According to the π -theorem, the n = 5 variables can be reduced by the k = 3 dimensions to form p = n − k = 5 − 3 = 2 independent dimensionless numbers. Usually, these quantities are chosen as R e = ρ ρ n D 2 μ μ , commonly named the Reynolds number which describes the fluid flow regime, and N p = P ρ ρ n 3 D 5 , the power number, which is the dimensionless description of the stirrer.