The power number Np (also known as Newton number) is a commonly used dimensionless number relating the resistance force to the inertia force. The power-number has different specifications according to the field of application. E.g., for stirrers the power number is defined as:
N p = P ρ n 3 D 5 {\displaystyle N_{\mathrm {p} }={\frac {P}{\rho n^{3}D^{5}}}}
with
P: power ρ: fluid density n: rotational speed in revolutions per second D: diameter of stirrer
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