Horseshoe vortex formation around a rotationally oscillating cylinder: Experimental investigation with PIV

This study aimed to experimentally investigate the horseshoe vortex region of a round cylinder positioned on a flat surface in a water channel at different rotational speeds and different frequencies. Experiments were carried out at three distinct oscillation amplitudes (theta A = 60 degrees, 120 degrees, and 180 degrees) and four different forcing frequencies (FR = 0.1, 0.3, 0.6, and 1.2) under a Reynolds number of 5000. Compared to the state of a stationary cylinder, applying a rotational oscillation to the cylinder changed the flow structure mostly in the wake region of the horseshoe vortex structure. Furthermore, as the rotation angle increased (theta = 120 degrees at FR = 0.1 and theta = 180 degrees at FR = 0.6), the vortex core progressively moved closer to the cylinder surface, leading to a reduction in the dead flow zone. Among the investigated cases, the condition of theta = 180 degrees with FR = 0.3 was particularly notable, as it eliminated the dead flow zone in the cylinder wake and demonstrated a clearer effect of the flow control method within the horseshoe vortex region. These findings suggest that rotational oscillation can effectively modify the flow structure and mitigate flow separation in the horseshoe vortex region.