We believe that without local grid refinement we might have to use level 5 or higher to produce nearly the same Nusselt numbers as the one produced by Le Quéré. This information shows us the expected result that local grid refinement helps a lot for this test configuration. The time step is not an issue as long as we put enough time steps over one period. 20 up to 40 time steps are already sufficient to produce excellent results for this problem, and no specific gain/loss in the quality of the Nusselt number has been observed if we increase/decrease the number of time steps in one period (see  ). Summarizing, we observe differences from the reference result with % for velocity and with % for temperature ; and we are very close with % difference for the Nusselt number.