Flake alignment in the secondary wave in a wide spherical Couette flow

In spherical Couette flow between rotating inner and fixed outer concentric spheres, the radius ratio determines the laminar-turbulent transition scenario. For small radius ratio, the transition triggereed by cross-flow instability around mid-latitude generates secondary wave with spiral vortices.

In the present study, we performed the experimental flow visualization for the secondary wave state in the case of half radius ratio, r_in/r_out=1/2. Small aluminium flakes suspended in working fluid, which are likely to aligned by coherent structures in the secondary wave, scattered a laser light sheet on the meridian cross section.

We numerically solved the algebraic equation for the exact rotating wave state of the Navier-Stokes equation, and investigated the time variation of the trajectory and alignment of a suspended small flake in the solved rotating wave state.  The experimentally recorded image is compared to the flake alignment reconstructed numerically.