LARGE-EDDY SIMULATION OF TURBULENT FLOW OVER A HEAVY VEHICLE WITH DRAG REDUCTION DEVICES

摘要

In order to improve the fuel efficiency of heavy vehicles, flow control devices aiming at aerodynamic drag reduction are often utilized. Computational simulation has been widely used in the investigation of fluid dynamics associated with aerodynamic drag over a heavy vehicle and control effects of many drag reduction devices. Most previous studies were, however, conducted using computational techniques based on the Reynolds-averaged Navier-Stokes equations and with rather simplified geometries (i.e., GTS, GCM, and Ahmed body), and therefore, the utility of the understanding of the drag-producing flow physics is often impractical and limited. In the present study, turbulent flow around a heavy vehicle with realistic geometric details is simulated using large-eddy simulation (LES), which is capable of providing unsteady flow physics responsible for aerodynamic in sufficient detail. The filtered Navier-Stokes equations (Eq. (1), Eq. (2)) are solved using Vreman-based dynamic subgrid-scale (SGS) model in unstructured grid. Fully-implicit fractional step method is employed as a time integration method and all terms of the N-S equations are advanced using the Crank-Nicolson method. A second-order central difference discretization scheme is implemented in space. Flow over a 15ton truck with and without drag reduction devices is simulated. The LES result is first validated against experimental measurement with respect to drag coefficient (Cd). The measured and simulated drag coefficient of a 15ton truck with and without side skirt and boat tail are shown in Tab. 1 and Tab. 2, respectively. Drag is caused by pressure difference and momentum loss along the streamwise direction. Thus, drag can be estimated by visualizing the pressure and streamwise velocity field over a vehicle. The time-averaged pressure and velocity fields around a truck model with and without drag reduction devices are plotted in Fig. 1-4.