| NSF Org: |
CMMI Division of Civil, Mechanical, and Manufacturing Innovation |
| Recipient: |
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| Initial Amendment Date: | July 23, 2015 |
| Latest Amendment Date: | July 23, 2015 |
| Award Number: | 1537917 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Joy Pauschke
jpauschk@nsf.gov (703)292-7024 CMMI Division of Civil, Mechanical, and Manufacturing Innovation ENG Directorate for Engineering |
| Start Date: | September 1, 2015 |
| End Date: | August 31, 2020 (Estimated) |
| Total Intended Award Amount: | $337,831.00 |
| Total Awarded Amount to Date: | $337,831.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
1350 BEARDSHEAR HALL AMES IA US 50011-2103 (515)294-5225 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
Ames IA US 50011-2207 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | NEES RESEARCH |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.041 |
ABSTRACT
Cables are often used in groups/bundles in a variety of engineering applications such as in suspension bridges, suspended roofs, guyed lattice towers, and power transmission. These cables are prone to large-amplitude vibrations in wind alone and in wind combined with precipitation (rain or ice). The vibrations can lead to fatigue damage and in some cases catastrophic failure of cables posing a threat to safety. Past investigations involving individual cables with smooth surface have resulted in an improved understanding of vortex-induced and rain-wind-induced vibration phenomena. However, there is a need for a credible wind load model that can be used to predict the dynamic response of bundled cables in turbulent and transient wind at moderate to high wind speeds. This research is to improve the resilience of cables used in cable-supported structures and power transmission lines to hazards of hurricanes and other windstorms. The study will facilitate development/evaluation of potential mitigation strategies leading to a reliable civil and power infrastructure.
In this project, a synergistic computational and experimental approach will study galloping of bare-cable and cable covered with ice to address the technology gaps. The specific objectives are (a) to improve understanding of aeroelastic (motion-induced) behavior of a single and bundled cables used in cable-supported structures and high-voltage power transmission lines in moderate to high wind speeds, (b) to understand the effects of upstream turbulence, non-uniform flow, transient flow, and wake-induced flow on cable response, and (c) to develop a robust time-domain aeroelastic load formulation to predict cable vibration amplitude. The study involves high-fidelity computational fluid dynamics using large eddy simulation, and wind tunnel experiments using section models of single and multiple cylinders and aeroelastic models. The simulation and experiments will be for bare-cables and cables covered with ice in single and grouped configurations. Comparison of computational simulation and wind tunnel experiment results will provide credibility to both procedures. The primary product of this project will be a wind-load model for cables and a methodology that can be used as a tool in structural analysis to identify the vulnerability of cables in a structure or power line at a given wind site.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
The primary objective of this project was to improve the resilience of cables, used in cable-supported structures and high-voltage power transmission lines, to hazards posed by moderate to extreme wind or ice-wind combination. These cables are prone to large-amplitude vibrations in wind due to their low inherent structural damping, which can lead to failure of the cables and/or the adjoining structure. The project tried to address ?galloping? of dry and iced cables using a synergistic approach including experiments and computations. It involved (a) wind tunnel experiments using section models and aeroelastic models of single and multiple cylinders representing bare-cables and iced-cables subject to yawed and turbulent inflow in the AABL Wind and Gust Tunnel located at Iowa State University, and (b) high-fidelity computational fluid dynamic (CFD) simulations with verification against wind tunnel data.
The parameters governing the turbulence-induced (buffeting) and motion-induced (self-excited) wind loads for stay-cables and power-transmission-line cables were identified through static/dynamic tests of rigid section models of smooth and grooved cylinders. These parameters facilitate the prediction of the cable response in turbulent wind and estimate the incipient condition for onset of cable galloping. This study mainly focused on the prediction of critical reduced wind speed (non-dimensional wind speed) for dry- and ice- galloping of cables as a function of yaw angle and Scruton number (non-dimensional mass-damping parameter) through measurement of aerodynamic- damping and stiffness. Empirical equations for mean drag coefficient, Strouhal number, buffeting indicial derivative functions and critical reduced wind speed for dry-cable galloping and ice-galloping were proposed for yawed cables. A simplified design procedure was introduced to estimate the minimum damping required to arrest dry-cable galloping from occurring below the design wind speed of the cable and transmission line. Furthermore, the results from this study can be applied to predict the wind load and response of a dry cable and a dry or iced conductor in time domain at a specified wind speed for a given yaw angle.
Wake-induced aerodynamics of non-yawed/yawed circular cylinders in a tandem arrangement was studied in a wind tunnel using pair of stationary cylinders that represent sections of stay-cables with smooth surfaces and high-voltage power conductors with grooved surfaces, often occurring in a bundled configuration. The results show that there was a reduction of drag coefficient of the downstream smooth cable model with increasing Reynolds number at each yaw angle and with increasing yaw angle at a fixed Reynolds number, but there was no change in the drag coefficient of the downstream grooved cable model with increasing Reynolds number at a given yaw angle.
Wind-induced response of an inclined smooth cable was studied through wind tunnel measurements using a flexible cable model for a better understanding of the vibration characteristics of structural cables in atmospheric boundary layer (ABL) wind. For this purpose, in-plane and out-of-plane responses of aeroelastic models of sagged and non-sagged flexible cables were recorded by accelerometers to investigate the wind directionality effect on its excitation mode(s) and response amplitude. Experimental results show that excitation mode(s) of a cable depend on wind speed, inclination angle, and sag ratio and cable tension. Finite element analyses of a stay-cable of a bridge in ABL wind show that the critical reduced wind speeds for dry cable galloping based on section models in smooth and uniform flow are more conservative.
A computational approach based on a k-ω delayed detached eddy simulation model for predicting aerodynamic loads on a smooth circular cylinder was verified against experiments. Comparisons with experiments were performed for flow over a rigidly mounted (static) cylinder and an elastically-mounted rigid cylinder oscillating in the transverse direction in non-yawed and yawed flow conditions. Three yawed flow cases with yaw angle of 15, 30 and 45 deg. were simulated and the results were found to be independent of yaw angle (independence principle) when the flow speed normal to the cylinder axis is selected as the reference wind speed. Good agreement is observed between the predictions and measurements for mean and rms surface pressure between experiments and computations for a static cylinder in a yawed flow angle of 30 deg. Dynamic simulations for an elastically-mounted rigid cylinder accurately predicted the displacement amplitude and unsteady loading over a wide range of reduced velocity compared with experimental results. It was found that simulations with span less than five-cylinder diameters gave erroneous results including, under/overprediction of displacement amplitude.
This project resulted in a better understanding of underlying physics of the dynamic flow field of wind and its interaction with cables that will lead to a more accurate prediction of wind-induced vibration of cables in the future. This project has raised public awareness of wind hazards to cable-supported structures and power transmission lines and benefitted graduate, undergraduate and K-12 students, as well as public in general through improved curricula, laboratory demonstrations, publications and news media outlet.
Last Modified: 11/30/2020
Modified by: Partha P Sarkar
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