Explicit treatment of drag term
 

Hi All

I am modelling solidification process and so far constructed a solver with phase change with fixed solid. Now I am interested in solid movement (solid grain motion ) and need to model drag term. The source term is written as follows -

Mld = (3*gs*rhom*Cd)/(4*dg) *mag(Ul-Us)* (Ul-Us)

gs - solid fraction
rhom - mixture density of solid and liquid
Cd - drag coefficient
dg - diameter of grain
Ul - liquid velocity
Us - solid velocity

I am pasting the partial code for UlEqn.H pertaining to the drag term

volScalarField drag("drag", ( (3.*gs)/(4.*dg) )*rhom*Cd);

and in fvVectorMatrix UlEqn

+ fvm::SuSp(drag*mag(Ul-Us), Ul)
- fvc::SuSp(drag*mag(Ul-Us), Us)

since I am placing mag(Ul-Us) in the fvm part, the calculation is being done implicitly leading to prohibitively slower calculation. The trick would be to treat the term mag(Ul-Us) explicitly. This could be done using fvc::, but I am not sure of how to do this.

Can anyone suggest me how to proceed with this?

Cheers
Akash