Research On DNS And LES Of Turbulent Flow Over A Backward-facing Step

A general computer code has been developed for direct numerical simulation(DNS) and large eddy simulation(LES) of turbulent flow over a backward-facing step in a two-dimensional duct. For DNS,the modified Marker-and Cell(MAC) method is adopted for solving time-dependent viscous incompressible Navier-Stokes equations on a staggered grid system and the time derivative is discretized using the second-order accurate implicit formulation. For LES,the k-equation sub-grid scale model was used to simulate the sub-grid eddy viscosity and SIMPLE algorithm and the hybrid difference scheme are applied to large eddy simulation for turbulent flows. Predictions of recirculation region length and axial velocity profiles are in fairly agreement with experimental data. It reveals that both two numerical methods can be used to predict transient turbulent flows and the accuracy of DNS is higher than LES,but computation time of DNS is more than LES.For DNS,the explicit finite difference methods based on second-order accurate hybrid and fourth-order accurate compact difference formulae are used to solve momentum equations. A Poisson equation for pressure correction is discretized by five-point and nine–point implicit central difference schemes. Effects of different difference schemes and inlet velocity profiles on turbulence flows over a backward-facing step are investigated by direct numerical simulation. By the comparison of calculation and measurements,Tangent mean velocity profile is better than Sine-shaped profile.