The calculation starts with the calculation of the normal vector of an element facet.If the facet is quadrilateral, then the perpendicular vector is derived by a vector product of the two diagonal vectors.If it is a triangular facet, then it is derived by the vector product of two vectors along its two edges.Afterwards it is normalized.There is one perpendicular vector calculated for each element and it is assigned to its centroid.Then the centroid velocity of the element is calculated as an average of the translational velocities of the corner nodes.Middle nodes and rotational velocities are not taken into account.