Vibrations acting on a fluid with density gradient induced by temperature variations can cause relative flows. High-frequency vibration leads to the appearance of timeaveraged (mean) flows (or streaming flows), which can essentially affect heat and mass transfer processes. This phenomenon is most pronounced in the absence of other external forces (in particular, static gravity). In this work, an extensive experimental and computational study of thermal vibrational convection in a reduced-gravity environment of a parabolic flight is performed. The transient evolution of the temperature field in a cubic cell subjected to translational vibration is investigated by optical digital interferometry. The mean flow structures previously reported in numerical studies are confirmed. The transition from four-vortex flow to a pattern with a large diagonal vortex and two small vortices is observed in the transient state. The experiments reveal a significant enhancement of heat transfer by vibrational mean flows with increasing the vibrational strength. Three-dimensional direct numerical simulation with real microgravity profile and two-dimensional numerical modelling based on averaging approach provide a very good agreement with the experimental results. The influence of residual gravity on heat transfer and bifurcation scenario is first investigated numerically and correlated with the experimental data. It is demonstrated that gravity effects on non-uniformly heated fluids can be reproduced in weightlessness by applying vibrations to the system.

Small particles transported by a fluid medium do not necessarily have to follow the flow. We show that for a wide class of time-periodic incompressible flows inertial particles have a tendency to spontaneously align in one-dimensional dynamic coherent structures. This effect may take place for particles so small that often they would be expected to behave as passive tracers and be used in PIV measurement technique. We link the particle tendency to form one-dimensional structures to the nonlinear phenomenon of phase locking. We propose that this general mechanism is, in particular, responsible for the enigmatic formation of the "particle accumulation structures" discovered experimentally in thermocapillary flows more than a decade ago and unexplained until now.

We report experimental evidence of convection caused by translational vibration of nonuniformly heated fluid in low gravity. The theory of vibrational convection in weightlessness is well developed but direct experimental proof has been missing. An innovative point of the experiment is the observation of a temperature field in the front and side views of the cubic cell. In addition, particle tracing is employed. The evolution of this field is studied systematically in a wide range of frequencies and amplitudes. The flow structures reported in previous numerical studies are confirmed. The transition from four-vortex flow to the pattern with three vortices is observed in the transient state.

The development of thermocapillary convection inside a cylindrical liquid bridge is investigated by using a direct numerical simulation of the three-dimensional ͑3D͒, time-dependent problem for a wide range of Prandtl numbers, Prϭ1,3,4,5 and Prϭ35. Above the critical value of temperature difference between the supporting disks, two counterpropagating hydrothermal waves bifurcate from the two-dimensional ͑2D͒ steady state. The existence of standing and traveling waves is discussed. The dependence of viscosity upon temperature is taken into account. The critical Reynolds number and critical frequency at which the system undergoes a transition from a 2D steady state to a 3D oscillatory flow decreases if the viscosity diminishes with temperature. The stability boundary is determined for Prϭ3-5 with a viscosity contrast (max / min) up to a factor 10. Near the threshold of instability the flow organization is similar for the constant and variable viscosity cases despite the large difference in critical Reynolds numbers. The influence of variable viscosity on the flow pattern is increased when going into the supercritical region. The study of spatial-temporal behavior of oscillatory convection for the high Prandtl number, Prϭ35, demonstrates a good agreement with previously published experimental results. For this high Prandtl number liquids instability begins as a standing wave with an azimuthal wave number m ϭ1 which then switches to an oblique traveling wave Ϸ4%-5% above the onset of instability.

It is common knowledge that light fluids rise while heavy fluids sink in the gravity field. The most obvious case is the isothermal Rayleigh-Taylor instability when a heavy fluid is placed on top of a light one. In the nonisothermal case, while heating from above, the density stratification is stable in a pure liquid. However, unstable density stratification might be established in a binary mixture with a negative Soret effect in the case of heating from above: the heavier liquid is accumulated on the top of the lighter one. Due to the large differences between viscous, thermal, and diffusion times the system has a tendency to fingering buoyant instabilities. At some moment the flow may be initiated. Near the onset of convection the flow pattern has a columnar convective structure: for a relatively low applied temperature difference Delta T the lighter and colder liquid is drawn up in the central part of the cell and the heavier liquid flows down along the walls. For finite size systems the situation is reversed at higher Delta T. Here we present results of three-dimensional direct numerical simulations of heat and mass transfer in a system with a negative Soret effect. While the development of Soret-induced convection is similar for a wide class of liquids: water based mixtures, colloidal, and polymer solutions, the parameters of the chosen system correspond to a realistic binary mixture of water (90%) and isopropanol (10%) enabling comparison of theoretical predictions with planned experimental studies.

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The paper presents a three-dimensional numerical study of the bifurcations and onset of chaotic regime for the thermoconvective oscillatory flow in cylindrical liquid bridge. Three-dimensional Navier-Stokes equations in Boussinesq approximation are solved numerically by finite volume method. Silicone oil 1cSt, with rather large Prandtl number, Prϭ18.8, is chosen as test liquid. The simulations are done at normal gravity conditions and unit aspect ratio. The dependence of viscosity of the fluid upon temperature allows us to be close to the real phenomenon. Both spatial and temporal changes occurring in the system are analyzed. The results are compared to the experimental data. A following sequence of well-defined dynamic regimes was detected when temperature difference between the supporting disks is increasing: steady, periodic, quasiperiodic, periodic, and chaotic. The observed succession of bifurcations on the way to chaos is similar to the one coming from experiments. Except for these dynamic bifurcations the system exhibits numerous transitions in spatial organization of the flow. Two-dimensional steady-state flow undergoes standing wave ͑SW͒ with azimuthal wave number mϭ1 as a result of supercritical Hopf bifurcation. Moving above the critical point the following succession of flow states has been numerically found: SW(mϭ1)→TW (mϭ1)→SW(mϭ1ϩ2)→TW(mϭ1ϩ2). The transition to chaos occurs while the flow pattern represents a traveling wave ͑TW͒ with a mixed mode mϭ1ϩ2, while the mϭ2 is dominant. Particular attention is paid to the analysis of special properties of the flow: entropy, net azimuthal flow, frequency skips, splitting of maxima, and related phenomena.

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