Roughness Based Passive Control Of Transition To Galloping Of A Circular Cylinder Undergoing Vortex Induced Vibrations

Flow induced motion (FIM) is a naturally occurring fluid-structure interaction phenomenon, typically considered harmful to a wide variety of engineering structures. As a result, the majority of research in the field has been geared towards mitigating the occurrence of FIM. In 2005, Michael Bernitsas at the University of Michigan identified the energy harvesting potential of FIM's, triggering a new line of research that instead focuses on augmenting it. Previous work by our group had tested the effectiveness of strips attached to the surface of a circular cylinder in augmenting FIM modes such as vortex induced vibrations (VIV) and galloping. The objective of the experimental results presented in this thesis is to develop a better understanding of the effect of strip surface on the incitation of galloping oscillations. Strip roughness ratios (ratio of the size of embedded roughness to the total strip thickness) ranging from 0% (smooth surface) to 100% (rough surface with zero thickness) were tested in addition to strip thickness ratios (ratio of strip thickness to cylinder diameter) ranging from 0.8% to 8.2%. At the higher roughness ratios, rough strips led to suppression of VIV and galloping amplitudes when compared to their smooth counterparts. However, with decreasing roughness ratio and increasing thickness, the effect of surface roughness became negligible in comparison to the smooth surface. We conclude that beyond a threshold values of strip thickness, the mechanistic impact of the strip (thickness) is the dominant parameter affecting the cylinder's FIM response with no additional passive flow control possible by varying the surface roughness. At the lower threshold of thickness, neither the smooth nor the rough strips experienced galloping.