Hysterical effects in flow structure behind a finite array of cylinders under gradually varying unsteady flow conditions

Recent investigations revealed that a group of cylinders offer more advantages than mono-pile when applied as a foundation for offshore wind turbines or pile-supported structures. In this context, the influence of gradually varying unsteady flow around a group of cylinder (Hexagonal arrays of circular cylinders, HACCs) was investigated by flume experiments. Reproducible gradually varying flow conditions were repeated in the flume for each point on the measurement grid distributed around the cylinder group. The dimensions of the recirculating flume, equipped with an electronically programmable discharge pump, were 30 m in length and 1 m in width. Spatio-temporal variations of the turbulence and mean flow quantities were obtained during the passage of the unsteady and steady flow. The experimental findings demonstrated that the velocity difference between contraction and wake regions is more significant during the falling stage of the unsteady flow than the rising stage for both arrangements of HACC. Besides, a hysterical counter-clockwise relationship between TKE and normalized streamwise velocity in the wake region of both cases of HACC was observed. Nevertheless, under steady flow conditions, while the regular array of HACC generates higher resistance compared to the unsteady case, the staggered array exhibited lower resistance.