Mesh Generation with Gambit

Read the documentation!Fluent has very good on-line documentation Gambit is not as goodDont just read the documentation but go to library and get hold of some of the journal references

Mesh generationGenerating a good mesh is a large part of the CFD problemGenerating a good quality mesh can be hard workWhat is a satisfactory mesh for a problem will not automatically be so when another model option is enabled

Grid TypesFluent can solve flow problems in 2d or 3dThe cyclone simulation discussed in the introductory lecture is a typical 3d problemThe 2d channel is a typical 2d problem

2d Gridsconsume much less memory and take much less time to solvea 2d grid intrinsically assumes that there are no velocity gradients in the direction normal to the gridThis assumption is really only valid is the channel is extremely wide in z

(u/(z =0

z

y

x

2d GridsCan also be used with an axisymmetric flow such as swirling flow in a centrifugeagain the assumption of zero velocity gradient normal to grid applies, but this is in the direction

(

z

y

x

2d grids - typesmapped (preferred if it possible to do so)

paved with triangles or quads (unstructured solver only)

2d quad mapped

2d tri paved

2d quad paved

2d grids2d grids are drawn on faces in Gambit2d face grids are used as a precursor to creating 3d grids

3d gridsThe grid you will generate for a full 3d simulation of the flowobviously consumes more memory and takes longer to solvegenerated for a volume in Gambitmapped preferred.complex 3d grids such as Cooper and t-grid are unstructured grids

Structured and Unstructured SolversStructured Solver (Fluent 4.5)uses i,j,k indexing to locate neighboring cellsgrid must be structured and based on a topologically rectangular block in computational spacebut this doesnt mean the grid cant be curvedall grid lines continue from one boundary to its oppositeUnstructured Solver (Fluent 5.0)uses an internal data structure to link cells and facesdoesnt force an overall topology on the grid so mesh volumes can be any shapedoesnt use indexing to locate neighboring cellscan read a structured grid

Structured and Unstructured solversStructured solvers were written first because they were easy to writeunstructured solves are more modernhowever Fluent 4.5 (structured) and Fluent 5.0 (unstructured) had different sets of modelseg Fluent 5.0 doesnt have the Eulerian granular flow model (4.5 does) but it is much easier to set up 5.0 for moving mesh problems. 5.0 has Large Eddy SimulationFluent 6 is supposed to merge the two approaches together

Structured GridsTopologically rectangularThis means that the mesh volume is a quadrilateral in 2d or a hexahedron in 3dEach mesh volume is linked only to its immediate neighborsBut the edges can be mapped around curves and mesh volumes dont have to be the same sizeReduces storage and CPU requirementsSolid regions like tank baffles can be generated by blanking those mesh points which overlap the solid region and making them dead zones. Fiddly but still generates a good griddead zones waste storage.

Unstructured GridsMesh volumes can be linked to any other volume in the domainAnd can be any shapeless computationally efficient than a structured gridbut can still read a structured grid topology (often still the best)can use non-conformal gridsintroduces flexbility but this flexbility creates problems

Unstructured Grid Topologies

3d hexahedron

2d Prism

2d Triangle

Tetrahedron Pyramid Prism

Unstructured grids - MappedIf you can generate a mapped mesh for the domain then do so

Unstructured grids - TgridUses a 3d paving algorithmuses the tetrahedron and pyramidsvery easy to generate a grid for complex geometrybut is inefficient and has problems of numerical diffusion and skewness

Unstructured grids - Cooper meshThis grid projects a set of face meshes from one side of the domain to anotherthe face meshes that are projected may be mapped or pavedbut the faces surrounding the ray of projection must be mapped and must have the same number of grid points along the raycan lead to very small mesh volumes at flow constrictionsbut a better alternative to the tgrid if you can work out how to generate it.

Pros and ConsYou will be using an unstructured solver (and grid) anywayThe unstructured tetrahedral mesh (tgrid) is the easiest mesh to generate for a complex geometryBut it is inefficient in that more mesh points are often generated compared to a hex meshit can also cause numerical problems because of skewnessand because the faces are not automatically aligned with the flow you can get false diffusion. This can reduce the accuracy of the problemEven if it requires more work to generate a quadrilateral or hexahedral (and cooper) mesh may give better results in the final simulation

What is a good grid?A high quality meshsmooth variations in grid spacingminimal grid skewness (cell interior angles equalgraded towards boundaries and other areas where rapid variations in parameters occursometimes all this is hard to achieve particularly with a tetrahedral mesh

Avoid:

Skew

Large Volume changes

High aspect ratio

And try to align the grid with flow streamlines

Assessing grid qualityDefault measure of quality is EquiAngle SkewThis should be minimisedDefinition depends on cell geometryHex and quad < 0.7Tets < 0.8Tri < 0.7The tetrahedral mesh will sometimes not meet this criteria in all parts of the domain

Hex or quads amax

amin

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

Tris or tets

Skew = Optimal face/cell size face/cell size

Optimal face/cell size

Optimal face

Actual face

Gambit - bottom up approachcreate vertexslink vertexs to make edgescreate faces from edgescreate volumes from faces (3d)mesh edgesmesh facesmesh volumes (3d)

Gambit - top down approachcreate volumes or faces directlygenerate complex 3d volumes by uniting and splitting volumesmesh volumes or faces directlythis can be a good shortcut way to create the geometry and/ora gridbut Gambit may default to the t-grid option when the grid is generated