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-   -   Using Periodic BC (http://www.cfd-online.com/Forums/flow-3d/109862-using-periodic-bc.html)

burbur November 28, 2012 10:32

Using Periodic BC
 
Hello,
I am a bit new in the software and tring to use periodic bc to obtain a fully developed flow in an open channel. But, I could not find a place to define my discharge value to this bc's input window.

Can anyone help me in dealing this bc? Where should I enter discharge value of the flow?

Thanks.

dmilano December 4, 2012 11:14

Periodic boundaries are always defined in pairs, so any fluid that leaves through one is reintroduced through the other.

burbur December 5, 2012 09:39

But, how about the flow properties entering to the inlet boundary at t=0?

Thank you for the reply.

JBurnham December 5, 2012 12:28

You can't specify discharge when using Pd boundaries upstream and downstream in a channel, because it would over-constrain the solution.

If you have gravity defined in both x and z (component gravity) to give a constant slope to the channel, the flow will evenually become steady and uniform. Then discharge is a function of the geometry and fluid only.

If you have not specified component gravity, then the fluid will eventually come to rest, since the inlet and outlet Pd boundaries must be at the same elevation.

If you calculate the steady-state discharge and mean velocity, you can apply this as an initial condition to speed the solution toward steady state.

burbur January 11, 2013 08:28

Hello again Jeff, thank you for your reply, but I am still thinking that a discharge value should be an input to the system. The gravity and the slope of the channel gives the relation between flow depth and discharge but one should be input.

Also I did several runs using this b.c. and I defined a fluid region and a velocity for this region, by this way the system got an initial discharge. But, looking at the results, I saw that the continuity equation did not satisfied in the system, the discharge is increased by 3 times after a 150 seconds run of a channel with a 7 m length.

Can you, or anybody, give any advice on this subject?

Thanks.


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