Changes between Version 436 and Version 437 of doc/app/initialization_parameters

Timestamp:

Oct 28, 2019 7:23:27 AM (5 years ago)

Author:

raasch

Comment:

--

doc/app/initialization_parameters

@@ -2205 +2205 @@
 {{{#!td
 Time step for the 3d-model (in s).\\\\
-By default, (i.e. if a Runge-Kutta scheme is used, see [#timestep_scheme timestep_scheme]) the value of the time step is calculating after each time step (following the time step criteria) and used for the next step.\\\\
-If the user assigns '''dt''' a value, then the time step is fixed to this value throughout the whole run (whether it fulfills the time step criteria or not). However, changes are allowed for restart runs, because '''dt''' can also be used as a [../d3par run parameter].\\\\
-In case that the calculated time step meets the condition\\\\
+By default, (i.e. if a Runge-Kutta scheme is used, see [#timestep_scheme timestep_scheme]) the value of the time step is automatically calculated after each time step in order to follow the Courant Friedrichs Levy (CFL) criterion. This value is then used for the next step.\\\\
+If '''dt''' is assigned a value, then the time step is fixed to this value throughout the whole run (whether it fulfills the time step criteria or not). However, changes are allowed for restart runs, because '''dt''' can also be used as a [../d3par run parameter].\\\\
+In case that the automatically calculated time step meets the condition\\\\
       '''dt''' < 0.00001 * [../d3par#dt_max dt_max] (with dt_max = 20.0)\\\\
 the simulation will be aborted. Such situations usually arise in case of any numerical problem / instability which causes a non-realistic increase of the wind speed.\\\\
-A small time step due to a large mean horizontal wind speed may be enlarged by using a coordinate transformation (see [#galilei_transformation galilei_transformation]), in order to spare CPU time.
+A small time step due to a large mean horizontal wind speed may be enlarged by using a coordinate transformation (see [#galilei_transformation galilei_transformation]), in order to spare CPU time.\\\\
+'''Warning:''' The simulation will crash in case of a fixed '''dt''', if the given value violates the CFL criterion at any time throughout the run. You should run the case without setting '''dt''' first, and check how small the timestep can become.  
 }}}
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