Vortex investigation over a rolling delta wing model in transonic flow by stereo PIV measurements

An investigation of the vortex behaviour over a delta wing model under different angles of incidence and roll has been performed using Particle Image Velocimetry (PIV) under transonic conditions at DLR Gottingen, Germany. These tests are important for the understanding of the leading edge vortex behaviour which is responsible for enhanced lift forces and CFD validation. The investigation used a stereo PIV arrangement, with both cameras in forward-scatter mode. For high-quality stereo PIV measurements of the flow around such models under transonic conditions, many details had to be optimised, especially as the light sheets were positioned in the cross-plane to the freestream over the model. Parameters like light sheet thickness and displacement with a certain overlap had to be considered carefully. It was also necessary to observe the optical system by means of additional cameras, as the starting and stopping of the tunnel affected the light sheet. Fortunately, readjustment of its position whilst the wind tunnel was running could be done by remote control and optical tools outside the tunnel. This had to be done in connection with a special layout for the trigger sequence, as lengths of wires or optical fibres for the transmission of TTL pulses became relevant for delays in the order of microseconds. It was expected that strong centrifugal and axial accelerations would be present and tiny particles had to be used for seeding. It could be established that the vortex structure is dependent strongly on the direction of change of the roll angle. A very impressive phenomenon was the so-called busting of vortices, which was accompanied by rapidly-changing pressure distributions.