This topic contains information about where you should pay close attention to your mesh definitions. The underlying theme of this discussion is that the mesh should be fine enough to capture gradients and changes in the flow. Gradients may be due to geometric features, boundary conditions, or distributed resistance areas.
Spatial gradients for velocity, pressure, turbulent kinetic energy and turbulent energy dissipation will generally be highest near a solid boundary, typically a containment wall or the surface of an immersed body. This is particularly true if the flow is constrained by a tight clearance, forced to turn around a sharp corner or suddenly brought to rest at a stagnation point. Accordingly, mesh density must be greatest in these regions.
When analyzing turbulent flow, the element size adjacent to a solid boundary is particularly important for accurate prediction of shear stress. This ultimately affects the calculation of pressure drop across the solution domain. The k-epsilon and RNG turbulence models compute a non-dimensional distance from the wall, y+, at all nodes adjacent to a solid boundary. This value is useful in determining whether the elements adjacent to solid boundaries are sufficiently sized.
The y+ values may be viewed as a results quantity. In general, they should be kept within the range 35<y+<350. It is impractical and unnecessary for all y+ values to be within this range, but it is a good general guideline. This range is most critical for flows that experience a great deal of pressure drop due to shear. Examples of such situations are the flow through long pipes and flow over aerodynamic bodies. In flows where form drag dominates the pressure drop, the y+ criteria is not nearly as important. The use of and Boundary Mesh Adaptation is strongly recommended to ensure that the mesh is fine enough near all walls of the domain.
In general, elements should be concentrated at inlet openings to allow solution gradients to develop. In some situations (compressible flows, for example), the regions near outlets should also have a fine mesh. If the outlet has been placed far enough out from the solution domain, no refinement is necessary. The goal is that the outlet should not strongly affect the solution.
Similar to the inlet passages, elements should be concentrated near walls with thermal boundary conditions. Usually near these boundaries, the heat transfer rate (which is the temperature gradient) is the highest. You should also try to concentrate nodes at the edges of these boundaries so the discontinuity in heat transfer can be captured accurately.
The area surrounding the separation point between two boundary condition types must have a refined mesh to adequately resolve the discontinuity. An example is the point at the intersection of an insulated wall and a specified heat flux boundary in a convection analysis.
It is good practice to concentrate the mesh on rotating regions and solids enclosed within a rotating region. The flow gradients are typically quite high within rotating regions, and the geometric shapes are often very intricate.