Three-dimensional numerical simulation of Marangoni instabilities in non-cylindrical liquid bridges in microgravity

Instability of Marangoni convection in non-cylindrical (convex or concave) liquid bridges of low Prandtl number fluids is investigated by direct three -dimensional and time-dependent simulation of the problem. Body-fitted curv ilinear coordinates are adopted: the non-cylindrical original physical doma in in the (r, z, phi) space is transformed into a cylindrical computational domain in a (xi, eta, phi) space, The geometry of the domain is transforme d using a coordinate transformation method by surface fitting technique. Th e field equations are numerically solved explicitly in time and with a fini te difference technique in a staggered grid. The numerical results are anal yzed and interpleted in the general context of the bifurcation's theory. The computations show that for semiconductor melts the first bifurcation is characterized by the loss of spatial symmetry rather than by the onset of oscillatory flow and that it is hydrodynamic in nature. The flow field azim uthal organization related to the critical wave number, depends on the geom etrical aspect ratio A = LID of the liquid bridge and on the shape factor S (convex S > 1, concave S < 1) of the free surface. The critical azimuthal wave number increases when the geometrical aspect ratio of the bridge is de creased and, for a fixed aspect ratio, can be shifted to higher values by i ncreasing the volume (convex bridges) or to lower values by decreasing the volume (concave bridges). This behavior is explained on the basis of the relation between the topolog y of the azimuthal disturbances and the structure of the fluid-dynamic held . A generalized law is found to correlate the critical azimuthal wave number of the instability to the geometrical aspect ratio and to the shape factor. A second oscillatory (Hopf) bifurcation occurs M hen further increasing the Marangoni number. Experimental results available in literature on this sec ond bifurcation are considered for comparison. The experimental and numeric al results show a good agreement. <(c)> 2001 Elsevier Science Ltd. All righ ts reserved.