Turbulent mixing in T-junctions: The role of the temperature as an active scalar

In this paper we present a detailed study of turbulent mixing between a cold crossflow and a hot incoming jet in a T-junction. Our study is based on wall-resolved and experimentally validated Large Eddy Simulations (LES) without invoking the Boussinesq approximation regarding density variations. Emphasis is placed on the role of the temperature as an active scalar and the impact of thermal stratification on the emerging structures in the flow field. Herein we discuss in detail the main features of the flow, such as the shear layer between the two streams, the regions of flow acceleration due to the Venturi effect, the recirculation regions, and the thermal mixing downstream the reattachment point of the flow. We also elaborate on the first and second-order statistics of flow quantities, including the turbulent heat fluxes, and on the near-wall behaviour of the streamwise velocity component. Our numerical predictions compare favourably with existing experimental measurements, as regards both the temperature field and the turbulent heat fluxes. Our study also includes a spectral analysis of the temperature signal at selected locations near the solid boundaries, which is helpful for assessing the risk of thermal fatigue at the walls of the T-junction.