The goal here is to set up an unsteady test case, that is, one for which we will pay attention to the results obtained during the transient. These must be correct, since here we are not only interested in what happens once the steady regime is established. We will seek to understand and analyze the results obtained during the establishment and the evolution of the physical phenomena.

To do so, we will start again from the stationary case with thermaleffects guide, transforming the square cavity into a rectangular cavity. The vertical enlargement of the cavity makes other counter-rotating vortices appear. These are not stationary and move constantly, which does not allow a stationary solution to this problem to be found.

Let us go over the construction of the dataset point by point:

The mesh: nothing changes, apart from an increase in the number of cells along Y.
The physics solved: we ask ourselves the same questions as for the stationary case with thermal effects guide: can we use the Boussinesq approximation? No, so we solve a quasi-compressible problem using the keyword Pb_Thermohydraulique_QC.
The fluid properties: these are defined in the same way as for the stationary case with thermal effects guide.
The time scheme: this must be changed, because it is no longer possible to work with an implicit scheme. We then switch to explicit schemes. Since the motions are incessant, it is necessary to use schemes of high order in time. All the schemes must have the same order (time scheme / diffusion scheme / convection scheme); otherwise, too strong a simplification is made on one of the terms, disturbing all of the results. The pressure gradient must have one order less than the others, so it is recommended to use a second-order scheme, and more particularly the second-order Runge-Kutta scheme.