3.II.18E

Fluid Dynamics
Part IB, 2005

Consider the velocity potential in plane polar coordinates

ϕ(r,θ)=U(r+a2r)cosθ+κθ2π\phi(r, \theta)=U\left(r+\frac{a^{2}}{r}\right) \cos \theta+\frac{\kappa \theta}{2 \pi}

Find the velocity field and show that it corresponds to flow past a cylinder r=ar=a with circulation κ\kappa and uniform flow UU at large distances.

Find the distribution of pressure pp over the surface of the cylinder. Hence find the xx and yy components of the force on the cylinder

(Fx,Fy)=(cosθ,sinθ)padθ.\left(F_{x}, F_{y}\right)=\int(\cos \theta, \sin \theta) p a d \theta .