Current Topics in Experimental Transonic Aerodynamics

Performing PIV measurements in trans- and supersonic test facilities successfully strong requirements concerning seeding, optical access, laser power and camera sensitivity need to be fulfilled. Problems like vibration of the wind tunnel walls, density and pressure variations and gradients around the test facilities and along the optical paths have to be taken into account. The PIV team of DLR Göttingen has shown the ability to measure instantaneous 3C- velocity vector fields in transonic flows up to Ma = 1.2 in many different applications. The quality of the data obtained in complex transonic unsteady flow fields by means of the PIV techniques allows detailed comparison with the results of numerical calculations and a study of the flow phenomena otherwise not possible. On-line evaluation, comprehensive post-processing and data reduction tools have been developed in order to compare and analyze experimental and numerical data during the test, and for easily understandable presentation of the results to the end user. A list of different application of PIV to transonic flows at DLR in the past is given in the references. The density distribution inside a free jet of air entering into ambient air was investigated. The measurements were taken using the BOS method. The convolution back projection algorithm was used to reconstruct the 3D density distribution. The technique was used on a steady, under-expanded and axis-symmetric jet and for the non-symmetric case on a steady, under-expanded double jet. The measurements were done with two different fields of view. The results of both measurements showed good agreement. With the reconstruction of the density field of the double jet it cold be shown, that even in non-symmetric cases one can get high quality results for the density field. It could be shown, that the combination of tomographic reconstruction techniques with the BOS-Method is a useful way of determining complete 3D-density fields in flows. The pressure sensitive paint (PSP) intensity and lifetime system is an optical measurement technique to investigate absolute pressure fields on model surfaces for basic research in laboratories, industrial wind tunnels or high speed rotating turbo machines. Detailed qualitative and quantitative information and understanding of flow phenomena can be obtained in speed ranges from U =20 m/s up to Ma= 5.0. A number of projects of industrial interest have been investigated in different wind tunnels covering low speed, transonic, trisonic and cryogenic facilities. The influences of the main error sources for the components of the PSP system have been checked. Comparison of experimental pressure fields obtained by means of PSP and the results of numerical calculations have been carried out. Different wind tunnel models ranging from basic configurations such as a cropped delta wing to a complex half model of a large propeller-driven transport aircraft with all flaps, rudders and shrouds, and rotating or oscillating models as well as Reynolds number effects on models have been investigated