Capturing Unsteady Flow Phenomena at High Speed Stall Conditions by Adaptation and Application of Cryogenic PIV

We measured and characterized the flow around a full-span, generic passenger aircraft model under buffet conditions at Mach numbers up to M = 0.9 and Reynolds numbers up to Re = 25 M by adapting and applying the cryogenic Particle Image Velocimetry (cryoPIV) technique in the European Transonic Windtunnel (ETW). Three different scenarios, involving buffet on the main wing, interaction of the buffet-wake with the horizontal tail plane (HTP) and buffet-phenomena in the vicinity of the engine nacelle were addressed in three dedicated wind-tunnel entries incorporating a variety of fields of view and angles of attack between -4.5° and +7°. To enable these measurements and to improve performance, reliability and data quality under the harsh experimental conditions at ETW, we carried out a comprehensive extension and upgrade of our cryoPIV measurement system. As a result, three different stereo particle image velocimetry (PIV) systems, involving one state of the art time-resolved and two novel multi-pulse PIV systems, were installed at ETW and operated during the first two of three wind tunnel entries in order to measure buffet phenomena on the upper wing side and in the wing wake downstream to the HTP. In the third entry, a system of three scientific CMOS cameras was combined to two different stereo PIV systems in order to measure buffet phenomena at the lower wing in the vicinity of the nacelles. The captured flow fields reveal the large- and small-scale flow structures involved in the highly three-dimensional flow around the aircraft model. Beside position, shape and dynamics of the shock waves, the turbulent wake of the wing as well as its interaction with the horizontal tail plane is disclosed by the instantaneous, mean and fluctuating velocity fields. The partially overlapping measurement fields show reasonable agreement among different measurement positions and systems.