Pressure sensitive paints (PSP) are optical sensors for surface pressure measurements. Traditional techniques for measuring surface pressure on models are limited to point measurements and to geometries where there is enough space to install them. Installation and instrumentation of a model with pressure taps and transducers is often costly due to the machining requirements and the sensors themselves. PSP is not limited by model geometry. It can measure pressure on model surfaces at every visible point with superior spatial resolution. Much like a paint coating, PSP is applied to a surface using an HVLP paint gun or airbrush.
A C-5M Super Galaxy Model Painted with Pressure Sensitive Paint (U.S. Air Force Photo)
Most often, PSP is used in wind tunnel research as a validation tool for computational fluid dynamics (CFD) models of certain flow conditions over a model of an aircraft. Wind tunnels from small, academic low-speed wind tunnels to large scale transonic research wind tunnels and hypersonic wind tunnels have utilized PSP for model testing and validation for over 20 years. Today, PSP continues to be a valuable resource in government and commercial testing of aircraft, helicopter, automotive, high-speed train, bridge and architectural models and their components. PSP is also utilized as a tool for film-cooling effectiveness measurements in gas turbine engine blade design.
How Does PSP Work?
A typical PSP is composed of an oxygen-sensitive fluorescent molecule embedded in an oxygen permeable binder. The PSP method is based on the sensitivity of certain luminescent molecules to the presence of oxygen. After application, the PSP is excited with a high-intensity LED, typically a UV 400-nm source. When a luminescent molecule within the PSP absorbs a photon from the LED, it transitions to an excited singlet energy state. The molecule then recovers to the ground state by the emission of a photon of a longer wavelength (red-shifted). When oxygen can interact with the molecule, the transition to the ground state is non-radiative. This process is known as oxygen quenching. The rate at which these two processes compete is dependent on the partial pressure of oxygen at the PSP surface. A higher oxygen quenching rate results in a lower intensity of light emitted from the PSP layer. Conversely, a lower oxygen quenching rate results in a higher intensity of light emitted. The result is an output from a model surface of varying intensities based on the local oxygen concentration which is directly correlated to the local barometric pressure. The output from the PSP is recorded with a sensitive scientific camera through a long-pass filter which separates the LED excitation from the PSP emission.
After the images are captured with the camera, they are stored for post-processing. Images are converted from images of intensity variations to images of pressure using a previously determined calibration of the same paint type. From there, false color maps are applied to better visualize the pressure gradients on the model surface. Data can be plotted and compared to pressure taps if present. Typical PSP test are within 5% of the pressure tap data.
So what can PSP do for you? PSP is utilized anywhere from small academic wind tunnels to large-scale commercial and government test facilities to deliver high-resolution pressure measurements for CFD comparison, loads calculations and model validation.
Delta Wing Painted with PSP (Left), PSP Data After Post-Processing (Right)
Wind Tunnel Model Painted with PSP at ARA’s Transonic Wind Tunnel (Left), PSP Data Compared to Pressure Taps (Right)
Preparation of SLS Rocket Model (Left), SLS Rocket Model Glowing Pink Under UV LEDs (Credit: D. Hart / NASA Ames Research Center)
Film Cooling PSP Setup in Small Wind Tunnel (Left), Film Cooling PSP Results (Right)
Film cooling effectiveness measurements from traditional gas analysis systems compare very well with measurements obtained using ISSI’s fluorescent film cooling system. Data Courtesy of Wright & Hahn, Texas A&M
Pressure sensitive paints come in a variety of quantities. ISSI sells in standard quantities of 200-, 400-, and 750-ml.
Paints ship in glass jars for application with pneumatic painting guns. Academic paints are available in aerosol spray cans for easy application.