[IADInocencio]

Is it possible to combine the inflow boundary condition B 60 1 with an initial water condition wherein only a portion of the domain is filled with water? Whenever I try to combine the two I get the "EMERGENCY STOP -- velocities exceeding critical value N 61" error.

So far I've only gotten it to work with either condition, but not both at the same time.

[kamath]

The reason why your simulation crashes is clear: the discharge.
If you set the discharge to 0.0, you will see that the simulation works.

Now as to why this is so, I would guess:
When you set a discharge outlet BC and there is no water at the outlet since you have a "dam break" scenario, the solver the crashes.

Is there a particular reason you stop the water right before the structure?
I also see (may be due to the grid res) that the set water level in the z-direction is lower than the outlet in your object, so there will not be a flow anyway.
What you might also notice is that if you have water filled across the entire x-direction to z=0.6 that you have chosen, the water level to the right of the structure will simply start to reduce to account for the discharge.
On the left hand side, the reinitialisation of the level set function will probably keep restricting the water level to before the structure. Eventually that simulation will crash too.

I hope this gives a different perspective regarding your setup.

[IADInocencio]

Hi Arun,

Yes I found that setting W 10 to 0.0 or just turning it off would allow the simulation to continue, however my intent was for water to flow out to simulate a drainage outfall. So that forebay in the picture is to simulate a manhole with water coming from an upstream pipe, and the opening is the pipe leading to a stepped outfall to dissipate the velocity.

Yes, the intent for the initial water to stop just at the structure was just to initialize that volume so I can have a shorter simulation duration than if it was empty. To illustrate, I have a smaller model (as attached) I used to learn and got a working result as shown below, where my initial water level was at A. And with the discharge Q it took some time T for the WSE in the upstream to raise B meters, and also took some time for the water downstream of the outfall to normalize because it was also wet at the start of the simulation.

So in an attempt to shorten the simulation time by T, I was trying to already have the WSE just below the opening and only upstream of it, so it discharges onto a dry stepped outfall.

I did some other tests (attached .txt's) with an even smaller domain and simpler structures and found water can inflow with a PatchBC, but I can't seem to get it to work correctly. After the initial pulse of water due to the numerical warm up causing some overtopping, it seems the forebay never fills up. https://youtu.be/7qS-H0HbP5A

[IADInocencio]

Here is a better screenshot of what I was trying to do, where the outfall is discharging but the downstream should initially be dry.

[kamath]

Interesting problem you are dealing with. My question is:
How essential is it to start from a dry condition?

The reason I ask this is:
If you look at literature on, say, dam break cases you will see that there is a certain (though very small) amount of water that exists downstream of the break on start up. eg. here and here

The reason being something that you have already experienced. The beginning of a "wet film" due to overflow of the fluid is hindered by the reinitialisation of the LS function. An extended BC on this would probably help somewhat, but I am unsure to what extent it would be good.

So, unless it is of utmost importance to have a dry start, I would recommend a wet start with an initial water level of about 1-2 cells on the downstream side.

[IADInocencio]

Hi Arun, it doesn't have to be 100% dry, having an initial downstream depth of around 0.2-0.5 is alright for me actually. My intent was to just have the steps I defined as dry. I realized from your previous reply too that the water should at least reach the downstream BC.

Thank you for the links, it's actually exactly like the initial water setup in Figure 2 of the 2nd article that I was trying to recreate but could not figure out the correct way to implement it using the F 51-F 56, F 70, and F 71 parameters. I was always ending up with just a flat initial water surface or just in the forebay; not both.

Again thank you for your time! There are lots engineering problems that I have been wanting to analyze using CFD and I am learning quite a lot on how to go about it.

[kamath]

Glad I was able to help.
Good luck ahead!

[valgrinda]

Hi Ismael,

maybe check B 77 2?

[IADInocencio]

Good morning Prof Hans, thanks for suggesting B77 2! I activated this and started off with a high initial water surface instead and the downstream water level is become lower as expected. (Side note, the entry in the manual for B 77 refers to a B 76 but is not listed?)

Thanks Arun; it looks like using the B 77 is another simple approach to my solution. Rather than my initial approach of letting the water in the forebay rise and spill to the steps, I'll let it work backward (in a sense) and fill the entire domain with a high initial water level and, hopefully, using the B77 will allow the downstream to normalize to a shallower depth.

Will let this run for a while and provide updates for science

[IADInocencio]

Originally Posted by kamath

Interesting problem you are dealing with. My question is:
How essential is it to start from a dry condition?

The reason I ask this is:
If you look at literature on, say, dam break cases you will see that there is a certain (though very small) amount of water that exists downstream of the break on start up. eg. here and here

The reason being something that you have already experienced. The beginning of a "wet film" due to overflow of the fluid is hindered by the reinitialisation of the LS function. An extended BC on this would probably help somewhat, but I am unsure to what extent it would be good.

So, unless it is of utmost importance to have a dry start, I would recommend a wet start with an initial water level of about 1-2 cells on the downstream side.

Hi Arun, apologies but this scenario has become an itch that I must scratch and have gone back to attempting simulating a domain where the water is higher on one side of a barrier with a discharge coming in. I have been able to successfully implement two different water levels as you recommended using the F 72 parameter but flows were still not going over the obstacle. So to compare I made two different simulations:
1) Scenario 1
- Inflow set by B 411 and B 442
- Constant water level for the whole domain set by F 72
2) Scenario 2
- Inflow set by W 10
- Initial water level set by F 60

Scenario 2 works as expected (although looks like accuracy could be better, likely due to a 0.02 grid size and sudden influx of water), but in Scenario 1 water just seems to disappear. The screenshot below is at 3 seconds, with the red surface set to Scenario 1, and the blue surface as Scenario 2.

I've also attached the ctrl files.

Which of these parameters am I implementing incorrectly?