Application of Doppler Global Velocimetry in Cryogenic Wind Tunnels

Abstract

A specially designed Doppler global velocimetry system (DGV, planar Doppler velocimetry) was developed and installed in a high-speed cryogenic wind tunnel facility for use at free stream Mach numbers between 0.2 and 0.88 and pressures between 1.2 and 3.3 bar. The necessary particle seeding was achieved by injecting a mixture of gaseous nitrogen and water vapor into the dry and cold tunnel flow which then immediately formed a large amount of small ice crystals. As operational and access conditions are very restrictive with respect to other facilities, DGV is currently considered the best choice for the non-intrusive measurement of flow fields. Three component velocity maps were achieved by simultaneously imaging a common area in the flow field from three different directions through the wind tunnel side walls by means of a multiple branch fiber imaging bundle attached to a common DGV image receiving system. The complete imaging system and fiber-fed light sheet generators were installed inside the generally inaccessible pressure plenum surrounding the wind tunnel’s test section. The control PC and frequency-stabilized laser system, requiring occasional re-adjustment, were placed outside of the pressure shell. The DGV system acquired flow maps with an area of 300 x 300 mm2 in the wake of simple vortex generators as well as from a half-span aircraft model with different wing tip devices. While the remotely controlled hardware operated reliably over the course of three months, a variety of problems mainly relating to light reflections and icing on the observation windows significantly impaired the measurements. These problems could partially be resolved by covering the view background with dull black paint.