Setting normal gradient as BC

Dozer_94 · May 3, 2019, 10:27

Hi all,

I am editing icoFoam for a specific calculation in which I want to set the normal gradient of a patch as a boundary condition.

So normally scalar field 'c' would be solved using the BCs for the values of 'c', but now I would like OF to use normal gradient 'dc/dn' as BC.

I know that 'c.boundaryField()[patch].snGrad()' would return the values of the normal gradient, but I'd like it vice versa.

Anyone know if this is possible at all?

Thanks in advance,

fumiya · May 7, 2019, 09:14

Hi,

You can use the fixedGradient boundary condition to set a patch normal gradient to a user-specified value.

Hope this helps,
Fumiya

Dozer_94 · May 7, 2019, 12:22

Hi Fumiya,

Thanks for your reply. I was aware that this was the way to go for the initial conditions. However, I would like OF to do this for me as a BC that changes each timestep.

To give a bit more information: I am using an external solver that calculates dc/dn from other quantities. The goal is to set this calculated dc/dn as BC for the next timestep. Therefore, I need a way (be it a provided member function or a more elaborate method) to incorporate this in the solver itself.

Thanks,

fumiya · May 8, 2019, 10:54

Hi Dozer_94,

The following thread will be of help:
Updating Boundary Conditions Each Iteration

Fumiya

Dozer_94 · May 21, 2019, 11:33

Thank you Fumiya, this was exactly what I was searching for. Seems like something I could have figured out myself, though.

gsam · August 1, 2024, 13:19

If I apply a fixed gradient condition on a patch under atmospheric conditions, will the gradient change over time as the liquid passes through this boundary? For example, if I use `zero gradient` as the boundary condition for the velocity ( U ), and the fluid passes through, would recalculating the gradient later necessarily result in a zero gradient again?

Tobermory · August 2, 2024, 08:11

I think you are misunderstanding the nature of the fixed gradient boundary condition ... it literally fixes the gradient. So yes, the gradient will be maintained as the flow passes through (the patch value will of course change).

gsam · August 5, 2024, 10:36

Thank you for the reply. Apparently, i am dealing with an Electrohydrodynamic solver. The problem is when I apply zero gradients at one of the boundaries as a voltage condition, my electric field changes, which is not supposed to since the electric field is -grad(Voltage). Since I am applying a zero gradient, it should maintain the Ey constant, which is a normal component of the electric field. I am unable to find what is the real issue occurring here.

May 3, 2019, 10:27	Setting normal gradient as BC	#1
Dozer_94 New Member Join Date: Dec 2016 Posts: 24 Rep Power: 9	Hi all, I am editing icoFoam for a specific calculation in which I want to set the normal gradient of a patch as a boundary condition. So normally scalar field 'c' would be solved using the BCs for the values of 'c', but now I would like OF to use normal gradient 'dc/dn' as BC. I know that 'c.boundaryField()[patch].snGrad()' would return the values of the normal gradient, but I'd like it vice versa. Anyone know if this is possible at all? Thanks in advance,

May 7, 2019, 09:14		#2
fumiya Senior Member Fumiya Nozaki Join Date: Jun 2010 Location: Yokohama, Japan Posts: 266 Blog Entries: 1 Rep Power: 19	Hi, You can use the fixedGradient boundary condition to set a patch normal gradient to a user-specified value. Hope this helps, Fumiya __________________ [Personal] Blog: https://caefn.com/ Twitter: https://twitter.com/FumiyaNozaki SlideShare: http://www.slideshare.net/fumiyanozaki96

May 8, 2019, 10:54		#4
fumiya Senior Member Fumiya Nozaki Join Date: Jun 2010 Location: Yokohama, Japan Posts: 266 Blog Entries: 1 Rep Power: 19	Hi Dozer_94, The following thread will be of help: Updating Boundary Conditions Each Iteration Fumiya Gerry Kan likes this. __________________ [Personal] Blog: https://caefn.com/ Twitter: https://twitter.com/FumiyaNozaki SlideShare: http://www.slideshare.net/fumiyanozaki96

August 1, 2024, 13:19	Doubt regarding fixed gradient boundary condition.	#6
gsam New Member ghanashyam k c Join Date: May 2022 Posts: 8 Rep Power: 4	If I apply a fixed gradient condition on a patch under atmospheric conditions, will the gradient change over time as the liquid passes through this boundary? For example, if I use `zero gradient` as the boundary condition for the velocity ( U ), and the fluid passes through, would recalculating the gradient later necessarily result in a zero gradient again?

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May 7, 2019, 12:22		#3
Dozer_94 New Member Join Date: Dec 2016 Posts: 24 Rep Power: 9	Hi Fumiya, Thanks for your reply. I was aware that this was the way to go for the initial conditions. However, I would like OF to do this for me as a BC that changes each timestep. To give a bit more information: I am using an external solver that calculates dc/dn from other quantities. The goal is to set this calculated dc/dn as BC for the next timestep. Therefore, I need a way (be it a provided member function or a more elaborate method) to incorporate this in the solver itself. Thanks,

May 21, 2019, 11:33		#5
Dozer_94 New Member Join Date: Dec 2016 Posts: 24 Rep Power: 9	Thank you Fumiya, this was exactly what I was searching for. Seems like something I could have figured out myself, though.

August 2, 2024, 08:11		#7
Tobermory Senior Member Join Date: Apr 2020 Location: UK Posts: 736 Rep Power: 14	I think you are misunderstanding the nature of the fixed gradient boundary condition ... it literally fixes the gradient. So yes, the gradient will be maintained as the flow passes through (the patch value will of course change).

August 5, 2024, 10:36		#8
gsam New Member ghanashyam k c Join Date: May 2022 Posts: 8 Rep Power: 4	Thank you for the reply. Apparently, i am dealing with an Electrohydrodynamic solver. The problem is when I apply zero gradients at one of the boundaries as a voltage condition, my electric field changes, which is not supposed to since the electric field is -grad(Voltage). Since I am applying a zero gradient, it should maintain the Ey constant, which is a normal component of the electric field. I am unable to find what is the real issue occurring here.