Airfoil Boundary-Layer Control through Pulsating Jets

Low Reynolds number flows prevail in turbomachinery applications like first stages of axial compressors, first stages of high pressure turbines and low pressure turbines. However, operation within low Reynolds number range yields to highly stable laminar boundary layers, which are more susceptible to separation than transitional or turbulent boundary layers. This statement corroborates the significance of research on active control at low Reynolds numbers flows. The present work continues the efforts made at the Technische Universitaet Darmstadt related to boundary layer control, such as the experimental work from Werden (1998) and the numerical simulations from Wang (2003). In the present work an open-loop scheme is implemented in an airfoil to control leading-edge stall at low Reynolds number. A symmetric NACA 633 − 018 is used to simulate separation over a blade operating at low Reynolds number, as adopted by Wang. Reynolds number is kept at 90.000 - representative of a real machine - while the pressure gradient is varied as the angle of attack is changed. Both steady and unsteady (pulsated) air injection are implemented to study the influence of the control parameters and to figure out how to reduce the necessary energy expenditure while keeping the flow attached under conditions where the flow would be otherwise detached. Also, it is sought to which extend such scheme can be effective. The interaction between the pulsating jets, the boundary layer and the separated flow is verified using three experimental methods: pressure taps, particle image velocimetry (PIV) and hot-wire anemometry. The main effort was to characterize the development of the boundary layer during different control conditions. The interaction between separated boundary layer, injection and main flow is presented and discussed. The results shall be used to evaluate the feasibility of such an active control design and to supply information to further research in the subject