problem in simulating waves in a channel

pyxbird · September 18, 2012, 04:28

Hi
All dears,
I am a newbie learning FLOW3D,and I wish to simulate waves in a water tank.
Unfortunately,I found the volume of fluid is not conservative. If I choose "outflow" as the boundary condition,the volume of fluid will reduce in a steady rate. Else if I change the boundary condition into "wall", then the volume of water will increase slowly.
So I really wish to know why does it happen and how to solve this problem, can any experienced user can help me out?

And please forgive me for my poor English.
pyxbird

JBurnham · September 18, 2012, 13:12

This is a known issue, and is because of the way the outflow boundary handles truly orbital motion (as opposed to gravity waves, which it handles quite well). The solution: use 'allow fluid to re-enter at outflow boundary' check box, and use multiple 'linked' mesh blocks, each coarser (cell size larger) by a factor of 3 or more so that the initial wave does not hit the outflow boundary downstream until the necessary data has been gathered. The total length of the domain should thus be # x L, where # is the number of waves that need to be modeled and L is the wavelength. Using multiple progressively coarser blocks will do two things: it will reduce the number of cells necessary to meet the criteria that the first wave not hit the outflow boundary until the simulation is complete, and it will diffuse the wave after it passes through the period of interest and thus damp reflections that would interfere with the solution in the region of interest.

pyxbird · September 18, 2012, 23:35

Thank you, I will try it soon.

September 18, 2012, 04:28	problem in simulating waves in a channel	#1
pyxbird Member bird Join Date: Sep 2012 Posts: 42 Rep Power: 14	Hi All dears, I am a newbie learning FLOW3D,and I wish to simulate waves in a water tank. Unfortunately,I found the volume of fluid is not conservative. If I choose "outflow" as the boundary condition,the volume of fluid will reduce in a steady rate. Else if I change the boundary condition into "wall", then the volume of water will increase slowly. So I really wish to know why does it happen and how to solve this problem, can any experienced user can help me out? And please forgive me for my poor English. pyxbird

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September 18, 2012, 13:12		#2
JBurnham Senior Member Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 17	This is a known issue, and is because of the way the outflow boundary handles truly orbital motion (as opposed to gravity waves, which it handles quite well). The solution: use 'allow fluid to re-enter at outflow boundary' check box, and use multiple 'linked' mesh blocks, each coarser (cell size larger) by a factor of 3 or more so that the initial wave does not hit the outflow boundary downstream until the necessary data has been gathered. The total length of the domain should thus be # x L, where # is the number of waves that need to be modeled and L is the wavelength. Using multiple progressively coarser blocks will do two things: it will reduce the number of cells necessary to meet the criteria that the first wave not hit the outflow boundary until the simulation is complete, and it will diffuse the wave after it passes through the period of interest and thus damp reflections that would interfere with the solution in the region of interest.

September 18, 2012, 23:35		#3
pyxbird Member bird Join Date: Sep 2012 Posts: 42 Rep Power: 14	Thank you, I will try it soon.