Solution
= Solution
For <irrotational flow>, $\mathbf u_j=\nabla\phi_j$. An impermeable stationary horizontal wall has zero normal velocity, so the <kinematic boundary condition> is
$$
\boxed{\partial_z\phi_1(x,L_1,t)=0,\qquad\partial_z\phi_2(x,-L_2,t)=0.}
$$
No tangential no-slip condition is imposed on these inviscid fluids.