Optical multi-point measurement of the acoustic particle velocity in a superposed flow using a spectroscopic laser technique

Abstract

Noise reduction in jet engines requires the optimization of sound absorbers, for example, bias-flow liners. To increase their efficiency, an enhanced understanding of the sound–flow interaction at liners is needed. Therefore, the simultaneous measurement of the acoustic particle velocity and the flow velocity is inevitable. Since the acoustic particle velocity is typically smaller than 100 mm s−1 and the flow velocity is about 100 m s−1, a high dynamic range of minimum 103 and a low measurement uncertainty of 1 mm s−1 are required here. To resolve the acoustic particle velocity in the audible frequency range up to 20 kHz without violating the Nyquist–Shannon sampling theorem, a measurement rate of more than 40 kHz is also demanded. Doppler global velocimetry with frequency modulation (FM-DGV) fulfils each of these requirements, which is demonstrated in this paper. Multiple sound frequencies were resolved with a measurement rate of 50 kHz at a simultaneous multi-point measurement showing excellent agreement with the theory. The measurement uncertainty of 4 mm s−1 on average was achieved for a measurement duration of 1 s and can be reduced further by increasing the measurement duration. Furthermore, it is shown that the acoustic particle velocity can be measured in a flow with FM-DGV providing a high dynamic range of 4 × 103.