Circulation and Vorticity

Circulation is the line integral around a closed curve of the velocity field.It is normally denoted by  $\tau$. $$\tau  =  - \oint_c {V.ds}$$

Circulation should not be confused by its general dictionary meaning, in aerodynamics it is the mathematical line integral around a closed curve of a velocity field - a technical term. Circulation is related to vorticity through stoke's theorem $$\tau  =  - \oint_c {V.ds}  =  - \iint\limits_s {\left( {\nabla  \times V} \right)}.ds$$ Example: Consider a velocity field where x and y components of velocity are $u = 5y/\left( {{x^2} + {y^2}} \right)$ and $v =  - 5x/\left( {{x^2} + {y^2}} \right)$. Calculate circulation around a circular path of radius $3m$ . Let the units of $u$  and $v$ be in $m/s$.

Solution: Let $x = r\cos \theta ,\,y = r\sin \theta$, therefore ${x^2} + {y^2} = {r^2}$. In polar- coordinates ${V_r} = u\cos \theta  + v\sin \theta$ and ${V_\theta } =  - u\sin \theta  + v\cos \theta$

Hence, 
$u = \frac{{5y}}{{{x^2} + {y^2}}} = \frac{{5r\sin \theta }}{{{r^2}}} = \frac{{5\sin \theta }}{r}$ ,  $v =  - \frac{{5x}}{{{x^2} + {y^2}}} =  - \frac{{5r\cos \theta }}{{{r^2}}} =  - \frac{{5\cos \theta }}{r}$

${V_r} = \frac{{5\sin \theta }}{r}\left( {\cos \theta } \right) + \left( { - \frac{{5\cos \theta }}{r}} \right)\sin \theta  = 0$ , ${V_\theta } =  - \frac{{5\sin \theta }}{r}\sin \theta  + \left( { - \frac{{5\cos \theta }}{r}} \right)\cos \theta  =  - \frac{5}{r}$

$V.ds = \left( {{V_r}{e_r} + {V_\theta }{e_\theta }} \right).\left( {dr{e_r} + rd\theta {e_\theta }} \right)$$= \left( {{V_r}dr + r{V_\theta }d\theta } \right)$$= 0 + r\left( { - \frac{5}{r}} \right)d\theta$$=  - 5d\theta$

Therefore, $\tau  =  - \oint_c {V.ds}  =  - \int\limits_0^{2\pi } { - 5d\theta }  = 5\int\limits_0^{2\pi } {d\theta }$$= 5 \times 2\pi \ = \,10\,{m^2}/s$.
Here, value of circulation, is independent of diameter of circular path.