Wu number

The Wu number is defined as.[1]

${\displaystyle Wu={\frac {\omega D}{2u_{k}}}}$,

where ${\displaystyle \omega }$ is the circular frequency, ${\displaystyle D}$ the diameter of the axisymmetric body (like for instance a cylinder) and ${\displaystyle u_{k}}$ the local flow velocity at the top of the body. It is therefore the ratio of the velocities of the body edge and the surrounding flow.

Rotating bodies on boundary layers are a means of active flow control on wings, empennage or stabilizers on airplanes. For large Wu numbers (${\displaystyle Wu\gg 1}$), the instability mechanisms are governed by the rotation of the body. For small Wu numbers (${\displaystyle Wu\ll 1}$) the stability-related effects of the body itself may outweigh its rotatory effects[2]

• Strouhal number
• Reynolds number