So, for incompressible fluid because rho is a function of x y z and t for incompressible fluid it has to have d rho by d t is 0. It is not necessary that d rho by d t is 0, so for a compressible fluid we have once for an incompressible fluid we have divergent q is 0. Where, as for the compressible fluid we have d rho by d t plus rho into divergent q is 0. Now so, in Cartesian coordinate we can write at for the incompressible fluid in Cartesian coordinate system.