Measurements of hydrodynamic and acoustic wall pressure fluctuations using ultra-thin-precision microphone

Abstract

Wall pressure fluctuations can be measured with flush- and pinhole-mounted sensors. Typically, to install the sensors, the surface of test objectives needs to be drilled. However, this manner of installation is undesirable or impossible for many industrial applications. Therefore, a surface sensor that can be easily mounted on the surface could be very helpful in measuring wall pressure fluctuations. This paper presents wall pressure spectra measured with GRAS ultra-thin-precision (UTP) microphones which have a thickness of 1 mm. Three mounting configurations were used in the measurements: pinhole- and flush-mounted in a recess on the surface and mounted with a flow-optimized fairing (elevation angle of 7.5° ) taped on the surface, which can significantly simplify the mounting. The flush-mounting sensor-size-induced high-frequency spectral attenuation and the pinhole-mounting-induced Helmholtz resonance are corrected with published methods. The obtained wall pressure spectra are compared to the spectra measured with pinhole-mounted Kulite sensors. The effect of the mounting with farings on the measured spectra due to the disturbance on turbulent boundary layers (TBL) is quantified by comparing to the results measured with the flush-mounted microphone in the recess. For the acoustic measurement of incident sound on the wall, the pinhole-mounted configuration was discarded because Helmholtz resonance needs to be avoided for most acoustic tests. The sensor-size-induced high-frequency attenuation up to 100 kHz was measured with a laser-generated sound source. The directivity of the UTP microphones is quantified with reference to a flush-mounted Kulite without a protection screen. Furthermore, the interference pattern due to the faring with different angles of incident sound is demonstrated.