Environmental impact of aviation is measured in emissions and noise. The communities in the vicinity of airports bear the brunt of aircraft noise in takeoff, climb, flyover, approach and landing.  The exposure to loud noise is harmful to human physiological and psychological health and welfare. The problem in the military (Department of Defense) is even bigger since there are servicemen working in close proximity of advanced supersonic jets, in takeoff and landing. The research team at The University of Kansas has identified a novel and powerful means of mitigating jet noise by inducing shear layer swirl through embedded vanes near the nozzle exit lip. In this proof-of-concept study, the project team aimed to demonstrate the feasibility of the concept through both high-fidelity computational simulations and experimental investigations.

The role of swirl in free turbulent jets is to trigger centrifugal instability, set up a radial pressure gradient and thus promote mixing by both means. However, large-scale swirl in the exhaust jet creates thrust penalty. Thus, a targeted approach to limited swirl induction in the nozzle inner and outer shear layer is proposed that injects the benefits of centrifugal instability waves in the nozzle inner and outer shear layers without incurring significant loss of thrust.

A comprehensive approach was taken that uses 1) Computational simulation track that involves high-order Computational Fluid Dynamics and Computational Aero-Acoustics and 2) Experimental track that involves a free-jet facility for jet noise/acoustic measurement. A benchmark supersonic nozzle problem was selected as the base-line, which was also used to verify and validate the present computational and experimental results. After that, swirl-inducing vanes were added to the nozzle, and many vane configurations were investigated both computationally and experimentally for jet noise mitigation.

When the vanes were added in the subsonic converging section of the nozzle before the throat, no noise reduction was found. When the vanes were added near the nozzle exit in the supersonic region, noise reduction of various degrees were discovered depending on the configuration of the vanes: height, swirl angle and solidity. Eight design iterations were performed using three-dimensional printed nozzles, and the best design was able to achieve over 3dB reduction in noise. The results were reproducible in the free-jet facility.

If the swirl-inducing vanes are demonstrated to be effective in a real-world military aircraft engine, advanced fighters will operate with significantly reduced noise at takeoff and landing. With civil supersonic flight on the horizon, the same noise mitigation idea may be also applicable. The scientific community benefits from a novel noise mitigation tool that may be used as an element of a smart aircraft engine component.