Consider the velocities $u, v$, and $w$ in $x$-, $y$-, and $z$-directions, respectively. The vorticity expression in the $y-z$ plane is
- $\frac{\partial v}{\partial x}-\frac{\partial u}{\partial y}$
- $\frac{\partial v}{\partial y}-\frac{\partial w}{\partial z}$
- $\frac{\partial w}{\partial y}-\frac{\partial v}{\partial z}$
- $\frac{\partial u}{\partial z}-\frac{\partial w}{\partial x}$