Which one of the following is the expression of the rotational component for a two – dimensional fluid element in x – y plane?
(a) $\omega _{Z}=\frac{1}{2}\left ( \frac{\partial v}{\partial x}-\frac{\partial u}{\partial y} \right )$
(b)$\omega _{Z}=\frac{1}{2}\left ( \frac{\partial v}{\partial x} +\frac{\partial u}{\partial y} \right )$
(c) $\omega _{Z}=\frac{1}{2}\left ( \frac{\partial u}{\partial x} -\frac{\partial u}{\partial y} \right )$
(d) $\omega _{Z}=\frac{1}{2}\left ( \frac{\partial u}{\partial x} +\frac{\partial u}{\partial y} \right )$