# Advice on open channel flow for an water intake chamber

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August 21, 2014, 05:05
Advice on open channel flow for an water intake chamber
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Jonny Cox
Join Date: Aug 2014
Posts: 6
Rep Power: 5
Hello, I am new to CFX and need advice on how to start/continue a simulation to do with open channel flow.
In the attached file is the setup showing what I would like. There will be an input and output of water with the free surface in atmospheric conditions.
I need to find the velocity of the water entering the chamber.

What boundary conditions and general settings would you suggest?
Which analysis type would be best?

Sorry for the really simple questions and thanks for the help!

Jonny
Attached Images
 CFX Setup.jpg (86.3 KB, 27 views)

 August 21, 2014, 05:52 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,805 Rep Power: 107 What do you know about the flow? What are the fluids?

 August 21, 2014, 08:41 #3 New Member   Jonny Cox Join Date: Aug 2014 Posts: 6 Rep Power: 5 The fluids are water and air. The input and output flow is the same (mass flow rate 300). Thanks a lot. Jonny

 August 21, 2014, 20:37 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,805 Rep Power: 107 There needs to be something to drive the flow. Is the channel sloped? Pressure gradient? What is open to the atmosphere? Does anything control the water level?

 August 22, 2014, 04:21 #5 New Member   Jonny Cox Join Date: Aug 2014 Posts: 6 Rep Power: 5 The area open to atmosphere is the top face of the channel. In reality water comes from that open face and the channel is sloped, but I want to make as simple a model as possible. The water would collect in the channel and eventually due to gravity drop into the chamber. I know this won't work as I have modeled the water entering from the side face however. I just want to find out the behaviour of the water as it goes through the channel, into the chamber and out the pipe. A valve further down the pipe controls how much water goes through the system, but i thought it would be okay just to monitor the flow in and the flow out. What would you suggest? Thanks again

 August 22, 2014, 06:53 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,805 Rep Power: 107 It sounds like before you start anything you have to sort out exactly what your boundary conditions are. You say water flows into it - you have to define this. Do you know the flow rate? If so then this is easy to define the inlet boundary. You say the outlet is controlled by a valve. How is the valve controlled? Is it just at a fixed opening point to throttle the flow? In that case you should model the outlet as the valve discharge curve. Or is it controlled somehow - in that case how?

 August 22, 2014, 08:56 #7 New Member   Jonny Cox Join Date: Aug 2014 Posts: 6 Rep Power: 5 So I have played around with some tutorials and think I would be able to do it apart from knowing what boundaries to use! I know the flow rate going in which will be straight forward. The outlet valve will have the same flow going out as we are not modelling the valve. Trying to get a basic flow analysis done before we add more complex parts to it! The main problem was that I wasn't sure about was the top face opening boundary conditions and the general settings for water in atmospheric conditions. Cheers

 August 26, 2014, 06:27 #9 New Member   Jonny Cox Join Date: Aug 2014 Posts: 6 Rep Power: 5 Yes that makes sense. How would I set the outlet condition to be dependent on the static head? Would it be similar to the equations found in tutorial 7? how would they compare? Thank you for bearing through this!

 August 26, 2014, 22:56 #10 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,805 Rep Power: 107 If the outlet valve is fixed then it will have a pressure drop versus flow rate curve. Then apply this curve. If the valve moves or is under active control then you have to model the movement or control.

 September 4, 2014, 05:17 #11 New Member   Jonny Cox Join Date: Aug 2014 Posts: 6 Rep Power: 5 I have tried to search how to apply this curve but have been unsuccessful. How would you apply it?

 September 4, 2014, 07:28 #12 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,805 Rep Power: 107 There are two options: 1) Set the outlet to be a massflow outlet. Calculate the areaAve(p)@Outlet, and use a lookup table on the massflow versus p for the valve to set the massflow. 2) Set the outlet to be a pressure outlet. Calculate the massflow()@Outlet, and use a lookup table on the massflow versus p for the valve to set the pressure. They are just two different ways of doing the same thing. But I suspect you will find the first is more numerically stable than the second.

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