Following the same steps as in lecture 15, the line integral in equation (16.2) can be evaluated approximately over the segment AB, BC, CD and DA, by

(16.3)

Various techniques for evaluating etc. discussed by Fletcher (1988) and Peyret and Taylor (1983). Here is evaluated as the mean value over the area in Fig. (16.1).

Thus we can write (Fletcher, 1988).

(16.4)

(16.5)

where Green's theorem has been used to obtain the final line integrals on the right, and