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Mitch3007

Improve H Type (HT, HS, HX, HQ, QT, 2D) Open Boundary Stability

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Hi TUFLOW Users,

 

To improve model stability at 2D “H” boundaries or links (eg. HT, HS, HX, HQ, QT, 2D) that exhibit oscillations, you can include the term “BOUNDARY VISCOSITY FACTOR == ” in your TCF file (Build 2013-12-AC or later). This sets the factor to adjust the eddy viscosity coefficient at water level boundaries.  Note that 2D QT boundaries and 2D/2D links utilise HX links, and are therefore considered 2D “H” boundaries/links.

 

Prior to Build 2013-12-AC the eddy viscosity term was not applied at “H” boundaries/links.  The term was included for the 2013-12-AC build as it was found to provide improved stability in the case of 2D/2D multiple 2D domain links, along with other improvements to 2D/2D linking.   Therefore, for multiple 2D domain models from 2013-12-AC onwards the default factor is set to one as one of the enhancements in the new “Link 2D2D Approach == METHOD D”.  To maintain backward compatibility for single 2D domain models the factor is set to zero (i.e. no boundary viscosity term) for the 2013-12 builds.  For the 2015 release, the default factor will be a value of one whether single or multiple 2D domains as this is the mathematically correct approach.

 

Note that whether the factor is set to zero or one there will be little or no measurable difference in results if the model is running healthily.  However, for “H” boundaries/links that experience oscillations or instabilities, using a “BOUNDARY VISCOSITY FACTOR ==” of one or higher may provide improved performance.  Note that other checks such as ensuring the boundary is appropriately set up are more important than simple applying an increased “BOUNDARY VISCOSITY FACTOR ==”.  For example, the most common set up problem for 2D QT and HT boundaries are that they are not digitised approximately perpendicular to the flow direction.

 

Testing has shown that for circulatory or oscillatory 2D “H” boundaries, a “BOUNDARY VISCOSITY FACTOR ==” of up to 5 will have no measurable or adverse effect on model results, but can significantly improve the flow patterns at the boundary/link.  To ascertain the effect, run one or more sensitivity tests applying different factors. 

As an example, a model of the Bay of Fundy with a large ocean tidal range (16m!) and difficult tidal boundary location performed well with a factor as high as 5 without any detrimental effect on the general flow patterns within the model. 

 

For further reference in the 2013-12-AC release notes, please see Item 68(e).

 

Wishing you happy modelling. Regards the TUFLOW team. 

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Hi,

We have noticed that when modelling extreme depth tailwater conditions (i.e. depth > 15 m), we are getting oscillations across the floodplain. Review of the PO.csv file reveals that the flood height at the tailwater boundary is fluctuating below a fixed tailwater level. We don't understand how the water level can drop below the fixed global level.

We have tried adopting a boundary viscosity factor of 4 in lieu of the default 1, and it had no measurable effect. We have also tried reducing the timestep - which did not change the results.

We mapped the courant number for a problem timestep, and the maximum value was 4.9 - which would appear to be okay. 

We would appreciate some advice on how to minimise this oscillation. Is this a known issue when modelling extreme depths? We have had the same issue with models elsewhere in the same floodplain when modelling deep regional tailwater conditions.

Regards,

Francis

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Hi Francis, 

We'll likely need more information to be able to help out with your above issue. What TUFLOW Release Build are you using? A CPU or GPU simulation? Is there an initial water level set at the tailwater boundary?

Could you please send an email through to support@tuflow.com with the above info and a copy of your TLF log file to begin with. We will  probably need to look at a copy of your model or some input files, but we'll take a look at your TLF first.

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