The collision rate of monodispersed particles in turbulent flows with gravity

It is of great importance to investigate the collisions of monodispersed particles in turbulent flows both in the mechanical engineering and in the cloud microphysics. A number of previous studies investigated the collision phenomena but they neglected the effect of gravity acting on particles under the assumption of the isotropy of particle motions. However, it is not concluded whether the gravity effect can be neglected or not. In this study, therefore, we investigated the effect of gravity on the collision frequency by using a direct numerical simulation. The results show that the gravity acting on particles decreases the collision frequency, compared to the zero-gravity case. When the terminal velocity of a particle is comparable to or greater than the root mean square value of fluid velocity, the effect of gravity is significant.