Manipulating Flow to Reduce Drag of a Square Cylinder by Using a Self-Sustained Vibrating Rod

The flow, vortex shedding, and surface pressure of a square cylinder at incidence were manipulated by means of a self-sustained vibrating rod placed around the leading edge of the upwind-facing lateral face of the square cylinder. The flow patterns on the cylinder surface were studied by using the surface-oil flow method for a Reynolds number between 4.5 × 10⁴ and 1.1 × 10⁵ as the incidence angle varied from 0° to 45°. Vortex-shedding characteristics were measured by means of a single-component hot-wire anemometer, and surface-pressure distributions were detected by using a linear-pressure scanner. The results show that owing to the influence of the rod vibration, the flow pattern on the agitated face changed from its natural state of a dual-ring bubble to the mode of boundary-layer separation. The critical incidence angle separating the dual-ring bubble and single-ring bubble modes was advanced to 11° from its natural state of 15°. The locations of the characteristic points on the cylinder surface were altered by the rod vibration, implying that the whole flow field surrounding the square cylinder was modified by the vibrating rod installed around the leading edge of the upwind-facing lateral face. The Strouhal numbers of wake instability of the controlled and uncontrolled cylinders did not present significant difference. The variations of the pressure coefficients induced by the rod vibration were closely related with the modification of the flow field on the cylinder surface. The decreases in the pressure coefficients on the upwind-facing faces and on the leeward-facing faces lead to drag reduction of the controlled cylinder by ∼25% when compared with the uncontrolled cylinder.