Due to its unique pore structure, porous materials have the potential to be used in the fields of acoustic noise reduction and flow drag reduction control. In order to study their effects and mechanism of drag reduction on the flow around a circular cylinder, experiments are conducted in a low-speed wind tunnel with low turbulence intensity. The drag forces acting on a circular cylinder model are measured using wind tunnel balance when porous materials with different permeability are applied within different intersection angles on the trailing-edge and leading edge, and the flow fields are visualized with a particle image velocimetry system with high time resolution. The method of dynamic mode decomposition (DMD) is also used for reduced-order analysis of the vorticity field in the wake of the cylinder. The measured drag forces and wake flow fields are then compared with those of a smooth cylinder, and the results show that porous materials laid on the trailing-edge can reduce drag, when a porous material with 20 pores per inch is laid within 270° on the leeward side, the best effect of the drag reduction ratio of 10.21% is reached. The results of flow visualization indicate that after the porous material is applied, the vortex region in the wake of the cylinder is expanded; both the frequency of vortex shedding and the magnitude of vorticity fluctuation decrease; the Reynolds-shear-stress decreases significantly, and both indicate that vorticity is dissipated earlier. The results of DMD analysis show that porous materials can effectively relax the energy of vortices in different modes.

The complex surface of an aircraft generates a nonzero pressure gradient flow. In this study, the boundary conditions of favorable and adverse pressure gradients are constructed in a small low-turbulence wind tunnel test section. Hot-wire anemometers and time-resolved image velocimetry are used to analyze the flow structure in a fully developed turbulent boundary layer with porous media. The effects of the porous surface on the statistical characteristics of the turbulent flow field and turbulent flow structure are analyzed and discussed. The results show that porous media reduce the velocity gradient in the linear layer, and the friction drag reduction effect is higher downstream of the porous wall. The drag reduction effect decreases along the flow direction. A wall with a 10 pores per inch produces a slightly better drag reduction effect than smooth wall. The maximum local drag reduction effect of a 10-pores-per-inch porous wall is 43.7% under a favorable pressure gradient and 42.3% under an adverse pressure gradient. The velocity streaks in the inner layer show that the porous wall widens the low-velocity streaks, making them more stable, while the high-speed streaks decrease in size under the pressure gradient. In the case of the adverse pressure gradient, the structure of the streaks becomes blurred, and their strength weakens. Under both favorable and adverse pressure gradients, the porous media lift up the coherent structures near the wall, thus weakening the large-scale coherent wall structures.

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To improve the low aerodynamic efficiency and reduce the high energy consumption of a single-stage circulation control wing, a multistage circulation control wing was designed. By combining force measurement and particle image velocimetry (PIV), the aerodynamic and flow-field characteristics of an aerofoil were investigated with respect to the increase in the number of blowing slots, changes in the blowing coefficient, and different blowing ratios for three slots. The force measurement results revealed that the maximum lift-to-drag ratio resulting from simultaneous blowing into the three slots increased by 95.3% compared with that in the absence of circulation control. With an increase in the blowing coefficient, two stages were observed: separation control and supercirculation control. In the separation control stage, the lift and drag coefficients significantly increased and decreased, respectively. In the supercirculation control stage, the lift coefficient gradually increased with the blowing coefficient, whereas the drag coefficient remained unchanged. When the blowing ratio (blowing flow ratio of three slots) in the three slots was 3:1:2, the maximum lift-to-drag ratio of the wing could reach 143.48%. The effects of different slot positions on the aerodynamic control were found to vary. The effects of Slot.1 and Slot.3 in terms of the drag reduction and lift, respectively, were evident, and the influence of Slot.2 on blowing between these two slots played a role in jet relay. The PIV results revealed that multistage blowing circulation increased the curvature of the trailing-edge streamline, thus increasing the equivalent aerofoil camber and improving the wing lift. At a high angle of attack, this circulation demonstrated a flow separation control effect.

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