Computational Fluid Dynamics Investigation into High Mast Illumination Poles: Influence of Light Fixture Type

Abstract

High mast illumination poles (HMIPs) are commonly used throughout the United States to illuminate large areas such as highways and rest stops. HMIPs have proven to be susceptible to fatigue under wind loading, yet interactions between the structure and wind remain poorly understood. Aeroelastic phenomenon, such as vortex shedding, have been shown to induce larger than normal deformations and stresses that can lead to premature fatigue in structural details near the base of the pole. The geometry of the structure is a contributor to how it will respond to wind loading. While HMIP geometry remains relatively the same, the type and number of luminaires installed at the top of the pole can differ greatly. This study aims to analyze how differences in luminaire installations can affect flow around the lighting assembly at the top of the pole. Flow past two types of luminaires commonly used in Kansas, LED and incandescent, was analyzed using 2D computational fluid dynamics (CFD) modeling. In addition to luminaire type, angle of attack of the wind, number of luminaires, and wind speed were varied. Lift and drag force time-histories were extracted from the CFD simulations, and the power spectral density (PSD) of the data was found. Using +/-10 percent bounds, the natural frequencies of the HMIP were compared to the peaks in the PSD curves to formulate conclusions regarding the susceptibility of poles to lock-in vortex shedding. The results of the study showed: 1) both luminaire types were susceptible to inducing vibrations that could lead to lock-in behavior across the first three natural frequencies of the poles, 2) the number of peaks in the PSD curves developed from each simulation that fell within a natural frequency range increased as the wind speed increased, 3) more peaks fell within the mode 1 range for the incandescent luminaires than for the LED luminaires, and 4) there was no clear indication that LED luminaires might incite a greater response than incandescent luminaires in the first mode.