In aftertreatment simulations the cell selections for the catalyst, the reactive porosity (RPOR) and the particulate filter are defined internally as porosity blocks. It is no longer necessary (since FIRE v8.5) to activate the Porosity module in the GUI. Nevertheless, some mesh requirements have to be taken into account:
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4.1.8.1.1. Mesh Requirements for Catalyst and Particulate Filter
The flow through a catalyst or a particulate filter is determined by the channel shaped structure of the monolith. Thus, the monolith is modeled as directed porosity for which the following mesh requirements have to be taken into account:
• Arbitrary interfaces are not allowed within the porosity (catalyst or particulate filter) blocks.
• The distance between arbitrary interfaces and the porosity/fluid interfaces must be at least two cell layers (see the following figure).
• The catalyst or the particulate filter block must be a structured, direction-aligned grid.
Porosity/fluid interfaces must be plane and normal to the porosity direction.
Figure 49. Mesh Requirements for Catalysts and Particulate Filters
4.1.8.1.2. Mesh Requirements for Reactive Porosities
In general there is no preferential flow direction in reactive porosities. Thus, they are modeled by undirected porosities for which the Mesh Requirements can be set to Yes (=fulfilled) or No (=not fulfilled). For not fulfilled mesh requirements no special treatment is necessary. For fulfilled mesh requirements the following conditions must be taken into account:
• The distance between arbitrary interfaces and porosity/fluid interfaces must be at least two cell layers.
• The face selection determining the porosity/fluid interface must be a smooth surface without protruded cells.
However, it is recommended to create computational grids with fulfilled mesh requirements whenever possible.
4.1.8.2. MPI Decomposition
4.1.8.2.1. MPI Decomposition for Catalyst
In general the computational effort for catalyst cells is higher than that for ordinary fluid cells. In addition to the transport equations, the chemical reactions and the solid temperature equation have to be solved for the porous cells of the catalyst. To achieve a good load balance in MPI simulations, FIRE offers the capability of weighted MPI decomposition. This means that if the user specifies a cell selection with the name
<prefix>"_decomp_weight_"<value>,
then instead of the ordinary cell count, the cell count for the generated MPI domains is
determined by the factor specified at <value>. The weight must be an integer greater than 0. For all cells outside such cell selections, a weight of 1 is assigned.
For example, if there is a cell selection specified with the name "CAT01_decomp_weight_3", during the MPI decomposition the cells contained by this selection have the weight 3, while the cells outside of this selection have the weight 1.
For catalysts it is recommended to apply the weighting to all cells of the porous block. The quantity of the weighting factor depends on the setup of the simulated case, i.e. how many chemical reactions are active, how often the reaction solver is called (see Implicit solution of chem. kinetics in section Reaction Solver Parameters page [138]
), etc. For many cases a weighting factor of 3 seems to be a good choice.
Note:
Any "_indivisible" or "_domain_"-selections and arbitrary-interface-cell-layers are processed afterwards.
4.1.8.2.2. MPI Decomposition for Particulate Filter
Contrary to the catalyst selection the MPI decomposition of PF blocks is not arbitrary. The MPI interface must be aligned along the porosity block direction. This means that the PF cell rows in porosity direction (representing a certain number of PF channels) must not be located on different domains.
Note:
The MPI decomposition of PF blocks is not arbitrary. To avoid PF channels splitting onto different domains, the decomposition must be topologically normal to the front surface of the PF.
By specification of "_domain_"-selections one can influence the MPI decomposition. The following figure shows the specification of the selections for the domain decomposition of a simulation with 4 CPUs.
Figure 50. Example of Manual Domain Decomposition of a PF for 4 CPUs
If the cell selections are specified in that way, PF channels are not distributed onto more than one domain and the decomposition requirement for PF is fulfilled. The names of the selections are composed by the selection name plus the domain extensions "_domain_X" starting at index zero (e.g. mesh_domain_0, mesh_domain_1, mesh_domain_2, mesh_domain_3,..). This method is sophisticated and leads to an excellent load balance, since the user specifies exactly on which domain which PF channel row is calculated.
However, the creation the cell selections may be circuitous, and if one changes the number of processors of the simulation, one has to create new selections. Therefore, FIRE offers a more convenient way for the domain decomposition. If face-selections with the names
<prefix>"_decomp_struct_front_"<value> and <prefix>"_decomp_struct_end"
are found, then they are considered as the front and back sides of a structured cell-block, where the decomposition is performed only topologically normal to the front surface. If there is no "_decomp_struct_end" face selection, the structured block ends at the mesh boundary or as soon as a non-sweepable cell is encountered, the suffix <value> specifies the cell count weighting factor, similar to that already described in section MPI Decomposition for Catalyst page [176]
for the catalysts. The specified weight is assigned to the cells in the structured block. If no specific weight will be assigned, the suffix <value> can be omitted. The prefix may be any name, but any "decomp_struct_end"-selection must have the same prefix as the corresponding "decomp_struct_front"-selection. An arbitrary number of such
"decomp_weight"- or "decomp_struct_front"-selections may exist, but the structured cell-blocks must not overlap. Also here, any "_indivisible" or "_domain_"-selections and arbitrary-interface-cell-layers are processed afterwards.
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models are active (Depth filtration), etc. For many cases a weighting factor of 2 seems to be a good choice.
The following figure shows an example of the face selections applied for weighted decomposition. The prefix "DEC_DPF_0" and the suffix "2" determine the names of the front and backside selections "DEC_DPF_0_decomp_struct_front_2" and
"DEC_DPF_0_decomp_struct_end". For correct decomposition, these selections are located one cell layer before and after the PF block.
Figure 51. Face Selections for Weighted MPI Decomposition of a PF