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An experimental technique is developed for a vehicle moving steadily in the vicinity of the ground. A towing tank with a steadily advancing carriage is used and the unfavorable effects of the boundary layer on the ground which is inevitable in the case of a wind tunnel are fully removed. Experiments with a box-shaped model and a car model revealed some interesting features of lift, drag and side force at various clearances from the ground. Lift force is the most sensitive to the boundary layer and the lift measured in a wind tunnel may not completely represent lift on the road. The flow with vortices near the bottom surface of the body has one of the most important effects on the forces.

A newly developed finite-difference solution method TUMMAC-VII is applied to the flow about a road vehicle. It is a time-marching solution method for the Navier-Stokes equation in the framework of a fixed rectangular coordinate system. The no-slip boundary conditions are implemented in the body boundary cells. The use of a subgrid-scale turbulence model enables us to resolve the structure of the 3D vortical flows past road vehicles. Simulation is performed for the Ahmed model as well as the GEOSTORM model with a quarter and a half-million grid points, respectively, and the results are compared with experimental results.

The flow structures about a car-like bluff body with a variety of after-body configurations are numerically studied based on a finite-volume solution method with the sub-grid scale turbulence model. The numerical calculations are compared with the experiments in both the water basin and wind tunnel. Different detachment features are demonstrated in the case of different after-body configuration by the numerical analysis and the computer-graphics visualization. Some critical features such as high and low drag states are revealed in the numerical simulation in the case of critical-angle configuration and their mechanism are discussed.