Particle Reynolds number
= Particle Reynolds number
{title2=$\operatorname{Re}_p=2r\rho_fW_s/\mu$}
A <Reynolds number> based on particle diameter and relative speed in the carrier fluid. Small values justify <Stokes drag law>; a high-Reynolds-number regime with nearly constant <drag coefficient> instead gives <quadratic drag>. The drag law must be evaluated in the appropriate regime for every species.