Published online by Cambridge University Press: 28 December 2018
The flow induced by a disk rotating at the bottom of a cylindrical tank is characterised using numerical techniques – computation of steady solutions or time-averaged two-dimensional and three-dimensional direct simulations – as well as laser-Doppler velocimetry measurements. Axisymmetric steady solutions reveal the structure of the toroidal flow located at the periphery of the central solid body rotation region. When viewed in a meridional plane, this flow cell is found to be bordered by four layers, two at the solid boundaries, one at the free surface and one located at the edge of the central region, which possesses a sinuous shape. The cell intensity and geometry are determined for several fluid-layer aspect ratios; the flow is shown to depend very weakly on Froude number (associated with surface deformation) or on Reynolds number if sufficiently large. The paper then focuses on the high Reynolds number regime for which the flow has become unsteady and three-dimensional while the surface is still almost flat. Direct numerical simulations show that the averaged flow shares many similarities with the above steady axisymmetric solutions. Experimental measurements corroborate most of the numerical results and also allow for the spatio-temporal characterisation of the fluctuations, in particular the azimuthal structure and frequency spectrum. Mean azimuthal velocity profiles obtained in this transitional regime are eventually compared to existing theoretical models.
Movie of the experiment at G = 0.1856, Re = 30000 and Fr = 0.0335: top view of the water layer seeded with Kalliroscope flakes. The red point has been artificially inserted and rotates at the disk angular speed. A m = 3 pattern is clearly identified.
Same parameters as movie 1, viewed in a meridional plane. One can observe that the liquid surface is horizontal and remains flat. The turbulent recirculation region can be easily seen in the radial range r=0.6 to r=1, in fair agreement with the numerical results shown in movie3. The movie has been slowed down by a factor 1.5 compared to the real time experiment.
Temporal evolution of the azimuthal vorticity in a meridional plane, taken from the 3D numerical simulation at G=0.1856, Re=30000 and Fr=0. Time is scaled back to dimensional value and slowed down so that the movie is running at the same speed as in Movie 2.