Pressure Sensitive Paint
Pressure sensitive paints (PSP) are optical sensors for surface pressure measurements. Traditional techniques for measuring surface pressure on models are limited to point measurments 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 requirments 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.
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 architechtural models and their components. PSP is also utlized 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 whereas 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.
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.
Pressure Sensitive Paint Formulations
Major sources of error in PSP data are due to illumination variations and temperature changes during the data collection period while the wind tunnel is running. Blow-down type wind tunnels will change temperature during the course of a run while closed-circuit or continuous wind tunnels are more stable with temperature. Errors in pressure measurements taken at low wind speeds are largely the result of temperature gradients on the model surface. These temperature gradients can be the result of model construction, tunnel operation, or fluid dynamics. A rapid prototype model, for example, is constructed using an internal metal structure and a polymer resin. The thermal signature of the internal structure is apparent when the surface of the model is subjected to a heat flux. The model is exposed to a heat flux due to changes in tunnel Mach number. This condition is most apparent during tunnel startup. Temperature errors can be minimized by using temperature controlled tunnels and constructing the model from materials with high thermal conductivity like aluminum or stainless steel. Model construction and tunnel operation are key considerations for effective lows-peed PSP measurements. Illumination variations can be caused by non-stable output from LEDs and by vibrations of the model or camera relative to one another. To the camera, these changes are interpreted as pressure changes.
A way of dealing with the temperature errors is to add a second component to the PSP. This is known as Binary PSP. What Binary PSP adds that single-component PSP lacks is the ability to correct for temperature and illuminiation induced errors. Binary PSP works by acquiring data from both the oxygen sensitive component and the second component, known as the reference molecule. The emission from the reference and oxygen sensitive molecules are sepctrally independent. A color camera is used to separate the signals from the reference and oxygen sensitive components of the Binary PSP. From this, two images are acquired (one of the reference and one of the oxygen sensitive component). Taking the ratio of the oxygen sensitive image over the reference image eliminates dependence on temperature. This is due to the reference and oxygen sensitive molecules having the same sensitivity to temperature. By dropping the temperature dependence, an ideal PSP is created where the only dependence is on the pressure change.
Where temperature changes during the data acquisition are not a problem, single-component PSPs are still used. Higher resolution images can be obtained using single-component PSPs as they use a monochrome camera and don't reduce the resolution like a color camera does when it separates colors.
Traditionally, PSP was used to measure low-frequency or steady-state pressure changes on model surfaces. With LED and high-speed camera technology rapidly advancing, fast-response PSP has also rapidly advanced over the past decade. Rather than acquiring steady-state results with traditional PSP data acquisition, fast-response PSP utilizes paints which respond to pressure changes much more rapidly, leading to accurate, high-resolution time resolved pressure data. With the vast amount of data collected (typically thousands of images per test condition), fast PSP has produced unsteady aerodynamics data like never before. Today, fast PSP can measure pressure fluctuations up to 20 kHz. Data mining these thousands of data points leads to vast improvement of CFD models and complete spatial distribution maps of pressure at selected frequencies. Problems like buffeting and acoustic noise can be studied at depth and resolved with data of the caliber.
Fast PSP allows scientists to look at rapid pressure fluctuations on model surfaces like never before. Conventional methods of fast pressure fluctuation detection are limited to point measurements. If the sensor is in the wrong spot, it will be missed all together. Using the global data from fast PSP data collection, these fluctuations are brought into view rather easily. The above images show an F-22 model painted with fast PSP, the model under UV illumination (through a long-pass filter to separate the UV light from the paint emssion) and finally the procesed PSP data with a false color map applied. This is only one in a series of thousands of images.
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