Degree Type

Dissertation

Date of Award

2019

Degree Name

Doctor of Philosophy

Department

Aerospace Engineering

Major

Engineering Mechanics

First Advisor

Partha P. Sarkar

Second Advisor

Anupam . Sharma

Abstract

Wind-induced loads can cause moderate to large-amplitude vibrations of flexible structures with low inherent damping. Structural or fatigue failure that is induced by these vibrations pose a significant threat to the safety and serviceability of these structures, and hence important for their performance-based design. Flow-induced vibration can be classified as rain-wind induced vibration (RWIV), vortex-induced vibration (VIV), wake galloping, dry and ice galloping. This study focuses on understanding of the mechanisms that influence the wind-induced vibration (galloping and VIV) of structural cables, power-line cables (conductors), and traffic signal structures by identifying the aerodynamic or turbulence-induced (buffeting) and aeroelastic or motion-induced (self-excited) wind load parameters that facilitate the predictions of their response in turbulent wind along with the critical wind speed for incipient instability. In this regard, a series of wind tunnel experiments using static and dynamic section models of cylinders and signal-light units were performed to measure the aerodynamic and aeroelastic forces under uniform and smooth/gusty flow conditions using the Aerodynamic-Atmospheric Boundary Layer (AABL) Wind and Gust Tunnel located in the Department of Aerospace Engineering at Iowa State University. Different combinations of wind speed components, surface pressures, loads and response were measured. The mean aerodynamic force coefficients and vortex-shedding frequency (f_s) were measured. Self-excited and buffeting wind load parameters were identified for yawed and non-yawed single cable and conductor using a free-vibration suspension system and a gust generator setup. Static and dynamic measurements on cylinder models that are representative of cables and signal-light units resulted in proposing empirical equations for estimating the mean aerodynamic force coefficients, Strouhal number, flutter derivatives and buffeting indicial derivative functions for a specific yaw angle. A stability boundary for dry-cable galloping of a smooth cable was determined based on Scruton number and yaw angle that compares well with past studies. Additionally, a design procedure was introduced to predict the minimum required damping to arrest dry-cable galloping of cables and conductor from occurring below the design wind speed. Flow around a yawed circular cylinder with smooth and grooved surfaces in the wake of another cylinder in a tandem arrangement and with iced surface was also investigated. In another study, the response of an inclined cable in yawed wind was studied through wind tunnel test of an aeroelastic model of the cable for better understanding of the vibration characteristics of a structural cable in atmospheric boundary layer wind. Six cases with and without cable sag were studied to investigate the wind directionality effect on excitation mode(s) and response amplitudes of the cable model. Experimental results showed that vibration mode(s) of the cable mainly depend on wind speed, inclination angle, and sag ratio of the cable. While first, second, and third vibration modes of the cable contributed to the response at low wind speeds for different cases, higher modes contributed at high wind speeds. Moreover, measured cable force showed that the cable tension significantly increases with wind speed resulting in increased natural frequency of the cable. For the traffic signal structure, a method was developed in time domain to predict its 2DOF response in normal or yawed wind by using the wind loads that were generated with measured aerodynamic load parameters of the mast arm with circular section and signal-light units. The tip response of the mast arm and the maximum stress at its joint were calculated for a given signal structure and validated with field measurements. Numerical results indicated that yawed wind below a speed of 9 m/s from the backside of the signal light and vortex shedding of the mast arm are the primary reasons of large-amplitude vibration, and the calculated maximum stress exceeds the endurance limit that can cause fatigue failure over time. Additionally, a modified signal light design was proposed to mitigate the vibration.

Copyright Owner

Mohammad Jafari

Language

en

File Format

application/pdf

File Size

228 pages

Included in

Engineering Commons

Share

COinS