Studies in unsteady penetrative thermal convection

The dynamics and structure of turbulence in the interfacial region and the upper part of a mixed layer which is capped by a linearly stratified, nonturbulent region are studied. Measurements of the local, instantaneous values of the temperature, the vertical velocity and horizontal velocity were made using a thermocouple and a two component dual beam laser Doppler velocimeter. Vertical profiles of all relevant one point moment including joint temperature velocity moments up to fourth order were obtained. Mean temperature measurements were made for various kinematic heat flux settings to understand the thermal structure in the entrainment zone. Conditional averaging techniques and probability density functions were employed to infer the properties of fluid elements in the mixed layer. A length scale in the interfacial layer, based on the temperature gradient in the stable region is proposed. Power spectra for velocity and temperature were analyzed to check the possible existence of inertial subrange and estimate the noise level. With the aid of flow visualization, a phenomenonological model of penetrative convection for low entrainment rate was developed.