摘自Fluent Theory Guide（对RANS模型而言）：

High quality numerical results for the wall boundary layer will only be obtained if the overall resolution of the boundary layer is sufficient. This requirement is actually more important than achieving certain values. The minimum number of cells to cover a boundary layer accurately is around 10, but values of 20 are desirable. It should also be noted that an improvement of boundary layer resolution can often be achieved with moderate increase in numerical effort, as it requires only a mesh refinement in the wall-normal direction. The associated increase in accuracy is typically well worth the additional computing costs. For unstructured meshes, it is recommended that you generate prism layers near the wall with 10–20 or more layers for an accurate prediction of the wall boundary layers. The thickness of the prism layer should be designed to ensure that around 15 or more nodes are actually covering the boundary layer. This can be checked after a solution is obtained, by looking at the turbulent viscosity, which has a maximum in the middle of the boundary layer – this maximum gives an indication of the thickness of the boundary layer (twice the location of the maximum gives the boundary layer edge). It is essential that the prism layer is thicker than the boundary layer as otherwise there is a danger that the prism layer confines the growth of the boundary layer.

简单翻译：
想获得高质量的边界层数值结果，边界层必须要有足够的网格解析度。保证边界层的模拟的精确性所需的边界层网格数，需要看实际需求，而不是简单的达到某一个特定的网格数就足够，边界层最少需要10层左右的网格，但通常需要20层。适当提升边界层的网格解析度而获得更精确的数值解是值得的，因为边界层网格通常只在壁面法向加密，加密网格而引起的计算量的增加是适度的（moderate increase）。对非结构网格，壁面附近使用棱柱型网格时，建议在壁面生成10-20层，或者更多层的。棱柱层的厚度内要保证有15个左右甚至更多个节点（node）。

可以使用湍流黏性的分布来判断边界层内网格解析度是否足够，湍流黏性通常在边界层的“正中间”附近达到最大值，如果边界层网格没有覆盖住湍流黏性最大值所在的位置的2倍厚度的地方，那么边界层网格数是偏少的，它将会限制边界层的发展。