Drag Coefficients in Nonsteady Turbulent Flow

Drag coefficients for spheres in nonsteady turbulent flow are found to be functions of the acceleration number, and either the particle Reynolds number or the relative turbulence intensity or both. Dependence on all three parameters is the most likely alternative. Expressions for the force on a sphere in nonsteady turbulent flow are formulated by dimensional analysis. It is suggested that the drag coefficient depends upon: (1) Particle Reynolds number; (2) relative turbulence intensity; (3) turbulence scale to particle diameter ratio; and (4) the particle to fluid density ratio. Drag coefficients derived from a study of particle motion in water passing through a diffuser are analyzed statistically to determine their dependence upon the suggested parameters. A similar analysis of results obtained by other investigators, who fired solid particles into a steady air stream, shows that the drag coefficient depends upon the same dimensionless ratios in the same way as in the writers' experiments.