Towards DES in CFD-based optimization: The case of a sharp U-bend with/without rotation

Abstract Channels with a sharp U-bend are typical elements of convective cooling systems designed for blades of high-temperature gas turbines. The flow passing a sharp bend is characterized by a high level of pressure loss that is attributed to development of intensive secondary flows, as well as to formation of a large-size separation adjacent to the divider wall. A considerable reduction of pressure losses can be achieved by proper changes in geometry of the walls forming the U-bend, first of all, in the divider wall geometry. The paper presents results of CFD-based optimization of a stationary or orthogonally rotating U-bend of strong curvature. Baseline geometry is same as used by Cheah et al. (1996) for detailed measurements of U-duct isothermal turbulent flow at the Reynolds number of 105, with and without system rotation. The minimization of the total pressure loss in the bend is achieved by means of a hybrid numerical optimization method that uses design of experiments and a metamodel for fast surrogate modeling of the design space. Optimization databases are produced by incompressible fluid unsteady RANS-(k-ω)-SST calculations. Due to optimization of the divider wall profile, the total pressure loss coefficient has been reduced by 47-67 % depending on the rotation number (Ro = 0,±0.2). For the original and the optimized shapes of the bend, simulation is performed also with the eddy-resolving IDDES method validated against the velocity field and pressure measurement data for the original geometry. For optimized shapes, IDDES predicts somewhat lower pressure losses comparing to the RANS model, by 3 to 15 % depending on the rotation number.