Jasna Bogunović Jakobsen

Span-wise structure of lift and overturning moment on a motionless bridge girder

The span-wise correlation of buffeting lift and overturning moment on a bridge box-girder is estimated based on surface pressure measurements in a wind tunnel. The correlation in frequency domain is expressed in terms of normalized co-spectrum and a smooth surface is fitted to the calculated co-spectrum. A corresponding co-spectrum estimate is also obtained for the span-wise correlation of vertical velocity fluctuations which are assumed to be dominant in generation of lift and moment. The span-wise structure of buffeting forces is found considerably stronger than the structure of the oncoming turbulence. This suggests that the influence of structure (i.e. bridge girder) on the oncoming flow is not negligible. The assumption, used in the quasi-steady theory, that the span-wise structure of turbulence can represent the span-wise structure of the associated forces on a line-like structure may thus not be conservative.

Aeroelastic effects on a rectangular box-girder bridge

Abstract Rectangular box-girders are commonly used for prestressed concrete bridges constructed by the balanced cantilever method. The aerodynamic characteristics of such a girder, with width to depth ratio varying between 1.4 and 2.9, are investigated in a series of wind tunnel tests. The aeroelastic tests carried out with a low structural damping (damping ratio 0.2% of critical) revealed the presence of vortex shedding and galloping forces both in turbulent and smooth flow. Possible description of these effects in terms of a linearized aerodynamic damping is discussed. The RMS displacement estimate associated with such a damping parameter is compared to the monitored displacement data. Lift force in the response calculation is taken into account according to Vickerys stochastic model of across-wind loading, based on the data from pressure measurement tests with a non-moving model.

Measurements of Surface-Layer Turbulence in a Wide Norwegian Fjord Using Synchronized Long-Range Doppler Wind Lidars

Three synchronized pulsed Doppler wind lidars were deployed from May 2016 to June 2016 on the shores of a wide Norwegian fjord called Bjornafjord to study the wind characteristics at the proposed location of a planned bridge. The purpose was to investigate the potential of using lidars to gather information on turbulence characteristics in the middle of a wide fjord. The study includes the analysis of the single-point and two-point statistics of wind turbulence, which are of major interest to estimate dynamic wind loads on structures. The horizontal wind components were measured by the intersecting scanning beams, along a line located 25 m above the sea surface, at scanning distances up to 4.6 k m . For a mean wind velocity above 8 m · s - 1 , the recorded turbulence intensity was below 0.06 on average. Even though the along-beam spatial averaging leads to an underestimated turbulence intensity, such a value indicates a roughness length much lower than provided in the European standard EN 1991-1-4:2005. The normalized spectrum of the along-wind component was compared to the one provided by the Norwegian Petroleum Industry Standard and the Norwegian Handbook for bridge design N400. A good overall agreement was observed for wave-numbers below 0 . 02 / m . The along-beam spatial averaging in the adopted set-up prevented a more detailed comparison at larger wave-numbers, which challenges the study of wind turbulence at scanning distances of several kilometres. The results presented illustrate the need to complement lidar data with point-measurement to reduce the uncertainties linked to the atmospheric stability and the spatial averaging of the lidar probe volume. The measured lateral coherence was associated with a decay coefficient larger than expected for the along-wind component, with a value around 21 for a mean wind velocity bounded between 10 m · s - 1 and 14 m · s - 1 , which may be related to a stable atmospheric stratification.

Time-Domain Analysis of Wind-Induced Response of a Suspension Bridge in Comparison With the Full-Scale Measurements

The present study compares the buffeting response of a suspension bridge computed in the time-domain with full-scale measurement data. The in-service Lysefjord Bridge is used as a study case, which allows a unique comparison of the computational results with full-scale buffeting bridge response observed during a one year monitoring period. The time-domain analysis is performed using a finite element approach. Turbulent wind field is simulated according to the governing bridge design standard in Norway for three different terrain categories. The time-domain analysis indicates that the non-linear components of the wind loading are of limited importance in the present case, contributing by less than 5% to the standard deviation of the lateral displacement. The contribution of the buffeting loads on the main cables, hangers and towers to the lateral dynamic response of the bridge girder is about 6%. With the time-domain method, mode coupling as well as the influence of cables and towers are well captured in the simulation results. The buffeting response, estimated in terms of the standard deviation of acceleration, is found to be in good agreement with the field measurement data. Comparison suggests that the proposed numerical method, with the non-linear force model, is able to predict the bridge response reasonably well.

Velocity Spectra and Coherence Estimates in the Marine Atmospheric Boundary Layer

Two years of continuous sonic anemometer measurements conducted in 2007 and 2008 at the FINO1 platform are used to investigate the characteristics of the single- and two-point velocity spectra in relation to the atmospheric stability in the marine atmospheric boundary layer. The goals are to reveal the limits of current turbulence models for the estimation of wind loads on offshore structures, and to propose a refined description of turbulence at altitudes where Monin–Obukhov similarity theory may be limited. Using local similarity theory, a composite spectrum model, combining a pointed and a blunt model, is proposed to describe the turbulence spectrum for unstable, neutral and stable conditions. Such a model captures the

Vortex-induced vibration (VIV) effects of a drilling riser due to vessel motion

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Critical Reynolds number and galloping instabilities – Experiments on circular cylinders

-1 power law followed by the velocity spectra at an intermediate frequency range in the marine atmospheric boundary layer. For the Monin–Obukhov similarity parameter