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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 [ref1]. In this tutorial, we provide the Orr-Sommerfeld mode as an initial condition to the flow.

In the presence of particles, the vorticies 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 [ref2]. This effect is known as preferential concentration and is illustrated in the following sketch:

Figure 2. Preferential concentration mechanism: particles gather along stretching regions and get expelled from vortical regions.

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