Laboratory experiments on continually forced 2D turbulence

Mathew Wells
Eindhoven University of Technology, Holland

We present new observations from laboratory and numerical experiments of continuously forced quasi-2D turbulence. The laboratory experiments were performed in a square Perspex tank filled with water. The flow is made quasi-2D by a steady background rotation. The rapid rotation rate of the tank has a small (< 8%) sinusoidal perturbation, which leads to the periodic formation of eddies in the corners of the tank. When the oscillation period is greater than an eddy roll-up time-scale, dipole structures are observed to form. These dipoles migrate away from the walls, and the interior of the tank is continually filled with new vortices. The effects of Ekman pumping and stripping of vorticity by background shear, implies that once formed the maximum vorticity decays with time. Subsequent interactions between the weaker 'old' eddies with strong 'young' eddies results in straining and their destruction, rather than decaying to form larger eddy scales. This means that the length-scale of the vortices appears to be largely controlled by the initial formation mechanism and that large-scale structures are not observed to form at large times.