Hydrothermal-wave Instability and Resultant Flow Patterns Induced by Thermocapillary Effect in a Half-Zone Liquid Bridge of High Aspect Ratio
We focus on a transition of the convective fields of a thermocapillary convection in a half-zone (HZ) liquid bridge of high aspect ratio by numerical and experimental approaches. We have been inspired by the fluid physics experiments in a microgravity environment on a thermocapillary convection carried out on the Japanese Experiment Module ‘Kibo’ aboard the International Space Station since 2008. In these experiments, the thermocapillary-driven flow in a HZ liquid bridge of high Prandtl number (Pr) fluid in a range of the aspect ratio (height/radius) have been examined to firstly indicate the transition point of the flow field from the two-dimensional steady flow to the three-dimensional time-dependent ‘oscillatory’ one. These space experiments also indicate the characteristics of the hydrothermal waves (HTW) after the transition as a function of the aspect ratio; especially new knowledge on the flow fields in the liquid bridge at high aspect ratio, which would be so hardly performed under the normal gravity. The present research is intended to realize the flow fields in the high-aspect-ratio HZ liquid of rather high Prandtl number fluid by both experimental and numerical approaches, and make comparisons with the results obtained in the space experiments. In this numerical simulation, we carry out a series of simulation on the fluid of Pr = 28.6. Physical properties are decided by assuming 2-cSt silicone oil that is employed as the test fluid in the terrestrial experiments. The flow patterns before/after the onset of the transition as well as the transition points in the cases of aspect ratio greater than 2.0 are focused. In order to make comparisons with the results by the space experiments, we extract the spatio-temporal structures of the HTWs; especially the temperature variation over the free surface of the liquid bridge and its modal structure. Our transition points and the characteristics of the oscillatory flows are in good agreement with the results by the space experiments. It is noted that our simulation indicates there might exist an additional flow pattern between the transition point and the observed oscillatory flow in the space experiments in the case of the aspect ratio greater than 2.0, and this additional flow pattern will bring a reasonable explanation to a unique correlation between the fundamental frequencies of the oscillatory flow and the flow patterns detected in the space experiments.