= Three-dimensional point source
{title2=$\mathbf u=Q\mathbf r/(4\pi r^3)$}
A point injection of volume at rate $Q$ produces the displayed radial <potential flow> in three dimensions. Its <velocity potential> is $-Q/(4\pi r)$, and every enclosing <sphere> has outward <volume flux> $Q$. The <divergence> vanishes away from the source and is $Q$ times the three-dimensional <Dirac delta> distribution at it. The field is also a <Stokes flow> with constant <pressure> away from its singularity.
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