= Critical angle of a flux-normalized Stokes wedge flow
{title2=$\tan(2\alpha_c)=2\alpha_c$}
On the symmetric small-$Q/\nu$ branch of <radial viscous flow in a wedge>, the leading <Stokes flow> solves $G_0^{(3)}+4G_0'=0$, zero wall values and unit integral. Hence
$$
G_0(\theta)=\frac{\cos2\theta-\cos2\alpha}{\sin2\alpha-2\alpha\cos2\alpha}.
$$
For $\pi/2<\alpha<\alpha_c$, where $\alpha_c\simeq2.24670$, this is positive in $|\theta|<\pi-\alpha$ and negative near the walls. The normalization denominator tends to zero at the stated root. At the root the homogeneous angular mode has zero net flux, so it cannot be rescaled to carry a fixed nonzero source flux.
Approaching that angle makes large outward and reverse contributions cancel to leave the prescribed flux. The leading amplitude diverges and the nonlinear inertial correction ceases to be perturbative, however small its coefficient is at fixed angle. This is a failure of a regular expansion, not a proof of infinite speed in the full <Navier-Stokes equation>. Distinct inner scalings or different steady branches require separate analysis.
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