Experimental Evidence of Thermal Vibrational Convection in a Nonuniformly Heated Fluid in a Reduced Gravity Environment A. Mialdun, I.I. Ryzhkov, D.E. Melnikov, and V. Shevtsova Microgravity Research Center, Universite ´ Libre de Bruxelles, CP–165/62, Avenue F.D. Roosevelt, 50, B–1050 Brussels, Belgium (Received 17 December 2007; published 19 August 2008) 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. DOI: 10.1103/PhysRevLett.101.084501 PACS numbers: 47.20.Ky, 42.40.Kw, 44.27.+g, 46.40.f Vibrational convection refers to the specific flows that appear when a fluid with a density gradient is subjected to external vibration. The density gradient may result from the inhomogeneity of temperature or composition. The case of small amplitude and high-frequency vibration (when the period is much smaller than the reference hydro- dynamic times) is of special interest. In this case, the flow field can be decomposed into the ‘‘quick’’ part, which oscillates with the frequency of vibration, and the ‘‘slow’’ time-average part (mean flow), which describes the nonlinear response of the fluid to a periodic excitation [1]. This effect is most pronounced in the absence of other external forces (in particular, static gravity). The study of vibrational impact on fluids has fundamen- tal and applied importance. Vibrational convection pro- vides a mechanism of heat and mass transfer due to the existence of mean flows. In weightlessness, it is an addi- tional way of transporting heat and matter similar to thermo- and solutocapillary convection. Mean flows show some similarity with gravity-induced convection and might serve as a way to control and operate fluids in space [2]. Vibrations can suppress or intensify gravitational convec- tion depending on the mutual orientation of vibration axis and thermal (compositional) gradient [3,4]. When a fluid is subjected to high-frequency vibration and density inhomogeneity is caused by the thermal gra- dient, the vibrational and gravitational convective mecha- nisms are characterized by the dimensionless parameters Gs A T TL 2 2 ; Ra g T TL 3 ; (1) where A is the amplitude of vibration, 2f is the angular frequency, g is the gravitational acceleration, T is the thermal expansion, is the kinematic viscosity, is the thermal diffusivity, L is the characteristic size, and T is the applied temperature difference. In (1), Ra is the Rayleigh number and Gs is known as its vibrational ana- logue. We suggest calling it the Gershuni number to mark a significant contribution of G. Z. Gershuni to the theory of thermovibrational convection [1]. The ratio Gs=Ra de- scribes the relative importance of thermovibrational and gravitational convective mechanisms. There have been extensive theoretical studies of ther- movibrational convection in weightlessness and ground conditions. The fundamental treatise [1] comprises a sys- tematic study of convective flows induced by high and finite frequency vibrations in closed and infinite cavities. Thermovibrational convection in square, rectangular, and cubic cavities was widely investigated providing a variety of mean flow structures and bifurcation scenarios [5 – 8]. Influence of vibration on double diffusive convection with the Soret effect was analyzed in [9]. The experimental studies of vibrational impact on the fluids in ground conditions were performed in a number of works. The considered configurations include vertical and horizontal layers [10], cavities subjected to nontransla- tional vibrations [11], Hele-Shaw cell [12], etc. At the same time, experiments addressing thermovibrational phe- nomena in low gravity are very limited. The only known series of experiments was carried out with the ALICE-2 instrument onboard MIR station [13,14]. The influence of vibration on the propagation of a temperature wave from a heat source in a near-critical fluid was investigated. The thermovibrational flows were registered by observing the optical inhomogeneity caused by the distortion of the temperature field. It was not possible to reconstruct this field quantitatively. In this Letter, we report an experimental study of ther- movibrational convection in low gravity. The fluid is placed in a cubic cell with differentially heated walls and subjected to translational vibration perpendicular to the temperature gradient (Fig. 1). This configuration was studied theoretically in weightlessness as well as ground conditions in a large number of works; see [1,5,6,8] and references therein. However, to the best of our knowledge, no experiments have been performed to evaluate these PRL 101, 084501 (2008) PHYSICAL REVIEW LETTERS week ending 22 AUGUST 2008 0031-9007= 08=101(8)=084501(4) 084501-1 2008 The American Physical Society