Inertial flow around obstacles in microchannels

Formation of recirculating wakes is a prominent feature of inertial flow around bluff bodies. Below the onset of vortex shedding in uniform unbounded flows, the fluid in the recirculating wake region moves on closed planar orbits. The steady wake is thus an isolated zone in the flow and does not exchange fluid with the free stream. In this work, we utilize lattice-Boltzmann simulations and microfluidic experiments to demonstrate that in microchannel inertial flow of Newtonian fluids, the recirculating wake is replaced by a three-dimensional vortical flow. Spiraling streamlines generate a continuous exchange of fluid between the vortex behind the obstacle and the free stream. The flow inertia is represented by Reynolds number defined as $Re = \frac{u_{max}D_{y}}{\nu}$, where $u_{max}$ is the maximum fluid velocity in the channel inlet, $D_{y}$ is the characteristic obstacle length and $\nu$ is the fluid kinematic viscosity. We discuss the effects of $Re$, the obstacle shape and the wall confinement on the fluid entry into the vortex. Further, we demonstrate that in flow of a dilute suspension of particles around the obstacle, the fluid entry into the vortex can result in entrapment of particles as well.