VORTICAL NATURE OF THERMAL PLUMES IN TURBULENT CONVECTION

Thermal plumes in Rayleigh-Benard convection have been observed to occ ur with strong vertical component of vorticity, resulting in spiraling hot updrafts and cold downdrafts. Results from two different simulati ons at Rayleigh numbers 9800 and 33 000 times the critical Rayleigh nu mber close to the boundaries show rapid increase in the conditional av erages of vertical velocity and temperature perturbation at large valu es of vertical vorticity. This result along with the spatial distribut ion of vertical vorticity indicates a strong correlation between narro w regions of upmoving hot fluid (or downmoving cold fluid) and local v ertical vorticity. The horizontal dimension of these vortical plumes i s an order of magnitude smaller than the layer depth, but they extend up to half the convective layer in the vertical direction.