Kapil M. S. Bajaj

Shape of convective cell in Faraday experiment with fine granular materials

The shape of convective cell at the onset of heap formation in the Faraday experiment with fine granular materials is studied in a simple experiment. The motion of fine grains are found to be confined in a single thin conical cell instead of a pair of convective cells as considered previously. The subsequent pattern-selection is strongly affected by the shape and size of the grains as well as the presence of ambient air.

Rayleigh–Bénard convection in the presence of a radial ramp of the Rayleigh number

We present experimental results for pattern formation in a thin horizontal fluid layer heated from below. The fluid was SF6 at a pressure of 20.0 bar with a Prandtl number of 0.87. The cylindrical sample had an interior section of uniform spacing d = d0 for radii r r0. For Rayleigh numbers R0 > Rc in the interior, straight or slightly curved rolls with an average wavenumber (e0 ≡ R0/Rc - 1) with were selected. The critical wavenumber depended sensitively on the cell spacing. For the largest the patterns were skewed-varicose unstable and dislocation pairs were generated repeatedly in the interior and for all e. For slightly smaller time-independent rolls were stable for , but for larger e the skewed-varicose instability was encountered near the sample centre and dislocation pairs were formed repeatedly for all samples. When stationary rolls were stable, their slight curvature and the width of their wavenumber distribution slowly increased with e. This led to a complicated shape and overall broadening of the structure factor S(k). For the inverse width ξ2 of S(k) was roughly constant and presumably limited by the finite sample size, but for larger e we found .