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-   -   Free surface flow over a bump (http://www.cfd-online.com/Forums/openfoam-solving/58871-free-surface-flow-over-bump.html)

qtian December 10, 2007 16:02

Hello all, I am using rasIn
 
Hello all,

I am using rasInterfoam to simulation a flow over a bump. This picture shows the test condition. Water is about half height of the bump and the free stream velocity is about 5 m/s.

http://www.cfd-online.com/OpenFOAM_D...ges/1/6135.jpg

Here is the free surface simulation results when it reaches to steady case.

http://www.cfd-online.com/OpenFOAM_D...ges/1/6136.jpg

It seems like the free surface is over the bump crest. This is not right physically. I am trying to figure out what might be possiblly wrong.

The only thing I can think about is the boundary condition for the bump surface. I chose the typical wall function condition for the bump surface. I know there are some other option for the wall treatment such as wallContactAngle in rasInterfoam. However I can not find any reference about how to define wallContactAngle boundary condtion and also I am not quite sure that that is the problem.

Can anyone give me some help here? Thanks.

liu December 11, 2007 12:56

Maybe you can upload your case
 
Maybe you can upload your case so we can have a look.

qtian December 11, 2007 13:15

Liu, I can upload my case fil
 
Liu,
I can upload my case file. The size will be aournd 100 mb. Will this be a problem? Thanks.

QT

liu December 11, 2007 13:57

Don't include the time steps e
 
Don't include the time steps except "0".
Unless you have a very complicated mesh, I don't think the case file will be huge.

qtian December 11, 2007 14:37

Xiaofeng, Here is the link.
 
Xiaofeng,

Here is the link.
http://files-upload.com/files/672787...urfaceBump.tgz

Please let me know if you have any trouble to download. My mesh file size is bout 70 mb. I includes 0, constant and system folder.

As you might see, I used k-omega model for the turbulence. Thanks for your help.

QT

liu December 11, 2007 15:00

I can't finish the downloading
 
I can't finish the downloading.
Delete the mesh file and upload it here.

qtian December 11, 2007 15:23

Here it is, QT
 
Here it is,



QT

qtian December 11, 2007 15:32

Here it is, http://www.cfd
 
Here it is,

http://www.cfd-online.com/OpenFOAM_D...hment_icon.gif FreeSurfaceBump.tgz

QT

qtian May 8, 2008 10:19

Xiaofeng, did you happen to lo
 
Xiaofeng, did you happen to look at my case? I am revisiting my free surface simulation now,after half year http://www.cfd-online.com/OpenFOAM_D...part/happy.gif. Your input is greatly appreciated. If someone else know where I did wrong, please let me know also.

Anothter question, did someone simluate free surface flow around a cylinder before with OpenFoam? I thought this will be a great test case for rasinterfoam. I would like to see some results,if we have it. If no one did, I would like to give a try and update my results in this thread.

Thanks.

QT

soeren May 9, 2008 08:34

I'm also interested to have a
 
I'm also interested to have a look at your test case. But without your mesh file or a CAD geometry and a description how you generated the mesh I can't.

Are you aware of the fact that at small scales of the bump the kinetic energy of the fluid is large enough to cover the bump exactly as you observed?

In addition I have a question to you, because you are apparently more experienced with rasInterFoam than I. Are the small scale waves downstream of the bump physically existent? Or are they artefacts of the surface reconstruction? Or numerical oscillations - is the magnitude of these oscillations bounded?
The same small scale waves are visible at http://www.opencfd.co.uk/src/damBreakRefineUnrefine.gif.

Soeren

qtian May 15, 2008 16:59

Soeren, I solved free surfa
 
Soeren,

I solved free surface issue and the results looks more realistic now. The problem is that I have a wrong definition of the boundary conditions at the side walls. I will update my results later. For your question, the water height is about half of the bump height. The bump scale is not that small. We should observe hydrolic jump.Thanks for your interests.

Best Regard

Quinn

jaswi May 16, 2008 07:46

Hi Quinn Greetings Will
 
Hi Quinn

Greetings

Will it be anyhow please possible to get the corrected test case for learning purposes.

I would appreciate any reply. My email i.d. is jaswi.singh@gmail.com

Warm Regards
Jaswi

qtian May 16, 2008 10:35

Jaswi, I will be happy to
 
Jaswi,


I will be happy to make some contribution to our forum. Since this is
a 3d bump, the mesh size is too big. I am not sure that we can include
such information in the test case. If you don't think that will be a
problem, I will be glad to work on a test case.


Quinn

alexandrepereira May 16, 2008 11:27

Hi Jaswi, I have been follo
 
Hi Jaswi,

I have been following this topic with some interest since, i wanto to model the two phase flow around a boat...

Just give me a hint, how do you specify which part of the domain is to be filled with water, and which is to be filled with air?

How do you impose a freestream velocity on water and air on the same ( inlet ) patch...?

Do i need to specify some kind of boundary between water and air or is it implemented in rasInterFoam?

Do I have to specify noslip conditions on water and air separately in respect to the hull structure, or is it automatically done in RasInterFoam...?

Sorry for my nooish questions, but i am a newbie in rasInterFoam...

Thanks in advance,

Alex

soeren May 17, 2008 10:05

>Since this is a 3d bump, the
 
>Since this is a 3d bump, the mesh size is too big.

You have probably generated the mesh from some geometry file which is only a few kilobytes in size. If this geometry format is understood by blockMesh or Gmsh or other free meshers, just attach the geometry and everybody could generate the mesh.

If you have the geometry in a format which can't be converted to be read by free tools, please describe the dimensions, and I'd try to write a blockMeshDict.

Soeren

alexandrepereira May 18, 2008 20:50

Hi again... I have followed
 
Hi again...

I have followed the rasInterFoam tutorial, as well as a fine tutoriasl on rasInterFoam, interFoam and lesInterFoam that i found in

http://www.tfd.chalmers.se/~hani/kurser/OF_phD_2007/

My difficulty here in extrapolating from these tutorials to my case lies in:

which boundary conditions apply to the external patches... wall or wallFunctions...? since i am dealing with rasInterFoam, i guess that i will better use wallfunctions for my external ( parallel to flow ) patches, and for my internal ( hull boundary ) patch.

But that is the least of my problems:

I setFields, after creating a setFieldsDict specifying the gamma=1 below my draft plane ( i will simulate the seakeeping ability of my designed aircraft carrier in OpenFOAM... :-) ) , and gamma=0 as default, the default vectorfield U is set to ( -16.666 0 0 ) by default in either domain, and g is set to (0 0 -9.81 )

But i feel that specifying pd as uniform through the domain just isn't correct, as well as specifying an inlet gamma of 1...

is there any way of specifying pd varying with depth in water, inside the mesh, as well as specifying gamma as 1, below draft plane and 0 above draft plane in the inlet patch...?

i think that specifying fixedVelocity Outlet is correct for my problem...

brgds

alexandrepereira May 21, 2008 08:26

Ok ppl, I guess that i ( pa
 
Ok ppl,

I guess that i ( partially ) solved my problem :

I initialized gamma=1 below waterplane with setFields; as to the inlet, outlet and sidewalls of my domain i initialized gamma with funkySetFields, but in order to fet some realistic results, i used a mesh of about one and a half million elements, which combined with the magnitude of U gives a Courant number below 1 only for time steps of 0.001 in order of magnitude...

well, since i want to acheive stationary flow condition, i will have to integrate all along a 200 second time with that time step... meaning 200000 iterations with a 0.001 time step...

My choice to integrate in a 200s time period has to do with the time scale required to acheive steady condition, roughly 10*L/U, and i am considering U=16.666m/s ( 35 knots )...

... and i am just using interFoam... not rasInterFoam yet...

My domain is 330x100x100 m^3, with a hull of about 100m length inside, gamma is set to be 1 below my design waterline, inside the domain and in all patches except the bottom and top.

So tell me, if somebody knows... :-)

How should i reduce the computational burden...?

1. Use some similarity technique with Froude/Reynolds numbers so as to acheive a similar Courant number 0.2-0.9 with a bigger time step...?

2. Use Half a model ( although i think that this is not advisable, since i want to study the roll-trim coupling in a future analysis in a virtual wave tank ) ?

3. Make the whole analysis with single precision version of OpenFOAM 1.4.1 ?

Best regards

Alex

Arnoldinho August 10, 2010 10:15

Hi Alexandre,

this thread is rather old, so sorry for posting in here again. But I found your thread when looking for a solution for simulation free-surface flow.

I want to model free-surface flow in a (laboratory) channel with structures in it, so that I can model the flow around them. I need a free surface (water and air) so that I can later compare the results to flows due to waves.

Did you finally find a working solution for your modeling problem? I'm especially interested in your boundary conditions, so how did you set up the constant velocity water patch with air above it at the inlet and outlet boundaries?

Regards,

Arne


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