Consider the motion given by

[omega] = U(z + a²/z), (1)

so that

/ a²\ / a² \

[phi] = U ( r + -- ) cos [theta] = U ( 1 + -- )x, (2)

\ r / \ r² /

/ a²\ / a² \

[psi] = U ( r - -- ) sin [theta] = U ( 1 - -- )y.

\ r / \ r² /

Then [psi] = 0 over the cylinder r = a, which may be considered a

fixed post; and a stream line past it along which [psi] = Uc, a

constant, is the curve

/ a²\

( r - -- ) sin [theta] = c, (x² + y²)(y - c) - a²y = 0 (3)

\ r /

a cubic curve (C3).

Over a concentric cylinder, external or internal, of radius r = b,

/ a²\

[psi]´ = [psi] + U1y = [U ( 1 - --- ) + U1] y, (4)

\ b²/

and [psi]´ is zero if

Encyclopaedia Britannica, 11th Edition, "Hydromechanics" to "Ichnography" Volume 14, Slice 2 · 第 26 / 120 题