Novel geometries of serrated helical strakes to suppress vortex-induced vibrations and reduce drag

We present an experimental investigation on the effectiveness of unconventional geometries of serrated helical strakes to suppress the cross-flow vortex-induced vibrations (VIV) of a circular cylinder with low mass and structural damping. The VIV responses of five models of strakes are compared with that of a bare cylinder at moderate Reynolds numbers: one continuous helical strake, two serrated strakes and two inverted strakes. While the conventional strake suppressed 88% of the peak amplitude of response with a 48% drag reduction, the most efficient serrated strake reduced the peak amplitude of vibration by 95% producing 54% less drag than a bare cylinder. When the models were not allowed to respond to VIV they all increased drag in relation to that of bare cylinder. We verified that simply inverting the local angle of attack of the individual blades in relation to the helical pitch of the strake did not produce a favourable result in terms of VIV suppression, but reduced drag and fluctuating lift on a fixed body. Visualization of the flow around the blades helped to clarify the hydrodynamic mechanisms governing flow separation in the near wake, disrupting the formation of coherent vortices and reducing VIV.