# Contrib/swak4Foam/Example dropletVelocity

From OpenFOAMWiki

Here is a way of calculating the velocity of a drop falling. This is for a 2D case. First, one can average the velocities of all cells within the drop (for alpha>0.5 for example):

downAverage { type swakExpression; valueType internalField; variables ( "downDirection=vector(0,-1,0);"//select the downwards direction "thres=0.5;"//which cells to keep "liquidVol=sum(alpha1>thres ? vol() : 0);" //calculates the volume of the drop "downVel=alpha1>thres ? (U & downDirection) : 0;" //a & b:inner vector product. Keep the y component of U. ); expression "downVel*vol()/liquidVol";//vol():vol of the cell accumulations ( sum ); verbose true; }

Another possibility would be to track the evolution of the diameter in the x and y direction:

createInterface { type createSampledSurface; outputControl timeStep; outputInterval 1; surfaceName interface; surface { type isoSurface; isoField alpha1; isoValue 0.5; //select cells with alpha equal to 0.5 interpolate true; } } xDiameter { type swakExpression; valueType surface; surfaceName interface; verbose true; expression "pos().x"; accumulations ( min max ); } yDiameter { type swakExpression; valueType surface; surfaceName interface; verbose true; expression "pos().y"; accumulations ( min max ); }

Note: this will generate the horizontal and vertical diameter of the droplet, and by looking at the center of the "cross", one can track the evolution of the whole drop.