Calculating Forces

Friendly · November 28, 2017, 13:59

Hi,

I intend to calculate the drag and lift on a body. Therefore I would like to know how OpenFOAM calculates those forces. I have no troubles to make the solver calculate thoses forces but I would like to interpret them and therefore I need to know how they are calculated.

In forces.C it appears, that OpenFOAM calculates a total force by integrating the pressure over a surface. The projection in a given direction is the drag force. But what about viscious drag?

Can anybody help me with that?

Lisandro Maders · November 30, 2017, 22:08

Hi,

I took a quick look in the "forces.C" file and I think it should be something close to this:

In the "forces.C" file you will find the function

Code:

void Foam::functionObjects::forces::calcForcesMoment()

in which the pressure, viscous and porous forces are calculated separately. The viscous forces, for instance, are stored as a "vectorField", through an object called "fT" and it is calculated in its construction as follows:

Code:

vectorField fT(Sfb[patchi] & devRhoReffb[patchi])

Somewhere it should have a loop to sweep all surfaces from a given [patchId].

Hope it helps.

Regards, Lisandro Maders

Friendly · December 1, 2017, 06:26

Thank you very much, it does.

I have found this as aswell (and for fN aswell). I have two questions for the second line of code you have posted:

What excatly does "&" mean? And what is "Sfb"? B stands for boundary, but why excatly is Sf? I think it has to be the normal vector fo the surface times the are of the surface (surface of the face).

November 28, 2017, 13:59	Calculating Forces	#1
Friendly Member Join Date: Jun 2017 Posts: 73 Rep Power: 8	Hi, I intend to calculate the drag and lift on a body. Therefore I would like to know how OpenFOAM calculates those forces. I have no troubles to make the solver calculate thoses forces but I would like to interpret them and therefore I need to know how they are calculated. In forces.C it appears, that OpenFOAM calculates a total force by integrating the pressure over a surface. The projection in a given direction is the drag force. But what about viscious drag? Can anybody help me with that?

November 30, 2017, 22:08		#2
Lisandro Maders Member Lisandro Maders Join Date: Feb 2013 Posts: 98 Rep Power: 13	Hi, I took a quick look in the "forces.C" file and I think it should be something close to this: In the "forces.C" file you will find the function Code: void Foam::functionObjects::forces::calcForcesMoment() in which the pressure, viscous and porous forces are calculated separately. The viscous forces, for instance, are stored as a "vectorField", through an object called "fT" and it is calculated in its construction as follows: Code: vectorField fT(Sfb[patchi] & devRhoReffb[patchi]) Somewhere it should have a loop to sweep all surfaces from a given [patchId]. Hope it helps. Regards, Lisandro Maders

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December 1, 2017, 06:26		#3
Friendly Member Join Date: Jun 2017 Posts: 73 Rep Power: 8	Thank you very much, it does. I have found this as aswell (and for fN aswell). I have two questions for the second line of code you have posted: What excatly does "&" mean? And what is "Sfb"? B stands for boundary, but why excatly is Sf? I think it has to be the normal vector fo the surface times the are of the surface (surface of the face).