[snappyHexMesh] Add layers- symmetric boundary condition

prapanj · May 28, 2009, 06:38

Hi,

I have attached a rough picture of the domain I am planning to solve. The floor and ceiling mentioned in the fig are actually symmetry boundary conditions. So I want layers to be added all through the cylinder wall. But this doesn't happen( even after I played with a number of parameters). What I actually want is to have layers present from the floor till ceiling around the cylinder. For some reason the layers thin down towards the floor and ceiling and no layers are seen at the floor and ceiling.

Can any one tell me if I can achieve what i want?

Thank you

prapanj

santos · May 28, 2009, 07:27

Hi,

I assume you are using snappyHexMesh. As far as I know, what you want to do is still not possible with the current version of snappyHexMesh. I also have the same problem as you do, and if you search here in the forum you will notice we are not the only ones.

Maybe an option would be to add layers both to the cylinder and symmetry planes.

Regards,
Jose Santos

prapanj · May 28, 2009, 07:34

Hi Santos,

Good to know there are few more in the same boat

I considered using a complete blockMesh grid, like a H grid. But I am going to have another cylinder and will have to constantly rotate these. So it's going to be difficult. I need a y+ of 1.0 for my case. If I just go for surface-wise refinement, I will end up in too many cells. The best option for me is to add layers. Another problem is that I am unable to add more than 10 layers. Do you know of any parameters that relaxes snappy to add layers in difficult times too .(

)

Prapanj.

PS: sorry about not mentioning this as a snappyHexMesh issue though I was posting this under snappyhexmesh section.

santos · May 28, 2009, 09:26

Try playing around with expansionRatio, minThickness, finalLayerRatio and nGrow. Unfortunately, I am not totally sure what is the effect of each of these parameters.

Good luck!

Jose Santos

prapanj · May 29, 2009, 00:49

Santos,

I think I understand what those parameters do.

expansion ratio: is the relative thickness of the layers. If the inner most is one, the next will be 1.5 and the third will be 2.25 etc if it is set to 1.5.

minthickness: It is to prevent the layer becoming thinner than this value.

finalLayerratio: It is the ratio between outer most layer's width and the next outer cell. I understand that finalLayer ratio and expansion ratio are the two parameters that let us get the layer thickness we need.

I am not sure what nGrow does. I think I will be able to add more layers only by setting the advanced parameters or the mesh Quality parameters. Because layers are added after the domain is meshed. And during layer addition, the surrounding mesh is moved outward.

Thanks

Prapanj.

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May 28, 2009, 07:27		#2
santos Senior Member Jose Luis Santos Join Date: Mar 2009 Location: Portugal Posts: 215 Rep Power: 18	Hi, I assume you are using snappyHexMesh. As far as I know, what you want to do is still not possible with the current version of snappyHexMesh. I also have the same problem as you do, and if you search here in the forum you will notice we are not the only ones. Maybe an option would be to add layers both to the cylinder and symmetry planes. Regards, Jose Santos

May 28, 2009, 07:34		#3
prapanj Senior Member Prapanch Nair Join Date: Mar 2009 Location: Bangalore, India Posts: 105 Rep Power: 17	Hi Santos, Good to know there are few more in the same boat I considered using a complete blockMesh grid, like a H grid. But I am going to have another cylinder and will have to constantly rotate these. So it's going to be difficult. I need a y+ of 1.0 for my case. If I just go for surface-wise refinement, I will end up in too many cells. The best option for me is to add layers. Another problem is that I am unable to add more than 10 layers. Do you know of any parameters that relaxes snappy to add layers in difficult times too .( ) Prapanj. PS: sorry about not mentioning this as a snappyHexMesh issue though I was posting this under snappyhexmesh section.

May 28, 2009, 09:26		#4
santos Senior Member Jose Luis Santos Join Date: Mar 2009 Location: Portugal Posts: 215 Rep Power: 18	Try playing around with expansionRatio, minThickness, finalLayerRatio and nGrow. Unfortunately, I am not totally sure what is the effect of each of these parameters. Good luck! Jose Santos

May 29, 2009, 00:49		#5
prapanj Senior Member Prapanch Nair Join Date: Mar 2009 Location: Bangalore, India Posts: 105 Rep Power: 17	Santos, I think I understand what those parameters do. expansion ratio: is the relative thickness of the layers. If the inner most is one, the next will be 1.5 and the third will be 2.25 etc if it is set to 1.5. minthickness: It is to prevent the layer becoming thinner than this value. finalLayerratio: It is the ratio between outer most layer's width and the next outer cell. I understand that finalLayer ratio and expansion ratio are the two parameters that let us get the layer thickness we need. I am not sure what nGrow does. I think I will be able to add more layers only by setting the advanced parameters or the mesh Quality parameters. Because layers are added after the domain is meshed. And during layer addition, the surrounding mesh is moved outward. Thanks Prapanj.