Solution
= Solution
The unperturbed <fluid wake> is the horizontal strip $-h<y<h$: the fluid is stationary there and moves to the right with <velocity> $U\mathbf e_x$ above and below it. Its two <shear layers> are the straight lines $y=\pm h$. After the symmetric disturbance, draw the upper layer at $y=h+\eta(x,t)$ and the lower one as its reflection at $y=-h-\eta(x,t)$, so the local wake width is $2(h+\eta)$.