Solution
= Solution
The <vorticity> is
$$
\nabla\times u=(H-F,\ C-G,\ D-B).
$$
Hence the flow is irrotational exactly when
$$
\boxed{H=F,\qquad C=G,\qquad D=B},
$$
equivalently when $\Gamma$ is symmetric. Again, $U_0$ is unrestricted.
= Solution
The <vorticity> is
$$
\nabla\times u=(H-F,\ C-G,\ D-B).
$$
Hence the flow is irrotational exactly when
$$
\boxed{H=F,\qquad C=G,\qquad D=B},
$$
equivalently when $\Gamma$ is symmetric. Again, $U_0$ is unrestricted.