Calculating gradient only at patchInternalField

7islands · January 30, 2007, 20:19

Hi all,
I'd like to know how to calculate gradient (not surface-normal gradient) at patchInternalField within a boundary condition code in a computationally efficient way. Doing it simple programming-wise I could write

#include "fvcGrad.H"
const volScalarField& phi = db().lookupObject<volscalarfield>("phi");
const Field<vector> gPhip
= patchInternalField(Field<vector>(fvc::grad(phi)));

but obviously this is computationally too inefficient (calculating gradient of the entire field just for patchInternalField).

I appreciate any suggestions.

Thanks,
Takuya

akidess · February 2, 2011, 05:17

Takuya, did you ever find a more efficient way to do this?

7islands · February 2, 2011, 20:36

I'm afraid not.

T

eugene · March 17, 2011, 09:51

I just had a look at the grad calculation code and what you suggest should be relatively easy to implement (for an explicit gauss gradient at least, I didn't check the requirements for least squares and other gradient calculation methods).

The gradient is calculated from: grad(phi)_cell = sum_cellfaces(Sf*phi_f) / cellVolume;

Simple as that. So all you would need for the gradient at the patchInternalField is to loop through the cells, calculate the interpolated face value on each face in the set and compute the simple sum. There is no parallel complexity or anything required that I can see. You will just have to find an efficient way to limit the number of face interpolations so there is no duplicate calculation.

ata · March 20, 2011, 09:43

Hi
I have a question. May be it is not related. What is db() in db().lookupObject?
Thanks a lot

deepsterblue · March 20, 2011, 19:59

I actually think this is straightforward. Create a cellSet and use that to make an fvMeshSubset. This should also carry all necessary information to estimate the cell-centered gradient that you're asking for.

7islands · March 20, 2011, 22:06

Hi all, thanks for the input.

@Eugene: Yes you are right, that was exactly what came up to my mind, but I was so lazy that I thought there would have been a simpler (or more OpenFOAM-ish) way.

@ata: db() returns a reference to the object registry.

@Sandeep: That sounds like what I was looking for at that time if the subsetMesh and the field do not have to be constructed each time step. In fact I no longer have to do this, but shall try your proposal if I have to do this next time

Takuya

January 30, 2007, 20:19	Hi all, I'd like to know how	#1
7islands Super Moderator Takuya OSHIMA Join Date: Mar 2009 Location: Niigata City, Japan Posts: 518 Blog Entries: 1 Rep Power: 20	Hi all, I'd like to know how to calculate gradient (not surface-normal gradient) at patchInternalField within a boundary condition code in a computationally efficient way. Doing it simple programming-wise I could write #include "fvcGrad.H" const volScalarField& phi = db().lookupObject<volscalarfield>("phi"); const Field<vector> gPhip = patchInternalField(Field<vector>(fvc::grad(phi))); but obviously this is computationally too inefficient (calculating gradient of the entire field just for patchInternalField). I appreciate any suggestions. Thanks, Takuya

March 20, 2011, 09:43	What is db?	#5
ata Senior Member ata kamyabi Join Date: Aug 2009 Location: Kerman Posts: 323 Rep Power: 18	Hi I have a question. May be it is not related. What is db() in db().lookupObject? Thanks a lot

March 20, 2011, 19:59		#6
deepsterblue Senior Member Sandeep Menon Join Date: Mar 2009 Location: Amherst, MA Posts: 403 Rep Power: 25	I actually think this is straightforward. Create a cellSet and use that to make an fvMeshSubset. This should also carry all necessary information to estimate the cell-centered gradient that you're asking for. __________________ Sandeep Menon University of Massachusetts Amherst https://github.com/smenon

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Calculating Drag	Ajay Rao	FLUENT	8	February 15, 2010 10:15
Calculating torque	Andrew	FLUENT	5	September 4, 2007 12:10
calculating temp	student	Main CFD Forum	3	December 26, 2006 08:53
Calculating the average y+ value	Sebastiaan Bottenheim	FLUENT	1	May 11, 2004 10:25
errors in calculating	ustcer	FLUENT	1	April 4, 2004 14:08

February 2, 2011, 05:17		#2
akidess Senior Member Anton Kidess Join Date: May 2009 Location: Germany Posts: 1,377 Rep Power: 30	Takuya, did you ever find a more efficient way to do this?

February 2, 2011, 20:36		#3
7islands Super Moderator Takuya OSHIMA Join Date: Mar 2009 Location: Niigata City, Japan Posts: 518 Blog Entries: 1 Rep Power: 20	I'm afraid not. T

March 17, 2011, 09:51		#4
eugene Senior Member Eugene de Villiers Join Date: Mar 2009 Posts: 725 Rep Power: 21	I just had a look at the grad calculation code and what you suggest should be relatively easy to implement (for an explicit gauss gradient at least, I didn't check the requirements for least squares and other gradient calculation methods). The gradient is calculated from: grad(phi)_cell = sum_cellfaces(Sf*phi_f) / cellVolume; Simple as that. So all you would need for the gradient at the patchInternalField is to loop through the cells, calculate the interpolated face value on each face in the set and compute the simple sum. There is no parallel complexity or anything required that I can see. You will just have to find an efficient way to limit the number of face interpolations so there is no duplicate calculation.

March 20, 2011, 22:06		#7
7islands Super Moderator Takuya OSHIMA Join Date: Mar 2009 Location: Niigata City, Japan Posts: 518 Blog Entries: 1 Rep Power: 20	Hi all, thanks for the input. @Eugene: Yes you are right, that was exactly what came up to my mind, but I was so lazy that I thought there would have been a simpler (or more OpenFOAM-ish) way. @ata: db() returns a reference to the object registry. @Sandeep: That sounds like what I was looking for at that time if the subsetMesh and the field do not have to be constructed each time step. In fact I no longer have to do this, but shall try your proposal if I have to do this next time Takuya