Airborne Background Oriented Schlieren Technique

Supersonic flight over land is generally prohibited because sonic booms created by shockwaves disturb people on the ground and can damage property. Armstrong innovators are working to solve this problem through a variety of innovative techniques that measure, characterize, and mitigate sonic booms. The BOSCO technology is helping researchers understand how sonic booms travel through the air. How It Works Armstrong's patented system visualizes air density gradients generated by air compressing as it flows around an object. Researchers first obtain a celestial background image and then collect a series of images of an object in supersonic flow in front of the celestial object. The density change in the air refracts the light, shifting the background as compared to the undisturbed background image. The amount of movement corresponds directly to density gradients in the airflow. Using computer algorithms to analyze the images, resultant images essentially show the distortions caused by the aerodynamic flow of shockwaves passing between the camera and the celestial background. Why It Is Better Schlieren photography has been used for years in wind tunnels, where the environment is controlled. BOSCO enables its use in the real atmosphere with real propulsion systems. Studying life-sized aircraft flying through Earth's atmosphere provides better results than modeling and can help engineers design better and quieter supersonic airplanes. In addition to studying shock waves for aircraft, NASA's schlieren techniques have the potential to aid the understanding of a variety of flow phenomena and air density changes, such as investigating air flows around tall buildings and the tips of wind turbines and helicopter blades.

The Projected BOS imaging system developed at the NASA Langley Research Center provides a significant advancement over other BOS flow visualization techniques. Specifically, the present BOS imaging method removes the need for a physically patterned retroreflective background within the flow of interest and is therefore insensitive to the changing conditions due to the flow. For example, in a wind tunnel used for aerodynamics testing, there are vibrations and temperature changes that can affect the entire tunnel and anything inside it. Any patterned background within the wind tunnel will be subject to these changing conditions and those effects must be accounted for in the post-processing of the BOS image. This post-processing is not necessary in the Projected BOS process here. In the Projected BOS system, a pattern is projected onto a retroreflective background across the flow of interest (Figure 1). The imaged pattern in this configuration can be made physically (a pattern on a transparent slide) or can be digitally produced on an LCD screen. In this projection scheme, a reference image can be taken at the same time as the signal image, facilitating real-time BOS imaging and the pattern to be changed or optimized during the measurements. Thus far, the Projected BOS imaging technology has been proven to work by visualizing the air flow out of a compressed air canister taken with this new system (Figure 2).

Although polystyrene microspheres are often the seed material of choice for subsonic airflow studies. These seed materials, however, do not provide any benefit for near wall measurements compared to other state-of-the-art seed materials. Consequently, in this innovation NASA scientists have developed this method of generating dye-doped polystyrene microspheres using novel synthetic approaches. The novel features of this invention are the utility of specific chemical functionalities, monomeric species, environmental additives (buffers), and polyelectrolytes to promote incorporation of dye molecules into developing polystyrene microspheres while enabling control of the spectral properties of the dye relative to pH dependence. These particles will have great utility for wind tunnel measurements near the wall where the state-of-the-art seed materials are not able to collect data. Additionally, the incorporation of these dyes will offer other avenues of data collection including temperature and pressure of the airflows and wind tunnel regions. Likewise, the ability to selectively filter the data collected from these dye-doped polystyrene microspheres can have further applications including the direct visualization of 2 or more fluid flows mixing, among other applications.

Langley has developed various technologies to enable the portable detection system, including: - 3-inch electret condenser microphone - unprecedented sensitivity of -45 dB/Hz - compact nonporous windscreen - suitable for replacing spatially demanding soaker hoses in current use - infrasonic calibrator for field use - piston phone with a test signal of 110 dB at 14Hz. - laboratory calibration apparatus - to very low frequencies - vacuum isolation vessel - sufficiently anechoic to permit measurement of background noise in microphones at frequencies down to a few Hz - mobile source for reference - a Helmholtz resonator that provides pure tone at 19 Hz The NASA system uses a three-element array in the field to locate sources of infrasound and their direction. This information has been correlated with PIREPs available in real time via the Internet, with 10 examples of good correlation.

Traditionally, unsteady pressure transducers have been the instrumentation of choice for investigating unsteady flow phenomena which can be time-consuming and expensive. The ability to measure and compute these flows has been a long-term challenge for aerospace vehicle designers and manufacturers. Results using only the pressure transducers are prone to inaccuracies, providing overly conservative load predictions in some cases and underestimating load predictions in other areas depending on the flow characteristics. NASA Ames has developed a new state-of-the-art method for measuring fluctuating aerodynamic-induced pressures on wind tunnel models using unsteady Pressure Sensitive Paint (uPSP). The technology couples recent advances in high-speed cameras, high-powered energy sources, and fast response pressure-sensitive paint. The unsteady pressure-sensitive paint (uPSP) technique has emerged as a powerful tool to measure flow, enabling time-resolved measurements of unsteady pressure fluctuations within a dense grid of spatial points on a wind tunnel model. The invention includes details surrounding uPSP processing. This technique enables time-resolved measurements of unsteady pressure fluctuations within a dense grid of spatial points representing the wind tunnel model. Since uPSP is applied by a spray gun, it is continuously distributed. With this approach, if the model geometry can be painted, viewed from a camera, and excited by a lamp source, uPSP data can be collected. Unsteady PSP (uPSP) has the ability to determine more accurate integrated unsteady loads.