Viscous or skin friction drag is accountable for a majority of the total drag of most modern aircraft. It has a significant impact on specific fuel consumption, pollution and noise emission. For quickly and accurately skin friction measurements in wind tunnel tests a non-intrusive imaging technique, the Global Interferometry Skin Friction (GISF), is implemented by the DLR Institute of Aerodynamics and Flow Technology.
The difficulties of skin friction measurements with existing methods in interacting flows or in flows with thin boundary layers are well known. They are mainly related to the intrusiveness of the measuring device and the corresponding flow disturbance. The GISF technique overcomes many of these limitations, since it provides non-intrusive measurements. It is based on the relationship between the thinning of an oil film, placed on the test surface and exposed to the flow at the local surface shear. The rate of thinning of the transparent oil film in time is determined by using optical interference created when an incident light beam is partially reflected on the oil and on the test surfaces.
A software application 3DGISF for image acquisition, image processing and skin friction computations has been developed for optical skin friction measurements. The implemented marker-based 3-D reconstruction of the interferogram pictures enables the mapping of the resulting values onto the 3-D grid of the model surface, facilitating the analysis and comparison with results of complementary CFD-calculations.
An example of the measured skin friction distribution in cross section of the 3-D crossing SWBLI (shock-wave/boundary-layer interaction) flow in vicinity of a double fin configuration is shown right. The results in the areas of the secondary flow separation show an impressive spatial resolution and a high sensitivity of the GISF-technique.
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