Forced periodic rotation imposes vortex shedding in the wake of a cylinder at subcritical Reynolds numbers

We focus on the wake of a cylinder placed in uniform flow and forced to rotate periodically at subcritical Reynolds numbers, i.e. for Reynolds numbers smaller than 47 calculated based on the incoming flow velocity and the cylinder diameter, where vortices are not shed in the wake of a fixed cylinder. We show that in the near wake, the imposed periodic rotation causes the Föppl vortices (the symmetric steady vortices that are formed right behind a fixed cylinder within the Reynolds number range of $5\lt {Re}\lt 47$ ) to appear only momentarily during each rotation cycle until they disappear at higher rotation rates. In the far wake, vortices can be induced for certain values of rotation rate, $\alpha$ , and rotation frequency, $f$ . The shedding of these vortices in the wake results in a periodic lift force that acts on the cylinder. We have defined a new parameter $\omega /(f\alpha )\equiv 1/F$ , where $\omega$ is the angular velocity of the cylinder, which is significant in describing the system. For any values of angular velocity and the frequency of change in the rotation direction, the wake pattern remains the same if the value of $1/F$ stays constant. Subsequently, the fluctuating lift coefficient and the average drag coefficient peak at the same value of $1/F$ for any value of $\omega /f\equiv \alpha /F$ . The Reynolds number for the onset of shedding decreases with increasing rotation rate at a constant $\alpha /F$ . We have observed shedding at Reynolds numbers as low as ${Re}=1$ for higher rotation rates.