The following properties of these tunnels make the implementation of PSP more difficult:
These properties must be duly considered for the proper design of a PSP system.
Because of the short test times in these tunnels, the PSP paints must possess short fluorescence lifetimes, preferably in the microsecond range. Furthermore, after excitation by an appropriate light source (e.g. flash lamp, LED, laser), the fluorescence intensity must be high enough to deliver strong signals (more correct, to deliver good signal-to-noise ratios). The fluorescence detection system, consisting of camera and lens, must be sufficiently fast (high framing rate) and have a large enough light gathering power to enable the signals to be captured and processed to deliver quantitative pressure data. Different combinations of PSP paints, light sources and camera systems are being analysed and, in some suitable cases, tested in the laboratory and in a small test shock tube.
In the first experimental studies successful PSP measurements were carried out on two different Scramjet nozzles in the DLR Hypersonic Wind Tunnel H2K in a Mach 7 flow at two different Reynolds numbers and nozzle expansion ratios. The hypersonic tunnel H2K has long test times (up to 20 s), so that in these first tests the added complications derived from having to carry out short-term measurements were not present. Pressure distributions were measured on the nozzle ramps (see upper figure) and could subsequently be used to optimize the method of gas introduction into the nozzle forechamber. The agreement between the PSP pressure values and those obtained from pressure taps using a convential PSI Pressure Mesurement System was very good (see lower figure).