Free surface simulation

Dev4415 · January 11, 2018, 04:16

Hi everyone! This is my first post here.

I had a doubt regarding the bump2D tutorial in CFX.
For free surface simulation, is it always required to pre-define the DownH and DownPres?

Shouldn't the liquid surface develop on its own?

ghorrocks · January 12, 2018, 03:06

Yes, this example is a little unusual in that it defines these properties. In general they are not known and you use the simulation to determine it.

Boundary conditions are tricky in free surface flow and require careful thought for them to work properly. That is why there has been many questions on the topic on the forum. But the approach used in the tutorial is unusual and most people cannot use that approach.

Dev4415 · January 12, 2018, 23:57

Thanks ghorrocks! That gave some confidence.

I did a free surface simulation for water flowing through some chambers. (attaching some pics). Boundary conditions of Inlet and outlet (mass flow rate), opening and wall with initialization have been provided.

As in the pics, in the last chamber before outlet, though velocity vectors have covered the entire area in the chamber, the velocity streamlines are not throughout.

Is there any particular issue behind it?

ghorrocks · January 13, 2018, 17:27

In the final chamber the water just goes around and around in the vortex. Unless you seed a streamline in this vortex you won't get any streamlines in the vortex.

Dev4415 · January 14, 2018, 05:56

Thanks a lot!

Got the streamlines in the recirculating zone Now. We can also get them by seeding them from downstream with backward integration.

Dev4415 · January 26, 2018, 09:21

Hi Glenn again!

I am trying to do a free fall simulation with free surface development of water + air phases. My results (I have attached a pic) are a bit weird. I have given Inlet BC as mass flow (400 kg/s) through a rectangular face (0.9 x 0.8 m). If you see pic 1, the maximum velocity by CFX is shown to be very high.

Also even though I have given gravity -10 m/s2 in y-direction, the flow does not free fall. Outflow BC, also is a mass flow (400 kg/s distributed through 4 pipes)

Can you help on this?

Thanks!

ghorrocks · January 26, 2018, 17:21

Please show images of the volume fractions. Also post your CCL.

Dev4415 · January 29, 2018, 07:22

Hi Glenn! Am sorry for the late response, there was a problem with my ANSYS License.

I have done some changes (gave an inlet condition of velocity and outlet of mass flow in the setup, attaching the ccl file in .txt format and volume fraction hereby.

Thanks!

ghorrocks · January 29, 2018, 16:52

Your domain is almost completely filled with water. If you intended this to be a jet of water squirting into air you are going to have to define things so the main body is filled with air, not water.

In your case it appears your boundary condition settings mean the water level will rise to fill the volume. So you will need to define boundary conditions which define the correct water level height.

I also note you have the surface tension model on. I would not expect surface tension to be significant in this flow, and it is a very expensive model to use. Remove it, unless you know you need it.

Dev4415 · January 30, 2018, 10:04

Thanks Glenn! I figured those out, tried few iterations!

And yeah, I will put the posts on the dedicated forum!

Thanks for the help!

January 11, 2018, 04:16	Free surface simulation	#1
Dev4415 New Member Dev Join Date: Jan 2018 Posts: 8 Rep Power: 8	Hi everyone! This is my first post here. I had a doubt regarding the bump2D tutorial in CFX. For free surface simulation, is it always required to pre-define the DownH and DownPres? Shouldn't the liquid surface develop on its own?

January 29, 2018, 16:52		#9
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,819 Rep Power: 144	Your domain is almost completely filled with water. If you intended this to be a jet of water squirting into air you are going to have to define things so the main body is filled with air, not water. In your case it appears your boundary condition settings mean the water level will rise to fill the volume. So you will need to define boundary conditions which define the correct water level height. I also note you have the surface tension model on. I would not expect surface tension to be significant in this flow, and it is a very expensive model to use. Remove it, unless you know you need it. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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January 12, 2018, 03:06		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,819 Rep Power: 144	Yes, this example is a little unusual in that it defines these properties. In general they are not known and you use the simulation to determine it. Boundary conditions are tricky in free surface flow and require careful thought for them to work properly. That is why there has been many questions on the topic on the forum. But the approach used in the tutorial is unusual and most people cannot use that approach.

January 13, 2018, 17:27		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,819 Rep Power: 144	In the final chamber the water just goes around and around in the vortex. Unless you seed a streamline in this vortex you won't get any streamlines in the vortex.

January 14, 2018, 05:56		#5
Dev4415 New Member Dev Join Date: Jan 2018 Posts: 8 Rep Power: 8	Thanks a lot! Got the streamlines in the recirculating zone Now. We can also get them by seeding them from downstream with backward integration.

January 26, 2018, 17:21		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,819 Rep Power: 144	Please show images of the volume fractions. Also post your CCL.

January 30, 2018, 10:04		#10
Dev4415 New Member Dev Join Date: Jan 2018 Posts: 8 Rep Power: 8	Thanks Glenn! I figured those out, tried few iterations! And yeah, I will put the posts on the dedicated forum! Thanks for the help!