PIV Measurements of unsteady transonic flow fields above a NACA 0012 Airfoil

Particle Image Velocimetry (PIV) is increasingly used to investigate unsteady velocity field instantaneously. At DLR an experimental set-up for PIV has been developed which can be operatred under the rough environmental conditions of a large wind tunnel. This system has successfully been applied as well to low speed as to high speed flows (U &#61 10. 500 m/s). The evaluation and post processing of the PIV recordings runs fully automatical on a work station. The paper will describe investigations of instantaneous transonic flow fields above a NACA 0012 airfoil at different angles of attack. Some improvements of the PIV technique had to be carried out in order to be able to utilize the PIV technique in a high speed wind tunnel with a measuring time of less than 20 seconds. Among these improvements are a fas focusing device for the recording camera, on line observation of seeding with tracer particles, master control unit for automatic control of the whole measuring sequence and the data aquisition of pressure, temperature, anlge of attack etc. A high speed image shifting system is required in order to be able to measure even in reverse flows or in order to optimize the dynamic range of the evaluation. For this purpose a high speed rotating mirror system was developed. The frequency of rotation ranges from 1 Hz to 50 Hz, thus covering a range of shift velocities from 10 m/s to 500 m/s (at a ratio of reduction of 1:7 between oberservation plane in the flow to the recording plane)