forceModels RongDrag???

Hi Everyone,

When I am modifying the couplingProperties file in constant folder, I find the option in the forceModels called RongDrag, anyone can tell me where this option "drag force" come from, and how to set up its properties below:
RongDragProps
{
verbose true;
velFieldName "U";
voidfractionFieldName "voidfraction";
interpolation true;
implForceDEM true;
implForceDEMaccumulated true;
granVelFieldName "Us";
}
Also I find there are some options for the subModels which are not showing in the documents on the website. Can someone tell me about where I could get those information (Literature etc.) about how to set up parameters in those dictionary.

Thank you very much!