Acceptable residuals of continuity in open channel flow?
 

Hi all,

I'm currently trying to model what is essentially a broad crested weir. I feel that the resulting phase plot looks realistic (I am trying to study the waves/undulations that form after the obstruction), but due to the relatively high residuals in continuity I'm not so sure. So I'm wondering if this solution is reliable, and if not, what steps could I take to improve it? Any help appreciated! Also if this is the wrong place for this post please let me know.

Residuals and phase plots attached.

My settings are:
- models tab: multiphase VOF, 2 phases, implicit, open channel flow
- materials tab: water (primary) and air (secondary), surface tension enabled
- cell zone conditions: gravity enabled, specified operating density enabled
- BCs: velocity inlet, pressure outlet with free surface level specified

Air primary and water secondary

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Hi Ahmed,

Thanks for your reply. I was wondering about this too, in the manual it does say to use the heavier fluid as your second one (as you suggested). However, I have also got a pressure outlet for the top surface, with air backflow volume set to 1. I think this allows the model to work as a canal type flow, otherwise it just looks like a pipe full of water, with no air on top.

I have been following this tutorial as a guide:

https://youtu.be/WXgYASXefOk

Good news! Think I have figured out the problem. The trick is to use two sets of boundary conditions, in the same model (you change them halfway through).

So first of all, set up the model as follows:

• ensure that in models > multiphase model > "Open channel flow" is on
• velocity inlet for the left wall
• pressure outlet for the right wall with either air set to 0 or water set to 1 (only one of these will be available, depending on whether water or air is set as your primary phase)
• pressure outlet for the top wall, with either air set to 1 or water set to 0
• allow this to run until it appears to flat line, this happened at around 400 iterations for me

Stop the calculation. Now do the following:

• change the right wall to a pressure outlet. With the phase set to mixture, edit this outlet BC. In the multiphase tab, turn on "Open Channel" and set a suitable free surface height (1.85 metres for me).
• change the left wall to a pressure inlet. With the phase set to mixture, edit this outlet BC. In the multiphase tab, turn on "Open Channel" and set a suitable free surface height (2 metres for me).
• change the top surface to a wall
• right click on "calculate", and select "calculate" (NOT "Initialise and calculate"). This ensures that the new settings are applied to the existing model. After an additional 2000 or so iterations, it should show good convergence.

In the attached residual image, you can see where the change of BCs occurs due to the spike in the graph. Also attached: phase contours and geometry (with dimensions).

Hope this is of some use to others.

Reason to set two BCs?
 

Dear I.whelan 11

Do you know the reason to set different BCs?

I did a steady open channel flow case.

Inlet is divided into two face zones, namely water inlet and air inlet.
pressure inlet is set for air inlet, while mass flow is set for water inlet.

Residual decreased until 500 iteration, then it rose sharplyhttps://drive.google.com/open?id=0B8...F9ad29xVmpLR00.
the console show information as follows:

1)turbulent viscosity ratio is limited to 1e6 in *** cells.
2)reverse flow in *** cells.

Hi Kemin,

I don't know for sure why the use of two sets of BCs works. I feel though, that it helps to initialise the problem. The first set of BCs results in a high water level in the domain. Then, when the second BCs are applied, the water level drops off and the solution converges. See attached image for phase contour plot after first BCs.

I'm not sure I fully understand your setup - what is the BC at your outlet? Also what is the geometry like? I also tried a split inlet of air and water for a while but could never get that working. I would suggest looking at the phase plot before and after 500 iterations, to see what is going on. I found this lab demonstration of a weir helpful in understanding the physical meaning behind the solution at various stages:

https://youtu.be/VDkoWcD5RYM

Notice how long it takes for it to reach the steady solution, and all of the transient behaviour that occurs in between.

This process is not working in Fluent 18.2. Is there any other way?

This process is not working in Fluent 18.2. Is there any other way?