Large eddy simulations of two-dimensional and three-dimensional spatially developing mixing layers

A complete understanding of the noise generation mechanisms is prerequisite to reducing aircraft jet noise. A computationally intensive large eddy simulation (LES) can help directly predict the noise. For this purpose, the Ansys-Fluent LES tool is evaluated for its modeling accuracy and computational cost. A satisfactory computational methodology is developed first by studying a canonical problem. A mixing layer at a Reynolds number of 720 is simulated using LES in two- and three-dimensions without the splitter plate walls and is studied extensively to gain insights of the flow physics. The effects of inflow-forcing and the buffer-zone at the domain exit incorporated in 2-D LES are investigated. The flow is damped in the buffer zone without explicitly introducing the damping terms and is found to be satisfactory for the current work. The instabilities of the 2-D mixing layer are captured well using random inflow-forcing. The buffer-zone is found to help the prediction of the Reynolds stresses near the exit boundary. The sensitivity of the 2-D results to timestep, sampling time and grid is also studied. The 2-D grid is, then, optimized to construct a mesh for 3-D LES. The deficiencies of Fluent’s in-built vortex method (VM) algorithm to force perturbations in 3-D are discussed. An adapted VM algorithm is implemented and is found to predict the three-dimensional instabilities of the mixing layer well. The energy spectrum of the LES computations indicate that the second-order-accurate bounded central differencing scheme, used in the present study, is adequate for modeling turbulent flows. The computational resources to perform a realistic jet simulation are also estimated