摘要:

Wavy Taylor vortex flow between two concentric cylinders is simulated on molecular level by the direct simulation Monte Carlo method. The number of cells in circumferential direction is greatly reduced by introducing the idea oflocally axisymmetrical collision. As found in experiment, the wavy Taylor vortices are seen to move in azimuthal direction. The frequency of the azimuthal waves is in agreement with the experimental observation. It is the first time that wavy Taylor vortex flow is simulated by means of a molecular approach. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869178

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The diffusion term in the velocity gradient tensorAijevolution equation in homogeneous isotropic turbulence at moderate Reynolds number has been investigated using full direct numerical simulation (DNS) of the Navier–Stokes equations. This study also considered the diffusion terms associated with the strain rate tensorSijand the vorticity vector&ohgr;i.The statistics of the “diffusion frequency”&ohgr;0defined as&ohgr;0≡−&ngr;∇2Aij/Aij,which can be thought of as an inverse of a characteristic diffusion time scalet0,has been studied both for diagonal and off-diagonal elements of this tensor, forSijand for&ohgr;i.The probability density functions (PDF’s) of the diffusion frequency&ohgr;0for all the variables considered in this study have a distinct peak indicating a most probable positive value. It is found that the value of the frequency increases with larger absolute values of the velocity gradient. The conditional averages of&ngr;∇2Aij,&ngr;∇2Sijand&ngr;∇2&ohgr;iare found to be closely related by a cubic function ofAij,Sijand&ohgr;irespectively, which in the neighborhood of one standard deviation from the origin is very well approximated by the same linear relationship for all the variables. This latter result suggests that a linear approximation model [Dopazo &etal;,Ninth Symposium on Turbulent Shear Flows;J. Martin, Ph.D. dissertation] for the diffusion terms of velocity gradients may be appropriate in certain cases. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869179

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Bifurcation diagrams of nonaxisymmetric cylindrical volume liquid bridges held between nonconcentric circular disks subject to a lateral gravitational force are found by solving the Young-Laplace equation for the interface by a finite difference method. In the absence of lateral gravity, the primary family of liquid bridges that starts with the cylinder when the eccentricity of the disks,e, is zero first loses stability at a subcritical bifurcation point aseincreases. Further loss of stability is experienced by the already unstable primary family as a turning point is encountered at yet higher values of the eccentricity. However, the introduction of lateral gravityglchanges entirely the structure of the solutions in that instability always occurs at a turning point with respect toeno matter how small the magnitude ofgl.The stability limits calculated are compared with the ones obtained using asymptotic techniques by taking as base solution the cylinder of slenderness &Lgr;=&pgr;. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869181

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

An experimental study of the rivulet instability associated with spin coating a circular drop of fluid is conducted to examine the effect of elasticity on the onset and evolution of the instability. The spin coating experiments are conducted with viscoelastic drops consisting of a high molecular weight polystyrene in tricresyl phosphate (TCP), as well as the Newtonian solvent TCP. Results show an unequivocal delay in the onset of the instability when the appropriate Weissenberg number is sufficiently large, resulting in a larger coated area and more finger arms relative to Newtonian results. Experiments performed with the viscoelastic fluid at low Weissenberg number exhibit similar behavior to those performed with the Newtonian solvent as expected. Additionally, the growth rate of the instability is reduced for experiments in which the elastic forces are important, in agreement with the perturbation theory of Spaid and Homsy [Phys. Fluids8, 460 (1996)], demonstrating that elastic forces have a stabilizing influence on the contact line instability. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869480

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In order to characterize conductivity profiles due to drilling fluid invasion into a hydrocarbon formation, a model for radial fluid transport is presented. The model assumes a water-based mud and accounts for the convective movement of oil, water, and salt. A mathematical analysis of the model using the method of characteristics is given. An equivalent graphical construction is also provided. Computations of radial conductivity profiles for specified formation characteristics and total filtrate loss are given. For typical fractional flow curves, it is shown that three qualitatively different profiles may occur depending on the initial water saturation. These results are compared with numerical simulations that include capillary pressure and gravity segregation. Next, the important issue of the inverse problem is analyzed. It is shown that from a single snapshot of the conductivity profile, an exact calculation of filtrate loss and formation fractional flow curve is possible. The calculation is valid provided there is a resistivity contrast between the drilling fluid filtrate and the formation water. For practical application, we show the limits of applicability of this result with complete numerical calculations. Thus, if wireline logs of resistivity variation away from the wellbore are available, formation oil producibility and water cut at native conditions may be estimated. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869483

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We present the results of numerical simulations of the three-dimensional thermocapillary motion of deformable viscous drops under the influence of a constant temperature gradient within a second liquid medium. In particular, we examine the effects of shape deformations and convective transport of momentum and energy on the migration velocity of the drop. A numerical method based on a continuum model for the fluid–fluid interface is used to account for finite drop deformations. An oct-tree adaptive grid refinement scheme is integrated into the numerical method in order to track the interface without the need for interface reconstruction. Interface deformations arising from the convection of energy at small Reynolds numbers are found to be negligible. On the other hand, deformations of the drop shape due to inertial effects, though small in magnitude, are found to retard the motion of the drop. The steady drop shapes are found to resemble oblate or prolate spheroids without fore and aft symmetry, with the direction of elongation of the drop depending on the value of the density ratio between the two phases. As in the case of a gas bubble, convection of energy is shown to retard the thermocapillary motion of a viscous drop, as the isotherms get wrapped around the front surface of the drop and effectively reduce the surface temperature gradient which drives the motion. The effect of inertia on the mobility of viscous drops is found to be weaker than that in the case of gas bubbles. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869182

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

One of the possible phases of a sheared system of inelastically colliding rigid smooth disks is one in which relatively dense strips aligned at 45° to the streamwise direction are interspersed among similarly aligned dilute strips. The dense strips may have secondary microstructures in the form of elongated clusters. The latter are formed by an instability, following which they are convected, stretched, and rotated by the shear field. This process causes cluster–cluster collisions, a result of which is the partial destruction of the colliding clusters, followed by the emergence of new clusters. In addition, it is demonstrated that clustering dynamics can be responsible for hysteresis and multistability in granular systems. The studies presented in this paper involve molecular dynamics simulations complemented by theoretical analysis. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869183

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Fluid flow and fluid evaporation both contribute to the overall rate of thinning during spinning of a fluid on a disk. Laser interferometry of solvent thinning behavior on spinning silicon wafers was performed to yield plots of solvent thickness evolution. These plots of thickness versus time were then analyzed to understand the respective contributions of viscous flow and evaporation to the thinning. A technique is described for extracting both the viscosity and the evaporation rate from the interference data. Well understood solvent systems are examined as test cases for this deconvolution. It is also demonstrated that nonevaporating fluids can be analyzed, even though their thickness evolution has no easily referenced endpoint to the thinning, in contrast to the volatile solvents which are rapidly spun dry. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869519

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The growth of the gradient of a scalar temperature in a quasigeostrophic flow is studied numerically in detail. We use a flow evolving from a simple initial condition which was regarded by Constantin &etal; as a candidate for a singularity formation in a finite time. For the inviscid problem, we propose a completely different interpretation of the growth, that is, the temperature gradient can be fitted equally well by a double-exponential function of time rather than an algebraic blowup. It seems impossible to distinguish whether the flow blows up or not on the basis of the inviscid computations at hand. In the viscous case, a comparison is made between a series of computations with different Reynolds numbers. The critical time at which the temperature gradient attains the first local maximum is found to depend double logarithmically on the Reynolds number, which suggests the global regularity of the inviscid flow. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869184

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A self-sustaining process conjectured to be generic for wall-bounded shear flows is investigated. The self-sustaining process consists of streamwise rolls that redistribute the mean shear to create streaks that wiggle to maintain the rolls. The process is analyzed and shown to be remarkably insensitive to whether there is no-slip or free-slip at the walls. A low-order model of the process is derived from the Navier–Stokes equations for a sinusoidal shear flow. The model has two unstable steady solutions above a critical Reynolds number, in addition to the stable laminar flow. For some parameter values, there is a second critical Reynolds number at which a homoclinic bifurcation gives rise to a stable periodic solution. This suggests a direct link between unstable steady solutions and almost periodic solutions that have been computed in plane Couette flow. It is argued that this self-sustaining process is responsible for the bifurcation of shear flows at low Reynolds numbers and perhaps also for controlling the near-wall region of turbulent shear flows at higher Reynolds numbers. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869185

出版商:AIP    

年代:1997

数据来源: AIP