TUFLOW Message 2583

TUFLOW Message
WARNING 2583 - Material ID "<>" contains a manning's n value ("<>") greater than Wu n limit 0.100 - n value will be limited in Wu formulation.

Alternate Message
WARNING 2583 - Material ID "<>" has a manning's n value ("<>") greater than Wu n limit 0.100 - n value will be limited in Wu formulation.

Message Type
WARNING

Description
This warning message indicates that one or more of the manning's values used exceed the limit imposed for Wu viscosity calculation. The default limit is 0.1.

The Wu eddy viscosity formulation is based on bed friction velocity, which in turn is dependent on manning's number. High manning's numbers produce very high viscosity values, which in turn can limit the model timestep through the diffusion or Peclet number (reported as “Nd” in the HPC output). In general, in areas where very high manning's numbers are used (for example the slowing effect of buildings may be represented as increased surface roughness), the flow velocity is much slower and the elevated viscosity has little effect, other than to slow the simulation down. An upper limit to the manning's number used in the Wu viscosity calculation was introduced in the 2023-03-AA release to limit the computed viscosity in regions of unusually high roughness. A manning's value of 0.1 was selected based on tests that indicated this value provides a sensible compromise whereby the majority of models would not be diffusion number controlled while keeping changes in results to a minimum.

Suggestions
Consider if the use of materials with manning's in excess of 0.1 is really required. The manning's n limit of 0.1 only affects the viscosity calculation, the momentum equation will continue to use the original roughness value (or lookup table) as defined in the materials file.

If a manning's value in excess of 0.1 is required for the viscosity calculation, it is possible to increase the limit used in the Wu viscosity calculation with the Viscosity Coefficients == command. Note that this may result in increased model solution time. If time permits, the model could be run with the default limit, and with a higher value (the maximum manning's roughness used), and the two solutions compared for differences. If the differences are minimal then the default limit of 0.1 may be considered acceptable and this warning message disregarded.

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