Theminussignintheright-handsideofequation(1.22)reflectstherelationbetween the density gradient and the direction of motion (Fig. 1.7): if a medium is moving in the direction of decreasing density (i.e. v grad(ρ) < 0), then the density in an immobile observation point increases with time Let’s now derive the advective transport relation (Eq. (1.22)) for the simple 1D case shown in Fig. 1.7. A fluid with a linear density distribution moves at a constant velocity vx. At the moment of time t0 the density (ρA) in the Eulerian observation
When the density in all moving material points does not change with time the Eulerian continuity equation reduces to the Eulerian advective transport equation