The project applies a three – dimensional URANS computational fluid dynamics analysis to examine the performance of the Flettner rotors, with and without the addition of "Thom disks", in a range of rotor heights. The results obtained in the present study show that the Prandtl's limit can be exceeded at high spin ratios (which is defined as the ratios between rotor and wind speeds), regardless of the rotor heights.
Simulations on flow past bare cylinders confirm the level of oscillation in lift coefficient appears and in general is a function of cylinder height. The shorter the rotor, the lower is the amplitude of the corresponding oscillation in the lift coefficient. Higher – order vortex shedding seems to be appeared when the flow past the bare cylinder with short heights. The exact range of spin ratios lead to the occurrence of this higher – order vortex shedding cannot be confirmed in the present explorations.
The addition of the disks on the rotors can improve the rotor performance at high spin ratios in terms of lift and negative drag generating capability. The functions of the disks are prominent for cylinder with short heights but not useful in long cylinders. Although the phenomenon of vortex shedding is still appeared for some particular spin ratios and cylinder geometries, the addition of "Thom disks" on the rotors can stabilize the flow and, in general, less oscillation in lift force is observed.
Finally, the simulation result suggests that cylinder with height equal to half of its diameter and with "Thom disks" included has the best performance because of its oscillation – free lift generating capability over a wide range of spin ratios.
Fig 3 The streamline of flow past the bare cylinder at spin ratio of 1 at time step 20000. The cylinder is rotating in the anti – clockwise direction.