We examine the effect of baseline nozzle shape on the ability of fan flow deflectors to reduce downward-emitted turbulent mixing noise of coaxial jets simulating the exhaust condition of a turbofan engine. Several deflector concepts were tested on a nozzle with parallel exit flow lines and a nozzle with convergent flow lines. In every comparison the nozzle with convergent flow lines produced superior acoustic benefit. Mean flow measurements indicate that the convergent lines help the deflectors reduce velocity gradients uniformly on the underside of the jet. In contrast, the parallel flow lines contributed to reduction in gradients in a narrow direction, with occasional increase of gradients in other directions that influence emission towards the ground.
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