Dynamic pressure sensitive paint showcased at Arnold Air Force Base

ARNOLD AIR FORCE BASE, TENN. -- A test demonstrating Innovative Scientific Solution Incorporated's (ISSI) dynamic pressure sensitive paint (PSP) was recently conducted in the 16-foot transonic wind tunnel at Arnold Engineering Development Complex (AEDC).

The effort was funded by the Air Force through a Rapid Innovation Funding grant for Air-Delivered Weapon Certification Cost Reduction. The goal is to provide a capability that can improve computational fluid dynamics (CFD) modeling simulation of store separation in order to reduce the need for wind tunnel and flight test drop testing during the certification process.

ISSI researchers were joined by engineers from AEDC, Lockheed Martin and Euclidian Optics for the program. 

According to Jim Crafton, senior research scientist for ISSI, the concept of dynamic PSP, also known as fast PSP, has been around for about 15 years, but due to a few technical shortfalls it hasn't been implemented until recently. 

"A lot of the hardware [finally] caught up, and the camera technology is better now than it was even five years ago," he explained.

Because of these advancements, Crafton stated ISSI thought it was time for fast PSP to progress from the development and research stage to the testing stage.

"We wanted to take this technology from an academic application and use it in a production facility that's capable of looking at real problems," he said.

Though ISSI has completed other fast PSP tests before, Crafton stated there's a major difference in the data obtained by using a "big boy" facility like AEDC's 16T rather than a smaller wind tunnel.

And AEDC is no stranger to PSP testing. PSP is a technique often used in the Complex's wind tunnels to acquire full surface pressure data on test models.

"AEDC has developed a 'production PSP capability' for the wind tunnels that leads the world in capability," stated Marvin Sellers, PSP lead engineer. "We can provide steady-state surface pressure and pressure-integrated loads from PSP to customers immediately after acquisition. This capability is required to make real-time decisions about wind tunnel test results of flight vehicles." 

The number of tests using PSP has increased significantly in the last two years as acceptance of the technique has grown.

Crafton mentioned this experience was also a reason ISSI chose 16T as the site to perform the test and commended Sellers on his assistance during testing.

"Fast PSP is the next step of development for a capability customers have been asking about for years," Sellers said.

Dynamic surface pressure measurements provide information to vehicle designers that are missing in the conventional PSP. The current method of acquiring this data is with special transducers that measure the pressure fluctuations. These transducers are very expensive, are limited to a few to several hundred points and require special wind tunnel models.

"If the dynamic pressure data can be acquired with PSP on a conventional wind tunnel model, information can be acquired on the complete model surface and significant savings can be achieved for the test customer," Sellers said.

The fast PSP used in the recent 16T test is like traditional PSP in that it can be used to produce high spatial resolution measurements of surface pressure.

"The PSP is excited by light at a specific wavelength and gives off light at another wavelength with an amount inversely proportional to the surface pressure," Sellers said. "What this means is, areas of high pressure will be dimmer than areas of low pressure."

The major difference between the two is found in the physical properties of the paint binder.

For conventional PSP, oxygen molecules must permeate into the binder layer for oxygen quenching (reduction in light output) of the PSP molecules. The process of oxygen permeation in a polymer binder layer produces slow response times. On the other hand, the molecule in a porous PSP is exposed so that the oxygen molecules quench the PSP faster. A large effective surface area due to the porous surface improves luminescence intensity; therefore, a higher signal to noise ratio can be achieved.

ISSI had to develop a new power supply for the light emitting diodes (LEDs) AEDC uses for the conventional PSP data acquisition system. The new units enabled the light to stay on, and constant, for several seconds to excite the PSP.

ISSI also purchased two high speed cameras for use in recording the emitted light fluctuations (a result of pressure fluctuations) on the model surface. The cameras recorded data at rates of 2,000 to 4,000 images per second. AEDC developed new data acquisition software to control the cameras and acquire and store the images. New image processing software has also been developed by Euclidean Optics, Inc. to provide processed data. The large amount of information acquired does not permit real-time processing at this time but future improvements will speed up the process.

The dynamic PSP data was acquired on a Lockheed Martin V7 weapons bay model, a 1980s Advanced Tactical Fighter concept, to measure the acoustic pressure levels in the bay. The model was instrumented with several dynamic pressure transducers to provide a comparison with the PSP. Lockheed will use the dynamic PSP data to validate and improve CFD code for weapons bay simulations. Initial data comparisons of the dynamic PSP and pressure transducers indicate very good agreement between the two measurements.