The melt flows out of the ladle and into the tundish, a material buffer which links the discontinuous secondary metallurgy with the continuous casting process. An important task of the tundish is to separate non-metallic particles from the melt. Here, a simulation has big advantages as compared to water modeling. Approved numerical two-phase models allow the direct comparison of the separation efficiency of different tundish designs. Moreover, buoyancy effects can be taken into account. Undesired and so-called dead water regions can be identified and avoided by re-design of the interior refractory contour.