Accessing surface normal gradient

Jakob1 · December 10, 2013, 08:54

Hello,
I am trying to access the surface normal gradient of a certain variable (i.e. a) (unstructured grid). fvc::snGrad(a) only gives me the absolute value, right? (surfaceScalarField). How do I get the surfaceVectorField version of that?

And on top of that: How does this (hopefully existing) quantity behave on boundary surfaces of a cell? I imagine it being defined as (a(V1)-a(V2))/d. Now if a has a Diriclet condition on the boundary, how would that work? (Can't define a Neumann bc, the change of gradient on the boundary is crucial for my function).

Would be great if someone could give me a hint here

Thanks in advance.

akidess · December 10, 2013, 09:02

A surface*Normal*Gradient always only has a magnitude, because the direction is given by the face normal. snGrad(a) * mesh.Sf() I guess will give you the gradient vector.

Jakob1 · December 10, 2013, 11:11

mesh.Sf() also includes the face area rght? So I guess mesh.Sf()/mesh.magSf() should do it then? Thanks by the way!

Still though, how does it behave at boundaries? Would it use the value on the boundary instead of the neighboring volume?

akidess · December 10, 2013, 11:23

Right. About the boundary question - why would snGrad neglect your boundary conditions, no matter what you select?

fluidflowsteel · January 6, 2017, 06:22

Hi ,

I have a variable p for which I want to calculate the gradient normal to the surface in both x and y directions. Can anyone help me ?

With regards

PositronCascade · January 9, 2017, 08:22

I suggest you write your own tool by modifying gradU utility of OpenFOAM or you can use swak4foam, I guess.

Quote:

Originally Posted by fluidflowsteel

Hi ,

I have a variable p for which I want to calculate the gradient normal to the surface in both x and y directions. Can anyone help me ?

With regards

December 10, 2013, 08:54	Accessing surface normal gradient	#1
Jakob1 New Member Join Date: Nov 2013 Posts: 20 Rep Power: 12	Hello, I am trying to access the surface normal gradient of a certain variable (i.e. a) (unstructured grid). fvc::snGrad(a) only gives me the absolute value, right? (surfaceScalarField). How do I get the surfaceVectorField version of that? And on top of that: How does this (hopefully existing) quantity behave on boundary surfaces of a cell? I imagine it being defined as (a(V1)-a(V2))/d. Now if a has a Diriclet condition on the boundary, how would that work? (Can't define a Neumann bc, the change of gradient on the boundary is crucial for my function). Would be great if someone could give me a hint here Thanks in advance.

December 10, 2013, 09:02		#2
akidess Senior Member Anton Kidess Join Date: May 2009 Location: Germany Posts: 1,377 Rep Power: 29	A surfaceNormalGradient always only has a magnitude, because the direction is given by the face normal. snGrad(a) * mesh.Sf() I guess will give you the gradient vector. __________________ On twitter @akidTwit Spend as much time formulating your questions as you expect people to spend on their answer.

December 10, 2013, 11:23		#4
akidess Senior Member Anton Kidess Join Date: May 2009 Location: Germany Posts: 1,377 Rep Power: 29	Right. About the boundary question - why would snGrad neglect your boundary conditions, no matter what you select? __________________ On twitter @akidTwit Spend as much time formulating your questions as you expect people to spend on their answer.

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December 10, 2013, 11:11		#3
Jakob1 New Member Join Date: Nov 2013 Posts: 20 Rep Power: 12	mesh.Sf() also includes the face area rght? So I guess mesh.Sf()/mesh.magSf() should do it then? Thanks by the way! Still though, how does it behave at boundaries? Would it use the value on the boundary instead of the neighboring volume?

January 6, 2017, 06:22		#5
fluidflowsteel New Member fluidflowsteel Join Date: Jun 2016 Posts: 21 Rep Power: 9	Hi , I have a variable p for which I want to calculate the gradient normal to the surface in both x and y directions. Can anyone help me ? With regards