Postdoctoral researcher

## Dr. Volker Becker

**Function**

Postdoctoral researcher

European Physical Journal, **27**, 107-114

(2008)

We consider the collision of frictional granular particles where the normal part of the interaction force is due to viscoelastic spheres and the tangential part is described by the model by Cundall and Strack being the most popular tangential collision model in Molecular Dynamics simulations. Albeit being a rather complicated model, governed by 7 phenomenological parameters, we find that it depends on 3 independent parameters only. Surprisingly, in a wide range of parameters the corresponding coefficient of tangential restitution, ε_t, is well described by the simple Coulomb law with a cut-off at ε_t=0. A more complex behavior of the coefficient of restitution as a function on the normal and tangential components of the impact velocity, g_n and g_t, including negative values of ε_t is found only for very small ratio g_t/g_n.

Physical Review E, **77**, 011304

(2008)

The linear dashpot model for the inelastic normal force between colliding spheres leads to a constant coefficient of normal restitution, ε_n=const., which makes this model very popular for the investigation of dilute and moderately dense granular systems. For two frequently used models for the tangential interaction force we determine the coefficient of tangential restitution ε_t, both analytically and by numerical integration of Newton’s equation. Although ε_n=const. for the linear-dashpot model, we obtain pronounced and characteristic dependencies of the tangential coefficient on the impact velocity ε_t=ε_t(**g**). The results may be used for event-driven simulations of granular systems of frictional particles.

Granular Matter, **10**, 231-232

(2008)

In contrast to a still common belief, a steadily flowing hourglass changes its weight in the course of time. We will show that, nevertheless, it is possible to construct hourglasses that do not change their weight.