Cube root in OpenFOAM

zcheng · July 2, 2019, 08:38

Hello everyone,

Does anyone knows how to put cube root in OpenFOAM?

clapointe · July 5, 2019, 08:59

Yes, this is quite straightforward. Knowing that we can calculate the square root of something e.g. x with :

Code:

sqrtX = sqrt(x)

we might guess that foam would use similar abbreviation for the cube root. So we guess that cbrt is likely and search the code for "cbrt" and see many matches. You could try computing a cube root of x as :

Code:

cbrtX = cbrt(x)

I've not used this myself before, so be sure to make sure it does what we expect. Another straightforward option that I use sometimes is the pow function :

Code:

cbrtX = pow(x,1.0/3.0)

Caelan

zcheng · July 5, 2019, 09:42

Thanks a lot!

anon_q · July 5, 2019, 09:43

For a more safe result (including negative numbers) you can use the following:

Code:

 // calculate the cube root of abs(x)
cbrtAbsX = pow(abs(x), 1.0/3.0)
 // the cube root is returned according to the sign of x
cbrtX = (x < 0)? -cbrtAbsX:cbrtAbsX

zcheng · July 5, 2019, 10:24

Thanks for the suggestion!

July 2, 2019, 08:38	Cube root in OpenFOAM	#1
zcheng New Member Join Date: Jun 2019 Posts: 17 Rep Power: 6	Hello everyone, Does anyone knows how to put cube root in OpenFOAM?

July 5, 2019, 08:59		#2
clapointe Senior Member Join Date: Aug 2015 Posts: 494 Rep Power: 14	Yes, this is quite straightforward. Knowing that we can calculate the square root of something e.g. x with : Code: sqrtX = sqrt(x) we might guess that foam would use similar abbreviation for the cube root. So we guess that cbrt is likely and search the code for "cbrt" and see many matches. You could try computing a cube root of x as : Code: cbrtX = cbrt(x) I've not used this myself before, so be sure to make sure it does what we expect. Another straightforward option that I use sometimes is the pow function : Code: cbrtX = pow(x,1.0/3.0) Caelan

July 5, 2019, 09:43		#4
anon_q Senior Member Join Date: Mar 2018 Posts: 115 Rep Power: 8	For a more safe result (including negative numbers) you can use the following: Code: // calculate the cube root of abs(x) cbrtAbsX = pow(abs(x), 1.0/3.0) // the cube root is returned according to the sign of x cbrtX = (x < 0)? -cbrtAbsX:cbrtAbsX randon likes this.

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July 5, 2019, 09:42		#3
zcheng New Member Join Date: Jun 2019 Posts: 17 Rep Power: 6	Thanks a lot!

July 5, 2019, 10:24		#5
zcheng New Member Join Date: Jun 2019 Posts: 17 Rep Power: 6	Thanks for the suggestion!