Large-eddy simulation of flow over a square prism with moving belts

This three-dimensional (3D) numerical study investigates the feasibility of controlling the intermittently reattaching flow (IRF) by using the moving-surface boundary-layer control (MSBC) method. To this end, a fixed square prism was preferred due to its geometric simplicity and was equipped with moving belts that placed side surfaces close to the trailing edges. The effects of three dimensionless key parameters including velocity ratio (i.e., VR, the ratio of the velocity of the moving belt to the freestream velocity), length ratio (i.e., LR, the ratio of the belt length to the edge length of the square prism), and gap ratio (i.e., GR, the ratio of the spanwise distance between the neighboring belts to the edge length of the square prism) mainly on the aerodynamic characteristics were investigated using large-eddy simulation (LES). The findings suggest that the rise in VR and LR parameters leads to a proportional decrease in the time-averaged drag, 〈CD〉, attributable to their contribution to the linear momentum transfer mechanism by the moving surfaces. This mechanism, firstly, provides an additional thrust to the prism in the opposite direction of the main flow. Secondly, this mechanism increases the base pressure by stretching the shear layers from the leading edges of the prism in the streamwise direction. For LR = 0.4 and VR = 4, a 17 % reduction in 〈CD〉 was observed. Additionally, the root-mean-square (RMS) values of fluctuating drag and lift coefficients, CD-RMS and CL-RMS, have decreased by 71 % and 55 %, respectively. Conversely, the increase in the GR parameter led to an increase in the spanwise distance between each individual moving belt and a reduction in the positive effect of the linear momentum transfer mechanism on the 〈CD〉 reduction. The segmentation of moving belts along the spanwise direction also triggered the vorticity redistribution mechanism concurrently. The study also elucidated the underlying mechanisms that led to the failure of the vorticity redistribution mechanism to adequately reduce the 〈CD〉.