Discrete Particle Modeling of a Wurster Coater

The fundamental equation ofthe DPM is the momentum equation of each individual particle with mass Mj and volume Vj, that can be described by Newton s second law 

In the case of a collision between two particles, the contact forces are calculated according to a contact model based on the theory developed by Hertz (1882) for the normal impact and a non-slip approximation of the model by Mindlin and Dere-siewicz (1953) for the tangential part of the contact force, as proposed by Tsuji et al. (1992). The normal contact force is 

n is the normal unit vector and v j is the relative velocity at the contact point. The elastic part of the contact force is represented by a non-linear spring, assumed proportional to the spring stiffness and to s- (sn displacement). Additionally, to account for viscoelastic material properties that cause energy dissipation, a damping factor Pn related to the coefficient of restitution is included in the model  

The normal coefficient of restitution Cn is defined as the ratio of the rebound velocity to the impact velocity 

Usually this value can be obtained from experiments. The symbol M in Eq. 7.45 denotes a combined mass. The tangential component of the contact force EcomactT is calculated analogously, as presented by Tsuji et d. (1992). 

---