Solving Unsteady Separated Flow Using ADINA CFD
To solve unsteady separated flows, the Detached-Eddy Simulation (DES) turbulence model is available in ADINA 8.4. This recent development is devised to predict, at a manageable computational cost, unsteady separated flows. Industrial problems involving this type of flows are very expensive to solve using LES. On the other hand, neither the RANS nor the URANS models are accurate enough to successfully predict the flow behavior in the highly-separated regions.
DES is a hybrid model that uses a 1-equation model (Spalart-Allmaras in ADINA) which behaves as a RANS model in the boundary layers, and as an LES Sub-Grid Scale model in the free stream region where the flow is separated.
To illustrate the DES modeling capabilities in ADINA, we analyze the flow past a cubic obstacle. This problem has been widely used as a benchmark for LES and URANS models. The schematic and the mesh used for the computational domain are shown below. A relatively coarse mesh with 180,000 hexahedral elements is used, with 1,500 time steps for the transient response solution.
The movie above depicts the time-dependent vorticity contours about the Y-axis, at 0.25h from the base of the block. The Reynolds number based on the block height, h, and the inlet bulk velocity is 40,000.
The following graphs show the time-averaged streamwise velocity profiles, vertically, at the mid-plane of the computational domain for the different locations A, B and C. The DES results from ADINA are compared against experimental data, showing very good agreement even though the mesh used is coarse.