PSP, Pressure Sensitive Paint Application at Hypersonic Speed
Study model of the Aerospike exhaust pipe central body
Distribution of pressure on the Aerospike exhaust pipe
PSP (Pressure Sensitive Paint) is an optical method that can be used to measure the pressure field on models in a wind tunnel. With this technique, the models do not have to be equipped with pressure sensors, so the model production time is reduced. To apply PSP in hypersonic blowdown wind tunnels, the coating has to respond rapidly to sudden pressure variations, but a reduced acquisition time is also needed, because the stagnation conditions may vary over time. In the installation in question (R2Ch wind tunnel), the time during which the stagnation pressure is constant is limited to around fifteen seconds. Furthermore, the temperature variations generated by supersonic or hypersonic flows call for coatings that are perfectly insensitive to temperature, or accurate temperature measurements, in order to make the corrections. Given the duration of the test (30 seconds), which limits the number of recordings, we selected a coating based on pyrene, which practically insensitive to temperature.
The model studied was an aerospike exhaust pipe for a future launcher tested for the SEP (ESA’s ARPT program). The flow came from 24 circular nozzles arranged around a central body whose purpose is to control the triggering of the jets during the ascent phase (photo of setup). The PSP exposure time was limited to one second per image due to the use of a liquid optic fiber connected to a Mercury vapor lamp. The fiber routed the UV excitation (334±5nm) directly into the test section, which prevented the use of Pyrex viewing ports as these absorb this type of radiation. The data was analyzed by comparing the reference and test images. The third image, recorded seven seconds after the start of the blowdown when the stagnation conditions had stabilized, was used for the measurements. The reference image was the one recorded at the end of the test. By using the last images as references, both for calibration and interpretation of the results, we were able to eliminate the consequences of the pyrene evaporation at high temperatures.
The map of the pressure field on the central body, equipped with a lateral injection, shows the complexity of the interactions produced by the injection from 24 supersonic nozzles. Comparisons with readings by sensors demonstrated that the PSP measurements matched the instrument readings very accurately, which validated both the coating and the calibration law used.
