In fluid dynamics, the lift per unit span (L') acting on a body in a two-dimensional flow field is directly proportional to the circulation, i.e. it can be expressed as the product of the circulation Γ about the body, the fluid density ρ ρ , and the speed of the body relative to the free-stream v ∞ ∞ : L ′ = ρ ρ v ∞ ∞ Γ Γ