看一个简单的2D平面流，V就是速度，只有X,Y方向的，但是这个流会旋转，所以vorticity在x,y方向为0，绕Z轴旋转，所以W的Z分量不为0。
V * W = 0，很容易理解。
http://web.mit.edu/fluids-modules/www/potential_flows/LecturesHTML/lec07/kelvintheoproof/kelvintheoproof.html
Kelvin's theorem of the conservation of circulation states that for an ideal fluid acted upon by conservative forces (e.g., gravity) the circulation is constant about any closed material contour moving with the fluid. Physicaly, this happens because no shear stresses act within the fluid; hence it is impossible to change the rotation rate of the fluid particles. Thus, any motion that started from a state of rest at some initial time, will remain irrotational for all subsequent times, and the circulation about any material contour will vanish.
