Error computation for the continuity equation.

Re	ϕ	a = −0.4	a = −0.2	a = 0	a = 0.2	a = 0.4
100	0.15	−1.12 × 10−17	−1.59 × 10−17	−1.75 × 10−17	−2.39 × 10−17	−4.92 × 10−17
300	0.15	−1.46 × 10−17	−4.64 × 10−17	−6.37 × 10−17	−8.29 × 10−17	−6.83 × 10−16
500	0.15	−3.53 × 10−17	−8.94 × 10−17	−1.24 × 10−16	−1.57 × 10−16	−2.93 × 10−15
700	0.15	−8.40 × 10−17	−1.50 × 10−16	−1.98 × 10−16	−2.40 × 10−16	−4.80 × 10−15
800	0.15	−1.23 × 10−16	−1.88 × 10−16	−2.39 × 10−16	−2.82 × 10−16	−5.53 × 10−15