Doppler Imaging of the Onset of Turbulent Convection

Abstract

We demonstrate that large rectangular fluid layers have several essentially time-independent convective states above the critical Rayleigh number $R_c$ in which the rolls align perpendicular to all lateral boundaries. The pattern is hence splayed and contains defects. Using real-time laser Doppler imaging, we show that the onset of time dependence at $R_1\cong 5R_c$ arises from a structural instability of the rolls which can be suppressed by a local heat source.