[atul1018]

Dear Community

Let me describe the case briefly: I am modelling a hydroshaft (small
scale turbine system). In this case, a trash rack is included as porous media (instead of actual trashrack with bars) and a simplified fan-like structure to represent the turbine. I am using inlet to maintain fixed water height and outlet as discharge bc. The case is considered as laminar to avoid errors and complications related to turbulence. I have first modelled the hydroshaft without porous media and runner. Only after confirming that the simulation runs without any problem, I added porous media via fvOptions, and it also runs well. I have to accept that I am using topoSet to create a cellZone to include the trashrack as porous media (instead of doing it in snappyHexMesh), which doesnʼt look perfect (a bit of uneven surface and undulations), but so far, it works at least. The reason for doing it in topoSet is that it gives strange cellSets and somehow doesnʼt recognize the stl properly in snappyHexMesh. The porous media part is not perfect but it still works.

Once confirming that the case with porous media works (without rotating parts), I started by including a rotating object in my setup. I have a runner geometry (stl file). I include this runner stl in my snappyHexMesh, and refine the edges and surfaces around it. Also, I create a cylindrical cellZone around the runner (which is not perfectly cylinder due to less space available around the runner), and also the final mesh failed in two mesh checks. I can also see that the edges of the runner are not really meshed properly, and only one cell is covering almost the whole edge of each blade in the runner. As mentioned before, the cellZone is also not really cylindrical. Despite these errors and not-so-good mesh, I tried to check if the rotation setup is working or not (runApplication moveDynamicMesh -checkAMI). As expected, the simulation crashes at the beginning. I am more or less sure about the settings related to AMI and boundary conditions, but mesh seems to be the biggest problem for me.

I am trying to include a simplified runner (fan-like structure) into my case for a while but have not been able to successfully run the case. To my knowledge, the mesh seems to be the problem. I am using level 4 refinement around then runner and cellZone. But it doesnʼt create a good mesh, and this probably leads to fail the testing of AMI. I would be grateful and appreciate it if you could get some time to look into my case and give your feedback. I am struggling to make this case run. Please help!

PS: I have attached a picture of the runner and AMIs surrounding it. Also snappyHexMeshDict, and createpatchDict are provided. I can provide the case personally if required. I also know the NCC approach in new OF are more efficient, but I wanted to try it with installed OF and the workflow I have been using so far.

SnappyHexMeshDict:

// Which of the steps to run
castellatedMesh true;
snap true;
addLayers true;


// Geometry. Definition of all surfaces. All surfaces are of class
// searchableSurface.
// Surfaces are used
// - to specify refinement for any mesh cell intersecting it
// - to specify refinement for any mesh cell inside/outside/near
// - to 'snap' the mesh boundary to the surface
geometry
{
domain
{
type triSurfaceMesh;
file "solid.stl";

patchInfo
{
type wall;
}
}

runner
{
type triSurfaceMesh;
file "runner.stl";

patchInfo
{
type wall;
}
}
//porouszone for fvOptions
/*porousTrashrack //the stl file must be closed (closed surface)
{
type triSurfaceMesh;
file "tr_porous.stl";
}

porousTrashrack //stl file above seem to be open, therefore using one single box having dim of stl files
{
type searchableBox;
min (0.269 0.2 -0.08);
max (7.269 4.8 0);
}*/
//cylindrical rotatingZone around the runner
rotatingZone
{
type searchableCylinder;
point1 (6.8 2.52 -8.32);
point2 (7.9 2.52 -8.32);
radius 1.3;
}
};



// Settings for the castellatedMesh generation.
castellatedMeshControls
{

// Refinement parameters
// ~~~~~~~~~~~~~~~~~~~~~

// If local number of cells is >= maxLocalCells on any processor
// switches from from refinement followed by balancing
// (current method) to (weighted) balancing before refinement.
maxLocalCells 100000;

// Overall cell limit (approximately). Refinement will stop immediately
// upon reaching this number so a refinement level might not complete.
// Note that this is the number of cells before removing the part which
// is not 'visible' from the keepPoint. The final number of cells might
// actually be a lot less.
maxGlobalCells 2000000;

// The surface refinement loop might spend lots of iterations refining just a
// few cells. This setting will cause refinement to stop if <= minimumRefine
// are selected for refinement. Note: it will at least do one iteration
// (unless the number of cells to refine is 0)
minRefinementCells 0;

// Number of buffer layers between different levels.
// 1 means normal 2:1 refinement restriction, larger means slower
// refinement.
nCellsBetweenLevels 3;



// Explicit feature edge refinement
// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

// Specifies a level for any cell intersected by its edges.
// This is a featureEdgeMesh, read from constant/triSurface for now.
features
(
{
file "solid.eMesh";
level 1;
}

{
file "runner.eMesh";
level 1;
}
);



// Surface based refinement
// ~~~~~~~~~~~~~~~~~~~~~~~~

// Specifies two levels for every surface. The first is the minimum level,
// every cell intersecting a surface gets refined up to the minimum level.
// The second level is the maximum level. Cells that 'see' multiple
// intersections where the intersections make an
// angle > resolveFeatureAngle get refined up to the maximum level.

refinementSurfaces
{
domain
{
// Surface-wise min and max refinement level
level (1 1);
}

runner
{
// Surface-wise min and max refinement level
level (4 4);
}

rotatingZone //creating cellZone and faceZone from rotatingZone
{
level (4 4);
faceType boundary;
faceZone rotatingZone;
cellZone rotatingZone;
cellZoneInside inside;//mode inside;
}

/*porousTrashrack //creating cellZone and faceZone for porousTrashrack and not assign any boundary (or patch to it)
{
level (2 2);
faceZone porousTrashrack;
cellZone porousTrashrack;
cellZoneInside inside;//mode inside;
}*/

}

resolveFeatureAngle 45;


// Region-wise refinement
// ~~~~~~~~~~~~~~~~~~~~~~

// Specifies refinement level for cells in relation to a surface. One of
// three modes
// - distance. 'levels' specifies per distance to the surface the
// wanted refinement level. The distances need to be specified in
// descending order.
// - inside. 'levels' is only one entry and only the level is used. All
// cells inside the surface get refined up to the level. The surface
// needs to be closed for this to be possible.
// - outside. Same but cells outside.

refinementRegions
{
rotatingZone
{
mode inside;
levels ((1E15 4));
}
}


// Mesh selection
// ~~~~~~~~~~~~~~

// After refinement patches get added for all refinementSurfaces and
// all cells intersecting the surfaces get put into these patches. The
// section reachable from the locationInMesh is kept.
// NOTE: This point should never be on a face, always inside a cell, even
// after refinement.
locationInMesh (0 3 2);


// Whether any faceZones (as specified in the refinementSurfaces)
// are only on the boundary of corresponding cellZones or also allow
// free-standing zone faces. Not used if there are no faceZones.
allowFreeStandingZoneFaces true;
}



// Settings for the snapping.
snapControls
{
//- Number of patch smoothing iterations before finding correspondence
// to surface
nSmoothPatch 3;

//- Relative distance for points to be attracted by surface feature point
// or edge. True distance is this factor times local
// maximum edge length.
// tolerance 4.0;
tolerance 1.0;

//- Number of mesh displacement relaxation iterations.
nSolveIter 100;

//- Maximum number of snapping relaxation iterations. Should stop
// before upon reaching a correct mesh.
nRelaxIter 5;

nFeatureSnapIter 10;
}



// Settings for the layer addition.
addLayersControls
{
// Are the thickness parameters below relative to the undistorted
// size of the refined cell outside layer (true) or absolute sizes (false).
relativeSizes true;

// Per final patch (so not geometry!) the layer information
layers
{
domain
{
nSurfaceLayers 1;

}

runner
{
nSurfaceLayers 1;

}

}

// Expansion factor for layer mesh
expansionRatio 1.5;


// Wanted thickness of final added cell layer. If multiple layers
// is the thickness of the layer furthest away from the wall.
// Relative to undistorted size of cell outside layer.
// See relativeSizes parameter.
finalLayerThickness 0.7;

// Minimum thickness of cell layer. If for any reason layer
// cannot be above minThickness do not add layer.
// See relativeSizes parameter.
minThickness 0.25;

// If points get not extruded do nGrow layers of connected faces that are
// also not grown. This helps convergence of the layer addition process
// close to features.
// Note: changed(corrected) w.r.t 17x! (didn't do anything in 17x)
nGrow 0;


// Advanced settings

// When not to extrude surface. 0 is flat surface, 90 is when two faces
// are perpendicular
featureAngle 60;

// Maximum number of snapping relaxation iterations. Should stop
// before upon reaching a correct mesh.
nRelaxIter 5;

// Number of smoothing iterations of surface normals
nSmoothSurfaceNormals 1;

// Number of smoothing iterations of interior mesh movement direction
nSmoothNormals 3;

// Smooth layer thickness over surface patches
nSmoothThickness 10;

// Stop layer growth on highly warped cells
maxFaceThicknessRatio 0.5;

// Reduce layer growth where ratio thickness to medial
// distance is large
maxThicknessToMedialRatio 0.3;

// Angle used to pick up medial axis points
// Note: changed(corrected) w.r.t 17x! 90 degrees corresponds to 130 in 17x.
minMedianAxisAngle 90;

// Create buffer region for new layer terminations
nBufferCellsNoExtrude 0;


// Overall max number of layer addition iterations. The mesher will exit
// if it reaches this number of iterations; possibly with an illegal
// mesh.
nLayerIter 50;

// Max number of iterations after which relaxed meshQuality controls
// get used. Up to nRelaxIter it uses the settings in meshQualityControls,
// after nRelaxIter it uses the values in meshQualityControls::relaxed.
nRelaxedIter 20;
}



// Generic mesh quality settings. At any undoable phase these determine
// where to undo.
meshQualityControls
{
#include "meshQualityDict"
}


// Advanced

// Flags for optional output
// 0 : only write final meshes
// 1 : write intermediate meshes
// 2 : write volScalarField with cellLevel for postprocessing
// 4 : write current intersections as .obj files
debug 0;


// Merge tolerance. Is fraction of overall bounding box of initial mesh.
// Note: the write tolerance needs to be higher than this.
mergeTolerance 1E-6;

createPatchDict

pointSync false;

// Patches to create.
patches
(
{
// Name of new patch
name draftTubeOutlet;

// Type of new patch
patchInfo
{
type patch;
}

// How to construct: either from 'patches' or 'set'
constructFrom set;

// If constructFrom = set : name of faceSet
set draftTubeOutlet;
}

{
//- Master side patch
name AMI1;
patchInfo
{
type cyclicAMI;
matchTolerance 0.0001;
neighbourPatch AMI2;
transform noOrdering;
}
constructFrom patches;
patches (rotatingZone);
}

{
//- Slave side patch
name AMI2;
patchInfo
{
type cyclicAMI;
matchTolerance 0.0001;
neighbourPatch AMI1;
transform noOrdering;
}
constructFrom patches;
patches (rotatingZone_slave);
}


);

Best Regards
Atul Jaiswal