Measures for Defining the Turbulent/non-Turbulent Interface of a Steady Round Plume: A DNS Study*

Free shear flows such as plumes and jets are characterized by a turbulent/non-turbulent interface (TNTI) which forms a part of the convoluted thin boundary that separates the turbulent flow from the irrotational ambient. Existing literature delineates several methods to identify the TNTI in such types of flows. We performed direct numerical simulation (DNS) of a turbulent plume at a Reynolds number (defined using base scales) of 2000 to show that additional insightful measures can be used to determine the TNTI. A code named Megha 3, used for the simulation, solves the Boussinesq approximation of the three-dimensional continuity and Navier-Stokes equations. We identify the TNTI using independent thresholds on three different variations of the vorticity field, including scaled vorticity and the impact of strain field on the vorticity. Each one of such measures is not merely a number. Instead, they seem to capture subtle differences in the flow field, which can be used to capture the turbulent nature of the flow more accurately. For example, the scaling based on self-similarity seems to reflect some aspects of coherent structures present in the flow. In contrast, the scaling based on time-averaged geometric and kinematic quantities makes the TNTI well-defined at all vertical levels.