[M.Davoodi]

Hi guys,

I have been trying to make sense of the definition of relative velocity in the twoPhaseEulerFoam solver and am really confused. Can some one explain what is happening in the code below? Kd is the drag coefficient but why D1*U2 or D2*U1 comes to the equation? also the definition of relative velocity is a bit confusing....
Can someone direct me toward the mathematical definition of these equation? I cannot make sense of them at all....



volScalarField D1(rAU1*Kd); //what is happening here?
volScalarField D2(rAU2*Kd);

U = alpha1*(Us1 + D1*U2) + alpha2*(Us2 + D2*U1);
volVectorField Ur(((1 - D2)*Us1 - (1 - D1)*Us2)/(1 - D1*D2));

U1 = U + alpha2*Ur;
U1.correctBoundaryConditions();
fvOptions.correct(U1);

U2 = U - alpha1*Ur;
U2.correctBoundaryConditions();
fvOptions.correct(U2);

U = fluid.U();