Comparison between the Koopman modes for the flow around circular cylinder and circular cylinder fitted with helical strakes

Abstract

The helical strake is a device commonly employed for vortex-induced vibration attenuation or suppression. It works by changing and affecting the flow patterns around a circular cylinder and, thereby, reducing the interaction of the shear layers formed in thismassive separated type of flow. In this paper thewake of vortices generated by a bare circular cylinder is compared to the wake generated by a circular cylinder fittedwith helical strake with pitch equal to ten cylinder diameters and height equal to 0.2 of the cylinder diameter. Besides the analysis of the measured velocity field, a Koopman decomposition is made and the most energetic modes obtained for each condition are compared. The Reynolds number for the visualizations is Re = 10,000.