TUFLOW General Discussion: Difference between revisions
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Yes, this can be used by setting the following commands:
<br>The viscosity coefficients == command on its own is used to set the eddy viscosity. In this instance, you need to set the dynamic viscosity, which is a property of the fluid itself. This can be used by setting the following commands:
Viscosity Formulation == Non-Newtonian
Viscosity Coefficients == k, n, muLow, muHigh, tau0
Where K is the viscosity coefficient in Pa S when n = 1. Where n=1, this relates to a Newtonian fluid. You can then use an appropriate value for tau0, the stress required to make the fluid move. The Non-Newtonian model uses the Herschel-Bulkley approach which can be used to model Newtonian fluids with different viscosities. See section 5.4 of the new release notes here for more information.
The graphs below explains the Herschel-Bulkley approach and it’s flexibility.
[[File:Newtonian_Model.png]]
With a Newtonian fluid, the shear stress is linearly proportional to the shear rate.
τ=k(du/dy)
For a non-Newtonian fluid, the shear stress varies non-linearly with shear rate, this is what is represented with the power law model in the graph above.
τ=k(du/dy)^n
We then have Bingham plastics which have a linear shear stress to shear rate relationship but this time an offset which means there needs to be some force for the fluid to move, but then the shear stress varies linearly with shear rate.
τ=τ_0+k(du/dy)
The Heschel-Bulkley model takes aspects from both the Bingham Plastic Model and the Power Law Model resulting in the following.
τ=τ_0+k(du/dy)^n
It should be seen that where Ƭ0=0, the Herschel-Bulkley equation is the same as the power law model. If n is 1, then the Herschel-Bulkley equation is the same as the Bingham Plastic model. If Ƭ0=0 and n =1, then the Herschel-Bulkley equation is the same as the Newtonian fluid approach. By using the Herschel-Bulkley equation as a Newtonian model, it does allow you to define the viscosity coefficient for the fluid. In your case you’ll need to convert the viscosity from centiStokes to Pa S (if using an N value of 1). You can then define the fluid density using the Density of Water command you mention.
You can test this out using the following commands.
Viscosity Formulation == Non-Newtonian
Viscosity Coefficients == 0.001, 1, 0.0, 1000, 0
This uses a n value of 1, a Ƭ0=0 and a viscosity coefficient of 0.001 Pa S which is a typical viscosity of water at 20 ℃. This should give comparable outputs to a standard TUFLOW model without the Non-Newtonian functionality.
== How closely do TUFLOW results match other hydraulic software? ==
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