On the oscillatory hydrodynamic modes in liquid metal layers with an obstruction located on the bottom

Hydrodynamic disturbances represent the preferred mode of instability of thermogravitational flow for a relatively wide range of substances and conditions (essentially pure or compound semiconductor and superconductor materials in liquid state). As nowadays almost all modern technologies rely greatly on such crystallized materials, targeting an improved understanding of the convective phenomena which occur in the melt has become a subject of great importance. Here an "ad hoc" model is developed to inquire specifically about the role played in such a context by geometrical "irregularities" affecting the melt container. More precisely, results are presented for the case of a fluid with Pr = 0.01 (silicon) filling an open cavity with a single backward-facing or forward-facing step on the bottom wall or with an obstruction located in the centre. It is shown that the presence of sudden changes in the considered geometry can lead to a variety of scenarios with a significant departure from classical situations examined in the past. These configurations have different spatial symmetries and show different dynamics, including rhythmic roll expansions and contractions along the vertical and horizontal directions at different locations, roll nucleation, deformation, transport and merging phenomena. In some circumstances a travelling wave with front perpendicular to the imposed temperature gradient emerges, which has never been reported in the literature. A frequency spectrum analysis is used to support the identification of the multiple convective phenomena enabled by the new geometric features. (C) 2017 Elsevier Masson SAS. All rights reserved.