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A mixing layer is a flow where two parallel streams flow at different velocities, resulting in a non-zero velocity gradient. In the absence of any perturbation, a mixing layer diffuses under the action of viscosity to the mean value of top and bottom velocities. However, in a real life situation, the flow is subject to random perturbations, which causes it to destabilize and forms vorticies. The most potent perturbation can be computed using the Orr-Sommerfeld equation [ref11]. In this tutorial, we provide the Orr-Sommerfeld mode as an initial condition to the flow.

In the presence of particles, the vorticies that result from the unstable perturbations offer interesting interactions with the suspended particles. In a general sense, "light" particles get trapped in the flow vorticies, while "heavier" particles carrying more inertia might get expelled under a centrifugal-type force from the swirling regions to gather along stretching regions of the flow [ref22]. This effect is known as preferential concentration and is illustrated in the following sketch:

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