Mesh Generation with Mesh Generation with GambitGambit

Read the documentation!Read the documentation!

Fluent has very good on-line Fluent has very good on-line documentation documentation

Gambit is not as goodGambit is not as good

Don’t just read the Don’t just read the documentation but go to documentation but go to

library and get hold of some library and get hold of some of the journal referencesof the journal references

Mesh generationMesh generation

Generating a good mesh is a large Generating a good mesh is a large part of the CFD problempart of the CFD problem

Generating a good quality mesh Generating a good quality mesh can be hard workcan be hard work

What is a satisfactory mesh for a What is a satisfactory mesh for a problem will not automatically be problem will not automatically be so when another model option is so when another model option is enabledenabled

Grid TypesGrid Types

Fluent can solve flow problems in Fluent can solve flow problems in 2d or 3d2d or 3d

The cyclone simulation discussed The cyclone simulation discussed in the introductory lecture is a in the introductory lecture is a typical 3d problemtypical 3d problem

The 2d channel is a typical 2d The 2d channel is a typical 2d problemproblem

2d Grids2d Grids

consume much less consume much less memory and take much memory and take much less time to solveless time to solve

a 2d grid intrinsically a 2d grid intrinsically assumes that there are assumes that there are no velocity gradients in no velocity gradients in the direction normal to the direction normal to the gridthe grid

This assumption is really This assumption is really only valid is the channel only valid is the channel is extremely wide in zis extremely wide in z

u/z =0

z y x

2d Grids2d Grids

Can also be used Can also be used with an axisymmetric with an axisymmetric flow such as swirling flow such as swirling flow in a centrifugeflow in a centrifuge

again the again the assumption of zero assumption of zero velocity gradient velocity gradient normal to grid normal to grid applies, but this is in applies, but this is in the the directiondirection

z y x

2d grids - types2d grids - types

mapped mapped (preferred if it (preferred if it possible to do so)possible to do so)

paved with paved with triangles or quads triangles or quads (unstructured (unstructured solver only)solver only)

2d quad mapped

2d tri paved

2d quad paved

2d grids2d grids

2d grids are drawn on faces in 2d grids are drawn on faces in GambitGambit

2d face grids are used as a 2d face grids are used as a precursor to creating 3d gridsprecursor to creating 3d grids

3d grids3d grids

The grid you will generate for a full The grid you will generate for a full 3d simulation of the flow3d simulation of the flow

obviously consumes more memory obviously consumes more memory and takes longer to solveand takes longer to solve

generated for a volume in Gambitgenerated for a volume in Gambit mapped preferred.mapped preferred. complex 3d grids such as Cooper complex 3d grids such as Cooper

and t-grid are unstructured grids and t-grid are unstructured grids

Structured and Structured and Unstructured SolversUnstructured Solvers

Structured Solver (Fluent 4.5)Structured Solver (Fluent 4.5)– uses i,j,k indexing to locate neighboring cellsuses i,j,k indexing to locate neighboring cells– grid must be structured and based on a topologically grid must be structured and based on a topologically

rectangular block in computational spacerectangular block in computational space– but this doesn’t mean the grid cant be curvedbut this doesn’t mean the grid cant be curved– all grid lines continue from one boundary to its oppositeall grid lines continue from one boundary to its opposite

Unstructured Solver (Fluent 5.0)Unstructured Solver (Fluent 5.0)– uses an internal data structure to link cells and facesuses an internal data structure to link cells and faces– doesn’t force an overall topology on the grid so mesh doesn’t force an overall topology on the grid so mesh

volumes can be any shapevolumes can be any shape– doesn’t use indexing to locate neighboring cellsdoesn’t use indexing to locate neighboring cells– can read a structured grid can read a structured grid

Structured and Structured and Unstructured solversUnstructured solvers

Structured solvers were written first because Structured solvers were written first because they were easy to writethey were easy to write

unstructured solves are more “modern”unstructured solves are more “modern” however Fluent 4.5 (structured) and Fluent 5.0 however Fluent 4.5 (structured) and Fluent 5.0

(unstructured) had different sets of models(unstructured) had different sets of models eg Fluent 5.0 doesn’t have the Eulerian granular eg Fluent 5.0 doesn’t have the Eulerian granular

flow model (4.5 does) but it is much easier to set flow model (4.5 does) but it is much easier to set up 5.0 for moving mesh problems. 5.0 has Large up 5.0 for moving mesh problems. 5.0 has Large Eddy SimulationEddy Simulation

Fluent 6 is supposed to merge the two Fluent 6 is supposed to merge the two approaches togetherapproaches together

Structured GridsStructured Grids Topologically rectangularTopologically rectangular This means that the mesh volume is a This means that the mesh volume is a

quadrilateral in 2d or a hexahedron in 3dquadrilateral in 2d or a hexahedron in 3d Each mesh volume is linked only to its Each mesh volume is linked only to its

immediate neighborsimmediate neighbors But the edges can be mapped around But the edges can be mapped around

curves and mesh volumes don’t have to curves and mesh volumes don’t have to be the same sizebe the same size

Reduces storage and CPU requirementsReduces storage and CPU requirements Solid regions like tank baffles can be Solid regions like tank baffles can be

generated by blanking those mesh points generated by blanking those mesh points which overlap the solid region and making which overlap the solid region and making them “dead zones”. Fiddly but still them “dead zones”. Fiddly but still generates a good gridgenerates a good grid

dead zones waste storage. dead zones waste storage.

Unstructured GridsUnstructured Grids Mesh volumes can be linked to Mesh volumes can be linked to

any other volume in the domainany other volume in the domain And can be any shapeAnd can be any shape less computationally efficient less computationally efficient

than a structured gridthan a structured grid but can still read a structured but can still read a structured

grid topology (often still the grid topology (often still the best)best)

can use non-conformal gridscan use non-conformal grids introduces flexbility but this introduces flexbility but this

flexbility creates problemsflexbility creates problems

Unstructured Grid Unstructured Grid TopologiesTopologies

3d hexahedron

2d Prism

2d Triangle

Tetrahedron Pyramid Prism

Unstructured grids - Unstructured grids - MappedMapped

If you can generate a mapped If you can generate a mapped mesh for the domain then do somesh for the domain then do so

Unstructured grids - TgridUnstructured grids - Tgrid

Uses a 3d paving algorithmUses a 3d paving algorithm uses the tetrahedron and pyramidsuses the tetrahedron and pyramids very easy to generate a grid for very easy to generate a grid for

complex geometrycomplex geometry but is inefficient and has problems but is inefficient and has problems

of numerical diffusion and of numerical diffusion and skewnessskewness

Unstructured grids - Unstructured grids - Cooper meshCooper mesh

This grid projects a set of face meshes from one This grid projects a set of face meshes from one side of the domain to anotherside of the domain to another

the face meshes that are projected may be the face meshes that are projected may be mapped or pavedmapped or paved

but the faces surrounding the ray of projection but the faces surrounding the ray of projection must be mapped and must have the same must be mapped and must have the same number of grid points along the raynumber of grid points along the ray

can lead to very small mesh volumes at flow can lead to very small mesh volumes at flow constrictionsconstrictions

but a better alternative to the tgrid if you can but a better alternative to the tgrid if you can work out how to generate it.work out how to generate it.

Pros and ConsPros and Cons You will be using an unstructured solver (and grid) anywayYou will be using an unstructured solver (and grid) anyway The unstructured tetrahedral mesh (tgrid) is the easiest The unstructured tetrahedral mesh (tgrid) is the easiest

mesh to generate for a complex geometrymesh to generate for a complex geometry But it is inefficient in that more mesh points are often But it is inefficient in that more mesh points are often

generated compared to a hex meshgenerated compared to a hex mesh it can also cause numerical problems because of it can also cause numerical problems because of

skewnessskewness and because the faces are not automatically aligned with and because the faces are not automatically aligned with

the flow you can get false diffusion. This can reduce the the flow you can get false diffusion. This can reduce the accuracy of the problemaccuracy of the problem

Even if it requires more work to generate a quadrilateral Even if it requires more work to generate a quadrilateral or hexahedral (and cooper) mesh may give better results or hexahedral (and cooper) mesh may give better results in the final simulationin the final simulation

What is a good grid?What is a good grid?

A high quality meshA high quality mesh– smooth variations in grid smooth variations in grid

spacingspacing– minimal grid skewness minimal grid skewness

(cell interior angles equal(cell interior angles equal– graded towards graded towards

boundaries and other boundaries and other areas where rapid areas where rapid variations in parameters variations in parameters occuroccur

– sometimes all this is hard sometimes all this is hard to achieve particularly to achieve particularly with a tetrahedral meshwith a tetrahedral mesh

Avoid:

Skew

Large Volume changes

High aspect ratio

And try to align the grid with flowstreamlines

Assessing grid qualityAssessing grid quality Default measure of quality Default measure of quality

is is EquiAngle SkewEquiAngle Skew This should be minimisedThis should be minimised Definition depends on cell Definition depends on cell

geometrygeometry– Hex and quad < 0.7Hex and quad < 0.7– Tets < 0.8Tets < 0.8– Tri < 0.7Tri < 0.7

The tetrahedral mesh will The tetrahedral mesh will sometimes not meet this sometimes not meet this criteria in all parts of the criteria in all parts of the domaindomain

Hex or quads amax

amin

skew = max[ (amax – 90)/90, (90 – amin)/90]

Tri’s or tets

Skew = Optimal face/cell size – face/cell size Optimal face/cell size

Optimal face

Actual face

Gambit - bottom up Gambit - bottom up approachapproach

create vertex’screate vertex’s link vertex’s to make edgeslink vertex’s to make edges create faces from edgescreate faces from edges create volumes from faces (3d)create volumes from faces (3d) mesh edgesmesh edges mesh facesmesh faces mesh volumes (3d)mesh volumes (3d)

Gambit - top down Gambit - top down approachapproach

create volumes or faces directlycreate volumes or faces directly generate complex 3d volumes by generate complex 3d volumes by

uniting and splitting volumesuniting and splitting volumes mesh volumes or faces directlymesh volumes or faces directly this can be a good shortcut way to this can be a good shortcut way to

create the geometry and/ora gridcreate the geometry and/ora grid but Gambit may default to the t-grid but Gambit may default to the t-grid

option when the grid is generatedoption when the grid is generated