摘要:

The method of modulated ultrasonic radiation pressure, previously used to drive the capillary modes of drops and bubbles, is used to excite the capillary modes of a cylindrical oil bridge in a Plateau tank. Specific modes may be selected by adjusting the modulation frequency and the location or orientation of the bridge in the ultrasonic field. Mode frequencies were measured as a function of the slenderness for the lowest two axisymmetric modes and two nonaxisymmetric modes. The frequencies of the lowest modes agree with an approximate theory which neglects viscous corrections where the interfacial tension is a fitted parameter. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868809

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Deep bed filtration has been studied experimentally and numerically for small non‐Brownian particles flowing into a random packing of monosize glass spheres at low Reynolds number. It was discovered that packets of particles penetrated further than the same number of particles released one at a time. These collective effects are attributed to hydrodynamic phenomena, one plausible explanation being the existence of relaunchable ‘‘hydrodynamic captures’’ in addition to ‘‘geometric captures.’’ ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868810

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We describe laboratory experiments on millimeter‐sized drops of liquid in air which indicate that both thermocapillary and isothermal shear flows are able to prevent the coalescence of bodies of liquid which would occur readily in the absence of such flows. We have also carried out molecular dynamics computer simulations of nanometer‐sized drops, which show the same qualitative behavior in the case of an applied shear. At the other extreme, persistent non‐coalescence of larger drops was observed in microgravity conditions in a space shuttle experiment. We give an explanation of the experimental observations based upon lubrication theory and simple continuum hydrodynamics arguments, along with complementary microscopic insight obtained from the molecular simulations. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868811

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The head‐on collision of equal sized drops is studied by full numerical simulations. The Navier–Stokes equations are solved for the fluid motion both inside and outside the drops using a front tracking/finite difference technique. The drops are accelerated toward each other by a body force that is turned off before the drops collide. When the drops collide, the fluid between them is pushed outward leaving a thin layer bounded by the drop surface. This layer gets progressively thinner as the drops continue to deform, and in several of our calculations we artificially remove this double layer at prescribed times, thus modeling rupture. If no rupture takes place, the drops always rebound, but if the film is ruptured the drops may coalesce permanently or coalesce temporarily and then split again. Although the numerically predicted boundaries between permanent and temporary coalescence are found to be consistent with experimental observations, the exact location of these boundaries in parameter space is found to depend on the time of rupture. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868812

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Ultrasonic and electrostatic levitation techniques have allowed the experimental investigation of the nonlinear oscillatory dynamics of free droplets with diameter between 0.1 and 0.4 cm. The measurement of the resonance frequencies of the first three normal modes of large amplitude shape oscillations in an electric field of varying magnitude has been carried out with and without surface charges for weakly conducting liquids in air. These oscillations of nonspherical levitated drops have been driven by either modulating the ultrasonic field or by using a time‐varying electric field, and the free decay from the oscillatory state has been recorded. A decrease in the resonance frequency of the driven fundamental quadrupole mode has been measured for increasing oblate deformation in the absence of an electric field. Similarly, a decrease in this frequency has also been found for increasing DC electric field magnitude. A soft nonlinearity exists in the amplitude dependence of the resonant mode frequencies for freely decaying as well as ultrasonically and electrically driven uncharged drops. This decrease in resonance frequency is accentuated by the presence of free surface charge on the drop. Subharmonic resonance excitation has been observed for drops in a time‐varying electric field, and hysteresis exists for resonant modes driven to large amplitude. Mode coupling from lower‐order resonances to higher‐order modes has been found to be very weak, even for fairly large amplitude shape oscillations. Most of these results are in general agreement with predictions from recent analytical and numerical investigations. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868813

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Considered are the transitional instabilities of the flow inside three‐dimensional rectangular cavities that are differentially heated over two opposing vertical walls. The horizontal and lateral walls are adiabatic. Emphasis is on (though not restricted to) theair‐filled,cubicalcavity. For this configuration, it was found that the occurrence of unsteady oscillations in the flow was preceded by asteadyinstability (i.e. an instability resulting in a steady solution of the Navier–Stokes equations for large time) which originated in an internal, stratified shear layer that separates from the adiabatic horizontal walls of the cavity. This instability is inherently three‐dimensional and characterized by the presence of streamwise‐oriented, counterrotating vortices. It is probably caused by centrifugal forces. The subsequent, low‐frequency, unsteady instability is strongly influenced by this steady instability and as a result its frequency differs strongly from its counterpart in the two‐dimensional, square cavity. For larger Prandtl numbers, however, the frequencies in the two‐ and three‐dimensional cavities are almost equal since no prior steady instability occurs. The instability mechanism responsible for the unsteady instability is therefore the same in both configurations even though the instability in the three‐dimensional cavity shows a distinct wave‐like modulation in the third direction. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868814

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Material line folding is studied in two‐dimensional chaotic cavity flows. Line folding is measured by the local curvaturek=l×l′/‖l‖3, wherel(q) is an infinitesimal vector in the tangential direction of the line,qis a coordinate along the line, andl′ is the derivative oflwith respect toq. It is shown both analytically and numerically that folding is always accompanied by compression. The vectorl′ plays a crucial role as a driving force for the stretching and folding processes. A material line is stretched whenl′ is tangential to the line and it is folded whenl′ is normal to the line. The spatial structure of the curvature field is computed numerically. The short‐time structure of the curvature field is similar to the structure of unstable manifolds of periodic hyperbolic points, and closely resembles patterns observed in tracer mixing experiments and in stretching field computations. The long time structure of the field asymptotically approaches an entirely different time‐independent structure. Probability density functions of curvature are independent of both time and initial conditions. © 1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868815

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Ensemble‐averaging theorems are applied to derive transport equations for the fluctuating kinetic energy of a particulate mixture consisting of a continuous fluid and solid particles. The evolution of fluctuating kinetic energy in a homogeneous flow is examined and discussed. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868816

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The temporal evolution of Ka´rma´n vortex shedding patterns in the wake of a cylinder placed at right angles to a uniform flow is studied for Reynolds numbers (based on cylinder diameter) between 80 and 140. Focusing on the dynamics of the vortex shedding phase in the wake planview (the plane spanned by the free‐stream direction and the cylinder axis) we study experimentally and model the response of shedding patterns to time‐dependent boundary conditions imposed at the cylinder ends. By appropriate impulsive changes of end conditions, spanwise wave number ‘‘shocks’’ can be produced that travel along the cylinder span. These shock experiments, together with data from steady oblique shedding patterns, are used to determine the parameters for the spanwise Ginzburg–Landau model, which has already been used successfully to describe many of the phenomena observed in cylinder wakes. We then demonstrate experimentally that, in analogy to gasdynamics, it is also possible to produce ‘‘expansion waves’’ of the spanwise wave number, which are well described by the Ginzburg–Landau model without further adjustment of its parameters. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868817

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The classical Rayleigh–Taylor instability for an interface of finite extent is modified by two independently controlled perturbation effects. A component of gravitational acceleration tangent to the interface is imposed and the interface is subjected to a flow‐induced pressure field. The stability of the flat horizontal interface when surface tension holds heavier liquid above ambient gas is considered in a 2D model problem. The two perturbations are realized by tilting the interface to the horizontal and by inducing a flow with shear. It is found that the effect of tilt angle or shear on its own is destabilizing, while together, in the right combination, they can stabilize. It is thereby shown how to extend the stability limit over the classical Rayleigh–Taylor result. The framework for the analysis is the classical unperturbed pitchfork bifurcation (codimension 2). The coefficients in the unfolding are calculated by applying the Lyapunov–Schmidt technique to a pinned deformable interface that holds a shear‐induced lubrication flow. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868818

出版商:AIP    

年代:1996

数据来源: AIP