Is 2D axisymmetric interFoam accurate

lasherwc · February 22, 2019, 10:55

I'm solving a problem involving injection of a highly viscous fluid into a container. I've modeled the problem as 3D and I'm getting good results. When I modeled it as a 2D axisymmetric problem the solution is different. I've attached pics of both simulations.

The axisymmetric problem is a 5 degree wedge with one cell in the thickness direction and wedge boundary conditions.

My question is, is this accurate in interFoam? There will be curvature in the thickness direction and I don't think a 2D simulation will pick that up, thus no theta surface tension. Am I heading in the right direction?

frantov · July 26, 2020, 22:03

did you solve this problem? Im also finding some issues along the axis when using axisymmetry for InterFoam

You may need to delete one of the boundaries in the polymesh/boundary file

lasherwc · July 31, 2020, 17:27

No, I was not able to fix the problem. What boundary should be deleted? I have a few walls, an inlet, an atmospheric outlet, symmetry line, and two wedge patches.

acalado · November 10, 2020, 12:58

Are you using GMSH? You may have to go to the polyMesh/boundary file and delete the default faces part (usually at the end of the file). Also you need to change the number of boundary patches at the top of the file from N to N-1.

lasherwc · November 10, 2020, 17:51

I used Blockmesh and the only faces in the boundary file are those i created and named in blockmesh. There are no default faces.

Bodo1993 · January 9, 2021, 17:58

Quote:

Originally Posted by lasherwc

I used Blockmesh and the only faces in the boundary file are those i created and named in blockmesh. There are no default faces.

Hi, I am wondering if you have reached any conclusions. Thanks.

lasherwc · January 9, 2021, 18:14

No, I have not - still wondering what's going on.

Bodo1993 · January 9, 2021, 19:29

Thanks, I actually running a 2D planar and 2D axisymmetric simulations to compare the results. I did not get similar results and I am wondering of the reason. I am not quite sure of what settings should be changed, apart from the mesh/geometry, such that the two cases are comparable.

I am not an expert, but it seems from the pictures you posted that the axisymmetric model has weak inertia. Did you ensure that the velocity inlet condition is pointing in the right direction with the correct sign? Based on the pictures, it should be in the negative y.

lasherwc · January 10, 2021, 09:54

Hi:

A 2D simulation and an axisymmetric simulation will not yield the same results. The flows are fundamentally different - since the "thickness" of the wedge increases with radius from the axis of rotation, there is more flow area near the outside than near the center. Analytically, the equations are different.

My problem is a highly viscous fluid (think molasses although that's not what it is), so diffusion is the predominate mode of transport and convection is almost non-existent. In a full 3-D simulation there will be curvature on the outer surface and I believe surface tension will affect the results. I am theorizing that there is no theta-direction surface tension with an axisymmetric simulation, so the results will be different - just looking for confirmation.

ejaz8894 · October 22, 2021, 17:32

Have you been able to resolve the issue? I was doing 2D axisymmetric simulation of droplet evaporation & seeing some unphysical behaviors along the symmetry axis. I see vapor accumulation inside the droplet along the symmetry axis. I'm not sure if I have to put any special boundary condition for the symmetry axis. I've created a wedge & put the front & back surface as wedge boundary condition & other sides as wall. I have initialized the droplet center at the symmetry axis with the sphereToCell command so that it takes half of the droplet in the axisymmetric domain. I also see a notch formation from the top & bottom side of the droplet along the symmetry axis & vapor starts accumulating inside the droplet from those notches. I think the symmetry axis needs to be treated differently to resolve the issue but I'm not sure how to do that. I can share my case if you want.

lasherwc · October 22, 2021, 18:23

No, I have not resolved it. I don't know exactly how the axis of symmetry is used in the modeling but I haven't seen anything about specifying boundary conditions for it. If the surface of the droplet isn't being calculated correctly due to a lack of theta-direction surface tension (which is my guess as to what the problem is) then I could see a cavity possibly forming which might result in vaporization - although this is just a guess.

ejaz8894 · October 22, 2021, 18:50

How can I properly implement theta-direction surface tension? Currently surface tension force is being calculated based on gradient of alpha volume fraction - just the way it is implemented in InterFoam. And also, how about imposing empty boundary condition for the axis of symmetry? (refer to this link: https://openfoamwiki.net/index.php/M...s/AxiSymmetric)

lasherwc · October 22, 2021, 19:10

Actually I don't know how to implement anything in Openfoam. I am just learning C++ (I learned Fortran a very long time ago :-) and trying to understand how the code works. I guess you could try defining the axis as empty. I gave up on trying to solve this some time ago as the workaround was to build a 3-D model and it's a pretty unique problem.

November 10, 2020, 12:58		#4
acalado Senior Member André Join Date: Mar 2016 Posts: 133 Rep Power: 10	Are you using GMSH? You may have to go to the polyMesh/boundary file and delete the default faces part (usually at the end of the file). Also you need to change the number of boundary patches at the top of the file from N to N-1. __________________ Sapere aude!

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July 26, 2020, 22:03		#2
frantov Member Francisco T Join Date: Nov 2011 Location: Melbourne, Australia Posts: 64 Blog Entries: 1 Rep Power: 14	did you solve this problem? Im also finding some issues along the axis when using axisymmetry for InterFoam You may need to delete one of the boundaries in the polymesh/boundary file

July 31, 2020, 17:27		#3
lasherwc Member Bill Lasher Join Date: Jun 2009 Posts: 36 Rep Power: 16	No, I was not able to fix the problem. What boundary should be deleted? I have a few walls, an inlet, an atmospheric outlet, symmetry line, and two wedge patches.

November 10, 2020, 17:51		#5
lasherwc Member Bill Lasher Join Date: Jun 2009 Posts: 36 Rep Power: 16	I used Blockmesh and the only faces in the boundary file are those i created and named in blockmesh. There are no default faces.

January 9, 2021, 18:14		#7
lasherwc Member Bill Lasher Join Date: Jun 2009 Posts: 36 Rep Power: 16	No, I have not - still wondering what's going on.

January 9, 2021, 19:29		#8
Bodo1993 Senior Member Join Date: Jul 2019 Posts: 148 Rep Power: 6	Thanks, I actually running a 2D planar and 2D axisymmetric simulations to compare the results. I did not get similar results and I am wondering of the reason. I am not quite sure of what settings should be changed, apart from the mesh/geometry, such that the two cases are comparable. I am not an expert, but it seems from the pictures you posted that the axisymmetric model has weak inertia. Did you ensure that the velocity inlet condition is pointing in the right direction with the correct sign? Based on the pictures, it should be in the negative y.

January 10, 2021, 09:54		#9
lasherwc Member Bill Lasher Join Date: Jun 2009 Posts: 36 Rep Power: 16	Hi: A 2D simulation and an axisymmetric simulation will not yield the same results. The flows are fundamentally different - since the "thickness" of the wedge increases with radius from the axis of rotation, there is more flow area near the outside than near the center. Analytically, the equations are different. My problem is a highly viscous fluid (think molasses although that's not what it is), so diffusion is the predominate mode of transport and convection is almost non-existent. In a full 3-D simulation there will be curvature on the outer surface and I believe surface tension will affect the results. I am theorizing that there is no theta-direction surface tension with an axisymmetric simulation, so the results will be different - just looking for confirmation.

October 22, 2021, 17:32		#10
ejaz8894 New Member Ejaz Ahmed Join Date: May 2021 Posts: 7 Rep Power: 4	Have you been able to resolve the issue? I was doing 2D axisymmetric simulation of droplet evaporation & seeing some unphysical behaviors along the symmetry axis. I see vapor accumulation inside the droplet along the symmetry axis. I'm not sure if I have to put any special boundary condition for the symmetry axis. I've created a wedge & put the front & back surface as wedge boundary condition & other sides as wall. I have initialized the droplet center at the symmetry axis with the sphereToCell command so that it takes half of the droplet in the axisymmetric domain. I also see a notch formation from the top & bottom side of the droplet along the symmetry axis & vapor starts accumulating inside the droplet from those notches. I think the symmetry axis needs to be treated differently to resolve the issue but I'm not sure how to do that. I can share my case if you want.

October 22, 2021, 18:23		#11
lasherwc Member Bill Lasher Join Date: Jun 2009 Posts: 36 Rep Power: 16	No, I have not resolved it. I don't know exactly how the axis of symmetry is used in the modeling but I haven't seen anything about specifying boundary conditions for it. If the surface of the droplet isn't being calculated correctly due to a lack of theta-direction surface tension (which is my guess as to what the problem is) then I could see a cavity possibly forming which might result in vaporization - although this is just a guess.

October 22, 2021, 18:50		#12
ejaz8894 New Member Ejaz Ahmed Join Date: May 2021 Posts: 7 Rep Power: 4	How can I properly implement theta-direction surface tension? Currently surface tension force is being calculated based on gradient of alpha volume fraction - just the way it is implemented in InterFoam. And also, how about imposing empty boundary condition for the axis of symmetry? (refer to this link: https://openfoamwiki.net/index.php/M...s/AxiSymmetric)

October 22, 2021, 19:10		#13
lasherwc Member Bill Lasher Join Date: Jun 2009 Posts: 36 Rep Power: 16	Actually I don't know how to implement anything in Openfoam. I am just learning C++ (I learned Fortran a very long time ago :-) and trying to understand how the code works. I guess you could try defining the axis as empty. I gave up on trying to solve this some time ago as the workaround was to build a 3-D model and it's a pretty unique problem.