Line integral of the fluid velocity around a closed curve
Field lines of a vector field v , around the boundary of an open curved surface with infinitesimal line element d l along boundary, and through its interior with dS the infinitesimal surface element and n the unit normal to the surface. Top: Circulation is the line integral of v around a closed loop C . Project v along d l , then sum. Here v is split into components perpendicular (⊥) parallel ( ‖ ) to d l , the parallel components are tangential to the closed loop and contribute to circulation, the perpendicular components do not. Bottom: Circulation is also the flux of vorticity ω = ∇ × v through the surface, and the curl of v is heuristically depicted as a helical arrow (not a literal representation). Note the projection of v along d l and curl of v may be in the negative sense, reducing the circulation.
In physics, circulation is the line integral of a vector field around a closed curve embedded in the field. In fluid dynamics , the field is the fluid velocity field . In electrodynamics , it can be the electric or the magnetic field.
In aerodynamics, circulation was first used independently by Frederick Lanchester [ 1] ,Ludwig Prandtl [ 2] , Martin Kutta and Nikolay Zhukovsky .[citation needed ] It is usually denoted Γ (Greek uppercase gamma ).