Experimental Comparison of Steady and Sweeping-Oscillating Jets in Active Control of a Square-Cylinder Wake

Based on time-resolved particle image velocimetry (TR-PIV) experiments, this study systematically compares two active flow-control strategies—steady jet and oscillating sweeping-jet actuation applied at the rear stagnation point of a square cylinder—and quantifies their relative efficiencies over a broad range of momentum coefficients. The experiments were conducted at a Reynolds number of Re = 1.2×10 ⁴ . We assess the flow from multiple perspectives, including mean-flow quantities, turbulence statistics, modal coherence, and proper orthogonal decomposition (POD)-based modal coherence and energy distributions, thereby elucidating the underlying control mechanisms of the two jets. Results show that both strategies suppress the Kármán vortex street, push the shear layers outward, and establish a high-momentum pathway along the centerline, thereby reorganizing the wake and enhancing symmetry; however, their mechanisms and efficiencies differ markedly. The steady jet primarily operates by continuously replenishing centerline momentum, shortening and ultimately eliminating the time-averaged recirculation region while confining large-scale turbulence to a near-wake mixing layer; at high \(\:{C}_{\mu\:}\)​, it exhibits diminishing returns. By contrast, the oscillating sweeping jet, through unsteady modulation that directly targets the dominant instability mode, substantially weakens the energy of the first few POD modes and depresses far-wake turbulent kinetic energy (TKE) and reynolds shear stress (RSS), thereby confining the dominant periodic transport and mixing to a narrow near-field band downstream of the nozzle. Consequently, at moderate \(\:{C}_{\mu\:}\)​ it achieves suppression comparable to—or better than—that obtained by the steady jet at substantially higher momentum input. These findings provide experimental evidence and quantitative guidance for selecting control type and amplitude for bluff-body vibration/noise mitigation and wake symmetrization.