SPECKLE MEASUREMENTS OF CONVECTION IN A LIQUID COOLED FROM ABOVE.

Buoyancy-driven recirculation in a liquid-filled rectangular cavity cooled from above is shown to be locally modulated by an unstable thermal layer at the surface. Interferometric observations suggest that fluctuations that occur in a plume descending through the upper liquid layers are of the type described by L. N. Howard and by R. Krishnamurti & Howard. Temperature measurements across the surface layer are in reasonable agreement with the diffuse heat-conduction model, but indicate that near the plume the fluid is cooler than elsewhere. Quasi-steady measurements of the velocity distribution in the upper regions of the cavity were made using multiple-exposure laser speckle velocimetry. Interrogation of the specklegrams with a Young's fringe technique yields a velocity-vector field of about two thousand elements. These data are used to calculate the corresponding velocity components and estimates of the vorticity distribution. The results compare favorably with measurements recorded directly from Fourier filtering methods.