Pressure-sensitive paints

Pressure-sensitive paints (PSP) have been used since 1980 to display and measure the distribution of pressures throughout the model. The advantages of the method are:

■ continuous pressure measurements on the whole surface;

■ non-intrusiveness, since there are no probes to be introduced in the fluid flow;

■ reduced time and cost of preparing the model (there are no orifices to be made, transducers or pressure tubes or scanning valves to be inserted in the model);

■ high resolution, limited only by the size of the pixels of the camera system.

Disadvantages are:

■ sensitivity to temperature;

■ the need for prior calibration;

■ filming techniques and sophisticated data processing.

The paints are made of luminescent substances, embedded in a binder, that, when excited by light radiation (UV or visible light), re-emit (Figure 1.13) part of it as light with a different wavelength (fluorescence), trans­forms a part of it in internal energy (internal conversion), and a part of the energy goes to excite oxygen molecules (oxygen quenching).

Re-emission of a luminescent molecule

Activated oxygen

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Photon

Since the luminescence is reduced by the presence of oxygen molecules, increasing pressure, and thus the partial pressure of oxygen, tends to mitigate the intensity of the light emitted: a measure of the distribution of the latter on the surface of the model therefore allows the distribution of pressure to be traced.