Enhancement of flow-induced motion of rigid circular cylinder on springs by localized surface roughness at 3×104≤Re≤1.2×105

Passive turbulence control (PTC) in the form of selectively distributed surface roughness is applied on a rigid circular cylinder on two end-springs. The cylinder is placed perpendicular to a uniform steady flow and the cylinder response is measured experimentally for 3×104≤Re≤1.2×105 and broad ranges of the main PTC parameters. PTC consists of two roughness strips placed parallel to the cylinder axis and symmetrically to the flow with thickness on the order of the boundary layer thickness. Different flow-induced motion (FIM) is observed depending primarily on the circumferential location of the two strips. FIM enhancement is studied in this paper in the soft galloping and the two hard galloping zones identified in the PTC-to-FIM Map. In galloping, amplitudes of oscillation reach 2.9 times the cylinder diameter limited only by the free-surface and bottom-boundary effects of the experimental facility. The galloping range follows the VIV range thus expanding dramatically the FIM range. Enhancement of FIM is needed to convert more hydrokinetic energy to mechanical and subsequently to electrical energy over broad velocity range. Use of laser broad field-of-view visualization reveals very different vortex structures between VIV and galloping confirming the fundamentally different driving mechanism of these two FIM kinds.