Mixing mechanisms in an axisymmetric lobed mixer

Due to steadily-increasing pressure from the public and from air-traffic regulators, aircraft and engine manufacturers are aggressively researching methods for reducing the level of noise of aircraft during landing and take-off. Generally, exhaust noise reduction is achieved by mixing the high-speed exhaust stream with the lower-speed bypass stream exiting the engine. A lobed mixer enhances this mixing (thereby reducing the noise) by shaping the circular exhaust nozzle with axisymmetric lobed protuberances, referred to as a “lobed mixer”. Through a series of high-fidelity computational simulations and experiments, we identify the instabilities present in the flow and how their mutual interaction enhances the mixing process. The results are novel and represent a significant step forwards in understanding the fundamental processes occurring in a lobed mixer.

If swirl is left in the flow stream exiting the core of the engine, the swirling flow conditions may alter the mixing mechanisms of the lobed mixer. Fig. 1 illustrates swirling inflow conditions (Case Sw-Lam) results in the separation of the core flow from one side of the lobe wall. The instability and roll-up of the resulting separated shear layer has a strong impact on the mixing of the core and bypass flows downstream of the lobed-mixer discharge plane.

Fig. 1

The roll-up of the separated shear layer in the swirling-inflow case is shown in Fig. 2 through instantaneous iso-contours of the static pressure fluctuations, which also shows that the rolled-up vortices form Lambda-shaped vortices. These Lambda-shaped vortices strongly accelerate the mixing of the core and bypass streams.

Fig. 2

The interaction of the coherent vortical structures downstream of the discharge plane of the lobed mixer is shown in Fig. 3 through instantaneous iso-contours of the vorticity magnitude. The Lambda-shaped vortices in the swirling-inflow case induce motions that enhance the mixing of the core and bypass streams. The earlier break-down to small-scale vortices qualitatively illustrates the enhanced mixing.

Fig. 3