Given ϕ=3xyandψ=32(y2−x2), the discharge passing between the streamlines through the points (1,3) and (3,3)
dQ=|dψ|=|ψ2−ψ1|
at(1,3)ψ1=32(32−12)= 12 units
at(3,3)ψ2=0 units ∴ dQ = 12 units