59. As shown in the figure, in the right triangle $\triangle OAB$, $\angle AOB=30^{\circ}, AB=2$. If the right triangle $\triangle OAB$ is rotated $90^{\circ}$ clockwise around point $O$ to get the right triangle $\triangle OCD$, then the area swept by $AB$ is $\qquad$.
Answer: $\pi$