Effects of Geometry on the Flow Around Elongated Cylinders



Deck geometry is one of the crucial factors that influence the flow characteristics and aeroelastic response of long-span bridges. The wake flow patterns of bridge sections become increasingly important when exploring the interaction between the wind and the structure, as designs are becoming longer and lighter. Since the flow field and the vortex shedding activity produced by simple wind-structure interaction are often complex in nature, the ambiguous effects of bridge deck geometry cause the level of complexity to increase significantly. The effects of deck geometry on the wake structure and shedding activity have yet to be established, causing such study to be considered as a necessary addition to further research within the field of bridge aerodynamics.
Particle Image Velocimetry tests involving flow around elongated cylinders, of various geometries, have been carried out in a 0.5m x 0.5m x 2m open channel wind tunnel at the Boundary Layer Wind Tunnel Laboratory at The University of Western Ontario. The purpose of each test is to measure the wake structure resulting from the specific bluff body geometry, as well as the flow field exhibited at the trailing edge. The cylinders tested have an elongation ratio of 7 and are symmetric in cross-section with leading and trailing edges of distinct rectangular, triangular and circular geometry. The cross-section of the fourth cylinder is one which resembles the design of the existing Storebælt Bridge, found in Denmark, which experienced significant vortex-induced response just prior to opening and the subsequent installation of turning vanes. The overall objective of the present study is to provide a quantitative interpretation of the wake topology characterized by the existence of a geometry-dependent vortex street. A phase-averaging technique to identify the turbulent structure in the near wake was applied here to characterize the organized coherent concentrations of large scale vorticity, unique to the body from which they are shed. Vortex street comparisons amongst the bodies are accomplished on the basis of the mean vortex spacing ratios, convection speeds and circulation strengths.
At comparable convection speeds of approximately 85% of the freestream flow, visual differences in the vortex spacing, size and strengths amongst the wake flows are apparent. More specifically, it is the triangular edge model which sheds a vortex street with the largest spacing ratio; the circular edge model which produces the smallest vortices in the wake; and the Storebælt model which experiences differences in the strengths of the vortices shed from the top and bottom separated shear layers. However, the PIV measurements suggest that the bluff rectangular edge geometry generates a relatively disorganized wake, where no periodic shedding is found; at least for this particular elongation ratio. Upon reviewing these results, an additional study involving the Reynolds Stress distributions produced in the wakes of the four cylinder geometries is ongoing to investigate the relationship that exists between the presence of a vortex street and its influence on surrounding turbulent activity. The full conference paper will focus on the distinct features of the resulting trailing edge and wake flows due to the variations in model geometry, as well as their effects on the aerodynamic behavior of such models

The research is performed by Emanuela Palombi as part of her graduate work in collaboration with Prof. Kopp from the Department of Civil Engineering in UWO.



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