Boundary Layers

) to create a boundary layer mesh with dense element distribution in the normal direction along selected boundaries, as shown in Figure 8-29. This type of mesh is typically used for fluid flow problems to resolve the thin boundary layers along the no-slip boundaries.

Figure 8-29: A channel in which a fluid flows from the left to the right and is hindered by a tilted blade in the middle of the channel. A boundary layer mesh is added to the wall boundaries (blue quad elements) to resolve the regions closest to the no-slip boundaries. The image to the left shows the full geometry while the image on the right-hand side shows a smaller portion of the geometry, zoomed in on the top of the blade.


	In 2D, a layered quadrilateral mesh is used along the specified no-slip boundaries, as shown in Figure 8-29.


	In 3D, the boundary layer mesh is a layered prism mesh or a hexahedral mesh depending on whether the corresponding boundary-layer boundaries contain a triangular or a quadrilateral mesh. Additional elements of an arbitrary type can also be inserted into the layers if needed.

When a node is added, a Boundary Layer Properties node is automatically added as a subnode. Use this subnode to specify the boundary layers and the properties of the boundary layers. If you want to specify different boundary layer properties for more than one boundary selection, right-click the node and add additional subnodes.

In 3D, for a mesh that defines its own geometric model, it is possible to create a boundary layer mesh in meshed domains. If the selected domains are not meshed, use a Free Tetrahedral operation to add a volume mesh before building the node.


	See The Mesh Node to learn more about meshes that define their own geometric model.

To add a node, select boundaries (2D and 3D) or edges (for a boundary layer mesh on boundaries in 3D) in the graphics, then choose one of the following:

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Right-click in the window and select (

) from the menu. For more information, see The Graphics Context Menu.

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On The Mesh Toolbar click the button (

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Right-click a 2D or 3D node and select (

Then enter the properties for the boundary layer mesher using the following sections:

Geometric Entity Selection

Specify the entities where a boundary layer mesh will be created by first choosing an option from the list:

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Choose to specify boundary layer mesh for the entire geometry.

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Choose to specify the domains where you want a boundary layer mesh. Choose in the list to select the domains in the Graphics window, choose a named selection to refer to a previously defined selection, or choose to select all domains.

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Choose (available in 3D only) to select boundaries for which to generate a boundary layer mesh along its edges. Choose in the list to select the boundaries in the Graphics window, choose a named selection to refer to a previously defined selection, or choose to select all boundaries. Use a Swept operation to sweep the face mesh in adjacent domains to create a volumetric boundary layer mesh.

Corner Settings

The following options for handling boundary layers at sharp corners are available from the list (in 2D) and the list (3D):

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Select (the default) to introduce boundary layer splits at sharp corners, as shown in Figure 8-30. In the field you specify the minimum angle between adjacent boundary layer boundaries for a split to occur. Control the maximum angle of the elements in the split region by the parameter.

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Select to trim the boundary layer mesh at sharp corners. In the field you specify the minimum angle between adjacent boundary layer boundaries for trimming to occur. It is recommended to combine with a Corner Refinement node to refine the mesh further close to the sharp corner, as shown in Figure 8-30.

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Select to not use any special treatment at sharp corners, as shown below.

Figure 8-30: The different options for handling sharp corners: Splitting, Trimming with Corner Refinement applied, and No special handling.

In the field you can specify the maximum difference in number of boundary layers between neighboring points on boundary layer boundaries.

In the section , specify the . This setting controls the maximum angle between adjacent boundaries for trimming to occur, as shown in Figure 8-31. For angles between a boundary layer boundary and any other boundary, half the is used.

Figure 8-31: The boundary layer mesh in a 45 degree corner. With the Trim for angles less than set to 50 degrees, the corner is considered as narrow and trimming occurs. Setting the value to 40 degrees, avoids trimming in the corner.

Transition

Select the check box to smooth the transition in element size from the boundary layer mesh to the interior mesh. You can specify the number of smoothing iterations in the field. In the field you can specify the maximum element depth, from the boundary layer interface, for the mesh points to be smoothed.

Advanced Settings

Specify the boundary layer method to use. From the list, select (default) to use the most recent version of the boundary layer mesher, or select or to use similar methods as used in version 6.1 and version 6.0, respectively. For , the directions in the boundary layers are kept straight in all layers, otherwise they may bend to improve the quality. The option affects the deformation of the mesh to make room for the boundary layers; more recent versions of COMSOL Multiphysics provide a more sophisticated method that allows you to create thicker boundary layers with higher element quality.