Pattern Competition in Temporally Modulated Rayleigh-Bernard Convection.

Computer-enhanced shadowgraph flow-visualization studies and heat-flux measurements were used to study convection subjected to temporal modulation of the Rayleigh number R in the form epsilon (t) = R(t) sup (stat) sub c -1 = epsilon sub (omicron) + delta sin (omega t), where R sup (stat) sub c is the unmodulated threshold, and omega and t are scaled by the vertical thermal diffusion time. For omega = 15 and 0.7 ~ delta ~ 2.0 the predicted hexagonal patterns were observed for a range of epsilon sub (omicron) immediately above the convective threshold epsilon sub c . With increasing epsilon sub (omicron) there is a region exhibiting coexistence between hexagons and rolls, followed by roll-like patterns. The observed boundaries between these regions and the magnitude of the convective heat transport are consistent with theoretical predictions.