Effects of passive and combined aerodynamic control on the aerodynamic characteristics of an elliptical cylinder

Abstract

The flow separation during the process of flows around elliptical cylinders causes unsteady vortex shedding in the wake, which leads to the vortex-induced vibration of structures that can pose serious challenges across different engineering disciplines. Therefore, investigation on the suppression of the flow separation and the vortex shedding of elliptical cylinders so as to improve their aerodynamic characteristics has been one of the most active topics in recent decades. In this study, to investigate the effects of the passive and combined aerodynamic control on the aerodynamic characteristics of an elliptical cylinder immersed in uniform flow, wind tunnel tests of two 2D smooth elliptical cylinder models whose major axes are parallel or perpendicular to the incoming wind direction (i.e. the models S∥ and S⊥), two corresponding rough models with different surface roughness (i.e. the models R∥ and R⊥ with the relative roughness Sr of 5.3 × 10−4 and 1.15 × 10−3) and four corresponding combined aerodynamic controlled models consists of the surface roughness and air suction (i.e. the models RS1∼RS4) have been carried out. The Reynolds numbers for the test models are about 5.52 × 104–8.625 × 104. The surface roughness, suction flux coefficient CQ, azimuthal locations of suction holes and placement of the major axes of the elliptical cylinders are considered as the main experimental variables, and the aerodynamic characteristics in terms of both the wind pressure coefficient distributions and the aerodynamic force coefficients of the smooth models, rough models and combined aerodynamic controlled models are compared to analyze the effect of the passive and combined aerodynamic control. The results show that the aerodynamic characteristics improvements generated by the mere surface roughness are limited but accompanying with some negative effects, especially for the models with their major axes being parallel to the incoming wind direction. As the air suction combined with the surface roughness is implemented, it is found that considerable and steady control effects on the aerodynamic characteristics can be generated as CQ reaches 0.02. And the changes of the aerodynamic characteristics are gradually insignificant as CQ is further increased for most cases. Among all the cases with different combinations of aerodynamic measures, quite effective aerodynamic characteristics improvements are acquired for the combined aerodynamic controlled model RS2 (its suction holes are located at the radial angles of 130° and 230°, and its major axis is parallel to the incoming wind direction) with Sr = 1.15 × 10−3 and the model RS4 (its suction holes are located at the radial angles of 90° and 270°, and its major axis is perpendicular to the incoming wind direction) with Sr = 5.3 × 10−4. The conspicuous reductions of the mean drag coefficient CD for the two models mentioned above under CQ = 0.02 are 87% and 59% respectively. Even so, the drag force coefficients of the model RS4 are much larger than those of the model RS2. The objective of the present study is to better understand the aerodynamic control effects and inherent control mechanism due to the changes of surface roughness and air suction, and to explore a proper combination mode of the passive and active aerodynamic control measures to achieve more aerodynamic characteristics improvement for elliptical cylinders.