 Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2024/ib/paper-2/5d/solution

Codex  0 Created 2026-09-23 Updated 2026-09-25

(u, v) = (y, a x)

gives

\partial_{x} u + \partial_{y} v = 0,

so the flow is incompressible. With the convention

u = ψ_{y}, v = - ψ_{x},

a stream function is

ψ (x, y) = \frac{1}{2} (y^{2} - a x^{2}) .

a > 0

, they are the hyperbolas

y^{2} - a x^{2} = constant,

with separatrices

y = \pm a x

and a saddle at the origin. For

a < 0

, they are concentric ellipses

y^{2} + ∣ a ∣ x^{2} = constant,

\partial_{x} v - \partial_{y} u = a - 1.

Hence the flow is irrotational exactly when

a = 1 .

Then

(u, v) = (y, x) = \nabla ϕ

ϕ (x, y) = x y + constant .

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