Measurement of the acoustic particle velocity under grazing flow using A-PIV

Kurzfassung

For many aero-acoustic research applications the direct measurement of the acoustic particle velocity is of high relevance. Often times these applications exhibit a mean flow field superimposed by the acoustics wave propagation. This work demonstrates the application of the non-intrusive, laser-based Acoustic Particle Image Velocimetry (A-PIV) in duct flows with Mach numbers up to 0.2. In a first step the particular features of an acoustic field with non-uniform mean flow are discussed briefly. The experiments were conducted in the acoustic grazing flow facility DUCT-R at DLR Berlin. When mean flow is present, the A-PIV results show a strongly profiled distribution of the acoustic particle velocity in duct-height direction (y). Hence, a difficulty occurs in determining a representative value of the acoustic particle velocity for comparison with microphone measurements, since the applied microphone pressure analysis neglects the y-dependency (plug flow assumption). However, averaging of the acoustic particle velocities along the half duct height reveals a very close agreement with the microphone data. Along the lines of previous work under no-flow conditions [1], the acoustic plane wave decomposition was applied to the A-PIV results. The resulting amplitudes of the up- and downstream propagating waves match very well with the microphone-based data. In results, the ability of A-PIV to determine the acoustic particle velocity under mean flow conditions could be proven. Interestingly, with the A-PIV measurement and post-processing procedure much higher Dynamic Velocity Range (DVR) values could be reached than expected for the applied PIV systems following previous literature.