Solution Adaptive Meshing In Simcenter FLOEFD

Adaptive mesh refinement (AMR), also called Solution-adaptive meshing in Simcenter FLOEFD, is a feature where the solver splits mesh cells based on where high gradients are located. High gradients will occur if cell size is large and/or properties in the flow field changes rapidly, like around and behind a blunt object for example. This week at the Volupe blog, we are therefore describing the concept of AMR in Simcenter FLOEFD and how to set up a simulation using this handy feature.

Starting off using a coarse global mesh you can utilize the advantage of AMR where the solver will find zones with high gradients in the flow field and then refine cells automatically. This means that remeshing is needed, but since you will obtain an approximate solution quickly (using the coarser mesh) and then only get finer cells where needed, computational resources may be saved. Refinements of the cells can be performed in several sequences.

Settings for AMR in Simcenter FLOEFD

In the Calculation control options window, at the Refinement tab (see picture below), you find options for AMR settings. First, make sure that Refinements in the Finishing tab is enabled, so that the refinement numbers are used from the table of refinements. There are additional ways to set the method of refinement, for example manual input, but using a table will let the remeshing occur in a more intuitive and clear order.

The example we will follow in this blog post uses global mesh setting of level 3, which will be specified in the Global domain parameter in the Calculation control option (see picture above). Using the refinement strategy of tables, you can add refinement levels in a pop-up window (see the pop-up window to the right in the picture above) by clicking on the button with three dots (in the Calculation control option window). In the pop-up window you edit rows and specify numbers for which level of refinement you want to fulfill when the gradients become too high in the solution. In this example, the refinement will happen in four steps, adding one level of refinement each time, from level four to level seven.

Example setup before AMR is active

In the example we will follow in this blog post we have flow from right to left. The initial global mesh is coarse, as seen in the picture below, and the cylinder curvature is not resolved yet (the cylinders are not becoming completely round using this level of refinement).

The flow field in the first iterations is smeared out and providing a rough estimation of the solution, where the cell count only goes up to around 4’500 cells.

Activation of AMR

The first 100 iterations run quickly, this is since the mesh is still coarse, and therefore unacceptable high gradients between adjecent cells are still forming. AMR gets activated due to the high gradients and remeshing is triggered. The new cell count reaches around 28’000 cells. Resolution of the flow field is improved significantly, and the curvature of both cylinders (especially the smaller cylinder) becomes more accurate.

Looking at the mesh, you see that at the inlet cells (to the right) are still large since the flow field (velocity, and also pressure) is of the same value in this part of the computational domain. Finer cells are used in front and behind the cylinders, and the cells close to the cylinder walls are even more refined since the flow have both high and low values in these locations.

Improving the resolution further

Remeshing will continue to the next level of refinement, level 6 (indicated by 3 levels up from the Global domain setting of level 3). When looking at the mesh we now see fine cells in the wake region together with very fine cells close to the cylinders.

Gradients are still high even though the resolution of curvatures and flow field features start to become quite fine. The results below are from using around 150’000 cells.

Results with accurate resolution

When the AMR reaches the 7th level of mesh cells, refinement level of 4 in addition to Global setting of 3, the mesh cells become very fine close to the cylinders. The resulting mesh adapts to the solution, providing a resolution that varies depending on where fluctuations in the flow field are located in that specific time step.

Looking at the solution we now see fluctuations and non-symmetric flow around the cylinders. The cell count of the mesh is now around 750’000. Continuing to calculate will change the solution but not the mesh, so this is the final mesh, since the AMR is now complete and reached maximum level of refinement.

We at Volupe hope that this blog post on Adaptive mesh refinement has been useful for you as a user. AMR is available within other software as well, but the ease of use in Simcenter FLOEFD is something worth remembering. Meshing in general can be a time-consuming process, but in Simcenter FLOEFD this procedure is set up to be a user-friendly experience with accurate results. Using AMR, the optimal meshing procedure may be just a few clicks away when you set up your simulation. If you have any questions, please reach out to us at support@volupe.com.