Learning Forum

[Jjoonnaass]

Hi,

I'm trying to simulate a open channel water flow with particles. At first, i tried to represent the surface as a pressure outlet since it is the main goal to investigate the behaviour of the particles. I want to compare the results of the DPM and the Eulerian method. However, the pressure outlet at the free surface leads to unrealistic and unsteady velocity profiles. Via the VOF model and a bigger simulation area above the surface, the surface and its effects can be shown in a better way. Is there any way i can combine e.g. an eulerian-eulerian model for the water and the particles and a vof model for the free surface?

Thanks

[Rob]

Why not model E-E with a third phase? Not sure what you're trying to replicate so it's a bit hard to give an answer.

[Jjoonnaass]

I'm trying to replicate a sedimentation basin that is shaped like a long channel (8m x 0.34m x 0.2). On one side, there is the water inlet from 0 m to 0.11m in height, and on the other side, the outlet from 0.32m to 0.34m. I tried modeling only the water domain with the particles and set the water surface as a pressure outlet. Due to e.g. wave formation, water is leaving the simulation domain through this pressure outlet. Therefore, there are quite some velocity fluctuations in the whole fluid domain. The magnitude of these fluctuations is too high to be realistic in my opinion.
I tried to simulate a larger domain above the surface with the E-E model (and 3 phases). To obtain the correct water level, I patched the respective volume fraction of air to the regions above and below the surface level. However, if water is entering the channel through the inlet, the surface level will rise as not enough water is leaving the channel through the outlet (another pressure outlet). The surface level is then rising until the water can leave the channel through the pressure outlet on top of the channel. If I switch to the VOF model with the same setting (without particles), the water level is quite constant at 0.34m and the velocity profile looks good. Do you have any advice on how I can properly simulate the free surface with the E-E model? Are there some learning aids available?

[Rob]

There's not an open channel option for E-E at present. What did you set the downstream boundary as in the E-E model?

[Jjoonnaass]

The outlet is a pressure outlet with 0 Pa of gauge pressure as there is no massflow outlet available for E-E.
So would you advice to simulate the free surface as a wall?

[Rob]

If you know the free surface height then yes, use a wall or symmetry. What I'm not getting is why the Eulerian phase isn't flowing out of the domain, it's usually OK with pressure boundaries.

[Jjoonnaass]

Okay, thank you.
There is outflow at the downstream outlet but it is not the same amount as the inflow because the water is leaving through the pressure outlet at the free surface.

[Rob]

Outflow or pressure outlet?

[Jjoonnaass]

Pressure outlet; the wording was a bit unclear

[Rob]

No worries. Water should leave through the outlet, what are the relative boundary sizes?

[Jjoonnaass]

The inlet and outlet are rather small compared to the volume of the basin. The inlet is 0.11m x 0.2m and the outlet 0.02m x 0.2m. The water domain is 0.34m x 0.2m x 8m. This leads to high outlet velocities compared to the velocities in the middle of the basin.

[Rob]

So it'll be a lesser pressure drop to over flow the domain: does it overflow in reality?

[DrAmine]

Are u using open channel flow with VOF? If yes then the Eulerian model requires paper pressure profile at the outlet.

[Jjoonnaass]

In reality, the channel has a height of 0.6m and the water level is at a height of 0.34m. So, there is wave formation and but water can only leave the domain through the downstream outlet

[Jjoonnaass]

I tried it with the open channel function of the VOF model and this has led to a nice representation of the water surface. However, the most important part here is to simulate the sedimentation process. The VOF model is not ideal for this purpose. Therefore, I use the euler-euler model.

[DrAmine]

Then in Eulerian try to have pressure orofile at the outlet abs same initialization as in the case with open channel.

[Jjoonnaass]

This might sound like a stupid question but what is the difference between this kind of initialization and patching the volume fraction of air to the respective regions?
The result should be the same, right?

[Jjoonnaass]

So, I have tried simulating this case with the free surface as a wall boundary. However, changing the mesh size or turbulence model leads to completely different results in the velocity profiles. Do you have any advice on what I can change to get more stable results?

[DrAmine]

With Open Channel Flow I assume the pressure profile is "described" by Fluent using the neighborhood VOF. In Eulerian that does not exist and you need to do that on your own.