An investigation on the “Onset” of oscillatory Marangoni flow

It is known that thermocapillary convection in differentially and uniformly heated liquid bridges shows a transition (Marangoni instability) from an axisymmetric to an oscillatory regime. The “Onset” experiment, performed during the D2 Spacelab mission on the Advanced Fluid Physics Module (AFPM), was aimed to investigate this transition at various bridge geometries; in particular the goal was to determine the critical temperature difference applied to the end disks supporting the liquid bridges, and to study the structure of the flow at the onset of the Marangoni instability. The paper reports the main results, obtained analyzing the experimental data (temperatures and velocities). The evaluation of the transition (onset) condition obtained from temperature data was achieved with the aid of signal processing techniques. The onset determinations have been also validated by analysis of flow field velocities. The results concerning frequency and amplitude of temperature oscillations in the neighbourhood of the onset of Marangoni instability are reported. We provide an evaluation of the heat fluxes through the plates. A comparison between numerical and experimental results related to the two-dimensional flow field is made, a satisfactory agreement is reached.