Aerodynamic Characteristics of Yawed Inclined Circular Cylinders
Creators: name: Hoftyzer, Michael Shane
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2016
Subjects: Cables
Inclined Cylinders

description: The wind-induced vibration of bridge stay cables has been a long studied and documented topic including a vast literature presenting experimental and numerical investigation results. There are several aerodynamic phenomena which can be associated with the wind-induced vibrations of bridge stay cables, such as vortex-induced vibration, rain-wind induced vibrations, buffeting phenomenon, dry cable galloping, and high-speed vortex excitation, to name a few. One of the most critical types of vibrations for slender structures exposed to wind is the galloping instability. This is typically not encountered for round structures, like circular cylinders or cables, due to their symmetrical nature, and therefore a lack of negative slope in the lift coefficient. However, vibrations of inclined cables of cable-stayed bridges have been noticed for several bridges, and were associated with partial damage of the cable stays, and damaged noted to cable anchors. It is still unclear if these cable vibrations are caused by dry inclined cable galloping, or by high speed vortex excitation. For this reason, stay cables construction guidelines (FHWA, 2005) have not been able to clearly identify the aerodynamic instability resulting in the recommended use of high structural damping, and high Scruton numbers used to limit bridge stay cable vibrations. The current research addresses these issues by performing numerical CFD (computational fluid dynamics) simulations of wind flow around inclined and yawed cables in order to identify the flow behaviour around the circumference and downstream of the cable. Such numerical models provide a new understanding regarding the flow conditions around an inclined cable and the beginning of dry galloping instability. The simulation was performed for full scale cables in the form of cylinder models with high aspect ratios. The arrangement for the cable was considered as a combination of the inclination and yaw angles, in such a way that it should match the experimental setting considered by Cheng et al (2003), based on which a validating comparison of results was performed. A LES (Large Eddy Simulation) model was developed with a constant Smagorinsky model for simulating the turbulent flow around the cylinders. Reynolds numbers (Re) ranging from 1.1 × 105 to 6.7 × 105 were investigated for various combinations of the inclination angles of 0° to 60° and yaw angles of 0° to 40°. The diameter of the circular cylinder was set to D = 0.089 m and the length of the cable was 2.67 m (30D). Pressure on the surface of the cylinder was monitored on 5 rings arranged along the circular cylinder at equal intervals and velocities were recorded for intervals of 0.1 – 0.5D downstream the cylinder. Also pressure, vorticity and streamlines distributions were recorded for several plans along and across the cylinder. The flow pattern visualisations were clearly established and wind speed profiles were presented. An axial flow along the leeward side of the cylinder was identified for inclined circular cylinders. The predominant axial flows were noted at intervals of 0.1D to 0.3D downstream of the cylinder. As the distance from the leeward side of the cylinder increased, the effect of the far field flow increased as well, for the flow around the leeward side of the cylinder. The drag crisis encountered as a sudden drop in the drag coefficient CD, with the increase of Re number, was confirmed. The preliminary results for inclined cylinders showed good agreement with the experimental results available in the literature. Slight discrepancies for the upper and lower branches of the drag crisis were found between the published data and results obtained in the current study. A new flow classification for inclined and/or yawed circular cylinders was proposed based on the velocity profiles, eddy viscosity, and swirl threads formations, as a combination of the TrSL and TrBL regimes similar with the ones defined by Zdravkovich (1997), for flow perpendicular to cylinders. Four cases showed a potential aerodynamic instability when results of the current study were employed into the theoretical aerodynamic damping equation derived by MacDonald and Larose (2006). Three of these cases demonstrated a similar flow phenomenon to the TrSL-Short flow phenomenon defined in this study, which occurs when the major axis of the ellipse is close to the direction of flow, and the turbulent shear layers detach almost on the leeward side of the cylinder. The coherence, cross-coherence and cross-bicoherence were calculated for the frequency components of the coefficient of lift, the pressure coefficient along the leeward side of the cylinder, and the total velocity along the leeward side of the cylinder, and it was found that three cases of low non-linear interaction, intermediate non-linear interaction, and high non-linear interaction could be identified. Also it was concluded that the interaction between the lift and pressure coefficients monitored for the cylinder and the variation of the total velocity component, did not have a significant influence on the flow regimes, or on the transition between the flow regimes. The high-nonlinear interactions relate more to the potential coupling between the frequencies of the parameters mentioned above, especially for the critical case of 60° relative angle.