Drag on a two-dimensional circular bubble from outer-flow dissipation
= Drag on a two-dimensional circular bubble from outer-flow dissipation
{title2=$D=8\pi\mu U$}
For the zero-circulation flow around a circle, $e:e=8U^2a^4/r^6$. 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.