Sub-critical, Critical, Super-critical flow

robin781105 · May 16, 2014, 07:17

Dear Expert:

I want to simulate a rectangular channel and I set "Volume flow rate" at upstream boundary.The discharge is steady and there is no initial fluid in the channel.

As I set a sub-critical flow about Fr=0.3 into the channel, the flow will speed up and become a super-critiical out the channel.

How can I slow down the flow velocity and let all fluid stay in sub-critical flow or stay in steady Froude number ?

JBurnham · July 5, 2014, 22:52

If you set Outflow boundary downstream, it acts as a critical control point, like a weir. So water accelerates toward the Outflow boundary unless it is already super-critical. Use Outflow boundaries only where flow is super-critical. Where outlet of domain is sub-critical (like your case), use a Pressure boundary, Fluid Fraction = 1, Surface elevation (or fluid height in some versions) = Z coordinate of Free Surface at that point.

If you don't know the downstream elevation then make a guess and set the surface elevation a little lower than your guess. If there is too much upstream flow the rest will go out over the top of what you set, so you still get a pretty good answer.

Put the downstream boundary location a little farther away from where you want to measure, because the boundary may interfere with the results near it. For example, stagnation pressure boundaries set velocity = 0 just outside the mesh, so there is deceleration near the boundary. This is because boundary conditions are numerical approximations, not real things.

May 16, 2014, 07:17	Sub-critical, Critical, Super-critical flow	#1
robin781105 New Member tsung han yang Join Date: Dec 2013 Posts: 12 Rep Power: 12	Dear Expert: I want to simulate a rectangular channel and I set "Volume flow rate" at upstream boundary.The discharge is steady and there is no initial fluid in the channel. As I set a sub-critical flow about Fr=0.3 into the channel, the flow will speed up and become a super-critiical out the channel. How can I slow down the flow velocity and let all fluid stay in sub-critical flow or stay in steady Froude number ?

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July 5, 2014, 22:52		#2
JBurnham Senior Member Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 17	If you set Outflow boundary downstream, it acts as a critical control point, like a weir. So water accelerates toward the Outflow boundary unless it is already super-critical. Use Outflow boundaries only where flow is super-critical. Where outlet of domain is sub-critical (like your case), use a Pressure boundary, Fluid Fraction = 1, Surface elevation (or fluid height in some versions) = Z coordinate of Free Surface at that point. If you don't know the downstream elevation then make a guess and set the surface elevation a little lower than your guess. If there is too much upstream flow the rest will go out over the top of what you set, so you still get a pretty good answer. Put the downstream boundary location a little farther away from where you want to measure, because the boundary may interfere with the results near it. For example, stagnation pressure boundaries set velocity = 0 just outside the mesh, so there is deceleration near the boundary. This is because boundary conditions are numerical approximations, not real things.