A mobile PIV system fulfilling the requirements for successful applications in trans- and supersonic flows and measuring the velocity fields accurately has been developed in the past decade. In order to cope with seeding problems, vibrations, optical access, synchronisation triggering, high mass flow rates, the pressure gradients and temperature range in such facilities several technical extras for controlling the measurement have been adapted in the PIV system. Especially the homogeneous seeding of the flow with suitable tracer particles and sufficient density is one of the major problems in high-speed air-flows. In order to minimize the velocity lag in regions with strong positive or negative accelerations of the flow e.g. across shocks, in turbulent shear layers or in vortical flows very small tracer particles with a diameter of 1 µm or smaller are required.
In the framework of the Vortex Flow Experiment 2 (VFE-2) project a subsequent application of PSP (Pressure Sensitive Paint), CFD and Stereo PIV on a generic delta wing with rounded leading edge in transonic flows provided a valuable data-set for validation of numerical codes and the understanding of the complex flow phenomena and topologies over the wing at different angles of incidence. After a PSP measurement in the transonic wind tunnel TWG of DNW in Göttingen has been performed the resulting pressure distribution over the surface of the delta wing was used as boundary condition for numerical simulations with the FLOWer code. This numerical result shows for a certain test case two parallel co-rotating vortices with different strength and origins. The Stereo PIV measurement has been performed immediately after the release of the numerical results in the TWG at various chord length, angles of incidence, Mach and Reynolds numbers (see Figure 1). For the certain parameter case, the "curious" double vortex has been found “again” and characterized with more details. The quantitative characterisation of the unsteady quality of these vortices, especially during the break-up process is not available otherwise. An example result of these Stereo PIV and PSP campaigns is shown in Figure 2. The local minima of the surface pressure distributions are oriented along and parallel to the vortex cores over the delta wing. The start of the secondary vortex structure with counter rotation between leading edge and main vortex is also visible.
Animation 1(click here to enlarge)