ABSTRACT
A scalar is denoted by italic Roman letters, vectors by bold-face capital Roman letters and tensors by capital Greek letters. r is the position vector, 1 is the unit tensor and d is the dimensionality of the problem. We have the synonyms div for the operator ”· and rot for ”×, but these are not used here. The rotation of a Áow, or a vector Àeld in general, can be interpreted as the rotation velocity of a small “mill” immersed in the Áow, best illustrated in two spatial dimensions.
