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In the casting of aluminum alloys and, in general, in the casting of fi lm-forming alloys, the entrainment of oxides into the bulk liquid severely reduces the strength of the cast part. To avoid this, the melt velocity must be kept below a certain value, namely critical velocity, which is widely assumed to be 0.5 m/s. In this paper the authors investigate, by means of fl uid-dynamic computer simulation, the dependence of critical velocity on geometrical features of the running channels and thermophysical properties of the molten metal. For each of the geometries studied, once the critical velocity is exceeded, the amount of oxide entrained in the liquid is quantifi ed. The analysis of the results reveals that surface entrainment is much more related to the non-dimensional Webber number than to melt velocity.