Abstract
Thermal buoyancy driven flows in confined vertical cylinders filled with melts of the material system Bi 2Te 3Sb 2Te 3 were investigated experimentally and numerically. In dependence on the height of the molten zone and on the Prandtl number the onset and further transitions of temperature fluctuations in the melt were detected by in-situ temperature measurements using a digital data acquisition system. The spectral features of the unsteady flows were analyzed by Fast Fourier Transformation and revealed two fundamentally different periodic oscillation modes. Based on the phase relations and amplitudes of the signals measured in axial and azimuthal direction spatio-temporal oscillating flow structures can be carefully interpreted. “Mode switching” and the rotation of a vortex configuration are assumed to be the prefered oscillation mechanisms. Numerical results have been obtained using a three-dimensional, time-dependent finite volume method. Steady axisymmetric convective flow has been found for ar<1.5 and non-axisymmetric flow for ar≥1.5. After a spatial symmetry-breaking the asymmetric single roll pattern shows a time-dependent motion. The transition point for the onset of flow oscillations agree well with experimental results.
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