Development and application of full-field optical techniques for unsteady/turbulent supersonic flow diagnostics (DLR.6198)

Summary
The research program covers the area of high-speed flows with emphasis on the development of experimental techniques for the investigation of unsteady/turbulent flows in supersonic and hypersonic wind-tunnels. The main focus of the research program consists in the extension of the quantitative application of full-field optically based methods such as Particle Image Velocimetry (PIV) to the study of supersonic and hypersonic flow regimes. Moreover, the program proposes to develop some innovative variants of the schlieren visualisation method so as to develop a technique for full-field densitometry. The combination of PIV with the latter methods will provide an integrated measurement technique aiming at a further improvement of the PIV performance. The challenge of the present research program is twofold: first the performance of the PIV technique will be assessed for the case of supersonic (hypersonic) wind-tunnel flows; second, an advanced version of the technique will be applied in real-life conditions demonstrating the possibility to scale-up the results from the research facilities to industrial wind-tunnels.
From the fluid dynamic viewpoint the research will focus on the benchmark compressible flow problems of known complexity such as compressible turbulence, shock wave-boundary layer interaction and separated shear flows. These topics have been studied for several decades alternating promising results with frustration in the scientific community, conducting either quantitative point-wise measurements (e.g. pitot probes, laser doppler velocimetry) or flow visualization (shadowgraphy, schlieren). A reliable quantitative whole field measurement approach is still missing, which is the objective of the present research program. On the other hand the computational approaches to these types of flows (compressible LES, DNS) is still under development and it would strongly benefit from the availability of extensive experimental databases.
The experimental approach is chosen such that the instantaneous flow pattern is frozen during the short measurement time of the PIV technique, yielding evidence of the unsteady flow organisation. The research will return the development and assessment of the quantitative PIV technique in supersonic and hypersonic flow conditions, addressing as main issues the seeding particles physical properties and aerosol generation-supply in supersonic wind-tunnels. Moreover the aspect related to compressibility effects (non-homogeneous optical medium) will be covered for what concerns particles illumination.

More information can be found on the website of teh aerodynamics lab.

Four companies and two universities are involved in this project.