摘要:

A new kind of net‐like structure formed by the metal spheres suspended in a metal/oil electrorheological (ER) fluid is reported for the first time in this Letter. The experimental results show that this structure is totally different from that formed by dielectric particles. After comparing the two formation patterns, it is found that the dielectric particles in certain ER fluids align themselves into chains or columns in the direction of the external electric field. The metal particles in a metal/oil ER fluid, on the other hand, form a net‐like structure of chains and no column could be observed even when the ER fluid has been exposed to a high electric field. We attribute the net‐like structure of the metal ER fluid to the strong interaction between dipole fields induced on the metal particles under the external electric field. Finally, the dielectric properties of the different structures were measured and are discussed in this paper. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869096

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The rheology of a dilute suspension of charged fibers with a large ratio of lengthLto diameterdis examined. The fibers may possess both a net charge and a charge dipole and it is assumed that the Hartmann number is small. The double layer thickness &lgr; is large compared with the rod diameter and may be comparable with the fiber length. Although the shear rate is sufficiently small so that the deformation of the double layer is small, no restriction is placed on the rotary Pe´clet number of the rods. The velocity disturbance caused by the rods is neglected when calculating the double layer deformation. This approximation is accurate when &lgr;/L=O(1) andL/dis very large, but leads to an overestimate of the ion cloud distortion for small values of &lgr;/Lor moderate values ofL/d. The counterion cloud affects the stress both directly through a primary electroviscous stress and indirectly by exerting an electric torque that changes the fiber orientation distribution. The additional stress caused by electrostatics shear thins for a fiber with a net charge. However, when the fiber has a charge dipole, the electrical stress may increase with increasing shear rate at sufficiently high Pe´clet numbers. At very high Pe´clet numbers, the electrical torque can lead to a fixed orientation of the fibers near the flow direction of the simple shear flow. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869085

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Sound driven gas bubbles in water can emit light pulses. This phenomenon is called sonoluminescence (SL). Two different phases of single bubble SL have been proposed: diffusively stable and diffusively unstable SL. We present phase diagrams in the gas concentration versus forcing pressure state space and also in the ambient radius versus gas concentration and versus forcing pressure state spaces. These phase diagrams are based on the thresholds for energy focusing in the bubble and two kinds of instabilities, namely (i) shape instabilities and (ii) diffusive instabilities. Stable SL only occurs in a tiny parameter window of large forcing pressure amplitudePa∼1.2–1.5 atm and low gas concentration of less than 0.4% of the saturation. The upper concentration threshold becomes smaller with increased forcing. Our results quantitatively agree with experimental results of Putterman’s UCLA group on argon, but not on air. However, air bubbles and other gas mixtures can also successfully be treated in this approach if in addition (iii) chemical instabilities are considered. All statements are based on the Rayleigh–Plesset ODE approximation of the bubble dynamics, extended in an adiabatic approximation to include mass diffusion effects. This approximation is the only way to explore considerable portions of parameter space, as solving the full PDEs is numerically too expensive. Therefore, we checked the adiabatic approximation by comparison with the full numerical solution of the advection diffusion PDE and find good agreement. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869131

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The dynamics of capillary pinching of a fluid thread are described by similarity solutions of the Navier–Stokes equations. Eggers [Phys. Rev. Lett.71, 3458 (1993)] recently proposed a single universal similarity solution for a viscous thread pinching with an inertial–viscous–capillary balance in an inviscid environment. In this paper it is shown that there is actually a countably infinite family of such similarity solutions which are each an asymptotic solution to the Navier–Stokes equations. The solutions all have axial scalet′1/2and radial scalet′, wheret′is the time to pinching. The solution obtained by Eggers appears to be special in that it is selected by the dynamics for most initial conditions by virtue of being less susceptible to finite‐amplitude instabilities. The analogous problem of a thread pinching in the absence of inertia is also investigated and it is shown that there is a countably infinite family of similarity solutions with axial scalet′&bgr;and radial scalet′, where each solution has a different exponent &bgr;. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869086

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A set of one‐dimensional models, previously derived for liquid jets, is generalized to viscous liquid bridges by applying suitably modified boundary conditions at the anchoring disks. A linear analysis for small‐amplitude perturbations around the cylindrical static solution is performed. The oscillation frequencies and growth factors so obtained are compared to the already known linear three‐dimensional results for a wide range in both the slenderness and viscosity. The relative error of each model is studied in terms of the typical axial length. Good agreement is found for slender enough bridges. The existence of boundary layers for weakly dissipative liquid bridges in the context of one‐dimensional models is also discussed. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869087

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In this study we examine laminar velocity profiles of freely suspended flowing soap films. We introduce a new device which supports large uniform films for indefinite periods of time. The geometry of the flow is two‐dimensional (2D), yet the measured velocity profiles depart from ideal 2D behavior. The main reason for this departure is that the soap film experiences an air drag force across its entire surface. Describing the air with Prandtl boundary layer theory, we predict the observed flow patterns with good accuracy. The downstream development of the profiles is self similar. Our models set an apparent upper limit on the film 2D viscosity of 5⋅10−6surface poise for dilute soap concentrations. This measurement implies that the surfactant layers on the film may not contribute measurably to the 2D viscosity. For higher soap and glycerol concentrations the opposite appears to be true. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869105

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Oceanic observations [Atmos. Ocean29, 340 (1991)] have revealed small‐scale thermohaline plumes near the surface of a calm sea under warming conditions. The stratification was favorable for the double‐diffusive salt finger instability, though a previously unreported up–down asymmetry was found in which narrow downward cells are balanced by a broader, weaker upwelling. The scales of the thermal structures are consistent with asymmetric hexagonal salt‐finger modes [J. Phys. Oceanogr.24, 855 (1994)], but no selection mechanism for the asymmetry has previously been identified. This paper explores the influence of nonlinear profiles of temperature and salinity, as might arise due to surface evaporation or warming, on the linear stability problem in a salt‐fingering regime. Three models are considered. In the first, a sharp, nonlinear solute‐concentration gradient is applied at the upper boundary, as might arise by surface evaporation. A Be´nard mode appears, driven by the destabilizing density gradient in the thin boundary layer and influencing motion only within the boundary‐layer thickness. In the second model, a weak salinity gradient is introduced below the boundary layer; double‐diffusive bulk modes influence the motion across the entire fluid. Nonlinear interaction of the boundary layer and bulk modes provides a mechanism for maintaining salt fingers with up–down asymmetry. The third model contains a large temperature gradient at the surface, as might arise from warming by solar radiation, overlying a quasi‐isothermal region above a region of moderate gradient. The largest‐growth modes are found to be salt fingers that extend throughout the middle region and disappear in the top and bottom regions. This vertical structure is close to that of the asymmetric salt fingers described in Osborn [Atmos. Ocean29, 340 (1991)]. The differing length scales of the regions impress an up–down asymmetry on plumes; this is expected to yield a hexagonal pattern at the onset. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869067

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A numerical program has been developed to simulate an assembly of inelastic, frictional hard spheres inside a control volume undergoing a steady‐state rapid Couette flow induced by the top and bottom bumpy walls. The bumpy walls are made of hemispheric particles fixed onto flat plates. The flow particles can collide with the wall particles and the exposed flat areas of the walls. The macroscopic flow properties are found to depend on a number of material and geometric properties of the granules, the bumpy walls, and the control volume. These properties include the overall solids fraction of the system, the height of the shear gap, the wall‐particle concentration, the wall‐particle distribution, the diameter ratio of the wall particle to the flow particle, the coefficients of restitution, the friction coefficients, and the sticking tangential restitution coefficients between the flow particles, the wall particles, and the flat walls. A parametric study is undertaken to examine the effect of some of the interesting factors identified above. A new definition for the slip velocity yields positive values consistently, and it represents a significant improvement over the previous ones. By exposing the flat areas of the bumpy walls for collisions, the transfer of energy and momentum from the driving surfaces to the flow medium can be enhanced. Depending on the wall‐particle distribution, there exist optimal wall‐particle concentrations at which the stresses may be maximized or the slip velocities may be minimized. For hemispheric wall particles arranged in an equilateral triangular lattice, the optimal wall‐particle area fraction for maximizing the stresses is about 0.44 while the one for minimizing the slip velocity is about 0.36. The simulation results also show that there exists for gravity‐free Couette flow of inelastic, frictional spheres a critical solids fraction of about 0.5 beyond which the stresses are found to decrease with increasing solids concentration. In general, there is reasonable agreement between the simulation results for stresses and the experimental measurements. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869068

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

This paper describes two‐dimensional computer simulations of granular flow in plane hoppers. The simulations can reproduce an experimentally observed asymmetric unsteadiness for monodispersed particle sizes, but also could eliminate it by adding a small amount of polydispersity. This appears to be a result of the strong packings that may be formed by monodispersed particles and is thus a noncontinuum effect. The internal stress state was also sampled, which among other things, allows an evaluation of common assumptions made in granular material models. These showed that the internal friction coefficient is far from a constant, which is in contradiction to common models based on plasticity theory which assume that the material is always at the point of imminent yield. Furthermore, it is demonstrated that rapid granular flow theory, another common modeling technique, is inapplicable to this problem even near the exit where the flow is moving its fastest. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869069

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The stable, time‐periodic flow between a porous injector disk and an impermeable substrate disk is explored for the case where fluid is injected into the gap region with a spatially uniform time‐periodic velocityV(&tgr;)=V0(1+&agr; cos(&sgr;&tgr;)), whereV0is the mean injection velocity, &agr; is the flow modulation amplitude, and &sgr; is the flow modulation frequency. Fourier series expansions in time (&tgr;) are combined with regular perturbation expansions of the Fourier coefficients in powers of &agr; to describe the linear and nonlinear frequency dispersion in the system when 0<&agr;<1. Generalized analytical expressions are obtained for the quasisteady response, and the method of matched asymptotic expansions is used to analyze the high frequency linear response of the system. Finite difference methods are used to calculate the frequency dispersion of the system for a wide range of modulation frequencies (&sgr;) and Reynolds numbers. Oscillating harmonics are shown to interact destructively (via nonlinear inertial terms), resulting in the nullification of certain Fourier modes in the flow field. The Reynolds number and frequency dependence of harmonic nullification events are explored and their implications for creating multilayered alloys are briefly discussed. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869129

出版商:AIP    

年代:1996

数据来源: AIP