Mapping of Passive Turbulence Control to Flow Induced Motions of Circular Cylinders.

Abstract

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 horizontally with its axis perpendicular to a uniform steady flow and is allowed one degree of freedom in the vertical direction. 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. Broad field-of-view flow visualization is used to study the wake vortex patterns.

Amplitude and frequency response are measured experimentally in the range of 3×10^4≤Re≤1.2×10^5 for broad ranges of the main PTC parameters. Lift force and force-displacement lag are calculated from the time history of the displacement. Different flow induced motion (FIM) is observed depending primarily on the circumferential location of the two strips. A PTC-to-FIM Map is developed showing six distinct FIM zones: two weak-suppression and one strong-suppression zones in vortex induced vibration, a soft galloping zone and two hard galloping zones in galloping. All zones exhibit robustness with respect to roughness strip width and thickness, and even change in strip configuration. In galloping, amplitudes of oscillation reach 2.9 times the cylinder diameter limited only by the free-surface and bottom-boundary of the experimental flow channel. Visualization shows some of the conventional, low Reynolds number patterns like 2S and 2P, as well as more complex patterns with up to ten vortices per cycle.

The developed PTC-to-FIM Map is useful in suppressing FIM to prevent structural damage as well as enhancing FIM to convert more hydrokinetic energy to mechanical and subsequently to electrical energy. Based on the PTC-to-FIM Map, suppression models using PTC are designed for flow-direction dependence and independence for a single cylinder. Both the amplitude and synchronization range were reduced. Two cylinder systems were also tested for FIM interference.