Connection Between Flow Structure and Separation Control for Various Fluidic Oscillator Spacings

The influence of actuator spacing on separation control using fluidic oscillators is experimentally investigated on a wall-mounted hump. Momentum inputs up to C_μ 2.5% are tested at four different spacings (Δz∕c 2.27, 4.55, 6.82, and 9.09%), at Re 1.0 ⋅ 10⁶ and M_∞ 0.09. For all spacings, the control authority is characterized by two distinct regimes associated with boundary-layer control and circulation control. The two narrow spacings show steep control authority gradients in the transition between boundary-layer control and circulation control. The time-averaged flowfields from spanwise stereoscopic particle image velocimetry (PIV) vary significantly between the presented spacings, but the circulation normalized by mass flow rate per jet remains constant for a given momentum input in the boundary-layer control regime. When the spacing is too small for a given oscillator design, the introduced vorticity is unorganized and actuation is less effective for a given momentum input. Once the flow is attached, the circulation reduces for larger momentum inputs (circulation control). This indicates a connection between increased control authority and decreased circulation in the circulation control regime. These findings are supported by novel circulation metrics that describe the level of organization in the flowfield.