Although most aircraft and automobiles are currently designed using computational fluid dynamics (CFD), wind tunnel testing is still necessary to validate CFD models. In the past, pressure measurements were limited to a few key locations on the model where mechanical pressure transducers could be placed.
Now optical measurements of pressure can be taken using pressure-sensitive paint (PSP). A wind tunnel model is covered with a paint formulation that fluoresces under short-wavelength excitation. As pressure is increased, collisions between oxygen and fluorescing paint quench the fluorescence. The change in fluorescence intensity can be used to calculate a pressure map of the surface.
Both steady-state as well as lifetime (time-resolved) imaging techniques are used in wind tunnels around the world.
Steady State PSP Imaging
In steady-state measurements, a continuous illumination is used to excite the paint surface. The resulting phosphorescence from the sample, after passing through appropriate band pass filters, is captured by a slow-scan, high-dynamic range CCD camera. After applying the calibration routines, the image is converted in order to demonstrate the pressure profile of the model.
Fig 1*. Typical PSP experimental setup
Fig 2*. Excitation-Emission characteristics of a pressure sensitive paint
Time Resolved PSP Imaging
Lifetime-based PSP imaging is a relatively new technique which
uses pulsed excitation and a gated CCD camera for capturing luminescence.
Two images are used for the measurement. The first image is taken immediately
after the excitation, while a much longer second exposure is taken at the
end of the emission, as shown in the figure below. The ratio of the two gated
exposures is proportional to the pressure differential.
Fig 3*. Typical setup for time resolved PSP
Fig 4*. Timing diagram for a typical "gating" sequence
Solutions from Princeton Instruments
Back-illuminated CCDs with large-pixel full well capacity are a common requirement
for steady state PSP imaging. This allows for maximum signal-to-noise ratio,
which is necessary to measure small changes in pressure. Also, PI/Acton
was one of the first companies to develop gated interline cameras for high-spatial
and time resolution for lifetime PSP applications. Most recently, PI developed
CoolSNAP: K4, a large format (4-mega pixel) interline camera with a special
on-chip accumulation capability. By accumulating multiple gated exposures,
the camera is capable of delivering unprecedented performance.
Multiple-camera control from a single host computer, accurate and calibrated
response and a reputation for reliability in demanding environments are just a
few reasons why PI
cameras are utilized in premier wind tunnel testing facilities around the world.