Transitions in convection driven by a horizontal temperature gradient

We report the first detailed experimental study of transitions in the convection of a low Prandtl number fluid driven by a horizontal temperature gradient. The observed states, from time independent to one frequency with noise, to pure noise, to two frequencies with noise, can be related to the two secondary flows predicted for a cavity with large lateral extent, transverse stationary and longitudinal oscillatory rolls. Measured wavelengths and frequencies for the longitudinal rolls are in agreement with theoretical values, while the critical Grashof number is much higher than expected. Our results call for a new theoretical approach which takes both instability mechanisms into account.