TUFLOW Stability Troubleshooting: Difference between revisions

** The new cell size insensitive Wu turbulence scheme in TUFLOW HPC has become the default from 2020 release. The previous default, Smagorinsky viscosity formulation, could produce unrealistic eddies in the model when cell size got smaller than flow depth.

** Since the 2020 release all H boundaries including QT boundary adjusts the water level by the dynamic head when water is entering the model (a common approach in CFD modelling), which reduces the chance of spurious circulations caused by the boundary assumptions and/or schematisation of the boundary.

* Boundaries should be digitised perpendicular to the flow and well away from the area of interest.

* Every flow path should have its own boundary.

** Set an initial water level so the model doesn't start completely dry. This can be done globally with <font color="blue"><tt>Set IWL </tt></font> command or locally with 2d_iwl polygon.

 

* Split or merge open channel sections - the general advice is to have the length of open channels about 3 to 5 times of their width. Merging very short channels will provide sufficient storage. Splitting long channels into shorter ones with more 1D nodes might smooth the water level slope and the simulated flow rate can become more stable.

* Increase the storage area where required with 1d_na table or 1d_nwk ANA attribute with type NODE. The extra storage can cause a lag in a the hydrograph and sensitivity testing should be undertaken.

* Connect cells that have a constant water level (perpendicular to flow) as the water level boundary will be averaged from the connected cells. Connecting cells that are not perpendicular to the flow could mean the average water level upstream of the culvert to be higher that in reality and downstream average water level will be lower.

* Multiply default SX boundary storage with the 'a' attribute.

* Use Sub-Grid Sampling (SGS) to improve volume calculations upstream and downstream of the structure.

 

* Check that a sufficient number of SX cells are selected for the magnitude of the inflows. If it’s a relatively small 2D cell size and a large pit inflow (negative flow) or surcharge (positive flow) is applied to too few SX cells this will may cause instabilities (which is why TUFLOW selects more than one cell at a pit SX if the width of the pit exceeds the 2D cell size). Increasing the number of SX cells may help.

 

* Double precision might be used, however this would cause models running slower and other measures are recommended to try first.

* Lower 1D timestep can be sensitivity tested, however the timestep should still remain within reasonable bounds. If the 1D timestep is required to be very small for stability reasons, usually other measures can be taken to improve the stability and consequently run time of the model.

 

== My model reports very high mass balance error at the beginning, then it lowers. Is it unhealthy? ==