Changes between Version 412 and Version 413 of doc/app/initialization_parameters

Timestamp:

Nov 22, 2018 8:43:39 AM (6 years ago)

Author:

suehring

Comment:

--

doc/app/initialization_parameters

@@ -873 +873 @@
 Moreover, if you use older revisions (<1931), it is noted that the naming convention for the multigrid solvers was different, 'multigrid_noopt' (now) was 'multigrid' (<1931), and 'multigrid' (now) was 'multigrid_fast' (<1931). \\ \\
 
-'''Warning: ''' In case of complex topography and '''masking_method''' = ''.FALSE.'', the wind velocity may possibly blow up at certain grid points. Hence, so far it is recommended to use '''masking_method''' = ''.TRUE.'' in case [#psolver psolver] = 'multigrid_noopt'.  
+'''Remark 1''' \\
+
+In case of very complex topography, including narrow street canyons or complex shapes resolved by only a few grid points, the use of [../../app/inipar#psolver psolver] = 'multigrid_noopt' in combination with [../../app/inipar#masking_method masking_method] = .F. can lead to a blow up of wind velocity. So far, this never happened if masking_method = .T. or [../../app/inipar#psolver 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. 
+
+
+'''Remark 1''' \\
+
+If topography is prescribed and [../../app/inipar#psolver psolver] = 'multigrid_noopt', the mean velocity-divergence reduction (see RUN_CONTROL) is smaller in case of [../../app/inipar#masking_method masking_method] = .T. compared to [../../app/inipar#masking_method 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.  
 }}}
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