Flow-Induced Flutter for Enhanced Mixing at Inertial Microscales

Scalar mixing in a channel enhanced by a flag undergoing flow-induced flutter has been investigated using two-dimensional, fully-coupled fluid-structure interaction (FSI) simulations. We find that the addition of a self-oscillating flag in a scalar mixer at inertial-scale Reynolds numbers (50<Re<200) leads to significant enhancement in mixing with a relatively low associated head loss. We examine the sensitivity of the system to both Reynolds and Schmidt numbers to better understand the impact of the flapping on the mixing performance, and to assess the relative importance of advection and diffusion in the mixing process. Large amplitude flutter occurs over the entire range of Reynolds numbers studied and leads to persistent vortices that generate significant cross-stream advection and stretching of the interfaces, across which, diffusive mixing can occur.  Finally, we compare the mixing performance of the flags to that due to vortex shedding from a cylinder; this serves as an analog for more conventional passive mixers in the inertial microfluidic regime. Our simulations show that flutter enhanced mixing offers mixing performance that significantly exceeds that of passive mixers but without the attendant design complexity of active mixers.