= Image vortex at a plane wall
{title2=$U=\frac{\Gamma}{4\pi b}$}
The <method of images> enforces impermeability at a plane wall by reflecting a <line vortex> across it and reversing its <circulation>. The two logarithmic <stream functions> cancel on the wall, which is consequently a <streamline>. For a vortex of <circulation> $\Gamma$ at $(0,b)$ above $y=0$, the image of <circulation> $-\Gamma$ at $(0,-b)$ induces velocity $(\Gamma/(4\pi b),0)$ at the real vortex. The signed drift parallel to the wall is therefore $\Gamma/(4\pi b)$.
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