[Nic86]

Hello everyone,
I am a new openFoam user and i am trying to figure out which solver should I use for my hydraulic applications.

In particular, I need to simulate a system of interconnected culverts( i.e. closed conduits of rectangular shape) where water flows at a specified inlet flow rate. The water in these closed channels can be pressurised or free surface (i.e. the water level can either be below the culverts' soffitt or reaching it determining a pressure increase).

I want to run 3D steady-state turbulent simulations with uncompressible newtonian fluid.
I don't know if I have to work with a single phase fluid (water) or if instead I have to work with 2 fluids (water +air) because the channel could be partially filled by water and air on the top.

The simpleFoam and the interFoam seemed to meet my needs (although I don't know which one to prefer in my case) but I have other requirements in terms of boundary conditions.
In fact, these solvers require velocity and pressure at the boundaries, while I need to set a water flow rate (m³/s) at the inlet and a constant water level at the downstream boundary (outlet).
Also, in the simpleFoam I could not find where to specify the gravity vector which is fundamental for my applications (water moves downstream due to gravity).


Can anyone assist me with this?
Is there any tutorial similar to my case?
Apologies if my question is confusing but I am confused myself.
Thank you,
Nic

[Tobi]

Hi Nic,


actually you answered your question yourself:

I don't know if I have to work with a single phase fluid (water) or if instead I have to work with 2 fluids (water +air) because the channel could be partially filled by water and air on the top.

If you have partially filled shapes, you need to have a 2-phase solver (actually you can also work with one and dynamic meshes but this would be too much here). So interFoam is the one you should start if you have partially filled domains (water + air as you mentioned). If you are interested in the steady-state condition, the LTS option would be nice for you.

[Nic86]

Hi Tobias and thank you for your answer.

I am focusing on interFoam as i understood is the solver for my applications, but I couldn't find any tutorial or example where the inlet and outlet boundary conditions are the following:

Inlet : constant or time dependent volumetric flow rate
Outlet : constant or time dependent water level

Do you know where can I find such an example or could you guide me on how to male this setup?
Cheers,
Nic

[Santiago]

Originally Posted by Nic86

Inlet : constant or time dependent volumetric flow rate
Outlet : constant or time dependent water level

Nic

For the inlet, where you know the discharge, you can just use fixedValue for velocity, zeroGradient for alpha (you might want to clip alpha between 0..1) and for dynamic pressure.

for the outlet, as you want to keep the water level you'll have to force a certain total pressure in the water column, while letting the pressure 'unrestricted' on the air side. A mixed (robin) condition is needed. The velocities can be inletOutlet, and alpha1 zeroGradient.

[Nic86]

Thank you Santiago.
That means that at the inlet i will have to set a constant velocity equal to the FlowRate/(Area x Alpha) ?

Once the simulation will run, is there a way in ParaView to calculate the flow along a cutting plane to ensure that the inlet flow is correct?

Do you have any example similar to my case?

Thank you.
Nic

[Santiago]

That means that at the inlet i will have to set a constant velocity equal to the FlowRate/(Area x Alpha) ?

Almost! V = flowRate*alpha/(Area + small)

Originally Posted by Nic86

Once the simulation will run, is there a way in ParaView to calculate the flow along a cutting plane to ensure that the inlet flow is correct?

Yes, there is a filter called 'integrate variables'. You'll just have to integrate the velocity field in the water phase.

Do you have any example similar to my case?

Yes, I have, but these will be useless since you'll need to implement some of the BCs anyway.

[Nic86]

What is it "small"?

[Santiago]

Originally Posted by Nic86

What is it "small"?

A small number that is. You don't want divisions by zero, I assume.

[Nic86]

Thank you !