Morphological evolution of particle accumulation structures induced by thermo-vibrational effects in non-uniformly heated systems under microgravity conditions

With the recent discovery of solid particle attractors in thermo-vibrational flow driven by the interplay of inertial and convective effects in a constrained 3D physical space (as confirmed by dedicated experiments conducted on board the International Space Station), a need has emerged for additional numerical studies aimed at acquiring additional data to improve our understanding of these phenomena and their evolution in the space of parameters. The thermal configuration considered in this study is more complex than that used for the space experiments and consists of two differentially heated opposite walls containing central square patches that are of opposite temperatures (i.e. a cold spot located at the center of the hot wall and vice versa). The driving force is represented by periodic vibrations of specific frequencies, which are applied in such a way that they are perpendicular to the gradients of temperature established between the thermal features pertaining to different boundaries. Two different types of particle structures were reported in a recent study published by the same authors for similar boundary conditions; namely, tubular formations such as those revealed by the space experiments (where no thermal patches were present, i.e. there was a uniformly cooled wall located in front of a uniformly heated wall) and a previously unseen quadrupolar set of structures aligned along a direction perpendicular to such tubular formations. The present study takes it further ahead by increasing the range of considered values of the Rayleigh number to assess the effect of the magnitude of the thermo-vibrational flow field on the morphology and co-existence of these structures. The characteristic numbers, Prandtl number, angular frequency, particle Stokes number and particle-fluid density ratio are fixed at 6.11 (water at ambient temperature), w=10 ³, St= 5×10 ^-6 and x =2 respectively whereas the vibrational Rayleigh number spans the interval 5×10 ³< Raw<10 ⁵ It is shown that when the Rayleigh number is increased, the quadrupolar formation, which previously was a secondary effect (having a limited spatial extension) becomes so dominant that the tubular formations cease to exist.