Database Full Index

1. Prism

1.1. How do I create a prism mesh?

ICEM CFD's Prism mesher requires the following to obtain a legitimate prism grid:

A tetra domain file with following characteristics:

good quality -typically, the quality of the Tetra_4, and Tri_3 elements should be above 0.3
no grid problems -there should be no errors with the volume mesh.
the surface mesh should project to the geometry appropriately.
as a rule of thumb, two or more tetras through any thickness that prism will be generated into (four elements for a thickness where both sides are to have prisms generated). Note: When prisms are generated, the surface mesh is extruded into the volume, if a tetra is engulfed in the new prism that will be created, the tetra is removed from the model. If the tetra can be compressed it will attempt to do this. The volume mesh has to stay consistent, however, so it is important that there are enough cells through the thickness, so that prism will not fail.

1.2. What is the use of Growth law?

This is used when the initial height and the height ratio are given. The Growth law determines which formulation is used for the thickness of a particular layer.

For instance:

If linear is used, the prism height of a particular layer is defined by h(1+(n-1)(r-1))
If exponential is used, the prism height of a particular layer is defined by h(r)^(n-1)

Where h=height, r=ratio, n=layer number

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1.3. How do I ensure that there is a smooth volume transition from prism to tetras?

Since tetra attempts to generate isotropic elements, the only way to have the volumes comparable is to have the last prism layer about as tall as it is wide. You can do this by one of the following:

by default, if no initial height is specified, ICEM CFD Prism will generate a prism layer which varies in thickness depending on the local surface element size. The local thickness of the prism layer is established automatically so that there is a smooth transition in cell volume size between the last layer of prisms and the attached tetrahedra.
by adjusting the number of layers from place to place on the surface. To adjust the number of layers use the parameter max_prism_height_ratio. This will adjust the number of layers at each place on the surface so that the height over base of the last prism is equal to the requested value. Using this parameter will cause pyramids to be generated where the number of prism layers changes.
by adjusting the growth ratio from place to place. To adjust the ratio use the parameter prism_height_limit_factor. This will adjust the growth ratio at each point on the surface so that the height over base of the last prism is equal to the requested value.

A value of 1.0 for either of last two parameters, will give you prisms as tall as they are wide in the last layer.

NOTE: ICEMCFD PRISM automatically does the refinements on the prism-tetra interface to have as much of a smooth transition as possible. This, of course, results in more elements.

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1.4. How do I calculate first cell height for my prism mesh?

When you have turbulence, you need to know the first cell height of the prism layer so that your Y+ values remain within 20-100 region.
You might want to use following set of equations:

u_tau = U * sqrt(C_f/2)
Y⁺ = y * u_tau * Rho/Mu

Based on this,

Y⁺	= y * U * sqrt(C_f/2) * Rho/Mu
	= y * Re * sqrt(C_f/2)/L
Hence,
y	= (Y⁺) * L/(Re * sqrt(C_f/2))

Here L is the characteristic Length, for example the Diameter in case of a circular pipe
This will only give you only an approximate idea of the value of y. Please note that this is the height of the center of the cell and thus the prism first height should be twice this.
Value of C_f/2 for various kind of geometries is as follows:

In a pipe, C_f/2 =0.039 * (Re^-0.2)

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1.5. Why I am getting stuck elements/Tetras after prism run and how do I get rid of them?

Sometimes you get stuck tetra/elements if you have a geometry corner as shown in the following figures and you have generated tetra mesh as shown in the second figure below. The third figure shows the stuck elements in pink.
Geometry with TRI mesh having no intermediate node

Zoom of above picture

Stuck elements

One possible reason could be the surface mesh is not good at the junction where you are getting stuck elements. If you see the closeup of the mesh (second figure) there is no node between the two corners of the junction. Here if you can get at least one intermediate node then the problem of stuck elements is solved.
To create the intermediate node without running tetra - split the corner edge and adjust the mesh as shown in the figure below or you can give a finer tetra size at that location and run tetra so that you get atleast one intermediate node.
The mesh after splitting and editing

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1.6. What are all parameters which can be set for prism mesher, as an expert?

Following are the different parameters which can be passed to the Prism mesher:

        print_memory_usage
        parallel_verbose
        refine_prism_boundary
        extend_pyramids
        auto_reduction
        checksum
        min_prism_quality
        prism_threshold
        boundary
        into_orphan
        delete_base_triangles
        law
        new_volume
        top_family
        side_family
        max_jump_factor
        ortho_weight
        max_prism_angle
        prism_height_limit
        max_prism_height_ratio|max_aspect_ratio
        do_not_allow_sticking
        tetra_smooth_limit
        graphics
        input_file|DomainInputFile
        output_file|DomainOutputFile
        surface_file|SurfaceFile
        family
        max_binary_tetin
        n_tetra_smoothing_steps
        n_triangle_smoothing_steps
        intermediate_write
        n_processors
        triangle_quality
        max_height
        total_height
        max_size_ratio
        area_power
        min_smoothing_steps
        min_smoothing_val
        smoothing_steps
        n_layers|layers
        ratio_multiplier
        ratio_max
        height
        ratio
        fillet

Most of these are explained on this website.

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1.7. How is aspect ratio for Prism defined?

There are lots of different definitions. As far as the prism mesher is concerned it is the minimum of

the quad face warpage
the prism determinant
the top and bottom triangle quality.

The triangle quality is by default the area of the inscribed circle divided by the area of the circumscribed circle and then normalized. You can change the prism warpage weight and it will favor the determinant more or less than then determinant.

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1.8. What is the best way to use smoothing to get a good quality prism mesh?

Following steps for smoothing can give a good prism mesh:

Smooth the tetra and tri mesh,
Smooth the tri mesh with tetra set to float with the "LAPLACE" option turned on in the Advanced options menu. This is very important since the prism mesh is dependent on the shape of the base triangle,
Smooth the tetra and tri mesh with "Laplace" switched off (very important since step 2 gives low-quality tet cells near the prisms),
In the prism panel, set 10 or more iterations for tetra smoothing. You can give 0 for tri smoothing since that is done already. Tetra smoothing should be performed since tetras get modified during prism creation,
Create only 1 prism layer with a height equal to the total prism layers height,
Load the mesh in MED and smooth only the tetras (fix prisms). If you get prism quality better than 0.1 from the prism mesher, never smooth prisms (not even set to float),
Even if the prism quality is low, they should not be smoothed more than a few iterations since it can cause severe prism warpage. It would be better to determine from the geometry or tetra mesh why low-quality prisms are created.
Once you have good prism and tetra quality, split and redistribute the prisms to get the desired number of layers and first layer thickness. Split/redistribute are in MED->Edit Mesh.

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1.9. I would like to vary my prism height (boundary layer) based on the size of the surface triangles. How do I do that?

If you don't wish to specify initial height
Please refer to the first point of topic "How do I ensure that there is a smooth volume transition from prism to tetras?"
If you wish to specify initial height
To illustrate this better please refer to the pictures below. Here we have a wing configuration where the surface mesh is 0.3 in the middle of the wing and it goes to 0.05 towards the trailing (and leading) edge. We would probably prefer that the boundary layer be much smaller at the trailing (and leading) edge as compared to the middle of the wing.
To achieve this, grow only 1 layer of prism with the height (or first height) much bigger than the biggest height you would like to have. We would give it 0.5 as an example. Then put the Prism height limit factor as 1. This value of Prism height limit factor would ensure that the prism height would never go beyond 1 times its base triangle size.
This would create prisms roughly 0.05 thick at the trailing (and leading) edge while the prism height would be as big as 0.3 at the middle of the wing. And this happens despite prism height being set to a uniform 0.5!! You could make this prism height limit factor anything you want.
We generally grow only 1 prism layer and then split it. This would result in the total thickness of prisms being the height of the boundary layer.

Effect of Prism Height Limit Factor for one layer

Effect of Prism Height Limit Factor after splitting into 5 layers with 1.5 growth ratio

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1.10. How is the Aspect ratio of a Pyramid calculated?

Basically, it is a circumsphere ratio, similar to the method for calculating aspect ratio of Tetras. Here are the basic steps for calculating Pyramid determinants. 1) Subdivide the Pyramid diagonally along its base to create 2 Tetrahedrons. 2) Calculate the normalized ratio of the diameter of a sphere entirely within (inscribed) the tetrahedron over the diameter of a sphere surrounding (circumscribed) the tetrahedron for both tetrahedron. 3) Repeat steps 1 and 2 for the other possible diagonal. 4) take the lowest of the 4 results and report that as aspect ratio. Most CFD codes will run happily as long as most of the Pyramids' quality is above 0.05.

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