In a common situation, the diffusion coefficient is constant, there are no sources or sinks, and the velocity field describes an incompressible flow (i.e., it has zero divergence). Then the formula simplifies to: ∂ ∂ c ∂ ∂ t = D ∇ ∇ 2 c − − v ⋅ ⋅ ∇ ∇ c .