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Subsection 8.7.1: Degrees of Freedom and Boundary Conditions Up Section 8.7: Guidelines for Fluid Analyses Subsection 8.7.3: Computational Efficiency: Broyden's Method

### 8.7.2 Biased Meshes for Boundary Layers

In fluid analyses, boundary layers will form in the vicinity of no-slip boundaries, where the velocity magnitude varies rapidly with the distance from a no-slip surface. The thickness of boundary layers decreases with increasing Reynolds number. To capture these boundary layers accurately in a fluid finite element analysis, it is necessary to refine the mesh to produce thinner elements closer to the no-slip boundaries. This is done most conveniently by using mesh biasing tools, available in various meshing software programs, or post-hoc boundary-layer meshing tool that modify an existing mesh. Both of these options are available in FEBioStudio. Boundary layer meshing should always be used in fluid analyses in FEBio. Whereas other finite element codes employ numerical stabilization techniques that partially alleviate the need for biased meshes, FEBio does not employ such techniques. Therefore, using uniform finite element meshes may fail to produce numerical convergence, even in some seemingly simple problems. Users should keep in mind that biased meshes don't necessarily require more nodes and elements, therefore there is no automatic computational cost for using biased meshes.
Subsection 8.7.1: Degrees of Freedom and Boundary Conditions Up Section 8.7: Guidelines for Fluid Analyses Subsection 8.7.3: Computational Efficiency: Broyden's Method