On the large scale evolution of rotating turbulence

For a number of years the fundamental processes behind rotating turbulence have been debated. Whilst the Coriolis force appears as a linear term in the Navier-Stokes equation, many believe that the columnar eddies, which are so evident in experiments and simulations, are created by non-linear mechanisms. However, new findings have recently re-established the importance of linear processes, suggesting that, under typical laboratory situations, linear inertial waves play an important role in the formation of columnar eddies. These findings are both analytical and experimental. Analytical work, conducted in the limit Ro -> 0, shows that an initially compact eddy evolves into two separate elongated structures, which propagate along the rotation axis. This change in the morphology of the eddy is achieved through inertial wave propagation, a prediction that has now been confirmed experimentally. Our laboratory experiment consisted of a single grid oscillation in the bottom of a rotating tank of water. This created a cloud of turbulence from which elongated columnar structures emerged. These propagate linearly with time into the quiescent region above. The distance travelled by these structures was then tracked over a sequence of images, and the speed of propagation is found to be proportional to both the speed of rotation and the bar size of the grid, which is consistent with structure formation due to linear wave propagation.