Transitory Control of a Pitching Airfoil using Pulse Combustion Actuation

The transitory response of the flow over a stalled, 2-D airfoil to pulsed jet actuation is investigated experimentally. Actuation is provided by momentary [O(1msec)] pulsed jets that are generated by combustion-based actuators integrated in a span-wise array in the center section of the airfoil. The flow field in the cross stream plane above the airfoil and in its wake is mapped using highspeed PIV measurements that are obtained phase-locked to the actuation and allow for continuous tracking of vorticity concentrations. A single actuation pulse leads to a strong transitory increase in the circulation about the entire airfoil that is manifested by a partial collapse of the separated flow domain which is accompanied by the shedding of a large-scale clockwise vortex and attachment and accumulation of the surface vorticity layer behind it. The attached vorticity layer ultimately begins to separate at t/Tconv = 2.1 and the airfoil becomes fully stalled again within ten convective time scales. It is shown that repetition of actuation pulses on the convective time scale can increase the stream-wise extent of the attached flow domain and the trapped vorticity leads to a substantial increase in the peak transitory circulation before the flow separates again when the actuation is terminated. Pulsed actuation "bursts" that are repeated at low duty cycle are used to extend the duration of flow attachment.

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