How to divide a vectorfield by the magnitude if the vector can be zero

tschenkel · October 5, 2021, 18:24

Hello,

I am trying to do some postprocessing with normalised vectors. The vector can be zero, so the simple form:

Un = U/mag(U)

will break with segfault.

Un = U/(mag(U)+1e64)

doesn't work, because the dimensions won't match (1e-64) has dimension [0 0 0 ... 0]

Is there a trick to achieve that?

U is not the velocity, so a specific normalised velocity function wouldn't help. Would need to take any dimensionedVolVectorField.

mAlletto · October 6, 2021, 08:24

You should decide by a small dimensionized scalar. You can find an example here how to convert to dimensionless scalar in openfoam

tschenkel · October 6, 2021, 09:50

Quote:

Originally Posted by mAlletto

You should decide by a small dimensionized scalar. You can find an example here how to convert to dimensionless scalar in openfoam

Thanks, that's very helpful. I was starting to play around with .value() and .dimensions().

Good to know that that is the right path.

olesen · October 9, 2021, 06:09

Need to check if the normalised(..) function works on GeometricFields or only on primitive fields, but could start looking there. It haves a check for ROOTVSMALL to protect the division.

tschenkel · October 9, 2021, 10:16

Quote:

Originally Posted by olesen

Need to check if the normalised(..) function works on GeometricFields or only on primitive fields, but could start looking there. It haves a check for ROOTVSMALL to protect the division.

normalised() isn't defined for volVectorFields, it seems.

tschenkel · October 9, 2021, 10:19

Code:

dimensionedScalar epsWSS = dimensionedScalar("epsWSS",WSS.dimensions(), 1e-64);

creates a very small scalar with dimension of the vector field, which can then be used as:

Code:

normalisedWSS = WSS / (mag(WSS)+epsWSS);

October 5, 2021, 18:24	How to divide a vectorfield by the magnitude if the vector can be zero	#1
tschenkel Member Torsten Schenkel Join Date: Jan 2014 Posts: 69 Rep Power: 12	Hello, I am trying to do some postprocessing with normalised vectors. The vector can be zero, so the simple form: Un = U/mag(U) will break with segfault. Un = U/(mag(U)+1e64) doesn't work, because the dimensions won't match (1e-64) has dimension [0 0 0 ... 0] Is there a trick to achieve that? U is not the velocity, so a specific normalised velocity function wouldn't help. Would need to take any dimensionedVolVectorField.

October 9, 2021, 10:19	SOLVED: create a dimensionedScalar for the very small value epsilon	#6
tschenkel Member Torsten Schenkel Join Date: Jan 2014 Posts: 69 Rep Power: 12	Code: dimensionedScalar epsWSS = dimensionedScalar("epsWSS",WSS.dimensions(), 1e-64); creates a very small scalar with dimension of the vector field, which can then be used as: Code: normalisedWSS = WSS / (mag(WSS)+epsWSS);

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October 6, 2021, 08:24		#2
mAlletto Senior Member Michael Alletto Join Date: Jun 2018 Location: Bremen Posts: 615 Rep Power: 15	You should decide by a small dimensionized scalar. You can find an example here how to convert to dimensionless scalar in openfoam

October 9, 2021, 06:09		#4
olesen Senior Member Mark Olesen Join Date: Mar 2009 Location: https://olesenm.github.io/ Posts: 1,685 Rep Power: 40	Need to check if the normalised(..) function works on GeometricFields or only on primitive fields, but could start looking there. It haves a check for ROOTVSMALL to protect the division.