Experimental and numerical investigation of the flow structures at the rear of three-dimensional bluff bodies

Abstract

Flows around bluff bodies have complex structures, which create drag, surface contamination and stability issues for transportation systems. The standard Ahmed body (SAB) is a simplified representative three-dimensional (3D) bluff body that is known to produce the essential features of complex bluff bodies. This thesis studies the flow structures at the rear end of 3D bluff bodies to aid the development of flow control strategies. In the first method, a modified SAB with a 25° slant angle is proposed that uses elliptical curvature at the rear end and is denoted as the elliptical Ahmed body (EAB). The particle image velocimetry (PIV) technique is used to provide the detailed flow structure. The PIV study is conducted at a Reynolds number of 4.31 ×104 based on the model height. This experimental study is complemented by detached eddy simulations at Reynolds numbers of 1.47 × 104, 4.31 × 104 and 1.90 × 105. In the second method, the effect of a hydrophobic coating on the flow structure of the SAB and EAB is investigated experimentally using the PIV technique and for the same Reynolds numbers stated above. For both methods, the coherent structures are evaluated using advanced analysis techniques, such as frequency analysis, proper orthogonal decomposition, dynamic mode decomposition, Q-criterion and λ2-criterion. For the Reynolds numbers and specific conditions investigated, the results show that the elliptical curvature creates significant reorganization of the flow structures, where the slant separation bubble, longitudinal C-vortices and lower recirculation bubble are eliminated, whereas the upper recirculation bubble shifts toward the slant surface. This flow restructuring provides ~10.4% drag reduction and reduces surface contamination. In addition, the hydrophobic coating increases the slant separation bubble and the Strouhal number at the slant surface of the SAB, while the wake recirculation length is not significantly affected. However, the shear stress, turbulent kinetic energy, and Strouhal numbers are reduced over the EAB with the coating. Overall, the results show that elliptical curvature and hydrophobic coating have the potential for drag reduction and the mitigation of surface contamination. However, further investigation is required before generalized conclusions can be drawn.