Drag on a two-dimensional circular bubble from outer-flow dissipation ($D=8\pi \mu U$)

For the zero-circulation flow around a circle, $e:e=8U^2{a}^4/r^6$. Integration over the exterior plane gives dissipation $8\pi \mu U^2$ per unit axial length and hence drag $D=8\pi \mu U$ per unit length.