Knowledge of the time-varying pressure field acting on an aerodynamic surface is insightful in understanding complex flow structures. For example, interactions between the fan and downstream stator inside a jet engine contribute to high-cycle fatigue and engine tonal noise, and unsteady inlet distortions have a significant impact on engine operability and efficiency. Helicopter blades operate under the influence of their own unsteady wake, which complicates the accurate prediction of lift and induces undesired noises and vibrations. Conventional techniques for measuring surface pressures in these instances, such as pressure taps or transducers, can be quite cumbersome to install on very thin leading edges and rotating surfaces. In addition, pressure information obtained by these conventional techniques is limited to point-wise measurements which may not capture subtle flow phenomena with sufficient spatial resolution.
Pressure-sensitive paint (PSP) is an optical measurement technique that is particularly suited for acquiring surface pressure maps. PSP consists of a special luminescent coating that responds to local wall pressure changes after being excited with a short-wavelength illumination source (typically LEDs or lasers). The local emitted light intensity is inversely proportional to the local surface pressure; thus, by recording the light distribution over the surface with a scientific-grade camera, the corresponding pressure field can be calculated. Each pixel on the camera chip essentially acts as a pressure transducer by sampling the paint luminescence – a major advantage over conventional techniques consisting of relatively sparse point-wise measurements.
In our research group, we continue to make developments in improving the frequency response of PSP to enabletime-resolved measurements of the pressure map in unsteady flows. Our research group has played a major role in developing PSP data acquisition techniques which effectively enable “point-and-shoot” sampling of unsteady surface pressure maps. Some examples of our past work have included applying our fast-responding PSP to transonic and low-speed flows with significant unsteady flow structures. In particular, we have applied our PSP to the investigation of unsteady transonic flow past a hemispherical turret in the Air Force Research Laboratory’s Trisonic Gasdynamics Facility. The sample data at left emphasize the utility of PSP in detecting the three-dimensional structure of the shock wave formed at the turret apex. Movie files created from the PSP images are extremely useful in visualizing the unsteady motion of the shock wave – an advantage over transducer arrays whose output is a series of electrical signals.
Our fast-responding PSP has been applied to a helicopter rotor blade in forward flight conditions where the surface pressure is a rapidly-varying function of time and angular position of the blade. Our group has led the way in developing image processing tools to correct the PSP data for camera artifacts such as image blurring due to surface movement. These contributions have the potential to change the way that helicopter loads testing in industry is conducted by using an all-optical system.
Another significant aspect of our PSP research is to improve the accuracy of PSP measurement by minimizing errors caused by temperature effects and model vibrations. PSP is inherently sensitive to temperature, and the temperature induced errors can be large for common fast PSPs. A 2-color fast PSP has been developed, which emits a red pressure signal and a green temperature signal simultaneously. The green signal is only sensitive to temperature and it is used to correct for the errors in red pressure signal due to temperature change. Either a color camera or a dual monochrome camera system is required for this paint. The same idea can be applied to removed the errors caused be model movements and vibrations.
An in-house suite of tools for paint mixing and characterization are available in our laboratory, including a full chemical preparation bench and spray hood. We have the capability of performing static PSP calibrations in a controlled sample chamber, as well as dynamic calibrations using acoustic resonance and shock tubes.
- Gregory, J.W., Asai, K., Kameda, M., Liu, T., and Sullivan, J.P., 2008, "A Review of Pressure-Sensitive Paint for High Speed and Unsteady Aerodynamics," Proceedings of the Institution of Mechanical Engineers, Part G, Journal of Aerospace Engineering, vol. 222, no. 2, pp. 249-290. doi: 10.1243/09544100JAERO243
- Fang, S., Long, S.R., Disotell, K.J., Gregory, J.W., Semmelmayer, F.C., and Guyton, R.W., 2011, “Comparison of Unsteady Pressure-Sensitive Paint Measurement Techniques,” AIAA Journal, vol. 50, no. 1, 209-222. doi: 10.2514/1.J051167
- Juliano, T.J., Kumar, P., Peng, D., Gregory, J.W., Crafton, J.W., Fonov, S., 2011, "Single-Shot, Lifetime-Based Pressure-Sensitive Paint for Rotating Blades," Measurement Science and Technology, vol. 22, no. 8, 085403. doi: 10.1088/0957-0233/22/8/085403