The extra term, v grad(ρ) in the Eulerian continuity equation is an advective trans-
port term that reflects changes of density in an immobile (Eulerian) point, due to the movement of an inhomogeneous medium with existing density gradients relative to this point (Fig. 1.7). Obviously, the advective transport terms in the Eulerian continuity equation are only relevant (i.e. nonzero) in situations when both the velocity of the medium and density gradient are nonzero. On the other hand, substantive changes of density (Dρ/Dt) in the moving Lagrangian point do not depend on density gradients and the Lagrangian continuity equation (1.3) thus does not contain advective terms.
When the density in all moving material points does not change with time the Eulerian continuity equation reduces to the Eulerian advective transport equation