Meshing is always a compromise between computational efficiency and the needed accuracy. We need the smallest possible mesh to capture the effects that we consider as the most important in our simulations. Oftentimes, this means defeaturing the geometry, choosing to ignore specific elements that will make the solver’s work hard or choosing to ignore elements that will make our life easier when meshing.
One such element that is often disregarded when creating periodic meshes for turbomachinery, mostly because the solver cannot handle it but also, by extension, the associated mesh tools cannot represent properly, is the bulb. Bulbs are ubiquitous in all kinds of applications. A classic example is a marine propeller which normally features a bulb downstream of the blades (see Fig1 above)
Bulbs are hard to work with because they can be modelled in ways which prove to be troublesome for most solvers. Either, we have to create a singular line where elements close to the bulb connect, or a butterfly topology where we have a series of cells that belong to both periodic faces of the mesh as shown in figure 2.
Figure 2. Different bulb topologies: Butterfly topology (left), Singular line topology (right)
OMNIS™/Turbo, NUMECA’s turbomachinery solver, and OMNIS™/AutoGrid, NUMECA’s turbomachinery mesh generator, have been developed from the beginning to take into account a geometry with zero radius. OMNIS™/AutoGrid has the capability to apply a bulb topology automatically when it detects that the edge of the hub is at zero radius. OMNIS™/AutoGrid then chooses which specific bulb topology to apply based on the geometry features, sharp, round or radial.
Figure 3. Different bulb topologies: Sharp topology (left), Rounded topology (middle), Radial Topology (right)
The sharp topology can be modelled only by a singular line while the round and radial topologies can be modelled either with a singular line or a butterfly topology. The butterfly topology in most cases results in higher mesh quality and it is the default.
There are more controls that help the user get a very good mesh while keeping all the features of the bulbs properly captured. The mesh can be then used for simulations without any compromise on the features of the geometry!
Figure 4. Flow solution of a propeller, showing the tip and inner vortices