How do you calculate the fluctuating pressure coefficient: by N or N-1
 

Just like the topic as follows

${C_p}^\prime (\theta )= \sqrt {\frac{{\sum\limits_{t}{{{\left( {{C_p}(\theta ,t) - {{\bar C}_p}(\theta )} \right)}^2}} }}{{N - 1}}}$

or

${C_p}^\prime (\theta )= \sqrt {\frac{{\sum\limits_{t}{{{\left( {{C_p}(\theta ,t) - {{\bar C}_p}(\theta )} \right)}^2}} }}{{N }}}$

Which item will you choose? Thanks