It is recommended to use this correlation in the vicinity of the critical velocity (from mixture velocities slightly below to three or four times greater). This correlation was then modified to take into account the specific mass of the particles, as reported by Newitt et al. [4]. In the case of particles of a few millimeters flowing in water, the particle Reynolds number is sufficiently high to assume that the settling velocity is given by Eq. (2) with a constant drag coefficient cd. This form of the correlation reads: