Experimental Research on Flow Separation Control using Synthetic Jet Actuators

Airplane wings can suffer from flow separation, which greatly decreases their aerodynamic per-formance. The flow separates due to the bound-ary layer possessing insufficient momentum to engage the adverse pressure gradient along the airfoil surface. Flow separation control actively influences the flow such that flow separation is delayed and airfoil performance is improved. In this research flow separation control is ap-plied on a 2D wing with a NACA0018 section with a 0.165 m chord using tangentially direct-ed synthetic jets. The actuators are located in-side the wing and the jet exits from a slot in the upper wing surface. The synthetic jet inhales the low momentum air in the boundary layer during instroke and during the outstroke the air adds momentum to the boundary layer. The actuator, with a piezo-electric diaphragm, has a slot width of 0.25 mm. With this design jet velocities up to 65 m/s have been achieved at an optimum actuation frequency of 900 Hz. A spanwise row of ten actuators is placed inside the wing, such that the slots cover 66% of the wing's span. During wind tunnel experiments forces have been measured using a balance. The tests have been performed at a fixed free stream velocity of 25 m/s (Rec = 2.73×105) and for various actuation frequencies and jet velocities. It is shown that for given actuation frequency a higher jet velocity results in a higher maximum lift coefficient and a corresponding higher stall angle. However, for the performance of the syn-thetic jet actuation, actuation frequency proves to be of greater importance than jet velocity. The best actuation frequency in combination with the maximum jet velocity possible with the present actuator is a dimensionless frequency F+ of 5.9 (1300 Hz) and a momentum coefficient cμ of 0.0014 (maximum jet velocity 32.9 m/s and Velocity Ratio of 1.32). Using these actuation parameters the lift coefficient is increased by 12% and the stall angle by 22%.