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.
© 2022 Elsevier. Received 2 August 2021, Revised 24 November 2021, Accepted 27 December 2021, Available online 18 January 2022, Version of Record 18 January 2022. We gratefully acknowledge support of the RCGI Research Centre for Gas Innovation, hosted by the University of São Paulo (USP) and sponsored by FAPESP (2014/50279-4) and Shell Brasil. GRSA acknowledges the support of FAPESP (2011/00205-6), CNPq (306146/2019-3) and the Brazilian Navy. We are grateful to Prof Beverly McKeon for providing the use of the Noah Flume at Caltech to perform the flow visualizations. CRediT authorship contribution statement. Gustavo R.S. Assi: Conception and design of study, Acquisition of data, Analysis and/or interpretation of data, Writing – original draft, Writing – review & editing, Approved the version of the manuscript to be published. Tommaso Crespi: Acquisition of data, Analysis and/or interpretation of data, Writing – review & editing. Morteza Gharib: Interpretation of data, Writing – review & editing. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.