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Meshing - with Mesh Sim

Note: This is only available for CRIMSON Pro, the open source version of CRIMSON uses the Tetgen mesher.

CRIMSON Pro uses MeshSim to mesh the model with linear tetrahedral elements.

Boundary Layers


Due to the no-slip condition (i.e. relative zero velocity between a viscous fluid, in this case blood, and solid it interacts with, in this case the vessel wall at the fluid-solid interface) the region close to the vessel wall is typically the region with the highest velocity gradients in the domain. Therefore, it may be desirable to refine the mesh close to the vessel walls. This may be done with the use of Boundary Layers.

Under Global Options click "Edit" to bring up the Global Options Dialog. There are multiple Boundary layer types that can be specified via the dropdown list


Geometric Growth: This works by specifying the thickness of the First Layer (the layer closest to the wall), the number of layers required (this should be greater than 1) and the total thickness of the region of the Boundary Layers (BLs) (this thickness should be larger than that of the first layer).

Relative (by first layer): This works by specifying the thickness of the first layer (the layer closest to the vessel wall), this should be less than the global size, and the graduation factor (the factor of increase of the succeeding layer). For example, if the global size is 3.0 specifying 0.5 for the first layer and a graduation factor of 0.5 will result in three boundary layers of thickness 0.5 at the vessel wall, 0.5 / 0.5 = 1, and 1/0.5 = 2 towards the vessel interior. This is the final layer as 2/0.5 = 4 is greater than the global size.

Custom: Requires manual specification of size of each Boundary Layer (use the "plus" and "minus" toggles to the side of the boundary layers properties list to add or remove boundary layers.

Relative (by layer count): This works by specifying the Graduation Factor (the relative difference from one layer to the next) and the number of layers with the smallest elements at the vessel face. For example, if the global size is 3.0, specifying 3 for the layer count and a graduation factor of 0.5 will result in layers of decreasing size from the vessel interior of 3*0.5 = 1.5, 1.5*0.5 = 0.75, and 0.75*0.5 = 0.375 at the wall.

  1. Under Global Options click "Edit" to

  2. Enter 3.0 as the Absolute Mesh size, as before

  3. For now turn curvature refinement off (this is not curvature refinement but will work with boundary layers as shall be shown subsequently but will remain off for now)

  4. In this instance Relative (by first layer) will be used. Select Relative (by first layer) in the dropdown list

  5. Enter 0.5 for first layer and 0.5 for the graduation factor

  6. When done click "OK"

  7. Ensure there are no faces listed in Local Refinement. If there are, select and delete them from the list

  8. Run the mesher by clicking on the "Mesh!" Button

  9. To compare the output from the mesh with and without boundary layers examine the inflow and outflow faces

    Comparison of a mesh without (left) and with (right) Boundary
layers. As can be seen the boundary layers increase the mesh density
towards the vessel

Curvature Refinement


Taking this geometry:


We can see the effect of curvature refinement:

No Refinement With Curvature Refinement
image image

Curvature refinement is a tool which increases the surface mesh in regions of high curvature. Therefore in regions where there are high curvatures, such as small vessels or at branching points, the complex flow profiles might be captured better.

  1. Click "Edit" to display the Global Mesh Parameters dialog. Enter previous Global Options (i.e. Global Mesh size of 3.0 with all other options disabled).

  2. Turn on Curvature Refinement by ticking the checkbox

  3. Curvature refinement can have a value of 0 (maximum effect) to 0.5 (minimal effect). Move the slider to 0.05 (alternatively enter 0.05 in the relevant textbox).

  4. Click "OK"

  5. Run the mesher via the "Mesh!" Button

  6. Examine the difference in the two meshes, particularly around the blend at the bifurcation. It should be clear that the mesh is less course around the regions of high curvature, particularly around the blend between the two vessels.

    Mesh without (left) and with (right) curvature refinement. Note
the greater number of elements around the regions of high