Doctor of Philosophy
Civil and Environmental Engineering
Dr. Gregory Kopp
Dr. Roi Gurka
As the spans of suspension bridges increase, the structures become inherently flexible. The flexibility of these structures, combined with the wind and particular aerodynamics, can lead to significant motions. From the collapse due to flutter of the Tacoma Narrows Bridge to the case of vortex-induced vibrations (VIV) of the Storebælt Bridge, it is evident that a better understanding of the aerodynamics of these geometries is necessary. The work herein is motivated by these two problems and is presented in two parts.
In the first part, the focus is on the physical mechanisms of vortex shedding. It is shown that the wake formation for elongated bluff bodies is distinct from shorter bluff bodies due to the leading edge separating-reattaching flow. Pressure data are then used to propose a mechanism of competition between the flow at the leading and trailing edges rather than synchronization which occurs at low Reynolds numbers. Within the context of this framework, the wakes are orthogonally decomposed and it was discovered that new modes appear not previously observed for shorter bluff bodies. In Part II, a time-resolved Particle Image Velocimetry (PIV) system is developed. This system is used to capture both the high and low frequency dynamics of flutter due its uniquely long recording length. It is shown that, contrary to conventional understanding, the vortex shedding does not significantly change during flutter. Thus, the fact that these bodies shed vortices is only a secondary effect in relation to the flutter instability.
There is a distinct contrast between flutter and VIV: the latter is known to be governed by the vortex shedding wake and it has been shown herein that the former is not. Regarding the problem of VIV, it is shown that the wakes of these bodies are formed due to interaction with the leading edge separating-reattaching flow. As the leading edge separation angle grows, it is shown to disturb the trailing edge vortex shedding altering many of the key parameters including fluctuating lift force and shedding frequency.
Taylor, Zachary J., "Vortex shedding from elongated bluff bodies" (2011). Electronic Thesis and Dissertation Repository. 264.