Closed-loop rotary control of flow past a circular cylinder: An experimental study

An experimental investigation of closed-loop rotary control for flow past a circular cylinder has been conducted at a Reynolds number of $Re=100$. To support this study, an experimental platform is developed that integrates image pair acquisition, real-time particle image velocimetry (PIV) processing, and actuator driving, employing multithreading technology for enhanced performance. When the cylinder is towed in the water tank at a constant velocity, the downstream wake pattern is recorded to determine the feedback velocity signal via real-time PIV processing. Subsequently, the cylinder is driven to rotate by a motor, with the rotational velocity derived from proportional control. Experimental results for various proportional coefficients $k_P$ indicate that the closed-loop control significantly influences the flow field. When $k_P>0$, the downstream velocity fluctuations at the sensor location exhibit a reduction. The implementation of optimal control with $k_P=1.0$ leads to a substantial decrease in the amplitude of transverse velocity fluctuations, reducing it to 52%. Additionally, this control strategy results in an elongation of both the recirculation region and the vortex formation region. The reduction in velocity fluctuations, as indicated by the normal Reynolds stress, suggests effective control of vortex shedding. However, in scenarios where $k_P<0$, the downstream wake pattern transitions to the “C(2S)” mode, resulting in a significant increase in transverse velocity fluctuations. Local linear stability analysis demonstrates that control mechanisms can effectively affect the wake stability, elongating the absolutely unstable region by 38.6% for the most effective control parameter while reducing the sensitive disturbance frequency. In addition to presenting experimental evidence for closed-loop rotary control, the current study introduces a methodology for employing real-time PIV to generate velocity fields for the purpose of regulating flow around a bluff body.