For a fluid flow, the partial derivative ∂/∂t measures the rate of change at a fixed point as different material flows through that point. Also known as the Lagrangian derivative or the material derivative, the convective derivative measures the change of a quantity q while following the fluid. It is given by $\frac{\text{D}q}{\text{D}t}=\frac{\partial q}{\partial t}+u\cdot \nabla q$ where u is the fluid’s velocity.