DOI link for Introduction
A PDE that expresses the variation of ρ with x and t for a fluid, the equation of continuity,
∂t = ∇ · (vρ) (1.1)
states a basic physical principle, conservation of mass, where v fluid velocity vector ∇ divergence operator
∇ is a vector differential operator that has three components in specific coordinate systems, for example, see Table 1.1.