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Optical Skin Friction Measurements in Short-duration Facilities

Background

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.

Measurement Technique

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.

Cross Sectional Skin Friction Distribution of 3D Crossing Shock Waves / Turbulent Boundary Layer Interaction Flow

This technique is successfully applied to different wind tunnel facilities covering the entire spectrum from hypersonic down to low speeds: the super- and hypersonic Ludwieg-Tube DNW-RWG, transonic wind tunnel DNW-TWG, low-speed wind tunnel DLR-1MK at DLR Göttingen, as well as the external low-speed facilities LSWT (Airbus Bremen) and LLSWT (TU Hamburg-Harburg). Here a selection on flows and phenomena investigated by GISF:

• Longitudinal vortices at turbulent 2-D compression corner flow
• 3-D wake flow behind a vertical fin on a flat plate
• 3-D side jet / turbulent boundary layer interaction
• 3-D swept SWBLI flow in vicinity of a vertical fin on a flat plate
• 3-D crossing SWBLI in vicinity of a double fin configuration
• Shock-wave control on a transonic swept wing by bumps
• Active laminar flow control on a supersonic swept wing by suction
• Qualification of the flow on the wind tunnel side walls

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.

Publications on this topic:

• Schülein E. (2004) “Development and Application of the Thin Oil Film Technique for Skin Friction Measurements in the Short-Duration Hypersonic Wind Tunnel”, in: New Results in Numerical and Experimental Fluid Mechanics IV, edited by C. Breitsamter et al., Vol. 87, Notes on Numerical Fluid Mechanics and Multidisciplinary Design, Springer-Verlag, Berlin Heidelberg, 2004, pp. 407 – 414
• Lüdeke H., Radespiel R., Schülein E. (2004) "Simulation of Streamwise Vortices at the Flaps of Re-entry Vehicles", Aerospace Science and Technology, 8 (8), 703-714
• Schülein E. (2004)  "Optical Skin Friction Measurements in Short-duration Facilities (Invited)", AIAA-2004-2115, 24th AIAA Aerodynamic Measurement Technology and Ground Testing Conference, 28 June - 1 July, 2004, Portland, Oregon, 18p
• Schülein E. (2006) “Skin Friction and Heat Flux Measurements in Shock Boundary Layer Interaction Flows”, AIAA-Journal, Vol.44, No.8, Aug. 2006, pp.1732-1741
• Kovar A. and Schülein E. (2008) “Visualisation and Prediction of Heat Flux Measured on the Example of Side Jet Control,” 13th International Symposium on Flow Visualisation, 01.-04.07.2008, Nizza, Frankreich
• Schülein E. (2008) “Experimental Investigation of Laminar Flow Control on a Supersonic Swept Wing by Suction”, AIAA Paper 2008-4208, 4th AIAA Flow Control Conference, Seattle, Washington (USA), 23. – 26.06.2008, 14p