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Pressure Sensitive Paint (PSP)

PSP will be used on the upper and lower surface of the sail to determine the pressure distribution around the section, and also by integrating the pressure distribution, to calculate the section lift coefficient. PSP is applied to a highly reflective surface and luminesces when excited molecules in the coating return to a lower energy state due to the collision with an oxygen molecule in a process known as oxygen quenching. Excitation is commonly provided by a UV lamp. The rate of quenching is proportional to the partial pressure of oxygen, which is in turn proportional to total pressure. PSP is therefore most sensitive to changes in pressure when the dynamic head is high and the percentage change in pressure is high with respect to atmospheric pressure.

The ratio of wind on to wind off intensity of the emitted light from the PSP is proportional to the ratio of respective pressures under conditions of constant excitation. The constants in the governing equation are derived either before the experiment by measuring the intensity of the PSP for various pressures and temperatures and/or during the experiment by using a reference pressure on the model such as that provided by a pressure transducer. Photogrammetry may then be required to associate the points in the PSP image with those on the model.

Uncertainty in the measurement of pressure can be due to errors in the calibration of the response of the paint to pressure, spatial and temporal variations in illumination and errors in the data processing. By far the largest source of error comes from an uncertainty in the paint’s temperature.

Sullivan (2001) states that the accuracy of PSP is 1mbar with a resolution of 0.5mbar, and that the typical time response is 0.5 seconds, although 1μs has been demonstrated. For further information on PSP, the reader is referred to Bell et al. (2001), Mehta et al. (2000) and Sullivan (2001).

An illustration of the capabilities of PSP to capture the pressure distribution with high resolution over a large area and range of pressure is given in Figures 6a and 6b.

 

 

 

 

 

Figure 6: Pressure distribution over the upper surface of (a) an F-16 and (b) a high-lift wing (Bell et al. (2001))

 

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