Optical Skin Friction Measurements in Short-duration Facilities
Department High Speed Configurations
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 accuratelly skin friction measurements in wind tunnel tests a non-intrusive imaging technique, the Global Skin Friction Interferometry (GISF), is implemented by the DLR Institute of Aerodynamics and Flow Technology.
The Technique
 |
| Cross Sectional Skin Friction Distribution of 3D Crossing Shock Waves / Turbulent Boundary Layer Interaction Flow | |
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 for image acquisition, image processing and skin friction computations has been developed for optical skin friction measurements. The efficiency of known GISF-methods for measurements under specific conditions of a short duration wind tunnel has been analytically investigated on the example of different 2D and 3D flows with given shear stress distributions. The tests showed good results for all GISF-methods that are based on the general thin-oil-film equation. The comparison of the results obtained by different optical skin friction methods demonstrated good agreement and reliability of all these methods.
Over the last few years some successful investigations could be conducted in the Ludwieg-Tube Göttingen at DLR with the developed GISF-Meter. The abilities of this technique and its application in the Ludwieg tube is demonstrated on skin friction measurements in some 2D and 3D shock wave / turbulent boundary layer interaction (SW/TBLI) flows:
- Longitudinal vortices at turbulent 2-D compression corner flow
- 2-D incident SW/TBLI
- 3-D wake flow behind a vertical fin on a flat plate
- 3-D side jet / turbulent boundary layer interaction
- 3-D swept SW/TBLI flow in vicinity of a vertical fin on a flat plate
- 3-D crossing SW/TBLI in vicinity of a double fin configuration
The results confirm the statement that all indirect skin friction measurement methods, determining wall-shear through the similarity laws, such as Reynolds analogy (heated film technique, buried wire, etc.) or the law of the wall (velocity profile technique, Preston or Stanton tubes, etc.), are unsuitable for measurements in SW/TBLI regions, as the applied similarities break down. The oil film interferometry is free from similar restrictions since it consists of measurements of the oil-film deformation, which present the direct impact of the wall-shear-stress and is independent of other flow quantities. This is a very important advantage of this technique.
An example of measured skin friction distribution in cross section of the 3D crossing SW/TBLI flow in vicinity of a double fin configuration is shown right. The results in the areas of the secondary flow separation show a very good local resolution and the high sensitivity of the GISF-technique.