Detached-Eddy Simulation of the Ground Transportation System

Abstract

Predictions of the flow around the Ground Transportation System (GTS) are obtained from Detached-Eddy Simulation (DES) and from the Reynolds-averaged Navier-Stokes (RANS) equations. Two methods are compared with experiment, but only one grid is used, as the work is in progress. Grid variations will be needed. The computations are performed at a Reynolds number based on body width and inlet freestream velocity of 2 × 10⁶, at 0° and 10° yaw. Solutions are obtained using unstructured grids comprised of approximately 6 × 10⁶ elements (prisms and tetrahedra) and the commercial flow solver Cobalt. No attempt is made to model laminar-to-turbulent transition. Instead, the predictions are of the fully turbulent solution obtained via prescription of a small level of eddy viscosity at the inlet to the computational domain. The RANS predictions are of the steady-state solution using the Spalart-Allmaras model. For 0° yaw the predicted drag coefficient Cd from the RANS of 0.370 is substantially larger than the measured value of 0.249. DES yields a more complex and three-dimensional structure in the separated regions. One of the improvements over RANS is a more accurate prediction of the back pressures, resulting in the DES prediction of Cd = 0.279. At 10° yaw, DES predictions of the body-axis drag are again closer to measurements than obtained using RANS, but substantially larger than the measured value. One source of the discrepancy is a more significant separated region near the front leeward corner than observed in experiments, resulting in tangible differences in the pressure distribution along the lee side.