Oscillations of a planar impinging jet induced by a potential-core confined airfoil

The flow features of a planar jet modified by an airfoil located within the potential-core region have been investigated experimentally by hot-wire velocity measurements and the flow control mechanism is then analyzed using linear stability theory. The jet–airfoil impinging configurations are varied by changing the angle of attack (αα) of the airfoil and adjusting the impingement distance (LL). The test results show that, at small angles of attack, the impinging jet displays two independent instability regions, i.e. the jet shear layer and the airfoil wake, in which the jet fundamental instability wave and the airfoil wake instability wave evolve downstream respectively. However, at large angles of attack, the impinging jet exhibits self-sustained oscillations where the jet shear layer is characterized by the wavelength and frequency of the airfoil wake. As a result, both the momentum thickness and the volume flow rate increase significantly. Linear stability analysis using a local concept of absolute/convective instability suggests that such self-sustained oscillations can only be observed when there is a sufficiently large absolutely unstable flow region behind the airfoil. The measured resonant frequency matches well with the absolute instability frequency at the streamwise location where the flow transition from absolute instability to convective instability occurs.