Pattern Competition in Temporally Modulated Rayleigh-Bénard Convection

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 ε(t)=R(t)Rcˢᵗᵃᵗ-1=ε₀+δsin(ωt), where Rcˢᵗᵃᵗ is the unmodulated threshold, and ω and t are scaled by the vertical thermal diffusion time. For ω=15 and 0.7δ2.0 the predicted hexagonal patterns were observed for a range of ε₀ immediately above the convective threshold εc. With increasing ε₀ 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 theory.