Influence of ground effect on air drag of car model

In order to obtain the influence of ground effect on air drag of car model, for a 1∶3 scaled MIRA car model, a wind tunnel test was carried out inΦ3.2 m wind tunnel in China Aerodynamics Research and Development Center. A moving belt was taken as a unified research platform, and the influence of ground's fixed and moving states, wheels' motionless and rotating states, and different clearances of car body to ground plane on air drag was studied. The working condition of no cross wind that yaw angle is 0°was simulated, the fixed test wind velocity was 25 m·s^-1, and the changing Reynolds number's test wind velocity was 15-26 m·s^-1. The aerodynamic force of car model was only measured, air drag was mainly focused, and the test result was expressed by the dimensionless air drag coefficient. Analysis result shows that when the ratio of boundary layer thickness of fixed ground and the clearance of car bottom to ground is equal to or less than 0.32, the clearance ratio of wheel bottom and body bottom to ground is equalto or less than 0.37, the air drag for fixed ground is less than the value for moving ground, and the difference is less than 1.1%, so the impact of ground state on air drag can be ignored. The air drag for motionless wheel state is less than the value for rotation wheel, and the difference is less than 2.1%. So in the engineering application, when wheel rotation condition can't be simulated, correcting (increasing) air drag should be considered, but the allowance should be no more than 2.1%. With the increase of the clearance from wheel bottom to ground, air drag gradually decreases on the whole, and when the dimensionless clearance is 0.069-0.370, the difference of air drag is less than 2.0%. Thus in the wind tunnel test of car model using the moving belt, the clearance of wheel bottom to belt surface should be as small as possible under ensuring the contactless state of wheel bottom and moving belt surface.