Experimental Validation of Unsteady Pressure-Sensitive Paint for Acoustic Applications

Abstract

Investigation of acoustic pressures on a surface is traditionally performed using microphones either placed in discrete locations on the surface or with microphone arrays situated around the model. Pressure-Sensitive Paint (PSP) is an optical pressure measurement technique based on a photo-physical phenomenon of luminophores, where when exited with a light with a certain wavelength the luminophores in the paint emit light with longer wavelength, Liu and Sullivan (2005). The intensity of the emitted light is directly related to the oxygen concentration around the luminophores and according to Henrey’s law, the concentration of oxygen in the paint layer is proportional to the partial pressure of oxygen of the gas above the surface. With this, the surface pressure is a function of luminescent intensity of the PSP luminophore. In order to measure acoustic phenomena a pressure sensitive paint designed for unsteady measurements (iPSP) featuring very short response time is used, Hilfer et al. (2017). The iPSP can be coated on many different surfaces regardless of geometry or material offering an advantage for measurements where a placement of a surface microphone is not possible. Furthermore, by using iPSP combined with a high definition camera each pixel in the image is a separate pressure sensor resulting in a high spacial resolution. A new test facility based on the design introduced by Ali et al. (2016) is build which allows evaluations of acoustic pressure distributions with frequencies up to 5 kHz and amplitudes between 0 and 400 Pa (or up to 146 dB). In this work frequencies and amplitudes comparable to the conditions found inside the Aeroacoustic Wind Tunnel (AWT) at Leibniz University of Hannover, Bartelt et al. (2013) are investigated to evaluate the capability of our iPSP system for acoustic measurements. Single mode acoustic excitation is performed to investigate the limits of our system showing a minimal detectable amplitude of 5 Pa (108 dB). Further, acoustic white noise signal was used to excite multiple modes to investigate the capabilities of our system to separate and detect specific modes. For post-processing phase averaging, proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) are investigated showing a clear advantage of DMD for signal to noise ratio improvement and filtering of noise with specific frequencies such as camera noise. From white noise signal measurements using DMD, modes with amplitude above 5 Pa were identified and spatial distribution extracted.