A fluid experiment was conducted on China’s SJ10 satellite to assess the instability of thermocapillary convection under microgravity. Oscillations of thermocapillary convection in an annular pool were investigated when the difference in temperature exceeded a certain threshold. Dynamics and transitions of the wave pattern were observed using an infrared camera. This work is concerned with the effect of the surface configuration on the wave patterns. There are 3 types of surface configurations that depend on the volume ratio: Vr < 0.77 (zone 1), 0.77 < Vr < 1 (zone 2), and Vr > 1 (zone 3). An irregular local wave occurs at Vr < 0.65, and a regular azimuthal wave occurs at Vr > 0.65. We observed the regular wave with the wavenumber m = 3 in zone 1 and zone 3 and the competition between m = 3 and m = 4 in zone 2. The nonlinear dynamics of traveling waves, standing waves, and counterpropagating waves were studied. The standing wave appears near the onset and transitions to the traveling wave. With a similar mechanism, counterpropagating waves with m = 3 and m = 4 occur in some cases at the onset. Additionally, counterpropagating waves also act as a transitional mode when the traveling wave transitions from m = 4 to m = 3. The traveling wave is very stable, but it develops into standing waves or Benjamin-Feir instability under highly supercritical conditions.
Bénard-Marangoni convection can be used to self-organize hexagonal convective cells, but defects easily emerge in the hexagonal pattern, which hinders its application in industry. The dynamics of front propagation and defect generation are studied in this paper. We focus especially on the onset process of a local disturbance of a hexagonal pattern, named the “nucleus.” The front propagation of the nucleus has been researched through numerical simulations of a model equation and experiments. In the numerical simulations, a single nucleus can evolve into a perfect hexagon pattern under critical or subcritical conditions, and a random disturbance can generate multiple nuclei which evolve into grain boundaries. In addition, under supercritical conditions, defects also emerge as a single nucleus grows. The instability of front propagation is considered to be the mechanism for the generation of irregular patterns. The curvature effect makes the protrusion of the front have a larger velocity in supercriticality, which results in a wavy front, and defects are generated in the concave portion of the front. Also, because of the curvature effect, the front of an irregular pattern has a larger velocity than that of the regular pattern since the protrusion of the front in the irregular pattern increases the average velocity. Experiments have also been carried out by using an infrared camera to analyze front propagation. The results are qualitatively in agreement with the results of numerical simulations. Through the study of defect generation in front propagation, we put forward a method for generating a hexagon pattern which greatly reduces the number of defects.
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