Defining a boundary layer mesh using a Fluid Domain

The boundary layer mesh is one or more layers of 3D wedge elements extruded from a temporary triangular mesh created on a surface on which you have defined a Fluid Surface Mesh type of Fluid Domain simulation object. The 3D elements in the boundary layer mesh are highly anisotropic and are ideal for capturing strong gradients near a surface.

With a boundary layer mesh:

• The software extrudes the 3D wedge elements layer by layer.

• Each layer can have a different thickness.

• Generally, the layer closest to the boundary surface needs to be thinner than any upper layers. This allows for a gradual transition between the size of the elements in the boundary layer mesh and the size of the elements in the fluid mesh.

Understanding the boundary layer thickness

In the Fluid Domain dialog box, the Boundary Layer Mesh Definition options let you specify the thickness of the layers and control how they transition to the surrounding fluid mesh.

The following figure shows an example of a boundary layer mesh and the individual layers that comprise it:

In the figure:

• shows the thickness of the first layer.

• shows the total thickness of the boundary layer mesh.

• shows the number of layers of elements in the boundary layer mesh.

The Layers Thickness options let you specify whether you define the thickness of the layers as an absolute or a relative value.

• If you select Total Thickness, you can specify options that control the size of the elements in the first layer of elements (First Layer Thickness), the total thickness of all the layers (Total Thickness), as well as the total number of layers in the boundary mesh (Number of Layers).

• If you select Relative Thickness Ratio, you can specify options that control the rate at which the thickness of the layers (size of the elements) increases in each successive layer.

Specifying a relative thickness ratio

If you select the Relative Thickness Ratio option, the Thickness Ratio value you enter, along with a value for either First Layer Thickness or Number of Layers controls the extrusion of the 3D wedge elements that form the boundary layer mesh.

• If you select Specify First Layer, you can specify a First Layer Thickness value. The Thickness Ratio controls the thickness of each successive boundary layer according to the following:Ratio = T n-1 / T nWhere:T = Layer thicknessn = Layer numberThe software continues to add successive layers of elements until the until the layer thickness (H) satisfies the following:(local triangle mesh size) > T ≥ Ratio × (local triangle mesh size)

• If you clear the Specify First Layer check box and enter a value for Number of Layers, the value you enter for Thickness Ratio apportions thickness to the specified number of layers according to the following:Ratio = T n-1 / T nWhere:T = Layer thicknessn = Layer numbersuch that the top layer thickness (T last layer) satisfies the following:(local triangle mesh size) > T last layer ****≥ Ratio × (local triangle mesh size).

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