Application of Doppler global velocimetry in cryogenic wind tunnels

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 bar and 3.3 bar. 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. Given the limited physical and optical access for this facility, DGV is considered the best choice for non-intrusive flow field measurements. A multiple branch fiber imaging bundle attached to a common DGV image receiving system simultaneously viewed a common area in the flow field from three different directions through the wind tunnel side walls. The complete imaging system and fiber-fed light sheet generators were installed inside the normally inaccessible pressure plenum surrounding the wind tunnels test section. The system control and frequency-stabilized laser system were placed outside of the pressure shell. With a field of view of 300×300 mm², the DGV system acquired flow maps at a spatial resolution of 3×3 mm² in the wake of simple vortex generators as well as in the wake of different wing-tip devices on a half-span aircraft model. Although problems mainly relating to light reflections and icing on the observation windows significantly impaired part of the measurements, the remotely controlled hardware operated reliably over the course of three months.