High Dynamic Range Measurements of Acoustic Particle Velocity and Flow Velocity for the Application at Liners

Abstract

Today, acoustic liners contribute significantly to the reduction of aircraft engine noise. To maximize the damping performance of so-called bias flow liners, a deeper understanding of the interaction between the acoustic wave and the bias flow is demanded. Therefore, contactless field measurements of the acoustic particle velocity and the flow velocity at a bias flow liner are necessary. Since the acoustic particle velocity is several orders of magnitude lower than the flow velocity, a low measurement uncertainty and a high dynamic range are required. In order to analyze the transfer from acoustic energy to flow turbulence, the velocity spectrum has to be resolved, which necessitates a high measurement rate. Doppler global velocimetry with sinusoidal frequency modulation fulfills these requirements, which is demonstrated in this paper. The application of the measurement method at a bias flow liner in a flow duct at a Mach number of 0.1 is presented for the first time. The measurement uncertainty of the acoustic particle velocity amplitude is 5 mm/s and a dynamic range of 7000 is achieved. The velocity measurement rate of 50 kHz allows to resolve the turbulence spectrum up to 25 kHz. The measurement method is validated by reference measurements with established microphone-technique and a comparison with results of acoustic particle image velocimetry measurements is shown.