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  Products Glossary of Technical Terms  



Traditional measurement techniques for acquiring surface pressure distributions on models have utilized embedded arrays of pressure taps. This requires much construction and setup time while producing data with limited spatial resolution. An alternative approach is to use pressure (oxygen) sensitive paint to measure surface pressure. Pressure measurements using pressure sensitive paints have been demonstrated in several challenging flow fields such as an operating compressor blade and an aircraft wing in flight. The advantages of pressure sensitive paint include non-intrusive pressure measurements and high spatial resolution when compared to conventional measurement techniques.

A typical pressure sensitive paint is comprised of two main parts, an oxygen sensitive fluorescent molecule, and an oxygen permeable binder. The pressure sensitive paint method is based on the sensitivity of certain luminescent molecules to the presence of oxygen. When a luminescent molecule absorbs a photon, it is excited to an upper singlet energy state. The molecule then typically recovers to the ground state by the emission of a photon of a longer wavelength. In some materials oxygen can interact with the molecule so that the transition to the ground state is radiationless, this process is known as oxygen quenching. The rate at which these two processes compete is dependent on the partial pressure of oxygen present, with a higher oxygen pressure quenching the molecule more, thus giving off a lower intensity of light.

Unfortunately, pressure sensitive paints are also sensitive to temperature. A rise in temperature will increase the probability that the molecule will transition back to the ground state by a radiationless process. This process is known as thermal quenching. A second source of temperature sensitivity occurs when the binder for pressure sensitive luminescent molecule has a permeability that is a function of temperature. This is often the case for the polymer based binders used for pressure sensitive paint. Temperature sensitivity can lead to many problems in converting the intensity distributions to pressure if not taken into account. Effective implementation of a pressure sensitive paint, therefore, requires that temperature effects be characterized and corrected, or the paint be used in an isothermal environment. This problem is especially apparent in flows with small pressure (1 [psi]) changes in the presence of a moderate (1 [K]) temperature gradient. Here the temperature dependent intensity changes would be on the order of the pressure dependent intensity changes. Once again, non-uniform illumination, paint thickness and concentration, and camera sensitivity are eliminated by a ratioing process involving the luminescent intensity at a known condition (Iref, Pref).

Lu, X., Winnik, M.A., “Luminescent Quenching by Oxygen in Polymer Films”.

Liu, T., Campbell, B.T., Burns, S.P., Sullivan, J.P., “Temperature- and Pressure-Sensitive Luminescent Paints in Aerodynamics”, Appl. Mech. Rev. v 50, n 4, 1997, 227-246.

McLachlan, B.G., Bell, J.H., “Pressure-Sensitive Paint in Aerodynamic Testing”, Exp. Thermal Fluid Sci., v 10, 1995, 470-485.

Moshasrov, V., Radchenko, V., Fonov, S., “Luminescent Pressure Sensors in Aerodynamic Experiments”, Central Aerohydrodynamic Institute, (TsAGI), Moscow, 1998.