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-   -   boundary conditions outlet-free surface flow (http://www.cfd-online.com/Forums/cfx/81400-boundary-conditions-outlet-free-surface-flow.html)

 antonio October 26, 2010 06:40

boundary conditions outlet-free surface flow

Hi all.

I am trying to simulate a free surface flow and I am having some problems in what concerns the convergence (the plot of the residuals is highly oscillating). According to the documentation one possible reason is incorrect boundary conditions right?. In some threads some people argue that free surface flows are very sensitive to the outlet height (that we should use experimental data for this height and I donīt have this data in my case). What`s the reason of this? Why the code doesn`t adapt to the specified height by default?:confused:

Can someone clarify this point to me?

Best Regards

 michael_owen October 26, 2010 08:53

You just said you don't know the height of the free surface at the outlet, and then you said it is specified. Which is it?

The problem is that the hydrostatic pressure is a function of the depth of the fluid, which is an implicit result of the solution. Often in CFD we are specifying the pressure at the outlet boundary condition to create a well defined problem for the solver. There is a conflict here. If you know the height of the fluid at the outlet, then you can specify the pressure via a CEL expressions like -Density*g*(y-Height)*Water.vf or some such. When you don't know the height a priori, it becomes trickier. Have you looked at the tutorial "Free Surface Flow Over a Bump"?

 antonio October 26, 2010 09:21

Hi. Thanks a lot for your comment.

Sorry if i wasn`t very clear. I wanted to say that I do not know exactly the water height at the outlet (I do not have any experimental results). However, as in cfx tutorials they say that specifying velocity at inlet and pressure at outlet it`s the most robust way of specifying boundary conditions, I have made an estimation of this height.

What I wanted to ask is if the solver is somehow extremely sensitive to this initial guess.

P.S. yes i have also looked to the tutorial
Regards

 michael_owen October 26, 2010 23:05

Yes, free surface flows can be extremely sensitive to the initial guess. Have you initialized a free surface and the hydrostatic pressure?

 antonio October 27, 2010 07:26

Yes, I did. However like I had said itīs hard to achieve convergence.

 vishal.indomitable August 24, 2011 06:49

Dear,
I am also facing the problem of convergence in my case. As per the talks with the guy from ANSYS, it is not possible to get convergence in flow involving multi-fluid flow.

 ghorrocks August 24, 2011 07:43

Yes, it is possible to get convergence. But it can be harder for some flows, free surface flows especially.

 Amir1 August 11, 2014 19:24

Hi guys,

I am wondering what you have selected as the boundary condition for a free surface? can you help me with that?
Thanks

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