Implementation of a Jet Collector and Dissipation Cavity into a Closed Anechoic Chamber for Jet Noise Studies

This paper discusses the integration of an air collector and dissipation cavity into a closed anechoic chamber for jet noise studies. Having the jet issued directly in the closed chamber introduces air recirculation effects, which influence the jet plume, producing a different flow compared to a free jet emitted in an open environment. The implementation of the jet collector aims to redirect, dissipate and extract effectively the airflow in the downstream region of the chamber. The fact of being a closed-section facility provokes air recirculation, which influences the potential core and jet plume. However, any physical changes made to the interior of the anechoic chamber would alter its acoustic performance. Therefore, an acoustic qualification is carried out before and after the implementation to ensure that the chamber is capable to accurately emulate acoustic free-field conditions for reliable jet-noise measurements. The acoustic characterization involves a theoretical analysis and an extensive measurement campaign, including impulse response and divergence loss measurements. Conveniently, the partial separation wall leads to decoupling the room's natural frequencies, preventing standing wave degeneracies. Additionally, since the nozzle's exit is fixed, the boundaries of the free-field region can be established, which allows defining a portion of space where microphone measurements can be assumed to be free from reverberation effects. Overall, the article presents a comprehensive approach to physically modifying the anechoic chamber for advanced jet noise research, ensuring the accuracy and repeatability of experimental results.