the vortex structure. Similarities between the entrainment mechanisms of vorticity in the simulation and dye in the experiment from the surface layer are identified. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869173

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Rolls in finite Prandtl number rotating convection with free‐slip top and bottom boundary conditions are shown to be unstable with respect to small angle perturbations for any value of the Crotation rate. This instability is driven by the horizontal mean flow whose estimation requires a special singular perturbation analysis. ©1997 American Institute of Physics

ISSN:1070-6631    

DOI:10.1063/1.869176

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A numerical study of laminar incompressible flows in symmetric plane sudden expansions was carried out. Computations were performed for various Reynolds number and expansion ratios. The results revealed that the flow remains symmetric up to a certain Reynolds number depending on the expansion ratio, while asymmetries appear at higher Reynolds numbers. The computations indicated that the critical Reynolds number of the symmetry‐breaking bifurcation reduces when increasing the expansion ratio while the flow regains symmetry downstream of an initial channel length. The flow asymmetries were verified by comparing several discretization schemes up to fourth order of accuracy as well as various iterative solvers. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869174

出版商:AIP    

年代:1997

数据来源: AIP