Let’s now analyse the Eulerian continuity equation (Eq. (1.1)) which contains both vector (velocity) and scalar (density) variables. This equation establishes the balance of mass in an elementary observation volume.It implies,in particular,that if mass is leaving (fluxing out of) the volume (i.e., div(ρ v)>0), the local density (i.e., the amount of mass per unit volume) decreases with time