Weird velocity at the air-water interface in olafoam

Jung hoo · June 19, 2017, 22:29

Hi, foamers. I'm using olafoam to simulate waves above a structure(a simple mound) like below.

As you can see, there exist some weird huge velocities at the start point and end point of the structure even though waves didn't approach the structure yet.

The surface is air-water interface(alpha.water=0.5) and the structure was inserted by using snappyHexMesh.

Can anyone explain to me why that some noises exist?
Thank you in advance.!

Phicau · June 19, 2017, 23:07

Hi Jung Hoo,

what you see there are spurious velocities (look for it in the forum) when the free surface reaches a non-conformal mesh section (i.e. change in refine level).

There is usually no solution, so what I tend to do is to refine the cells in which the free surface is going to be during the whole simulation to the same level beforehand. Another alternative is to use dynamic mesh refinement.

Before that, check whether this effect influences your results. It sometimes only decreases dt due to the large velocities and you may learn to live with it...

Best,

Pablo

Jung hoo · June 21, 2017, 21:15

Thank you for your fast reply. Now I can understand why the spurious velocities exist where the mesh refine level changes.

Thank you for your help!

June 19, 2017, 22:29	Weird velocity at the air-water interface in olafoam	#1
Jung hoo Member Lee Jung Hoo Join Date: Dec 2015 Posts: 37 Rep Power: 10	Hi, foamers. I'm using olafoam to simulate waves above a structure(a simple mound) like below. As you can see, there exist some weird huge velocities at the start point and end point of the structure even though waves didn't approach the structure yet. The surface is air-water interface(alpha.water=0.5) and the structure was inserted by using snappyHexMesh. Can anyone explain to me why that some noises exist? Thank you in advance.! lourencosm likes this.

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June 19, 2017, 23:07		#2
Phicau Senior Member Pablo Higuera Join Date: Jan 2011 Location: Auckland Posts: 627 Rep Power: 19	Hi Jung Hoo, what you see there are spurious velocities (look for it in the forum) when the free surface reaches a non-conformal mesh section (i.e. change in refine level). There is usually no solution, so what I tend to do is to refine the cells in which the free surface is going to be during the whole simulation to the same level beforehand. Another alternative is to use dynamic mesh refinement. Before that, check whether this effect influences your results. It sometimes only decreases dt due to the large velocities and you may learn to live with it... Best, Pablo

June 21, 2017, 21:15		#3
Jung hoo Member Lee Jung Hoo Join Date: Dec 2015 Posts: 37 Rep Power: 10	Thank you for your fast reply. Now I can understand why the spurious velocities exist where the mesh refine level changes. Thank you for your help!