Aerodynamic noise reduction using dual-jet planar air curtains

In this paper, dual-jet planar air curtains are demonstrated to be able to successfully remove aerodynamic noise radiated from tandem rods in a crossflow. Single-jet air curtains have shown significant promise as a low noise technology but can introduce additional noise sources such as lip and mixing noise. In this work, dual-jet air curtains are shown to address these obstacles, achieving the same shielding height with a significantly lower overall system noise. Providing high amplitude tonal and broadband noise, tandem rods are chosen as the test case and the fluid mechanics and acoustics associated with their shielding by air curtains are examined both experimentally and numerically. Particle image velocimetry and flow visualisation methods allow the flow fields to be examined, and a numerical analysis with computational fluid dynamics to be validated. Barely examined in the literature to date, the trajectory of planar jets in crossflow are studied here and compared to a basic semi-empirical model. The accurate identification of the trajectory allows a method for an optimum shielding height and dual jet velocity combination to be proposed and tested. Acoustic beamforming and 1/3 octave band analysis are performed for diagnostic noise source localisation and comparative evaluation. It is demonstrated that a judicious choice of flow parameters can result in the complete removal of the aerodynamic noise with almost no acoustic penalty.