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Recent Progress in Transition Behaviors of Thermocapillary Convection under Microgravity
Surface tension-driven convection plays a vital role in the natural heat and mass transfer under microgravity. Understanding the basic laws governing this process will deepen our understanding of the fluid motion in extreme space conditions, and facilitate the efficient development and utilization of space resources. Consequently, it has remained an essential research area in microgravity fluid physics. With China's rapid development in aerospace technology, particularly in advanced space fluid management, as well as the growing exploitation of space materials and extraterrestrial resources, research in this field has become a key focus and frontier of microgravity science. Besides, China’s space station provides high-quality experimental conditions, which makes research in this field a focus and frontier of microgravity science.
Recently, researchers from the CAS Key Laboratory of Microgravity, Institute of Mechanics have disclosed the dependency of the transition of thermocapillary convection in annular layers on the volume ratio under microgravity, this progress builds upon the "volume ratio theory" initially proposed by Academician Wen-Rui Hu and illustrate the spatial-temporal evolution characteristics of thermocapillary convection, providing novel perspectives for examining flow transition and serving as a source of inspiration for future research on turbulence. The relevant results were published in International Journal of Thermal Sciences[179 (2022) 107707] and International Journal of Heat and Mass Transfer [208 (2023) 124059]. The corresponding author is Prof. Kai Li and the (co-) first author is Ziyi Guo.
The researchers developed the numerical model of thermocapillary convection in fluid with curved free surface, which was used in direct numerical simulations (DNS) of the convection in annular layers with various volume ratios (Vr). The spectrum analysis and the dynamic mode decomposition (DMD) were adopted to extract both the temporal and spatial characteristics at the critical (onset of the oscillation) and supercritical (destabilization of the oscillation) transition stages. Results showed that for different volume ratios, transition behaviors of thermocapillary convection with the increasing temperature difference is different. When Vr<1.0, the convection remains periodic flow even under very high temperature differences, while when Vr≥1.0, the oscillation destabilizes to quasi-periodic, irregular and even chaotic flows under high temperature differences. The various temporal and spatial coherent structures of the characteristic modes exhibit distinct bifurcation patterns, which reveals the dependency of transitions of thermocapillary convection on the volume ratio. By stimulating more fundamental frequencies and produce higher order small-scale structures, larger volume ratios make the fundamental mode unstable, and thus generate complex flow patterns.
This work is supported by National Key R&D Program of China and the National Natural Science Foundation of China.
Fig.1 Transition behaviors of thermocapillary convection for different volume ratios
Fig.2 (a0) ～(c0) Frequency spectra of the azimuthal velocity oscillation under ΔT = 10K, 20K, 50K respectively; (a1) ～(a4) four stable azimuthal velocity fluctuation characteristic modes under ΔT = 10K; (b1) ～(b4) four stable azimuthal velocity fluctuation characteristic modes under ΔT = 20K; (c1) ～(c4) four stable azimuthal velocity fluctuation characteristic modes under ΔT = 50K (Vr = 1.113).