摘要:

We study the scaling behavior of the pressure structure function for isotropic turbulence. This function is given exactly by Hill and Wilczak [J. Fluid Mech.296, 247 (1995)] in terms of the three independent fourth order velocity structure functions,L(r)=⟨&Dgr;u4(r)⟩,T(r)=⟨&Dgr;v4(r)⟩,andM(r)=⟨&Dgr;u2(r)&Dgr;v2(r)⟩.We show from direct numerical simulation (DNS) that the cancellation between the positive terms proportional toL(r)andT(r)and the negative terms proportional toM(r)is almost complete. This suggests that the pressure structure function is extremely sensitive to recently observed small differences in scaling among the three quantitiesL(r),T(r), andM(r). We illustrate this sensitivity by calculating the pressure structure function in the atmospheric boundary layer using the recent data of B. Dhruva, Y. Tsuji, and K. R. Sreenivasan [Phys. Rev. E56, R4928 (1997)]. The cancellation among the three terms persists, and gives an effective scaling exponent for the pressure structure function,⟨(&Dgr;p(r))2⟩∝r1.17,which is smaller than the scaling exponent for any of the three velocity structure functions. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869731

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Given (and confirmed numerically) that the exponents&agr;and&bgr;in the passive scalar and velocity structure functions⟨(&Dgr;T3)a⟩∼r&agr;,and⟨(&Dgr;u3)a⟩∼r&bgr;are anomalous, the scaling of&ggr;in⟨(u(x+r)−u(x))a(T(x+r)−T(x))2a⟩∼r&ggr;is investigated. Analytical estimates show that&ggr;cannotbe as anomalous as&agr;or&bgr;.Numerical computations(Pr=1,Re&lgr;=141)show that&ggr;is closer to&bgr;than to&agr;.In addition, the statistical dependence of the velocity and passive scalar differences leads to anenhanced anomalyin&ggr;.©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869732

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Massively parallel computers are now large enough to support accurate direct numerical simulations (DNSs) of laboratory experiments on isotropic turbulence, providing researchers with a full description of the flow field as a function of space and time. The high accuracy of the simulations is demonstrated by their agreement with the underlying laboratory experiment and on checks of numerical accuracy. In order to simulate the experiments, requirements for the largest and smallest length scales computed must be met. Furthermore, an iterative technique is developed in order to initialize the larger length scales in the flow. Using these methods, DNS is shown to accurately simulate isotropic turbulence decay experiments such as those of Comte-Bellot and Corrsin [J. Fluid Mech.48, 273 (1971)]. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869733

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We describe a new dynamic light scattering technique for measuring diffusion in sheared suspensions. It involves a scattering geometry with two crossing laser beams. A detailed analysis of the correlation function of scattered light is given. The viability of our method is demonstrated in an experiment where the effect of Taylor diffusion on the scattered light correlation function is measured. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869734

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Solutions of the Brinkman equation for the arbitrary motion of a circular disk are obtained which examine for the first time the effect of particle orientation on the particle drag and torque. Four elementary motions are studied analytically: broadside translation, edgewise translation, rotation about the axis of symmetry, and rotation about the diameter. These motions are closely related to the analogous unsteady oscillation of a disk in Stokes flow [Zhang and Stone, J. Fluid Mech. (1998)]. However, our solution procedure differs in that the problems are formulated using a general solution of the Brinkman equation and are solved by reducing the dual integral equations arising from the mixed boundary conditions in the plane of the disk to a Fredholm integral equation of the second type. Asymptotic results for the drag and torque are derived for both small and large values of the permeability parameter &agr; defined bya/Kp,whereais the radius of the disk andKpthe Darcy permeability. In contrast to the Stokesian motion of a disk, where the drag differs by only a factor of 1.5 for broadside and edgewise translational motion, and is isotropic for rotation about any axis through its center, there is a large difference in the drag and torque with increasing &agr;. In a Brinkman medium, the drag on the disk is proportional to &agr; for edgewise motion and to&agr;2for broadside motion and the torque is proportional to&agr;2for out-of-plane rotation and to &agr; for in-plane rotation. For intermediate values of &agr;, the integral equations are solved numerically for the drag and torque exerted by the porous medium on the disk. These results are of importance in probing the microstructure of the porous medium and thus provide a way to test the validity of the effective medium approach. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869735

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The aim of the paper is to get insight into flow patterns visualized by suspended anisotropic reflective particles. The motion of triaxial ellipsoids embedded in a three-dimensional flow, i.e., which cannot be reduced to a local plane Couette flow, is calculated. Both the asymptotic trajectory and the transient time to reach it are discussed. These results are used to simulate laser sheet visualizations of two classical three-dimensional flows (Taylor–Couette vortices and flow between rotating disks) where the particle history is shown to be negligible. The simulated visualizations are well compared to experimental ones but the paper addresses the fact that the legitimate question of what shows the visualization does not have a simple answer. Nevertheless, these results open the way for quantitative comparisons between computational fluid dynamics and experimental visualizations. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869736

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Experiments on a nonstationary separation of nanometer-sizedMn0.5Zn0.5Fe2Oparticles of hydrocarbon-based ferrocolloids in a flat vertical thermal diffusion column are performed. By using a modified separation theory which accounts for a one-dimensional mixed (thermal and concentration) convection in the column, the Soret coefficient of lyophilized nanoparticles from the separation curves are calculated. It is shown that in a zero magnetic field particles are transferring toward decreasing temperatures. The thermal diffusion ratio&agr;Treaches a value&agr;T≈+20.A significant influence of a uniform magnetic fieldBon particle separation is observed. IfBis oriented along the temperature gradient&bnabla;T,a strong decrease in thermal diffusion coefficient takes place whereas the transversal fieldB⊥∇Tcauses an intensification of particle thermophoretic transfer. Both effects qualitatively well agree with theoretical predictions based on a hydrodynamic theory of particle thermomagnetophoretic motion. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869737

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The heat transfer near the critical point is governed not only by diffusion, convection, and radiation, but also by a thermomechanical coupling called the Piston Effect. This fourth mode of heat transfer is responsible for the so-calledcritical speeding up, which contradicts the first expectation of a critical slowing down of the heat diffusion. So far, the viscosity has been neglected in all the existing theoretical models of the Piston Effect. The aim of this paper is to present a comprehensive model of the Piston Effect, written for a real-fluid equation of state and including the critical divergence of the bulk viscosity. It is shown in particular that when the critical point is neared, the heat transfer goes faster and faster, until a point is reached where viscous stresses are no longer negligible. When going closer to the critical point, the heat transfer then slows down again; a regime ofcritical slowing downis entered. This phenomenon should happen sufficiently far from the critical temperature to allow experimental checks. Moreover, it could be used as an indirect way of measuring the critical divergence of the bulk viscosity. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869738

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

In this paper, we examine the problem of hemispheroid that is compliantly hinged to a plate oscillating with infinitesimal motions in fluid that can be modeled as irrotational. We use this model to understand the effect of the size and shape of hair bundles of sensory cells in the inner ear on hair bundle hydrodynamics at high frequencies. In response to the oscillating plate, the hemispheroid translates and rotates in the plane of the oscillation. Since the equations of motion are linear, the solution can be described as the superposition of the response due to translation of the hemispheroid and the plate, and the response due to rotation of the hemispheroid. The solution can be found by solving Laplace’s equation in prolate and oblate spheroidal coordinates. The response due to translational motion is the same as that derived inHydrodynamics, 6th ed. (Dover, New York, 1945) for a full spheroid undergoing translational motion. The response due to rotational motion of the hemispheroid about its hinge has not been previously derived. The hydrodynamic pressure, torque, and drag on the hemispheroid as a function of hemispheroidal shape are presented. A good match was obtained for results of the model and measurements of a neural tuning curve of the alligator lizard. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869739

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The instability growth leading to a liquid sheet breakup has been studied with the objective of improving the understanding of the fundamental mechanisms of atomization. A three-dimensional Lagrangian code based on vortex dynamics methods has been implemented to track the air/liquid interfaces treated as inviscid vortex sheets. The results of these numerical simulations indicate a possible explanation for the presence of transverse and longitudinal filaments observed in liquid sheet air-assisted atomization experiments. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869740

出版商:AIP    

年代:1998

数据来源: AIP