Aeroacoustic near-field measurement with micrometer resolution

Abstract

For the investigation of sound-flow interaction in near-fields, like aeroacoustic damping or acoustic streaming, measurements of the acoustic particle velocity (APV) and the flow velocity field with a micrometer resolution are required. In addition, a high working distance is needed for contactless measurement. For this task, the laser Doppler velocity profile sensor is shown to be a predestined tool. First, the APV measurement is successfully validated in an aeroacoustic duct using a microphone-based measurement method as a reference. Here, a minimum APV amplitude of 4 mm/s was resolved in agreement with the reference measurements. Then, the profile sensor was applied for measurements at a perforated acoustic liner with bias flow. Acoustically induced flow vortex structures were resolved with a spatial resolution of 10μm with a minimum distance of 350μm to the liner perforation. A comparison to frequency modulated Doppler global velocimetry (FM-DGV) demonstrated the advantage of the profile sensor for spatially resolved measurements of small scale structures. In contrast, FM-DGV is beneficial due to its high measurement rate which enables the spectral analysis of the velocity in order to better understand the energy transfer from sound to flow.