摘要:

A phenomenological model for the inertial range scaling of passive-scalar turbulence is developed based on a bivariate log-Poisson model. An analytical formula of the scaling exponent for three-dimensional passive-scalar turbulence is deduced. The predicted scaling exponents are compared with experimental measurements, showing good agreement. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869265

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Two recent intermittency models by Chen and Cao [Phys. Rev. E52, R5757 (1995)] and Nelkin [Phys. Rev. E52, R4610 (1995)] stemming from Novikov’s criticism [Phys. Rev. E50, R3303 (1994)] on the She–Le´ve⁁que model [Phys. Rev. Lett.72, 336 (1994)] are discussed. Both models are rewritten using&mgr;6(deviation from Kolmogorov 1941 of the sixth order structure function exponent) values computed numerically(=0.23and0.43using longitudinal and lateral velocity differences). Scaling exponents are compared. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869266

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A solution to the conduction equation has been developed for two equal size, nonconducting spheres with the line between centers perpendicular to the applied field. The solution, valid when the gap between the spheres is small compared to their radius, is based on a matched asymptotic expansion. For the case when the conductivity is uniform everywhere (i.e., Laplace’s equation), the solution agrees well with numerical results obtained from an infinite series solution in bispherical coordinates. An example with a nonuniform conductivity in the gap is presented to demonstrate how the method can be extended to more general conduction problems. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869260

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The Stokes flow past the unusual geometry of two fused (overlapping) spheres, somewhat surprisingly, has an exact analytic solution, provided that the spheres intersect orthogonally. The problem of flow past a partially encapsulated droplet is considered, to illustrate the basic idea. The simple nature of the analytical solution is of some interest in biophysical models of molecular assemblies involving a combination of stick and slip surfaces. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869261

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Simulations of the conformation change of model polymers in various steady, anisotropic Gaussian random flow fields are presented. These flow fields have been chosen because they are models for the flow through porous media and have been predicted to be “stochastic strong flows” according to the criteria developed by Shaqfeh and Koch [J. Fluid Mech.244, 17 (1992)]. To be specific, beyond a certain Deborah number (based on the sampling time of a velocity fluctuation), large average conformation change in the polymer is predicted. In our simulations, the polymers are modeled as dumbbells, but beyond this restriction, the assumptions of the theory by Shaqfeh and Koch are removed. Many realizations of the Gaussian fields are synthesized spectrally following a modified version of the method developed by Kraichnan [Phys. Fluids13, 22 (1970)]. Moreover, the ratio of the mean “plug” flow to the amplitude of the fluctuations is varied from mean-dominant to fluctuation-dominant flows. The simulated conformation change shows that, in fact, these flows are “strong” in the sense that the average second moment of the end-to-end distance becomes large (relative to equilibrium) beyond a critical value of the fluctuation Deborah number. Although qualitatively capturing these trends, the theory by Shaqfeh and Kochunderestimatesthe strength of the flows and thusoverestimatesthe critical Deborah number. We present a new theory which includes spring relaxation and Brownian motion in the sampling of a velocity fluctuation (two factors which were neglected in the existing theory), thereby breaking the fore–aft symmetry of the sampling, thus increasing the average polymer stretch. The new theory quantitatively predicts the simulation results. To the authors knowledge, this is the first evidence via direct simulation that these random flows can produce large conformation change in model polymer molecules, even when the mean flow would produce no such change. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869262

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The local solution behavior near corners formed by the intersection of a slip surface with either a no-slip or a shear-free boundary is analyzed by finite element calculations of the two-dimensional flow of an inertialess Newtonian fluid in several model flow geometries; these flows are the flow in a tapered contraction, a sudden expansion and the extrudate swell from a planar die. Local finite element mesh refinement based on irregular, embedded elements is used to obtain extremely fine resolution of the velocity and pressure fields near the region where there is a sudden change in boundary condition. The calculations accurately reproduce the expected asymptotic behavior for a shear-free surface intersecting a no-slip boundary, where the solution is given by a self-similar form for the velocity and pressure fields. Replacing the shear-free condition with a slip condition yields a similar form for the local velocity and pressure fields and indicates that the slip boundary behaves, to leading order, as a shear-free surface. Calculations for a slip boundary intersecting a shear-free surface yield similar results, with the local behavior being given by asymptotic analysis for two shear-free surfaces intersecting to form a wedge. These results suggest that replacing the no-slip boundary condition in planar Newtonian die swell with a slip boundary condition can give rise to local behavior of velocity gradients and pressure which ismore singularthan the flow created with no-slip boundary conditions. This prediction is confirmed by calculations of Newtonian die swell with slip. These calculations also demonstrate that the local solution in Newtonian die swell is sensitive to the details of the numerical method. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869263

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The translational and rotational velocities of a positively buoyant sphere suspended in a parallel plate device are measured, and related to the inertial lift of the sphere from the upper plate. The experimental results obtained agree well with the corrected theoretical predictions of Krishnan and Leighton [Phys. Fluids7, 11 (1995)]. The motion of the sphere principally depends on the shear Reynolds numberRe≡&ggr;˙a2/&ngr;, where&ggr;˙is the shear rate,ais the particle radius and&ngr;the kinematic viscosity, and the sedimentation Reynolds numberRes≡Usa/&ngr;whereUsis the Stokes sedimentation velocity of the sphere. The transition from translation without slip to translation with slip along the plane is observed to scale as Re/Res,and occurs at order10−1. The separation of the sphere from the plane due to inertial lift at low Reynolds number is found to have the scalingRe2/Res,as predicted by theory. In experiments with two particles of diameter 1.5 and 2 mm in two glycerin/water solutions of different densities, the particles were found to lift off the plane atRe2/Res≈4. This value is consistent with measurements of the particle surface roughness. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869264

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The rate of damping of surface gravity–capillary waves is investigated, in a system which consists of a thin layer of a Newtonian viscous fluid of thicknessdfloating on a Newtonian fluid of infinite depth. The surface and interfacial tensions, elasticities and viscosities are taken into account. In particular, an approximate dispersion relation is derived for the case wherekdand(&ohgr;/&ngr;+)1/2dare both small, wherekis the wavenumber,&ohgr;is the angular frequency and&ngr;+is the kinematic viscosity of the upper fluid. Ifd→0while&ngr;+dremains finite, published dispersion relations for viscoelastic surface films of extremely small (e.g., monomolecular) thickness are reproduced, if we add the surface and interfacial tensions, elasticities and viscosities together, and then add an additional4&rgr;+&ngr;+dto the surface viscosity, where&rgr;+is the density of the upper fluid. A simple approximation is derived for the damping rate and associated frequency shift when their magnitudes are both small. An example is given of what may happen with a slick of heavy fuel oil on water: a slick 10&mgr;mthick produces a damping rate only slightly different from that of a film of essentially zero thickness, but the effect of the finite thickness becomes very noticeable if it is increased to 0.1–1 mm. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869240

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Particle contamination in a cavity, or a recessed region, on the slider surface is an important issue in slider designs for magnetic recording. In this paper, a model is developed for the simulation of particle motion in a recessed region, in which various forces such as viscous drag, Saffman lift, Magnus lift, and gravity are considered. It is found that the latter two forces have a very small effect on the vertical motion of the particles compared with the Saffman lift, and they can be neglected in the analysis. It is also found, through the simulation for various cases, that the Saffman lift is not so important for the motion of the smaller particles(d<100 nm)as for the motion of larger particles, and that the magnitude of the velocity of particles relative to the air flow affects the Saffman lift, or vertical motion of the particles, significantly. The air flow velocity is relatively low in a recessed region, and a particle often has a large relative velocity with positive sign when entering it, which causes the Saffman lift force to point to the surface of the slider. Therefore, particle contamination on the slider occurs at the edges of the recessed region. A similar result can also be obtained for the particles entering an air bearing under a taper, which explains why the contamination is concentrated on the surface of tapers. ©1997 American Institute of Physics. 

ISSN:1070-6631    

DOI:10.1063/1.869241

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The onset of thermal convection in an oscillatory shear flow is considered as an initial-value problem. For the case with zero mean shear, numerical results indicate that the instability is absolute in nature, just as for the case without any shear. If a mean flow component also exists, then the instability tends to have the same nature as that which occurs in the flow when no oscillation is present. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869242

出版商:AIP    

年代:1997

数据来源: AIP