Using a built in physics interface

Hi All,

I have a modified equation for the fluid continuity equation based on the perturbation expansion. The modified eqn is:

d(rho_1)/dt + rho_0 * div (v_1) = 0 (also attached the eqn in the png file)....

so according to the equation we have two density variables .......where rho_0 is a constant.....

But in any fluid model in COMSOL the continuity equation is d(rho)/dt + rho * div (v_1) = 0 , where the density is the same for the first and second term of the equation......

Should I make use of the PDE interface to model this physics?

Please help me with this as I am totally new to COMSOL and confused to start....

Thanks in advance

Regards,
Rahul Kishor