Pressure-Sensitive Paint

Fast-response pressure-sensitive paint (uPSP) is an optical diagnostic tool used to obtain spatially- and temporally-resolved surface pressure distributions on aerodynamic test articles in wind tunnels. Pressure distributions are achieved by exciting the oxygen-sensitive luminescent molecules using ultra violet light (~400 nm) that make up the top coat of the paint. As these molecules are transitioning back to their ground state, they emit a photon; these photons make up the light intensity that is collected by a high-speed camera. Some luminescent molecules will not emit a photon due to oxygen quenching, where some of the oxygen molecules in the air will react with the luminophore to capture or “quench” the energy from the luminophore, thus deactivating its ability to emit a photon. Where there is a higher partial pressure of oxygen, there will be less light intensity collected from that location on the test article. The base coat consists of a polymer ceramic, which improves the gas diffusivity by increasing surface area, thus increasing the reaction time of the paint to a maximum of approximately 20 kHz.

The light intensities collected by a high-speed camera are put through a Matlab code that takes the ratio of intensities at wind-off and wind-on conditions to convert them into pressure distributions.

One objective of this work is to optimize the application technique of the paint to minimize uneven paint work and surface roughness effects without eliminating the functionality of the paint.

Another objective is to test the application of fast-response PSP with respect to the boundary layer thickness after only minimal surface roughness effects were seen applying PSP to a model with a thick, turbulent boundary layer.

Comparison between a raw light intensity image (left) as collected by a Photron FASTCAM Mini AX200 high-speed camera and the corresponding pressure distributions (right) of uPSP of a fully turbulent, Mach 2 boundary layer. Flow is from top to bottom.

Qualitative comparisons demonstrating the surface roughness effects of uPSP. Flow is from left to right.
Top: Schlieren animation acquired at 100 kHz of a thin boundary layer (~2 mm) in a Mach 2 freestream with no uPSP.
Bottom: Schlieren animations acquired at 100 kHz (left) of a thin boundary layer (~2 mm) and 20 kHz (right) of a thick boundary layer (~12 mm) in the same Mach 2 freestream, both with uPSP present. Surface roughness effects are not as dominate with a thicker boundary layer.