Impact of Shear Layer Swirl on Near- and Far-field Noise Emissions from Twin-Engine Military Aircraft

Z.J. Wang | University of Kansas Center for Research, Inc.



The environmental impact of aviation is measured in emissions and noise. 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. Within DoD, the effect is even bigger since there are servicemen working in close proximity to advanced supersonic jets in takeoff and landing. DoD identifies the noise of the legacy engines, such as FA-18 and EA-18G, as both an environmental and operational concern. The project team identified a novel and powerful means of mitigating jet noise by inducing shear layer swirl through embedded vanes near the nozzle exit lip. To minimize the swirl vane impact on thrust, the project plans to use an actuation mechanism to reset the vanes to neutral position at cruise. This proof-of-concept study aims to demonstrate the feasibility of the concept through both high-order computational simulations and experimental investigations.

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Technical Approach

The role of swirl in free turbulent jets is to trigger centrifugal instability and set up a radial pressure gradient, thus promoting mixing by both means. However, large-scale swirl in the exhaust jet creates a thrust penalty. Therefore, the project will demonstrate limited swirl induction in the nozzle inner and outer shear layers. This targeted approach will inject the benefits of centrifugal instability waves in the nozzle shear layers without incurring significant loss of thrust. By necessity, the swirl vane height, h, is of the same order of magnitude as the boundary layer thickness, δ. Thus, embedded swirl vanes with h/δ ~ O(1) is targeted. The dynamics of the mixing layer at the boundary of the inner and outer nozzle flow is affected by the centrifugal instability of the inner and outer shear layer. In the initial proof-of-concept (POC) phase, the inner shear layer swirl that promotes a first-order effect on the mixing layer characteristics, e.g., entrainment E(x), will be studied. This approach will use:

  • a computational simulation track that involves high-order Computational Fluid Dynamics (CFD) and Computational AeroAcoustics (CAA) and
  • an experimental simulation track that involves fundamental flow physics research in a water tunnel and a free-jet facility for jet noise/acoustic measurement.

Four nozzle configurations will be studied both computationally and experimentally:

  • subsonic baseline nozzle
  • subsonic nozzle with embedded swirl vanes
  • supersonic baseline nozzle
  • supersonic nozzle with embedded swirl vanes

The project will make extensive comparisons of near and far field noise levels and the flow physics explored in the water tunnel to study the feasibility of the concept and answer fundamental questions regarding centrifugal instability and supersonic jet noise.

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This novel approach to noise mitigation, through induced swirl in the shear layer, alters the turbulence structure in the mixing layer and the broadband shock-associated noise (BBSAN). Advanced fighters could operate with reduced noise at takeoff and landing, when swirl vanes are activated. The scientific community will benefit from a novel noise mitigation tool that may be used as an element of a smart aircraft engine component.

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Points of Contact

Principal Investigator

Dr. Z.J. Wang

University of Kansas Center for Research, Inc.

Phone: 785-864-2440