Mixing at low Reynolds with NEKTAR++

Can hydrodynamic instability be used for mixing? And at the same time can conduits be shaped such that hydraulic losses are reduced? We think that it is Yes to both.

We used NEKTAR++ to quantify mixing rates obtained for unstable flows through longitudinally grooved channel. This configuration is interesting because, aside from resulting in hydrodynamic instability it also allows for a significant reduction of hydraulic drag (You can find out more here). The unstable flow is shown in the colourful video below. You can see periodic action of vortices travelling downstream as well as turning of the velocity field:

Topology of the unstable flow. Streamwise velocity is on the right and helicity is on the left. The flow meanders from left to right and at the same time vortical structures travel downstream.

The left to right motion of the flow adds stirring to the flow. We measured rates at which concentration of a passive scalar homogenizes under this flow (You can find out more here). Video below shows evolution of the scalar interface surface and changes in the concentration of the scalar (the colourful plane). Note that the interface surface flexes and grows with regions of high concentration gradient reaching a greater portion of the channel signalling improvements in diffusive transport.

Colour shows scalar concentration and transparent surface the interface as it evolves under the flow.