DIRECT TRANSITION TO SPATIOTEMPORAL CHAOS IN LOW PRANDTL NUMBER FLUIDS

We present a large scale numerical simulation of three-dimensional Ray leigh-Benard convection near onset, under free-free boundary condition s for a fluid of Prandtl number sigma = 0.5. We find that a spatiotemp orally chaotic state emerges immediately above onset, which we investi gate as a function of the reduced control parameter. We conclude that the transition from conduction to spatiotemporal chaos is second order and of ''mean field'' character. We also present a simple theory for the time-averaged convective current. Finally, we show that the time-a veraged structure factor satisfies a scaling behavior with respect to the correlation length near onset.