Numerical analysis of the impact of exit conditions on low Mach number turbulent jets

Large-eddy simulations (LES) of axisymmetric turbulent hot jets at a Reynolds number Re = 316,000 and a Mach number M = 0.12 are computed using an unstructured hierarchical Cartesian mesh method. To analyze the impact of exit flow conditions on the turbulent free jet two formulations of the flow distributions at the nozzle exit are considered. First, the flow inside the nozzle is part of the overall nozzle-jet analysis such that the turbulent flow at the nozzle exit is determined by the solution of the full discrete conservation equations. This formulation is denoted free-exit-flow formulation. Second, the nozzle exit distributions which are used as inflow distributions for the free jet analysis are based on the mean flow profiles plus vortex rings. That is, for the latter case the impact of the nozzle geometry on the free jet is not directly taken into account, the nozzle exit flow distribution is imposed. This formulation is denoted imposed-exit-flow formulation. The LES analyses show that the flow field immediately downstream of the nozzle exit is strongly influenced by the flow structures induced by the nozzle geometry. Compared to the imposed-exit-flow case the free-exit-flow possesses an almost 30% higher peak turbulence intensity level which also results in a massively enhanced velocity decay. Moreover, the free-exit-flow case generates a spectral peak at St = 0.56, while no distinctive peak is observed for the imposed-exit-flow formulation.