The significance of engulfment in the process of turbulent entrainment by plumes

We present simultaneous two-dimensional velocity and scalar concentrations measured on a central vertical plane in a turbulent plume. We use an edge-detection algorithm to determine the scalar plume edge, and compare the data obtained in both a fixed Eulerian frame and relative to local plume coordinates defined in terms of the instantaneous plume edge. We observe that the time-averaged distributions of vertical and horizontal velocity are self-similar. Measurements in the plume coordinate show that there is significant vertical flow outside the edge of the plume, contrary to the classical notion of purely horizontal velocities at the plume edge. We observe large variations in the plume width and reason these to be due to the passage of large coherent eddies. The flow within the plume and in the nearby ambient fluid depends on whether an eddy is present or absent. When an eddy is present and the plume is wide, the vertical velocities near the plume edge are small, but in regions where the plume is narrow and there is no eddy, large vertical velocities are observed outside the plume.