PSP, Pressure Sensitive Paint Application at Low Speeds
Installation of the PSA 206 model
in the T4Po wind tunnel at ENSMA school in Poitiers
The principle of the PSP technique is simple. The difficulty with using PSP at low speed is the extreme accuracy requirement. In practice, the accuracy required is around 1mb for an absolute pressure of 1bar, i.e. a relative accuracy of 0.1%! Under these conditions, all the sources of error must be considered for applications at speeds of less than 50m/s.
A preliminary study on a delta wing in 1999 was used to validate a test procedure at low speeds. After it had been debugged and major improvements had been made to the software, this procedure was applied in 2000 during a test performed in collaboration with PSA Peugeot Citroën in the T4Po wind tunnel at the ENSMA school in Poitiers. The model, made by PSA and equipped with a few pressure taps, represented a Peugeot 206 with a simplified rear window. The figure above shows the installation of the model in the test section. Only the model's rear window was painted, as the purpose of the test was to characterize the air flow over this part, where separations may occur. The ultraviolet lighting was supplied by a liquid optical fiber affixed to a mast usually used to support sensors.
The next figure gives the map of pressure coefficients obtained at 40m/s. The overspeed created at the joint between the vehicle's roof and rear window can be seen. The overspeeds seen on each side of the rear window were generated by two vortices originating at the roof/side/rear window joint. These vortices are very intense and play a large part in the vehicle's drag balance. What you can see in the figure is in fact the trace left by these two vortices. The comparison between the PSP measurements and those of the pressure taps on the central part of the rear window is given in the graph. A 1 mb error bar has been added to the sensor results for easier comparison. The match is better than 1mb, which was our objective. This test demonstrated that PSP can be applied in low speed tests. The accuracy obtained was satisfactory and the processing tools developed can produce the results pressure maps just a few minutes after the measurements.
Map of pressure coefficients at 40 m/s, or 144 km/h.
