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- - Forces in V1.6 (https://www.cfd-online.com/Forums/openfoam-post-processing/67984-forces-v1-6-a.html)

I have to change the pRef value to calculate the forces.

I am adding this statement to functions forces: pRef 1000; This does not change anything in values, it looks like it is invariant to the pRef value. How can I adapt the pRef value :confused:

The flow field looked oke but given Cl value was not oke. So, I decided to calculate the Cl value by sampling pressure values. I get, Cl = 0.166, which looks oke , for airfoil at 0 angle of attack. ( I have not calculated shear stress, don't know how to do it yet. For now I assume it is small, icoFoam)

The OpenFoam gives these value:

Cl = 0.00166316;
Cd = 0.0000724

Strange, I see it now, it looks like Cl and Cd is missing a multiply with 100 :eek:

Still I haven't been able to specify the pRef value :o

I have tried this in the controlDict file, to calculate force coefficients: pref pRefPoint (x y z);

but openfoam doesn't see pRefPoint(x y z) as a scalar

I get this error: wrong token type - expected Scalar

Hi,

I would like to be able to take a look at the libforces.so file to understand how it calculate the forces and eventually modify the library. Does anyone knows how to do that?

Thanks

This thread http://www.cfd-online.com/Forums/ope...tml#post324445 is related to force calculations. Does any one know the answer? Thanks

Quote:

Originally Posted by santos (Post 240633)

Hi,

Put something like this in your system/controlDict, changing accordingly to your situation:

Code:

functions

{

forces

{

type forces;

functionObjectLibs ("libforces.so"); //Lib to load

patches (sphere_region0); // change to your patch name

rhoName rhoInf;

rhoInf 1000; //Reference density for fluid

CofR (2 0 0); //Origin for moment calculations

        outputControl   timeStep;

        outputInterval  1;

}

forceCoeffs

{

// rhoInf - reference density

// CofR - Centre of rotation

// dragDir - Direction of drag coefficient

// liftDir - Direction of lift coefficient

// pitchAxis - Pitching moment axis

// magUinf - free stream velocity magnitude

// lRef - reference length

// Aref - reference area

type forceCoeffs;

functionObjectLibs ("libforces.so");

patches (sphere_region0);

rhoName rhoInf;

rhoInf 1000;

CofR (2 0 0);

liftDir (0 1 0);

dragDir (1 0 0);

pitchAxis (0 0 1);

magUInf 1e-7;

lRef 0.06; // sphere diameter

Aref 0.0014137; //1/2 * projected area = pi*r²/2



        outputControl   timeStep;

        outputInterval  1;

}



}

Regards,
Jose Santos

Santos: I am running OpenFOAM 2.0.1. sloshingTank2D in interDyMFoam. I have rectangular tank half filled with water. Is there a quick way of getting the forces imparted by the fluid on the tank walls as the tank moves back and forth? I have some code in control dict that dumps cell alpha values and the corresponding pressure values into files that I have to extract and read.The only way I see of accomplishing this is first recording the cells that have alpha values of less than 1 indicating that the cell has fluid. Then look up the pressure in the cell. Then compute the force due to the pressure. is there a work around this so that I can, for example get the force due to fluid pressure on the left wall (or right wall) of the tank? What I am using right now is as follows:
wallPressure
{
type surfaces;
functionObjectLibs ("libsampling.so");
outputControl outputTime;
outputInterval 5;
surfaceFormat raw;
interpolationScheme cell;

fields ( alpha1
p
);
surfaces
(
leftwalls
{
type patch;
patches (leftWall);
interpolate true;
triangulate false;
}
rightwalls
{
type patch;
patches (rightWall);
interpolate true;
triangulate false;
}
);
}

Hi,

I have used for mine new simulation the solution of old case. Only the angle of attack ( so velocity and lift and drag coefficients ) is changed. The simulations went oke, however the calculated lift and drag coefficients are wrong. If I sample the pressure coefficients than I get the correct Cl , Cd values however not exact as I want. Because of the many simulations I do not want to run them all again. Is there any solution for this issue ?

Is it possible to calculate the aerodynamic coefficients just based on the last time step ?

Wall pressures in InterDyMFoam

Dear all:
I am simulating a small 2D tank with dimensions 8m long, 5m high and 0.01m in depth. I am subjecting the tank to a sinusoidal displacement only and have modified the controlDict to provide wall pressures for the left and right walls at every 0.02 secs. In my opinion, if I am supplying a sinusoidal displacement, the pressures should also be sinusoidal. However when I plot the pressures corresponding to fluid phase in the left and right walls, I get what looks like a very choppy representation, which does not make sense. Any help or advice on how to fix this would be greatly appreciated, Thanks. I have attached the input plot and the pressure plot.

My mesh size is 200x200x1

IRef Aref------> BI ELEMENT AIRFOIL

One question: If I had a bi element airfoil (main&flap), I put in the patches the name of the two walls (main, flap), but for lRef e Aref I need to put 2 different values because the main airfoil and the flap have different chord and so different area. Help me please

Hi Petter,

I know this is an old thread. But I want to know whether you have found a way to calculate the multi-phase torque? Right now I'm using interFoam as well. And I'm stuck with the multi-phase torque calculation. If you have any updates, please let me know. Thank you very much.

Penghcuan

Is there some way to introduce fluid / wall friction? I am running sloshingtank2d in interdymfoam. The sloshing I get goes on as long as the simulation continues. How can I introduce wall friction so that the sloshing will slowly dissipate once the displacement stops?

Hello, Santos,

I have confusions about the value of rhoInf, in this thread, different values appeared, e.g. 1.17, 1.225 and 1000. This difference confuses me.

Thanks for your attention.

Quote:

Originally Posted by paka (Post 276277)

I know the tool is not fully suitable for two-phase flows, but assuming that I only care for water domain (not air) I assume the tool gives acceptable solution.

So trying to use interFoam solver, the solver quits saying it cannot find "nu" in constant/transportProperties. I tried to redefine the code in forces.C, but for now I gave up. I found some work around, to add additional line at the beginning of transportProperties such as:

nu nu [ 0 2 -1 0 0 0 0 ] 1e-06;

Can any of OpenFOAM gurus comment on that? Is such work around acceptable? I think the other two nu-s in phase1 and phase2 are read correctly by the solver.

Thanks,
K

Hi Paka,

Could you find out if your nu nu ... approach at the top of the transport properties file is a valid workaround for the problem? Just FYI: for the turbulent interFoam, there is no problem in the force calculations, but the problem exists when using laminar interFoam. This problem is "solved" when I take your approach but I am not sure if it is a correct way of tackling this problem. I am using 221 version.

Cheers,
Hossein