The Blockage Effects of a Two-Dimensional Bluff-Based Body on Its Aerodynamic Drag Characteristic

Abstract

Measurements are performed on aerodynamic drag coefficients, C_D, of a two-dimensional bluff-based body over the range of blockage ratios of the body breadth to the wind tunnel width, d/W=0.1∼0.4, C_D increases with an increase of d/W, exponentially at d/W>3.5, being subject to the blockage constraint. A modified drag coefficient C'_D introduced by the clearance freestream velocity between the body and the tunnel side wall, U'_∞, remains a constant value, independently of blockage ratios of d/W<3.5. C'_D implys the base drag coefficient without the blockage effects, C_D0, and the latter may be predicted by an empirical formula C_D0K²·C_D and K=U_∞/U'_∞=1-d/W-2(δ^*_b+δ^*_w)/W, where δ^*_b and δ^*_w are the boundary layer displacement thicknesses produced on the body and the tunnel wall. The drag characteristic is also discussed with the results of the pressure and velocity distributions around the body and the wake, the prevailing frequency of shed vortices, and the flow visualization.