Coherent Tomographic Laser Interferometry for the Aero-acoustic Characterization of Cold Jets

This paper aims to describe the complete characterization of aero-acoustic sources, with specific attention to turbulent jet flows with a new approach. The traditional way to tackle fluid dynamic phenomena is to measure the velocity field by means of Laser Doppler Anemometer, Particle Image Velocimetry with the main drawback due to the need of seeding the flow. To overcome that problem the 3D spatial distributions of flow pressure fluctuation due to the flow turbulence can be measured via an interferometric technique and 3D field reconstruction by tomographic algorithms. That procedure employs a non-contact laser Doppler vibrometer in a non-conventional way. This instrument is able to measure the density oscillation within the medium traversed by the laser beam, with a bandwidth up to 200 kHz and with a very fine spatial resolution. The raw measured data, they being interpreted by the system as velocity of vibration and being due to the integral of the flow density variation over the whole laser optical path, need to be post-processed in order to obtain quantitative data about the 3D distribution of the flow density and, finally, the pressure fluctuation. The method is sensitive to both the aerodynamic “cause” of the noise, such as turbulence or vortex shedding, and to the “effect” of it, i.e. the acoustic waves that propagates from the jet to the far field. In order to distinguish between acoustic and purely aerodynamic phenomena, i.e. pressure fluctuation that generate or not an acoustic field, joined flow and acoustic pressure measurements were performed by using the proposed procedure and an omni-directional microphone. The two phenomena will be separated by applying de-correlation algorithms based on coherence function, it allowing to perform a complete aero-acoustic characterization of the fluid dynamic phenomenon.