Passive control of flow past a circular cylinder using control rods at high Reynolds number

Three-dimensional direct numerical simulations are conducted to investigate the effect of the transverse position of control rods on wake suppression at a subcritical Reynolds number. At a fixed streamwise position of x/D = 0.7, the transverse position of the control rods is varied from L/D = 0.60 to 0.80, covering the inner-to-outer region of the separated shear layers. The wake response is highly sensitive to L/D, and rich flow features emerge even within this narrow range. When the rods are embedded within the shear layers, the wake motion is intensified, leading to increased drag and lift fluctuations. When the rods are positioned near the outer side of the shear layers, the wake is effectively suppressed, with the lift fluctuation reduced by more than 95%. Further outward movement weakens the rod–shear-layer interaction and allows partial wake recovery. A relatively robust control range is identified for L/D = 0.70–0.75, where both drag reduction and lift-fluctuation suppression are achieved simultaneously. Within this range, the rod-induced wakes compete with each other and constrain the development of the separated shear layers, thereby reorganizing the near-wake structure. This process promotes rear-side pressure recovery, reduces wake fluctuation energy and Reynolds stresses, and shifts the generation of streamwise vortices from the main-cylinder surface toward the rod and gap regions. The alternate roll-up of the main-cylinder shear layers is interrupted, and the dominant wake oscillation is strongly attenuated. These findings clarify wake suppression mechanisms using control rods at higher Reynolds numbers and provide guidance for passive control design.