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PRESSURE SENSITIVE PAINT |
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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).
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Lu, X., Winnik, M.A.,
“Luminescent Quenching by Oxygen in Polymer Films”.
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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.
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McLachlan, B.G., Bell, J.H., “Pressure-Sensitive Paint in Aerodynamic Testing”, Exp. Thermal Fluid Sci., v 10, 1995, 470-485.
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Moshasrov, V., Radchenko, V., Fonov, S.,
“Luminescent Pressure Sensors in Aerodynamic Experiments”, Central Aerohydrodynamic Institute, (TsAGI), Moscow, 1998.
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