[Miguel Hernandez]

Hello to everyone,

In interFOAM simulations, the problems I find tricky are where there is higher tailwater than you get with zeroGradient. E.g. restrictive subcritical downstream system.

In these kind of problems how do you set up the boundary conditions?

e.g.

How to set up a fixed water level at the outlet higher than what you get with zeroGradient?

I searched in the forum and on the internet to find the right boundary conditions to use, without any success.

Thanks...

Miguel

[Santiago]

Originally Posted by Miguel Hernandez

How to set up a fixed water level at the outlet higher than what you get with zeroGradient?

The question shoudn't be about how to set a water depth, but how to set the correct specific energy that corresponds to a certain potential energy and a certain kinetic energy. Notice that, by corollary, a certain solution for specific energy proposes two water depths which, at the same time, have to respect mass conservation (obviously).

So you find yourself with 2 equations for 2 incognitas which can be exploited to enforce your specific energy at the outlet. There's one little problem: interFoam doesnt solve energy directly, so you must enforce energy at the boundaries via velocity or pressure.

In steady (as in d Q / D t = 0) simulations, the simplest form is to set up the velocity that corresponds to the "kinetic energy" contribution in the specific energy equation at the outlet (pressure and alpha remain Neumann). By corollary, mass conservation will enforce the water depth indirectly.

In transient (as in d Q / D t != 0) simulations, the total pressure (or specific energy times density) should be set along with alpha at the height that corresponds to the "potential energy" contribution to the specific energy IN WATER. The corrector step imposed at the outlet BC for velocity will adjust approximately to the pressure while enforcing mass conservation.

[Miguel Hernandez]

Thank you Santiago, very interesting… i have to manage transient simulations… can you “translate “ your answer in a more practical way? Which boundary conditions would you use to recreate that? I mean, for alpha, p_rgh and U…

Thank you very much…

[vatavuk]

Hi Miguel,

I think this thread can be useful for you:

Boundary Condtions for Open Channel Flow with interFoam

Have a look at post #11

I hope it helps