Function objects in multiphase flow

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 February 12, 2016, 09:40 Function objects in multiphase flow #1 Senior Member   ArielJ Join Date: Aug 2015 Posts: 127 Rep Power: 4 Sponsored Links 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: Code: ```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: Code: ```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: Code: ```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.

 February 17, 2016, 09:47 #2 Senior Member   ArielJ Join Date: Aug 2015 Posts: 127 Rep Power: 4 Hi, I'm posting again here because I'm still having the same issue! I am calculating hugely incorrect drag coefficients (I am expecting typical values for a cylinder in waves and am getting a fluctuating drag coefficient with an average at -2,6222! Some of my files are posted in the previous post so I won't repost. Can anyone please please offer advice on getting the drag coefficient?? I am so confused about why the values are so crazy! Please help!

 Tags force coefficients, forces, free surface flow, functionobjects, multiphase

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