Experimental Study of an Artificial Thermal Plume in the Boundary Layer. Part III: Dynamic Structure within the Plume

Abstract

An experimental study of the dynamics within artificial thermal plumes rising in the boundary layer is presented. In this third part, measurements just above the heat source and aircraft investigations in the plume aloft are used to reveal the internal structure of the airflow within the buoyant column. Analysis of the pressure perturbation obtained both by direct measurements and as a residual in the mean vertical motion equation for a plume, shows that the vertical pressure gradient accelerates the airflow near the heat source and then reduces the buoyancy in the upper levels. The pressure deficits, attaining maximum values of 1 mb in the core of the lower portion of the plume, are well correlated with large vertical velocities. During light ambient wind conditions, the reduced pressure near the heat source produces a large converging inflow sufficient to cause the lower portion of the plume to go into rotation as a whole. An analysis of the components of the velocity field and momentum fluxes within the column underscores the convergent and divergent characters of the flow, respectively, at the lower and upper portions of the plume. Strong vorticity concentration (?4 10?2 s?1) is associated with a reduction of entrainment into the column.