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-   -   free surface flow inside channel that gets narrower (http://www.cfd-online.com/Forums/star-ccm/118758-free-surface-flow-inside-channel-gets-narrower.html)

JohnAB June 3, 2013 17:34

free surface flow inside channel that gets narrower
 
2 Attachment(s)
Hello,

My system is pretty simple:
a classical channel, one inlet one outlet, everything else are walls. The inlet is larger that the outlet after a small reduction in width (type nozzle).

My goal seems pretty simple too:
visualize the free surface, i.e. the waves and elevation of water caused by that nozzle, if any.

What I did:
I have carefully read and tried to reproduce the boat tutorial. I have VOF waves that are very small but help me (I guess) generate the mean current in the channel without having to write a field function myself for the mean water depth. (I tried that too, it did not work).
What bothers me a lot with VOF waves is the parameter called "point on water level", which seems to be responsible for the water depth (I can put anything under "water depth", it is the "point on water level" that will define the water depth...) and I think this is the source of my problem but I do not see how to solve it.

When I run the simulation, the initialization goes well, but then, it looks like the water and the small generated waves from the inlet just perfectly slide on the walls (even if I have the non-slip condition) without being affected (no water elevation, no wave, nothing). The nozzle does not do anything and even more interesting, even if I change the Outlet to a Wall, the channel does not fill up, the water level do not change at all...

Could anyone please help me solve this and get the waves (generated from the nozzle) and the water elevation I am supposed to get ?

I have attached my case, if anybody wants to try to run it. It is ready to be surface&volume meshed, then hit run.

PS: I have a fine mesh and 0.01s time step, so this is not the issue I think.

The plot is a 0.5 volume of fluid isosurface.
the water speed is 2.5m/s along x.
the total length of the channel is 2m along x.
the water depth is 30cm.
the nozzle thickness is about 20cm.

JohnAB June 12, 2013 12:31

Hello again,

I still haven't figured out a way to get the correct free surface. Did anybody got the chance to take a look at it ?

I have also tried without the waves, i.e. only inlet (velocity inlet with a field function to define the water level) and outlet (pressure outlet), and in that case it looks like something happens, but it is still not good since I am losing water over time (empty channel after about 2 seconds of simulation) through the outlet, even with a refined mesh (base side would be like 0.01 for a 30cm*200cm channel) and small time step (0.05s).

Again, I have looked at the tutorials, and I am stuck here...

fuzel June 24, 2013 06:49

Hi John,

have you figured out what caused your problem? If not I am happy to have a look at it later this week...

Cheers

Kons

JohnAB June 24, 2013 14:43

Hello fuzel,

No I haven't figured it out yet.
I made this work without the waves, just with velocity inlet and it looks good, my issue was that I had to use a trimmer mesh. But regarding the case with the waves, it still does not work at all, the wave do not "see" the geometry change.

I would be very glad if you could made it work!

Thanks!

fuzel June 24, 2013 15:48

Hey John,

I ll have a go at it over the weekend. Regarding your point with the water depth:
The waterDEPTh is used by starccm for the respective formulation of wavetheory where as the actual water level is set by giving an initial point of the free surface. This gives you the advantage that you could basically model a deep water wave without having to have a really deep domain. So its not a bug it's a feature ;)

Cheers

Kons


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