摘要:

The experimental observation of the behavior of acoustically rotated and oscillated free drops in the microgravity environment of low earth orbit has yielded quantitative results on the gyrotational equilibrium shapes and the oscillation frequency of a liquid spheroid. Positioning techniques using the effects of acoustic radiation pressure were used during the Spacelab 3 flight to carry out this classical fluid mechanics experiment.

ISSN:0094-243X    

DOI:10.1063/1.38976

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The reduced gravity environment on board a spacecraft in low earth orbit gives materials scientists the opportunity to undertake experiments under conditions that reduce or eliminate buoyancy driven fluid convection in comparison to earth based conditions. As a consequence, the relative importance of heat and mass transport by diffusion is increased. In crystal growth this might be expected to lead to a more uniform crystal composition than would be obtained under terrestrial conditions. The process of crystal growth by the Bridgman technique is chosen as a case study. Two and three dimensional numerical models are used to examine the response of heat, mass and momentum transport to conditions characteristic of the microgravity environment. It is shown that the orientation of the experiment with respect to the steady component of the residual gravity is a crucial factor in determining the suitability of the spacecraft as a means to suppress or eliminate unwanted effects caused by buoyant fluid motion. The process is also extremely sensitive to transient disturbances. For example, a 3×10−3g impulse of one second duration acting parallel to the interface of a growing crystal produces a response in the solute field which lasts for nearly 2000 seconds. Consequently lateral and longitudinal compositional variations occur over a length of nearly 6 mm in the grown crystal.

ISSN:0094-243X    

DOI:10.1063/1.38977

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Calculations of convection driven by crystal and feed rod rotation and by surface‐tension‐gradients are presented for small scale silicon floating zones operating under microgravity and earthbound conditions. The analyses are based on a thermal‐capillary model of the floating zone process that presents a self‐consistent analysis of convection in the melt, heat transport in the melt, feed rod and growing crystal, the shape of the melt/gas meniscus and the shapes of the solidification and melting interfaces. Results for small‐scale silicon floating zones demonstrate show the intense convection driven by thermocapillary forces caused by the axial temperature gradients that are needed to sustain the molten zone. Sample results illustrate the difference between the shape and flows in floating zones on earth and in microgravity.

ISSN:0094-243X    

DOI:10.1063/1.38978

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

This paper reviews recently developed numerical methods for the solution of free‐boundary problems in fluid dynamics. A sample of recent CFD studies from our research is then presented, with the goal of illustrating the potential of ‘‘computational experiments’’ in elucidating the dynamics of bubbles and drops in viscous fluids at both zero and nonzero Reynolds numbers.

ISSN:0094-243X    

DOI:10.1063/1.38979

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A rigorous mathematical framework for studying the nonlinear dynamics of charged drops has been developed and summarized here. A combination of domain perturbation and multiple timescale methods are systematically used to compute the evolution of axisymmetric, inviscid and charged drops that exhibit a number of nonlinear phenomena.Nuclear physics has contributed theoretical analysis and impetus for experimental study of liquid drops, since Bohr and Wheeler began modeling atomic nuclei as uniformly charged liquid drops with surface tension. When moderate amplitude oscillations of charged drops are considered, it is shown that the increased inertia of the system slows down the motion by decreasing the frequency of the oscillation. The analysis also demonstrates the possibility of resonance between the fundamental mode of oscillation and one of its harmonics for particular values of the net charge on the drop. Both frequency and amplitude modulation of the oscillations are predicted for drop motions starting from general initial conditions. This effect cannot be anticipated from the linear analysis and proves that Rayleigh’s solution for small‐amplitude oscillations can actually be unstable.The dynamics of break‐up of a charged drop is a long standing issue, although the neutrally stable states have been known since the early sixties. Rayleigh calculated the maximum charge that a spherical drop can carry before it becomes unstable due to electrostatic repulsion. The present analysis shows that the first axisymmetric family that bifurcates from the spherical shape evolves transcritically, so that the drop will be either unstable for elongated prolate shapes or stable for flat oblate shapes. The evolution of drop shapes leading to break‐up is also analyzed, and the dependence of the amount of charge on the amplitude of the deformations is computed. The asymptotic analysis for the static shapes is in very good agreement with the finite element calculations for even large amplitude deformations for the drop. Recently, it has been shown that oblate spheroids are unstable with respect to non‐axisymmetric disturbances and, thus, are not observable.

ISSN:0094-243X    

DOI:10.1063/1.38980

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In most practical applications existing bubble dynamics models, either spherical or axisymmetric, are only more or less appropriate approximations. In this paper we will describe an on‐going project which considers the fully three‐dimensional bubble dynamics problem. The interaction between a growing, deforming and collapsing bubble near a boundary is simulated numerically using a Boundary Integral Method. The example of large bubble dynamics near a solid flat plate in a gravity field is considered. The plate orientation significantly influences the 3‐D bubble shape and behavior. The flow field due to the bubble dynamics is considered to be potential. To initialize the computations, the bubble is taken to be very small and spherical. From there on, no additional assumptions are imposed and the bubble surface is free to move under the influence of the pressure field, inertia forces, and the presence of body forces and a nearby wall. The presence of gas inside the bubble is accounted for using a polytropic law of behavior and surface tension is included in the model. This paper presents the method, addresses the numerical difficulties and shows the influence of the problem geometry on the bubble dynamics.

ISSN:0094-243X    

DOI:10.1063/1.38981

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Time dependent evolutions of the profile of free surface (bubble shapes) for a cylindrical container partially filled with a Newtonian fluid of constant density, rotating about its axis of symmetry, have been study. Numerical computations of the dynamics of bubble shapes have been carried out with the sinusoidal function vibration of gravity environment in high and low rotating cylinder speeds. The initial condition of bubble profiles was adopted from the steady‐state formulations in which the computer algorithms have been developed by Hung and Leslie, and Hunget al.

ISSN:0094-243X    

DOI:10.1063/1.38983

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Computations of large amplitude motion of free drops in zero gravity have been performed by an inviscid boundary integral method. Impulsive forces were applied to a body of liquid causing it to disintegrate into smaller drops. The break‐up process is studied in detail. We find that a narrow throat occurs between the main body of liquid and the developing droplet. When this throat is small we numerically cut off the droplet and follow its motion. It is found that fairly large oscillations of the droplet are caused by a jet of fluid which squirts into the droplet from the main body.

ISSN:0094-243X    

DOI:10.1063/1.38984

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The behavior of many dispersed two‐phase systems, such as bubbly flows and droplet mists, are strongly affected by interfacial effects. A statistical approach is presented here which may be used to model the global behavior of such geometrically complex systems. This approach describes the interfaces as a distribution of differential surface patches, each having two local, independent principal curvatures. Statistically‐averaged (mean‐field) physics describing the appropriate phenomena are formulated for the individual patches. These physics can include the volume‐fraction effects of the dispersed phase, as well as flow conditions and interfacial phenomena. The resulting local patch dynamics are then used in a distributional analysis to predict the ensemble behavior and evolution of the dispersed system. Application of this approach to diffusive coarsening in solid‐liquid systems will be described briefly, along with suggestions of applications to other two‐phase systems of interest.

ISSN:0094-243X    

DOI:10.1063/1.38985

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The axisymmetric creeping motion of a neutrally buoyant deformable drop flowing in a tube with periodically varying diameter is analyzed with a boundary integral equation method. The undeformed drop radius is comparable to the average tube radius. The fluids are immiscible, incompressible and the suspension flows at constant volume flux. Two tubes are considered. Tube I has a maximum to minimum radius ratio of 1.8 and a maximum radius to wavelength ratio of 0.3. Tube II has a contraction ratio of 3 and the same maximum radius to wavelength ratio. The effects of the capillary number, the drop to suspending fluid viscosity ratio, drop size and the contraction ratio on the extra pressure drop and drop speed are examined.

ISSN:0094-243X    

DOI:10.1063/1.38959

出版商:AIP    

年代:1990

数据来源: AIP