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Title:Computational Study on 3-D Aerodynamic Characteristics of Flow around a Yawed, Inclined, Circular Cylinder
Author(s):Yeo, DongHun; Jones, Nicholas P.
Subject(s):Yawed and inclined circular cylinder
Detached eddy simulation (DES)
Three-dimensional flow characteristics
Swirling flow
Cable vibration
Abstract:The excessive and unanticipated vibrations of stay cables in cable-stayed bridges around the world have been often observed with the occurrence of wind and wind-rain. These vibrations with low frequency and high amplitude are important for the safety and serviceability of cable-stayed bridges. There has been extensive research involving wind tunnel experimentation and field observations that is devoted to investigating the main factors contributing to the cable vibration and that propose potential mechanisms. A comprehensive understanding is, however, still not available. Moreover, there have been few studies involving in-depth investigation of the fluid flow around a stationary cable, which is fundamental to understanding the aerodynamic instability associated with the cable vibration. In order to investigate the fundamental characteristics of aerodynamic forces on a cable oblique to wind in this work, the 3-D DES (Detached Eddy Simulation) approach was applied to flow around a yawed and inclined circular cylinder in this study. DES enables simulation of unsteady three-dimensional flow at high Reynolds number while maintaining reasonable computational requirements. First, simulations of flow normal to a circular cylinder using 2-D RANS (Reynolds-averaged Navier-Stokes equation) were conducted for verification of numerical schemes/conditions and for validation of the simulated flow. In DES, the flow region near a wall is treated in the RANS mode whose solution becomes converged with grid refinement. Therefore, 2-D RANS could be used to examine the numerical effects of spatial/temporal discretizations and computational schemes on the simulated flow. The verification associated with three-dimensional effects, such as spanwise grid sizes, cylinder lengths and spanwise boundary conditions, was performed in flows normal to and oblique to a cylinder using 3-D DES and comparison to experimental data. The verified and validated 3-D DES was used to investigate characteristics of the flow and the associated aerodynamic forces on a yawed and inclined circular cylinder. The study shows that the flow around the cylinder is inherently three-dimensional. According to the proposed mechanism to explain the flow-induced forces, an important factor is a swirling flow, which has an axial velocity component as well as rotational one. The swirling flow is developed by the rolled-up shear layer flowing past the oblique cylinder, generating relatively large forces on the cylinder. Multiple forces with peaks of finite length occur at spatial intervals along the cylinder axis, alternating on both sides of the cylinder while moving at a fixed speed. The speed of these force peaks is approximately 90% of the velocity component of the oncoming upstream flow parallel to the cylinder axis. Three-dimensional characteristics of the aerodynamic forces on the cylinder are discussed in local coordinates as well as in global coordinates. While a few high frequencies are observed in the force coefficient (Cy) of the local y axis, the force coefficient (Cx) in the local x axis has a significant low-frequency component. It is much lower than that of Karman vortex-induced vibration in the same flow environment. The behaviors of Cx and Cy result from the periodic moving forces due to traveling swirling flows in equi-spaced intervals. This regular pattern of the multiple moving loads is a potential source for low-frequency and high-amplitude aerodynamic instability of a long circular cylinder oblique to the flow. Since the forces move along the cylinder, they were investigated in a Lagrangian reference frame as well as in an Eulerian frame. The moving force coefficients (Cmx and Cmy) have one dominant frequency for the fluctuations in the x and y axes, respectively when they travel along the cylinder. The dominant frequency of Cmx is twice as high as for Cmy, which implies that the moving forces are related to classical Karman vortices. The effects of the angle of the cylinder to the flow on characteristics of the flow and the associated forces were also examined in this study. As yaw angle increases, the peak frequency of Cy deviates from the prediction by the Cosine Rule. The dominant frequency of Cx has a relatively low-frequency component and increases with yaw angle, but it cannot be predicted by the Cosine Rule. The comparison results indicate that the three-dimensional characteristics of the forces cannot be precisely explained by a quasitwo- dimensional approximation, such as the Cosine Rule. While there have been many efforts to investigate the characteristics of cylinder flow and related forces, this is, to the author’s knowledge, the first numerical study to focus on the three-dimensional characteristics of the fully-developed flow at high Reynolds number and associated forces on a yawed and inclined circular cylinder. This study is a significant contribution for improved understanding of the complex threedimensional characteristics of the flow and flow-induced forces. The low frequency components of the forces provide insights into the fundamental mechanism of cable vibrations with low frequency and high amplitude. This study also can be applied not only to circular cylindrical structures under oblique flow such as electrical transmission lines under oblique wind and marine structures under inclined currents but also to any elongated bluff bodies under flow in three-dimensional environments.
Issue Date:2011-03
Publisher:Newmark Structural Engineering Laboratory. University of Illinois at Urbana-Champaign.
Series/Report:Newmark Structural Engineering Laboratory Report Series 027
Genre:Technical Report
Publication Status:published or submitted for publication
Peer Reviewed:not peer reviewed
Rights Information:Copyright held by the authors. All rights reserved.
Date Available in IDEALS:2011-04-06

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