which has the dimension of unit mass, fluxing through a unit surface, per unit time kg m2s. This definition follows from the fact that the velocity in a continuous medium can be considered as material volume flux (Fig. 1.5) i.e., unit volume fluxing through a unit surface per unit timem s = m3 m2s. Therefore, velocity (i.e. volume flux) multiplied by the density (i.e. mass per unit volume) gives the mass flux.