Solution (source code)

= Solution

Since $2p-q=1$, the leading <turbulent viscosity> is linear:
$$
\nu_T=C_\mu\sqrt{k_0}\,x.
$$
Using $U_0=C_\mu p\sqrt{k_0}/\sigma_k$ and $1/p=2-\sigma$ gives
$$
\boxed{\left.\frac{d\nu_T}{dx}\right|_{0^+}
=C_\mu\sqrt{k_0}
=\sigma_k(2-\sigma)U_0
=(2\sigma_k-\sigma_\epsilon)U_0.}
$$