Drag reduction of a downstream cylinder in a tandem arrangement using a flexible fin

In a two-tandem-cylinder system, the downstream cylinder experiences a sudden increase in drag when the gap distance between the two cylinders (G/D) exceeds a critical distance (co-shedding regime). To reduce such drag on the downstream cylinder when G/D = 4, we propose a new configuration of a downstream cylinder with an attached passive flexible fin, and the drag behavior is numerically assessed by varying the length (L/D) and bending rigidity (γ) for the fin. Three distinct flapping states of the fin emerge in the regime map of (L/D, γ): limit-cycle flapping, chaotic flapping and deflected mode. In the limit-cycle flapping mode under a small (L/D, γ), the drag acting on the downstream cylinder with a passive fin system is minimized, showing a 42.5 % reduction compared to that on a pure downstream cylinder without a fin due to the vortex shedding suppression. A further increase in drag reduction performance is achieved by applying active pitching motion to the flexible fin attached to the downstream cylinder. A maximum drag reduction of 48.1 % is obtained as the active fin generates thrust through a large curvature around its leading edge, although the wake structure behind the fin is characterized by a von Kármán vortex.