Surface pressure measurement in a cryogenic wind tunnel by using luminescent coating

In recent experiments, we demonstrated the feasibility of using luminescent coatings for surface pressure measurement in a cryogenic wind tunnel. This technique is based on a new coating technology in which luminescent molecules are directly deposited onto the model surface by an electrochemical process. The resulting coating has an extremely high oxygen sensitivity for mole fractions of oxygen is less than 0.1%. This capability allows us to measure the pressure field on the model surface in a cryogenic wind tunnel. To demonstrate this technology, a 14%-thick bump model was tested in the 0.1-m Transonic Cryogenic Wind Tunnel at NAL. Mach number was changed from 0.4 to 0.84 whereas temperature was maintained at 100 K. A small amount of oxygen was injected into the tunnel and the mole fraction of oxygen in the test gas was kept constant. We acquired two intensity images, one taken at low speeds and the other taken at high Mach numbers. By taking the ratio of these images, surface pressure distributions on the model were clearly captured. The result of the in situ calibration showed that effects of temperature dependence of the coating was negligible. The paint-derived pressure distributions are in good agreement with pressure tap measurements.