{latex}$$
{\mathop{\rm Re}\nolimits} = {{4Q} \over {\pi D\nu }}
$$
{latex}\\
{latex}
$$
{\rm{f}} = {{0.25} \over {\left[ {\log \left( {{\varepsilon  \over {3.7D}} + {{5.74} \over {{\mathop{\rm Re}\nolimits} ^{0.9} }}} \right)} \right]^2 }}
$$
{latex}\\
{latex}
 $$
h_{\rm{f}}  = {\rm{f}}{8 \over {g\pi ^2 }}{{LQ^2 } \over {D^5 }}
$$
{latex}


{latex}

<math>h_{\text{f}}=\text{f}\frac{8}{g\pi ^{2}}\frac{LQ^{2}}{D^{5}}</math>
{latex}