Clarify Re in exercise

notes.tex

@@ -4366,6 +4366,7 @@ \subsubsection{Governing equations and boundary conditions}
 
 \begin{equation}
   Re \equiv \frac{\left<\overline{u}\right> 2 \delta}{\nu}
+  \label{eq:Re_bulk}
 \end{equation}
 where $\left<\overline{u}\right>$ is the mean velocity in the channel (often also called
 \textit{bulk velocity}):
@@ -4849,7 +4850,7 @@ \subsection{Exercises}
   of a laminar channel flow. How large is it compared to the centerline velocity
   $u_0$? How about the bulk Reynolds number relative to the centerline Reynolds
   number $Re_0$?
-  \item Consider a fully developed turbulent channel flow at $Re = 10^5$.
+  \item Consider a fully developed turbulent channel flow at $Re = 10^5$ (Eq. \ref{eq:Re_bulk}).
   The fluid is water with $\rho = 10^3$ kg/m$^3$ and viscosity $\nu = 10^{-6}$ m$^2$/s.
   The channel half-height is $\delta = 0.1$ m,
   and the skin-friction coefficient is $C_f = u_*^2 / U_0^2 = 4 \times 10^{-3}$.