Application of planar Doppler velocimetry (DGV) in cryogenic wind tunnels - A progress report

Kurzfassung

Laser-based flow velocity measurements in cryogenic wind tunnel facilities are especially challenging due to the extreme operating conditions and have never been demonstrated or applied. One task within the M-DAW [1] Project – a 5th Framework EC-programme – is to provide flow field data in the wake of an airfoil with different wing tips at transonic speeds inside the European Transonic Wind tunnel (ETW) using Planar Doppler Velocimetry (equivalent to Doppler Global Velocimetry (DGV)). A joint work-package team consisting of members of DLR, ONERA and ETW was formed in November 2002 to conceive, install and bring into operation a DGV-system capable of providing three-component planar velocity data. DGV is an ideal choice for providing this type of data since – unlike Particle Image Velocimetry (PIV) – there is no need for high-powered pulsed lasers to resolve individual particles. This allows DGV to make use of laser light delivered through optical fibers, while the flow field can be viewed through flexible imaging fiber bundles. However, both PIV and DGV rely on the presence of seed material in the flow, such that a number of preliminary tests were conducted throughout 2002 to ensure the feasibility of this project. While natural seeding was found to be insufficient for the purposes of DGV, the introduction of heated water vapor with warm nitrogen as a carrier gas into the ETW provided significant light scattering in the light sheet plane. It is believed that the water vapor immediately forms ice crystals of currently unknown diameter after entering the dry nitrous atmosphere. No accumulation of ice on model surfaces was observed. Further tests within the Kryogener Windkanal Köln (KKK) – a facility similar to the ETW – showed equally encouraging results with seeding remaining visible in the tunnel for up to 15 minutes. In this context, qualitative DGV measurements were obtained in the wake of the a 2-D airfoil. The final DGV imaging configuration to be utilized in the ETW calls for two opposing light sheets aligned normal to the mean flow approximately ¼ span downstream of the airfoil’s wing tip. An imaging bundle with 4 receiving ends images the flow field from three different directions simultaneously through windows on the tunnel’s side walls. At the time of this progress report, the necessary hardware for was being manufactured; installation of the system is scheduled for June 2003 with actual tests following in July.