Self-organisation of decaying quasi-2D turbulence in stratified fluids in rectangular containers

Laboratory experiments on decaying quasi-two-dimensional turbulence have been performed in stratified fluid inside containers with length-to-width ratios delta up to 5. The Reynolds number Re of the horizontal flow, based on the r.m.s. velocity of the initial flow field and the half-width H of the container, was typically between 1000 and 3000. The turbulence was generated by towing an array of vertical cylinders through the container which was filled with a two-layer stratified fluid. By varying the grid configuration a different amount of angular momentum could be added to the initial flow. The evolution of the flow was visualized by two-dimensional particle tracking velocimetry. The observed decay has been investigated with the emphasis on the final states as function of delta, Re and the angular momentum initially added to the flow. In addition, numerical simulations were carried out for decaying two-dimensional turbulence on rectangular domains with delta = 2 and 3. In these runs zero net angular momentum was added to the initial flow field. The numerical study focused on the final states as a function of delta and Re. The numerically obtained final states appeared to agree with the experimental observations. Furthermore, they indicate a clear difference between the predictions of quasi-stationary final states from statistical-mechanical theories and the final states as found in the numerical simulations.