The effect of circumferential inflow swirl on the instability of the shear layer formed between the core and bypass flows discharged from an axisymmetric twelve-lobed mixer is studied through a combined experimental and computational investigation. A series of unsteady Navier-Stokes simulations are performed with 0 and 31 degrees of circumferential swirl specified in the core stream of the lobed mixer. Comparison of the axial- and swirling-inflow cases highlights the effect of swirl on the instability-driven transient flow structures that develop within and downstream of the lobed mixer. Medium- and large-scale unsteady motions are captured by the fine spatial and temporal resolution of the unsteady Navier-Stokes simulations. The simulations are validated against four-wire thermal anemometry measurements in a scaled lobed-mixer wind-tunnel model with turbulent, swirling inflow conditions. The simulation results illustrate that while the axial-inflow case develops layers of streamwise vorticity uniformly along the lobe walls, the core flow in the swirling-inflow case separates from the suction side of the lobe wall near the lobe trough. Roll-up and axial stretching of the separated flow produces Λ-shaped vortical structures upstream of the discharge plane. The Λ-shaped structures interact with the shear layers discharged from the lobe trailing edge and accelerate the breakdown of the shear layer in the swirling-inflow case relative to the axial-inflow case. The extent of this interaction is shown to strongly affect the streamwise mixing rate of the flow downstream of the discharge plane.

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