Preferred solution, K12 – Appendix E Aerodynamics
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This document contains a summary of aerodynamic assessment of the K12 alternative for the Bjørnafjorden crossing. Further details are given in the notes in the reference list. Aerodynamic coefficients, both static and flutter derivatives, was calculated by use of CFD. The CFD results were calibrated versus wind tunnel tests and take into account near ground effects for the low part of the bridge. For the high bridge part at zero degree angle of attack, Cd was found to be 0.68. Near ground effects increase these numbers by 20%. Traffic typically increased the drag factor with 70%. Aerodynamic coefficients is also given for cables (typically Cd=0.8), towers and columns (both typically Cd=1.5). Recommendations for wind parameter input is given and is mainly based on N400 values. For parameters not given by N400, ESDU values are used. The effect on the dynamic response of inhomogeneity in the wind due to aerodynamic forces has been studied in the frequency domain. Generally, maximum dynamic response is found for wind perpendicular to the main axis when the mean wind is uniformly distributed along the alignment. For cases with non-uniform distribution of mean wind, the dynamic response is smaller than for the case with uniform distribution. Sensitivity to dynamic response was checked for P10, P50 and P90 values of the following wind parameters: xLu, Au, Cux and Cuy. For xLu values in medium range gave the highest response. For Au increased values gave increased response. For Cux and Cuy low values gave highest response. The bridge system was checked for galloping, static divergence, classical flutter and torsion instability and it is found aerodynamically stable. A multimode approach was used when evaluating flutter, and the onset wind speed for flutter is calculated to be above 120m/s. Thus, fulfilling the criteria. Vortex shedding is not expected for the bridge system. To suppress cable vibrations external dampers are needed. The following phenomena was checked: Dry galloping, rain/wind galloping, ice/sleet galloping and vortex induced vibration. It is recommended to perform detailed wind tunnel test before concluding on the girder cross sectional design and cable design. The high turbulence of wind from the southern sector may lead to increased traffic closure of the bridge. We recommend that this is addressed in more detail.