This research project was done to investigate the effects of taper on vortex shedding coherence on High Mast Lighting Towers (HMLTs) with models of 8-, 12-, and 16-sided polygonal cross-section. The models are mounted on springs to recreate a flutter phenomenon seen on high mast lighting towers and left to move freely (with an extra "clamped" or stationary configuration) within the wind tunnel in the cross-wind direction. Later, the model was forced to oscillate at specified frequencies and amplitudes and the resulting wake and surface pressures were recorded and compared to the freemoving and clamped cases. The study aims to study the characteristics of a "lock-in" phenomenon, that is, a range of diameters where instead of different shedding frequencies for different diameters, there is a single shedding frequency for different diameters. This goes contrary to the standard for vortex shedding model characterized by the Strouhal relation. Results show the existence of vortex cell shedding for clamped models. Using a motor and a forcing cam to recreate the elastic movement of the HMLT in ambient conditions has yielded a specific range of diameters to determine the size of the locked in vortex cells. According to standard Department of Transportation manufacturing standards for taper, the lock in distance for small excitations (0.1 inches) would be approximately 10 feet in size in the spanwise direction. This study yields information on vortex cells, their change with forcing excitation, the boundaries between the vortex cells, the changes with Reynolds number and cross-sectional shape, and several other parameters.
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