[joey-mastlab]

Hello all,

I am working on studying hydraulic jumps and I am setting up a homogeneous free surface simulation. I have gotten other similar problems solved before but for some reason, this problem isn't producing results that make sense. Ultimately, I'd like to have an open container with a flow of 3m/s coming from the left. Currently, the boundaries are just no slip walls and an opening on the top. There is no velocity in the flow now and the volume fraction initial conditions are if(z<1.2[m],1,0) for water and if(z>1.2[m],1,0) for air. The opening is what is causing me issues. If I don't include the opening, the simulation produces the results I'd expect. When I add the opening, the volume fraction seems to not make sense. I need help figuring how why the volume fraction looks funky when I place an opening on the top.
I've attached images for reference.
Screenshot 2024-05-15 112908.png
this is how it should look. There is no opening on this simulation.

Screenshot 2024-05-15 150334.png
This is what happens if there is an opening placed.

Screenshot 2024-05-15 150359.png
Here is how the opening is specified. the pressure is 0 Pa.

Screenshot 2024-05-15 150421.png
Here is how the domain is set up.

[ghorrocks]

Openings (and outlets) which have both phases in them are always difficult to get working. It is much easier if you can rearrange your model so the opening (or outlets) only see one phase across their entire face. You may have to to construct artificial pipes, sumps or openings to do this.

For instance if you are modeling an open channel with 1 m3/s water flowing in it then have the water enter from a pipe at the bottom of the duct (so it is pure water) and boundary for the atmosphere at the top face of the domain (so it is purely air). Likewise for the exit condition you could have the flow running over a weir (to set the water surface level), falling into a sump and then being extracted from the bottom of the sump (so it is purely water).

[joey-mastlab]

I have changed the geometry and added an inlet "pipe" and a weir. I want to post the results so you can see them but when I go to add an attachment I get a screen that says "Forbidden". DO you know what causes that? I've had issues like that on this forum before when trying to post.

[ghorrocks]

This FAQ describes how to post an image on the forum: https://www.cfd-online.com/Wiki/Ansy...n_the_forum.3F

I suspect the forbidden image message is because it does not like the image format you are using. I am not sure about that but try a different image format and see what happens.

[joey-mastlab]

I've tried PNG and JPG image formats, it specifically says:

Forbidden
"You don't have permission to access /Forums/newattachment.php on this server."

[joey-mastlab]

I sent an email to webmaster@cfd-online.com so ill get the images uploaded as soon the forum lets me or they get back to me.

[ghorrocks]

Weird. If you want to try to debug the forum image issue then please PM me. But first try the process described in the FAQ and see if that works for you.

[joey-mastlab]

Here are the results from changing the geometry. This is still a steady state simulation also. The inlet velocity is 1m/s and the outlet condition is 0 Pa. All initial conditions remain the same.

This is the first iteration from problem
https://ibb.co/1ZwCqHS

This is iteration 1000.
https://ibb.co/t8dDZqM


NOTE: The only reason I am posting links to my images is because something is wrong with the image uploader on my end. I am working on getting it resolved. PLEASE DO NOT DO THIS

[ghorrocks]

Your first post said you are trying to study hydraulic jumps, so I presume you want to generate a hydraulic jump in this domain.

I think you have the right general idea here, you just need to adjust things to get it to work. I suspect the flow rate you are specifying results in the water level being high enough above the weir that it contacts the top boundary. So you probably need to make the top boundary higher.

[joey-mastlab]

I realized that after looking at the geometry and the development through the iterations. I just changed the geometry and made the top boundary higher. It seems to be working with a low velocity. I will switch it to my target velocity and see what happens.

[joey-mastlab]

There are the results Ive obtained with the proper mass flow rate. I believe the weir is causing that large rise in water depth. I am going to try and remove it and see if It runs.
https://ibb.co/5cxyCx5

for reference, these are the results I'm trying to obtain. Ive done this once and I can't seem to replicate it.
https://ibb.co/NrdTwsq

[ghorrocks]

I note that on your current setup with the weir the exit boundary is not entirely a single phase. So you have not achieved the point of the weir. But if it converging OK then it might not be a problem.

I suspect you will need some form of a weir, just smaller than you currently have. Won't you need something to trigger the hydraulic jump?