Constructing Embedded Boundaries in MFIX-Exa
MFIX-Exa allows 4 approaches to specify embedded boundaries (EBs).
Using native AMReX cpp functions.
Using CSG file format.
Using planar walls.
Using STL file format.
There are also two parameters (specified in the inputs
file) that influence
the level-set creation:
Parameter |
Description |
---|---|
|
If greater than 0, MFIX-Exa operates in multi-level mode. The level-set grids follow all other grids. If equal to 1, the level-set has two levels (one additional level with higher refinement). |
|
If |
EB Specification
Native AMReX functions
MFIX-Exa uses AMReX’s constructive solid geometry framework defined in the namespace
amrex::EB2
. See the AMReX EB documentation for more details. These are
defined in src/eb/mfix_eb.cpp
. The function mfix::make_eb_geometry
(also defined in src/eb/mfix_eb.cpp
) selects one of the following
geometries depending on the value of the mfix.geometry
setting in the
inputs
file.
Description |
|
---|---|
Box (up to six walls) |
|
Cylinder |
|
Hopper |
|
General |
|
Generic
geometry specification requires user implementation insidesrc/eb.mfix_eb_generic.cpp
. Also see the AMReX geometry documentation for information on how to construct new geometries using the method of constructive solid geometry.
CSG File Format
Alternatively, the constructive solid geometry can be created using
OpenSCAD’s CSG file format by installing the csg-eb
library.
To use this option, MFIX-Exa must be built with the flag MFIX_CSG=TRUE
and the options mfix.geom_type = csg
and mfix.geometry_filename
must be set in the inputs
. See the CSG-EB repository for more
details about this format.
Planar Walls
Planar boundary conditions can be specified in the inputs
file. Even if the user does not specify an mfix.geometry
or
mfix.geometry_filename
in the inputs
,
any no-slip or free-slip boundary conditions are expressed as EB walls.
STL File Format
A standard triangle language (STL) file created using numerous
CAD programs can also specify the EB geometry. To use this method,
the options mfix.geom_type = stl
and mfix.geometry_filename
must be set in the inputs
.
How MFIX-Exa Constructs the EB Geometry
Once a geometry is specified in the inputs
file, the procedure is
the same for all geometries.
Construct an implicit function representing the geometry.
Call
mfix::build_eb_levels(gshop)
this function builds the EB levels. Note that this also makes the particle EB levels point to the fluid eb levels.Call
mfix.fill_eb_levelsets
. This function fills the level-setMultiFab
.
MFIX-Exa’s EB Data Structures
The mfix
class stores the following EB data:
//! EB levels representing fluid boundary conditions
Vector<const EB2::Level *> eb_levels;
//! EB levels representing particle boundary conditions (same as
//! `mfix::eb_levels` but might include additional walls).
Vector<const EB2::Level *> particle_eb_levels;
//! EB factory that lives on the fluid grids
Vector< std::unique_ptr<amrex::EBFArrayBoxFactory> > ebfactory;
//! EB factory that lives on the particle grids
Vector< std::unique_ptr<amrex::EBFArrayBoxFactory> > particle_ebfactory;
As discussed in the previous sub-section, the difference between
mfix::eb_levels
and mfix::particle_eb_levels
enables the user to
specify a modified EB geometry for particles only. Whereas the fluid sees the EB
geometry in mfix::eb_levels
. If no addition particle EB geometry is
specified (point 4 in the previous section), then
mfix::particle_eb_levels
points to mfix::eb_levels
.
In the same spirit, the mfix::ebfactory
is constructed over the fluid
grid and using the fluid EB levels, whereas mfix::particle_ebfactory
is
constructed over the particle grid using the particle EB levels.
A note about constructing EB Levels
MFIX-Exa builds EB levels in mfix::build_eb_levels
(via
LSCore<F>::BuildEBLevel
)
EB2::Build(gshop, geom[lev], required_crse_lev, max_crse_level);
const EB2::IndexSpace & ebis = EB2::IndexSpace::top();
When building an EB level, the maximum coarsening level (int
max_crse_level
) and the required coarsening level (int
required_crse_lev
) need to be specified. The reason for this is that we need to
specify to which level of coarseness the EB is still defined. It might not be
immediately obvious, but the Poisson solver (used in the fluid solve) also
depends indirectly on these parameters. Thus changing these during EB level
creation might restrict how many levels the MLMG solver can use, and therefore
give slightly different answers in the fluid solve.
Local Mesh Refinement at Walls
MFIX-Exa has the capability of locally refining the computational grid near EBs.
This is done by tagging (in mfix::ErrorEst
) any cells with volume
fraction between 0 and 1. To enable local mesh refinement, set amr.max_level
to a value greater than 1. Note that the parameter mfix.levelset__refinement
is ignored on all cases except when amr.max_level = 1
.
MFIX-Exa Initialization Process
Since MFIX-Exa requires the volume fraction when building grids (because this is
needed by mfix::ErrorEst
), the EB geometries need to be built before
calling mfix::Init
. The recommended procedure therefore is
// Default constructor (geom[lev] is defined here)
mfix my_mfix;
// Initialize internals from ParamParse database
my_mfix.InitParams(solve_fluid, solve_dem, call_udf);
// Initialize memory for data-array internals
my_mfix.ResizeArrays();
// Construct EB (must be done _before_ mfix::Init)
my_mfix.make_eb_geometry();
// Initialize derived internals. Grids are create here.
my_mfix.Init(dt, time);
// Create EB factories on new grids
my_mfix.make_eb_factories();
if (solve_dem)
{
// Fill level-sets on each level (must be done _after_ mfix::Init)
my_mfix.fill_eb_levelsets();
}
// Finish constructing levels
my_mfix.InitLevelData(dt,time);
// Regrid (ensure all MultiFabs are on their correct grids)
my_mfix.Regrid();
Also note that mfix defines boundary conditions in Fortran also (via the
mfix.dat). Since these are potentially needed to build EB walls,
mfix::make_eb_geometry
also calls mfix_set_bc_type
.
The grids for each level are build in the mfix::Init
by invoking the
initialization functions inherited from amrex::AmrCore
.
// This tells the AmrMesh class not to iterate when creating the initial
// grid hierarchy
SetIterateToFalse();
// This tells the Cluster routine to use the new chopping routine which
// rejects cuts if they don't improve the efficiency
SetUseNewChop();
// This Builds the new Grids
InitFromScratch(0.);
The Level-Set Function
MFIX-Exa uses a level-set function to resolve particle-wall collisions. See the
AMReX Level-Set documentation for more details. The level-set function is
stored on the nodal Vector<std::unique_ptr<MultiFab>> mfix::level_sets
.
The level-set data is always stored on the particle grids. Depending on the
input amr.max_level
The level-set can be in one of two modes:
MFIX-Exa is running in single-level mode (
nlev == 1
). Thenmfix::level_sets[0]
will be at the same resolution as the fluid (except that it is stored on the particle grid). Even thoughnlev == 1
, there is a second level,level_sets[1]
. This level is the same aslevel_sets[0]
but refined bymfix::levelset__refinement
. This way the level-set always has the appropriate resolution to resolve structures in the EB, even if the fluid is defined on a fairly coarse grid.MFIX-Exa is running in multi-level mode (
nlev > 1
). The the parametermfix::levelset__refinement
is ignored.mfix::level_sets
then follows the rest of MFIX-Exa, i.e. it is defined on the particle grids on all levels.
The level-set is used in two places:
The function
MFIXParticleContainer::EvolveParticles
interpolates the level-set onto each particle’s position in order to resolve collisions with the EBs. Ifnlev == 1
,level_sets[1]
is used to evolve the particle positions. Otherwiselevel_sets[lev]
is used for each level.The fluid-particle coupling can sometimes rely on neighbor stencils where one or more cell is covered by an EB. In order to avoid values that do not conform with the boundary conditions, the fluid velocity is reconstructed in those cells. The algorithm relies on the level-set, and uses
level_sets[lev]
on each level.
Special Cases Involving Level-Sets
The level-set function is filled by the mfix::fill_eb_levelsets()
function.
There are two special cases involving level-sets:
Mass-Inflow boundary conditions are not given EB walls. However, we don’t want particles to fall out of a MI either, so at the very end of the
mfix::fill_eb_levelsets()
function we callmfix::intersect_ls_walls()
. This performs an intersection operation with the level-set representing a wall at each MI.Box geometries and regular geometries are comprised entirely out of planar surfaces. Therefore the levelset is not construction out of an EB factory (as would be the case for all other geometries). But out of an intersection with all planar surfaces. This has the advantage of correctly describing corners.