Collisional Restitution Dependence on Viscosity

Dense two‐phase flows occur in many material processing situations. In these flows, particle collision presents a mechanism for internal resistance. In the literature, restitution coefficients are given for collisions between dry objects. Little is known about two colliding objects placed in a fluid. Thus, although two‐phase flow models exist for dense mixtures, realistic values of the required restitution coefficient are not available. Recent theoretical and experimental analyses of colliding spheres in a viscous fluid show that collisional rebound depends greatly on the interstitial fluid. At very high viscosity, contact may not occur. This paper presents an experimental study of collisional restitution between steel and nylon, and a range of Newtonian viscous fluids. The fluids have a viscosity range from $27{\mathrm{mm}}^2/s$ to $2\text{,}600{\mathrm{mm}}^2/s\text{.}$ The normal impact velocity ranges from 0.015 m/s to 0.75 m/s. Preliminary results show that the behaviour of the impact and the resulting restitution coefficient depend on the interstitial fluid. These results will be useful to the mathematical modeling of dense two‐phase flows.