Let’s now analyse the Eulerian continuity equation (Eq. (1.1)) which contains both vector (velocity) and scalar (density) variables. This equation establishes the balanceofmassinanelementaryobservationvolume.Itimplies,inparticular,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