# Boundary Conditions Free Surface

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 January 4, 2010, 18:47 Boundary Conditions Free Surface #1 New Member   Cristina Join Date: Jan 2010 Posts: 4 Rep Power: 9 Sponsored Links Hello, I am trying to simulate a 3d free surface flow around a sphere. I am doing the same boundary conditions that the bump2d tutorial has but the free surface of the result is not getting what it should, with regular waves like a ship. In the tutorial does not mention the type of boundary condition in the top so this is my question. Thanks Pedro Lopes

 January 5, 2010, 06:12 #2 Senior Member   Roland Rakos Join Date: Mar 2009 Posts: 131 Rep Power: 10 Hi, if you have applied the same boundary conditions as in the tutorial, than look after the hydrostatic pressure distribution at OUTLET and INLET (UpH and DownH)!! I have studied the free surface flows CFD simulation and I have experienced that tha developed fluid surface is very sensitive to the UpH and DownH. By the way a complicated free surface flow (e.g. around a ship, sphere, cube etc.) generally contains a lot of transient phenomenons (e.g. hydraulic jump) which can be calculated by transient simulation...These transient phenomenons can not develop with steady state simulation, your calculations will not converge... Regards Roland

 January 5, 2010, 10:01 #3 New Member   Cristina Join Date: Jan 2010 Posts: 4 Rep Power: 9 Hello Roland, Thanks for the answer. In the top region of the domain i do not know the boundary condition of the bump2d tutorial so that i can match my top surface. I am trying several options. Can you tell me what is the best boundary condition? Its because I established the height of the free surface at the inlet and at the outlet and the flow at the inlet and at the outlet has that height but in the midle of the domain it has a huge hole and the free surface height does not reach the sphere. I think it has something to do with the boundary conditions of the top and the outlet of the domain. Thanks

 January 5, 2010, 12:04 free surface flow #4 New Member   Join Date: Jun 2009 Posts: 13 Rep Power: 10 hi,happy new year every body! so i´m simulating a free surface flow over a polymerfilm. i have to considere steady state and transient. my first question is:how to get the temperature of the cell on the surface,because is it not the same on the bottom? second :by transient flow i get after starting run a fatal error: Variable : water.Temperature Domain : Default Domain thank u very much

January 6, 2010, 10:03
#5
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Roland Rakos
Join Date: Mar 2009
Posts: 131
Rep Power: 10
Quote:
 Originally Posted by friducha Hello Roland, Thanks for the answer. In the top region of the domain i do not know the boundary condition of the bump2d tutorial so that i can match my top surface. I am trying several options. Can you tell me what is the best boundary condition? Its because I established the height of the free surface at the inlet and at the outlet and the flow at the inlet and at the outlet has that height but in the midle of the domain it has a huge hole and the free surface height does not reach the sphere. I think it has something to do with the boundary conditions of the top and the outlet of the domain. Thanks
Hello Christina!

Yes, I remember...I had this "huge hole" in my simulation too. In my case the flow domain had a simple ship body...and in the simulation the water don"t reach the body....so the ship has floated in the air

This was a long time ago ,I cannot say exactly what was the right solution, but I try to give some suggstions....

1. First, I can suggest the "wall" boundary condition with "free slip" option to the top surface. In the tutorial this surface is "opening" where the air volume fraction is 1, but I always define "free slip wall" to the top surface, in case of free surface flow...

2. Than check the surface tension value! In your simulation is this 0.072 N/m?

3. Can you say the height of the water at INLET and OUTLET? (UpH/DownH).

4. How many the Water/Air Volume Fraction in the Global Initialization?
(In case of free surface flow the results can contain large numerical errors if the initialization is incorrect)

Regards
Roland

 January 6, 2010, 11:53 #6 Senior Member   Roland Rakos Join Date: Mar 2009 Posts: 131 Rep Power: 10 By the way, I think that the main problem is the OUTLET in your case. It is possible, that you have defined a water height which can not develop in the real....or it can develop but you can represent it only with transient simulation....(I have mentioned already; the free surface flows generally contain a lot of transient phenomenons (e.g. waves on the surface, vorticies, hydraulic jumps etc.)) Have you defined the water height at INLET/OUTLET based on measuring? It is very significant what can occur in the real with your boundaries....a transient or a steady state flow phenomenon? Roland

 January 7, 2010, 20:14 #7 New Member   Cristina Join Date: Jan 2010 Posts: 4 Rep Power: 9 Hello Roland, I tried the top surface as wall free slip but it went overflow. About the surface tension, where can i see it? In the help menu it only explains what is it, nothing more. The height of the UpH and DownH is 5 meters to both. I have the sphere with 5 meter deep water and with 5 meter with air above it. What do you mean with "How many the Water/Air volume fraction in the Global initialization"? I have the global initialisation equaling the inlet, is it ok? I can change the simulation to transient instead of steady state by altering the timesteps correct? Thanks Pedro

 January 12, 2010, 08:13 #8 Senior Member   Roland Rakos Join Date: Mar 2009 Posts: 131 Rep Power: 10 Hello Pedro Your Global Initialization is right... The "overflow" can be because of many things but it's possible that your meshquality is not correct. The mesh has to be very fine near the sphere and the fluid surface! If the solver can not calculate the large gradient (density gradient, pressure gardient etc.) than you will get "overflow" (generally). Could you send a pictures about your mesh? Roland

January 31, 2010, 17:52
#9
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Cristina
Join Date: Jan 2010
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Hello Rolland,

I now have some pictures that i leave here as attachment.
What do you think of the mesh?

I am experiencing a regular pattern in the solver of the 7 lines RMS. This is due to the fact that the flow is transient? Should I stop in some particular time?

I re-mesh the domain and got a mesh of 93 megas which was quite a lot for my computer so I changed the memory allocation factor. This does not change the results right?

Thanks
Attached Images
 mesh center of sphere.jpg (99.6 KB, 88 views) Mesh free surface sphere.jpg (93.5 KB, 70 views) Mesh free surface view from top.jpg (95.1 KB, 72 views) Mesh Side Wall.jpg (86.2 KB, 73 views)

 February 19, 2013, 06:55 Ship On Free Surface #10 New Member   Join Date: Feb 2013 Posts: 6 Rep Power: 6 I am Sony, from Indonesia now i am doing my master thesis project by using CFD ansys 13 version. i intend to generate free surface simulation to predict ship resistance. the boundary condition in tutorial in term "free surface over bump" does not show good result. the simulation is always not finish. any body can help for explaining boundary condition used in free surface ship simulation in CFX? and may be also showing physical model that can be adopt? Thank you Regard Sony

 February 19, 2013, 18:03 #11 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,721 Rep Power: 106 First of all, do not spam the forum with the same post in multiple places - including private messaging me in the spam. It is very annoying. I have deleted the duplicate posts and deleted the PM. Just post your question once, that is all which is required. And your question is an FAQ anyway: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

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