Recent applications of PIV for investigation of unsteady flow fields in transonic flows

Abstract

The application of PIV for transonic flows became an important tool for the investigation of many unsteady flow phenomena which are in the focus of interest in recent research projects. The requirements for successful PIV applications that means for measuring the velocity fields accurately in trans- and supersonic flows are numerous. The seeding of the flow with suitable tracer particles and densities 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. Aerosol generators with multi hole nozzles of Laskin type or similar have been developed to provide a sufficient amount of seeding for industrial (transonic) wind tunnel applications with high mass flow rates. High power pulsed lasers, sensitive CCD cameras and exact triggering are also necessary requirements for PIV in transonic flows. Density gradients in the optical paths have to be considered. In the framework of the Vortex Flow Experiment 2 (VFE 2) project a subsequent application of PSP, CFD and Stereo PIV on a generic delta wing with rounded leading edge in transonic flows provides 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 DNW-TWG (see PSP part of this script) has been performed the resulting pressure distribution over the surface of the delta wing was used as boundary condition for numerical simulations with a combined RANS and DES 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 DNW-TWG at various chord length, angles of incidence, Mach and Reynolds numbers. 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 break-up process is not available otherwise.