Large Eddy Simulation of Turbulent Flow in a Confined Square Coaxial Jet

Large eddy simulation (LES) is performed to investigate the spatial evolution of the turbulent flow in a confined square coaxial jet. The turbulent inflow conditions in the jet are obtained from the LES in both square and annular ducts using the temporal approach. Such prescription of the inflow boundary condition faithfully represents the turbulent inlet conditions and makes it possible to realistically investigate two types of turbulent mixing mechanisms originated from the streamwise shear, caused by streamwise velocity difference, and the secondary shear, induced by the turbulence-driven secondary flows. The turbulent mixing properties in the confined square coaxial jet are studied by analyzing the spatial evolvement of the mean flow field and the second-order turbulence statistics. The simulation results present reasonable agreement with the experimental data from a square free jet and the measurements of a confined plane jet. The turbulent mixing phenomena are interrogated using the streamwise vorticity distributions on various section planes of the instantaneous flow field. The principle of ω x -dynamics in [K.B.M.Q. Zaman, (1996). “Axis switching and spreading of an asymmetric jet: the role of coherent structure dynamics”, J. Fluid Mech., 316, pp. 1–17] is used to understand the effects of the turbulence-driven secondary flow and to explain the observation found in the simulation.