A specially designed Doppler global velocimetry system (DGV, planar Doppler velocimetry) was applied in a high-speed cryogenic facility at Mach 0.2 to Mach 0.8 and pressures between 1.2 and 2.5 bar for the measurement of the wake flow of various wing tip devices in the framework of the M-DAW EC-project 1. The necessary seeding was achieved by injecting a mixture of gaseous nitrogen and water vapor into the dry and cold tunnel flow which then immediately formed large amounts of small (micron-sized) ice crystals. As operational and access conditions are very restrictive in comparison to other facilities, DGV is currently considered the best choice for the non-intrusive measurement of flow fields.
The DGV imaging system as well as the illumination system (laser light sheets) were mounted in various existing view ports on the side walls of the tunnel test section. As three viewing directions were required to achieve three-component velocity data, a specially designed fiber imaging bundle with multiple branches was utilized. This avoided the use of three separate, complex and costly DGV camera systems. All DGV subsystems were designed to be remotely controlled from the main tunnel control room and proved to be reliable throughout their 3 month period of use in ETW. Automation of the measurement procedure ensured that a complete three-component velocity map could be obtained within 5 seconds, which dramatically reduces the tunnel operational cost.
During test measurements prior to the actual M-DAW measurement campaign it was discovered that the gray paint of the tunnel test section produced a significant background signal that could not be accounted for with subsequent image processing. Painting the background of the viewing area with dull black paint was instrumental in reducing this problem. Unfortunately the viewing windows suffered form ice build-up during the high-speed operation of the tunnel, that is, sublimation of ice on the cold windows. Stray laser light reflected from the walls illuminated this ice such that the measured signal was biased to lower Doppler shifts and hence lower velocities. Therefore, the high speed data could only be processed in a qualitative manner. Low speed data did not suffer from this problem and produced very encouraging DGV measurement results.