Laboratory Simulation of Wake Effects on Second and Third Thermals in a Series

Abstract

Individual thermals rising through the atmosphere may encounter the wake of one or more thermals that started their rise at an earlier time. Laboratory simulations show that the growth rate of second and third thermals in a series is enhanced relative to that of a solitary thermal. The enhancement is due to a gain in momentum from the wake of a preceding thermal, and equations are developed for predicting the amount of enhancement as a function of the delay time between thermals. Agreement with experimental data is good for first and second thermals. The third thermal was found to have a growth rate very similar to that of a second thermal. Theory agrees to the extent that only a slight increase is predicted, and this amount could have been lost within the experimental error. Thus, it appears that by the time the third thermal in a series occurs, an equilibrium condition has been reached as far as the thermal growth rate and velocity field are concerned.