[arieljeds]

Hi everyone,

I'm running a multiphase case with regular waves (using waveFoam) passing a vertical surface piercing cylinder and I'm trying to calculate the forces and force coefficients. I have them set up and they are being calculated and in the right place, but the values look very very wrong.

1) First of all, the values for the drag and lift coefficients are oscillating... does this suggest that they are not converging? I expected to see one value after the wave has reached the cylinder. The values not only are changing, but massively increase.

2) Is it even possible to use the forces and forceCoeffs function objects as they are with a multiphase solver?? I'm wondering if I have defined the phases incorrectly? Or if I need to separate the U and p values somehow into U.water and U.air, etc?

For instance, I am confused in my transportProperties file because I have the transport model set to Newtonian, so I commented out all other information (BirdCarreauCoeffs, CrossPowerLawCoeffs), as shown below:

phases (water air);

water
{
    transportModel  Newtonian;
    nu              nu [ 0 2 -1 0 0 0 0 ] 1e-06;
    rho             rho [ 1 -3 0 0 0 0 0 ] 1028;
    /*
    CrossPowerLawCoeffs
    {
        nu0             nu0 [ 0 2 -1 0 0 0 0 ] 1e-06;
        nuInf           nuInf [ 0 2 -1 0 0 0 0 ] 1e-06;
        m               m [ 0 0 1 0 0 0 0 ] 1;
        n               n [ 0 0 0 0 0 0 0 ] 0;
    }

    BirdCarreauCoeffs
    {
        nu0             nu0 [ 0 2 -1 0 0 0 0 ] 0.0142515;
        nuInf           nuInf [ 0 2 -1 0 0 0 0 ] 1e-06;
        k               k [ 0 0 1 0 0 0 0 ] 99.6;
        n               n [ 0 0 0 0 0 0 0 ] 0.1003;
    }
    */
}

air
{
    transportModel  Newtonian;
    nu              nu [ 0 2 -1 0 0 0 0 ] 1.568e-05; //1.48e-05;  --> current measurement for kinematic viscosity found at engineeringtoolbox.com
    rho             rho [ 1 -3 0 0 0 0 0 ] 1;
    /*
    CrossPowerLawCoeffs
    {
        nu0             nu0 [ 0 2 -1 0 0 0 0 ] 1e-06;
        nuInf           nuInf [ 0 2 -1 0 0 0 0 ] 1e-06;
        m               m [ 0 0 1 0 0 0 0 ] 1;
        n               n [ 0 0 0 0 0 0 0 ] 0;
    }

    BirdCarreauCoeffs
    {
        nu0             nu0 [ 0 2 -1 0 0 0 0 ] 0.0142515;
        nuInf           nuInf [ 0 2 -1 0 0 0 0 ] 1e-06;
        k               k [ 0 0 1 0 0 0 0 ] 99.6;
        n               n [ 0 0 0 0 0 0 0 ] 0.1003;
    }
    */
}

sigma           sigma [ 1 0 -2 0 0 0 0 ] 0.00;

My forces file is:

forces_object
{
    type            forces;  
    functionObjectLibs ( "libforces.so" );

    enabled         true;
    outputControl   timeStep;
    outputInterval  1;

    pName           p;
    UName           U;	
    rhoName         rhoInf; 	// Incompressible solver
    log             on;

    
    rhoInf      1.0;    	// Fluid density
    patches     ( cylinder); 

    CofR        (0 0 0);
    pitchAxis   (0 0 1);	// Gives moment coefficient

}

And forceCoeffs file:

forceCoeffs_object
{
	type 	forceCoeffs;
	functionObjectLibs ("libforces.so");
	patches (cylinder);

	pName 	p;
	Uname 	U;
	rhoName rhoInf;				// Reference Density 
	rhoInf  1.0;				// Density/1000
	
	log 	true;				// Dump to file -- change to false to not save a file 

	CofR 	(0.0 0 0);			// Centre of rotation 
	liftDir (0 1 0);			// Direction of lift coefficient  ---> y-direction (transverse to flow)   ?????
	dragDir (1 0 0);			// Direction of drag coefficient  ---> drag acts in opposite direction to flow (-x) 
	pitchAxis (0 0 1);			// Pitch moment axis  --> changed from pitchAxis (0 0 1)
	magUInf 0.4622;				// free stream velocity magnitude 
	lRef 6.0;         			// reference length (cylinder diameter)
	Aref 1.488;         			// reference area: lref * zmesh (zmesh is approx. pi*D/76)

        outputControl   timeStep;
        outputInterval  1;
}

I would really appreciate any help on this, I know it's a basic application but I am still very new to Openfoam and cfd and am struggling to find an explanation to help me understand what is going on.