egin{cases}
   \displaystyle a '' + k_xa -\frac {4I} {I_a (\beta\gamma) ^ 3}\frac {1} {(a + b)} -\frac {\epsilon_x ^ 2} {a ^ 3} = 0,
   \\
   \displaystyle b '' + k_yb -\frac {4I} {I_a (\beta\gamma) ^ 3}\frac {1} {(a + b)} -\frac {\epsilon_y ^ 2} {b ^ 3} = 0.
 \end{cases}
\end{equation*}\end{split}\]</div>
</div>
</div>
<div class="section" id="Quantitative-criterion-for-the-applicability-of-the-flow-laminarity-approximation">
<h2>Quantitative criterion for the applicability of the flow laminarity