{latex}
\[
h_{\rm{f}}  = \frac{{32\mu LV}}{{\rho gD^2 }} = \frac{{128\mu LQ}}{{\rho g\pi D^4 }}
\]
{latex}
{latex}
$$
h_{\text{f}}  = \frac{{32\mu LV}}
{{\rho gD^2 }} = \frac{{128\mu LQ}}
{{\rho g\pi D^4 }}
$$
{latex}

{latex}
\huge\[
h_{\text{f}}  = \frac{{32\mu LV}}
{{\rho gD^2 }} = \frac{{128\mu LQ}}
{{\rho g\pi D^4 }}
\]
{latex}