The vorticity equation of fluid dynamics describes the evolution of the vorticity  of a particle of a fluid as it moves with its flow; that is, the local rotation of the fluid (in terms of vector calculus this is the curl of the flow velocity). The governing equation is:where  is the material derivative operator,  is the flow velocity,  is the local fluid density,  is the local pressure,  is the viscous stress tensor and  represents the sum of the external body forces. The first source term on the right hand side represents vortex stretching.
The equation is valid in the absence of any concentrated torques and line forces for a compressible, Newtonian fluid. In the case of incompressible flow (i.e., low Mach number) and isotropic fluids, with conservative body forces, the equation simplifies to the vorticity transport equation:
where  is the kinematic viscosity and  is the Laplace operator. Under the further assumption of two-dimensional flow, the equation simplifies to: