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TUFLOW Message 3043: Difference between revisions

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{{TUFLOW_Message
|tuflow_message=ERROR 3043: - Model unstable.
<br>
|alt_msg=ERROR 3043: - Too many consecutive failed step attempts.
 
|type=[[ERROR]]
|message_desc=
|message_desc=When using adaptive timestepping, should a timestep need to be repeated more than 10 times consecutively, the solution stops.
Either:
|suggestions=If using the adaptive timestepping (the default), the HPC timestep is calculated using the hydraulic conditions from the end of the previous timestep. If the hydraulic conditions have changed significantly it is possible for one or more of the Nu, Nc, Nd control number criteria to be violated at one or more locations within the model. For example, a sudden change in rainfall from one timestep to the next (which occurs with stepped rainfall boundaries) would potentially cause a violation. The HPC solver, by default, treats a 20% exceedance of a control number as a violation and will implement a repeat timestep feature.<br>
* A model running with fixed timestep has encountered NaNs.
* A model running with adaptive timestep has exceeded the maximum step retry count.
* A model running with adaptive timestep has dropped below the minimum allowable timestep.
 
|suggestions=
HPC uses a repeat timestep feature to maintain unconditional stability. The repeat timestep feature involves retaining the complete hydraulic solution from the previous (good) timestep. Should a control number anywhere within the model be exceeded by more than 20%, the solution reverts to the retained timestep, the timestep is reduced and then repeated. Should a timestep need to be repeated more than ten times consecutively, the solution stops. The simulation will also stop if the default minimum permissible timestep of 0.1 seconds has been reached. This value can be manually adjusted using 'Timestep Minimum' TCF command. These occurrences are rare and potentially an indication of poor model schematisation or poor data.
* If running a fixed timestep for a reason, consider first running in adaptive timestep mode and then identifying the minimum timestep used in the .hpc.dt.csv file, and then using this value (or slightly less) as the fixed timestep.
* If running with adaptive timestep and the model has exceeded the maximum step retry count, try running with a reduced "<font color="blue"><tt>HPC Control Number</tt></font>".
* If running with adaptive timestep and the model has dropped below the minimum allowable timestep, consider whether the minimum timestep is reasonable and reduce if necessary.
:* Also review the "<font color="blue"><tt>Minimum dt</tt></font>" map output and messages layer to see if a particular location is responsible, and then review model details in the region.
 
Each timestep is also tested for the occurrence of NaNs. A NaN is “Not a Number” and occurs due to undefined mathematical calculations such as a divide by zero or square root of a negative number. The occurrence of a NaN is also indicative of a sudden instability. Should a NaN occur, the repeat timestep feature is implemented.
 
To review and isolate the location of the minimum timestep, look at:
*Console window and .hpc.tlf file
*.hpc.dt.csv file (this file contains every timestep)
*_HPC.csv file output in the results folder (has the number of repeated timesteps for a time interval in the nRS_NaNs and nRS_HCNs column)
*_messages layer (location of repeated timesteps)
*“Minimum dt” map output (excellent for identifying the location of the minimum timestep adopted – add “dt” to “Map Output Data Types ==”)
 
 
|discussion_page=* [[Talk:TUFLOW Message 3043|Discuss Message 3043]]
|link3=''
|link4=''
|prelink=[[TUFLOW_Message_3042|Message 3042]]
|uplink=[[3xxx_TUFLOW_Messages|3xxx Messages]]
|nextlink=[[TUFLOW_Message_3044|Message 3044]]
}}
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