Cluster-based reduced-order modelling of the wake stabilization mechanism behind a twisted cylinder

The flow around a twisted elliptic cylinder is numerically calculated using large eddy simulation (LES) at the Reynolds number of 2 x 10(4), along with a circular cylinder for comparison. The mean drag coefficient and the fluctuating lift coefficient of the twisted cylinder are 27.99% and 94.01% lower, respectively, than that of the circular cylinder. The twisted surface leads to more stable wake, longer vortex formation length, higher base pressure and three-dimensional separation. In addition, cluster-based reduced-order modelling (CROM) is performed to analyze phase-dependent variations of the wake flow and evaluate the relationship between derived characteristic flow structures and their impact on forces. It captures the oscillatory behavior of Karman vortex shedding in the wake of the circular cylinder. Meanwhile, two flow regimes, anti-phased and in-phase-dominated vortex shedding, generated by the twisted cylinder are extracted and distinguished. The two regimes prevail alternately with changing probability of occurrence along the spanwise of the twisted cylinder, which can be attributed to the different separation angles. The in-phase-dominated vortex shedding inhibits the effect of Karman vortex shedding and is subjected to a lower force compared with the anti-phased vortex shedding.