Known bugs

Date	Reported by	Description
13/06/16	MS	Only if masking_method = .T.: Over-determination of Neumann boundary condition of perturbation pressure in poismg_noopt.f90 if a grid point within topography adjoins to two or more 'flow' grid points. This case, Neumann boundary conditions are not correctly set for the 'flow' grid points as the perturbation pressure inside topography is an average over all the adjoining 'flow' grid points. At this point it should be noted that is is not the reason for the velocity blow-up in case of complex topography.
13/06/16	MS	Less divergence reduction with masking_method = .T. compared to masking_method = .F. . Please see remarks below.
09/05/16	MS	For very complex topography, e.g. very narrow street canyons or corners that are resolved by only one or two grid points, the use of psolver = 'multigrid_noopt' and masking_method = .F. can lead to a blow up of wind velocity. Please see remarks below.

Remarks with respect to usage of psolver = 'multigrid_noopt'

In case of very complex topography, including narrow street canyons or complex shapes resolved by only a few grid points, the use of psolver = 'multigrid_noopt' in combination with masking_method = .F. can lead to a blow up of wind velocity. So far, this never happened if masking_method = .T. or psolver = 'multigrid' (where topography is always masked). A possible workaround to prevent such velocity blow-up is to preprocess the topography by filling holes and removing complex shapes on the small scale before running the LES. (This approach should have no significant effect on the flow field, as in LES the flow in such regions is poorly resolved and do not yield to reliable physical information.) The reason for this velocity blow-up is unclear so far.

Remarks with respect to divergence reduction

If topography is prescribed and psolver = 'multigrid_noopt', the mean velocity-divergence reduction (see RUN_CONTROL) is smaller in case of masking_method = .T. compared to masking_method = .F. (about one-order of magnitude). This smaller reduction can be attributed to wall-bounded grid points, where the divergence after pressure correction remains significantly larger in case of masking, while the divergence reduction for non-wall-bounded grid points is similar in both cases. However, validation tests revealed that this smaller divergence reduction has no effect on the resulting flow.