
When the flow is aligned with the grid (e.g., laminar flow in a rectangular duct modeled with a quadrilateral or hexahedral grid) the firstorder upwind discretization may be acceptable. When the flow is not aligned with the grid (i.e., when it crosses the grid lines obliquely), however, firstorder convective discretization increases the numerical discretization error (numerical diffusion). For triangular and tetrahedral grids, since the flow is never aligned with the grid, you will generally obtain more accurate results by using the secondorder discretization. For quad/hex grids, you will also obtain better results using the secondorder discretization, especially for complex flows.
In summary, while the firstorder discretization generally yields better convergence than the secondorder scheme, it generally will yield less accurate results, especially on tri/tet grids. See Section 25.22 for information about controlling convergence.
For most cases, you will be able to use the secondorder scheme from the start of the calculation. In some cases, however, you may need to start with the firstorder scheme and then switch to the secondorder scheme after a few iterations. For example, if you are running a highMachnumber flow calculation that has an initial solution much different than the expected final solution, you will usually need to perform a few iterations with the firstorder scheme and then turn on the secondorder scheme and continue the calculation to convergence. Alternatively, full multigrid initialization is also available for some flow cases which allow you to proceed with the secondorder scheme from the start.
For a simple flow that is aligned with the grid (e.g., laminar flow in a rectangular duct modeled with a quadrilateral or hexahedral grid), the numerical diffusion will be naturally low, so you can generally use the firstorder scheme instead of the secondorder scheme without any significant loss of accuracy.
Finally, if you run into convergence difficulties with the secondorder scheme, you should try the firstorder scheme instead.