A vector field can be represented in a number of different ways. The basis
vectors in terms of which the vector is defined may be local or global. In curvi-
linear coordinate systems, which are often required when the boundaries are
complex (see Chap. 8) one may choose either a covariant or a contravariant
basis, sec Fig. 1.1. The former expresses a vector in terms of its components
along the local coordinates; the latter uses the projections normal to coordi-
nate surfaces. In a Cartesian system, the two become identical. Also, the basis
vectors may be dimensionless or dimensional. Including all of these options,
over 70 different forms of the momentum equations are possible. Mathemat-
ically, all are equivalent; from the numerical point of view, some are more
diflicult to deal with than others.