Allan Larsen

Storebælt suspension bridge – vortex shedding excitation and mitigation by guide vanes

Abstract During the final phases of deck erection and surfacing of the suspended spans of the Storebaelt bridge, vertical deck oscillations of low frequency were observed by workers and supervision staff. The results of on-site monitoring of the oscillations and wind speed measurements at the nearby island of Sprogo correlated well with wind-tunnel test results obtained during design, indicating that the observed oscillations were due to periodic von Karman type vortex shedding in the wake of the girder. The present paper discusses the results of the on-site structural and wind monitoring programme, correlation to wind-tunnel tests and the design and erection of guide vanes to the main span girder for mitigation of the oscillations. The vortex-induced oscillations were found to be harmless from a structural point of view but unacceptable considering possible physiological impact on users of the bridge.

Aeroelastic analysis of bridge girder sections based on discrete vortex simulations

Two-dimensional viscous incompressible flow past bridge girder cross-sections are simulated using the discrete vortex method. The flow around stationary cross-sections as well as cross-sections undergoing cross-wind vertical (bending) and rotary (torsional) motions are investigated for assessment of drag coefficient, Strouhal number and aerodynamic derivatives for application in aeroelastic analyses. Good to excellent agreement with wind tunnel test results is demonstrated for analyses of forced wind loading, flutter wind speed and vertical vortex-induced response of four practical girder cross-sections. The success of the simulations is attributed to the bluff nature of the cross-sections and to the two-dimensional (2-D) nature of flow around bridge girders.

Discrete vortex simulation of flow around five generic bridge deck sections

Abstract Two-dimensional viscous incompressible flow past five generic bridge deck cross sections are investigated by means of the discrete vortex method. The analyses yields root mean square lift coefficients and Strouhal numbers for fixed cross sections and aerodynamic derivatives for the cross sections undergoing forced oscillatory cross wind and twisting motion. Fair agreement is established between the present simulations and wind tunnel test results reported in the literature.

Aerodynamic aspects of the final design of the 1624 m suspension bridge across the Great Belt

Abstract Long span suspension bridges are flexible structures which are highly sensitive to the action of the wind, hence aerodynamic performance often becomes a governing factor in the design process. The magnitude of the East Bridge, a 3 span suspension bridge of span lengths 535, 1624 and 535 m, has warranted an aerodynamic design process which involves state-of-the-art model testing and analytical procedures. The present paper focuses on the use of wind tunnel test results in the design of the East Bridge. Particular attention is given to the strategy adopted for wind tunnel testing, and to selected results obtained. Considerations for the bridge in service cover aerodynamic stability, dynamic wind loads and vortex shedding response of the girder. For the erection phase aerodynamic stability of the girder is treated in some detail.

Two dimensional discrete vortex method for application to bluff body aerodynamics

Two-dimensional viscous incompressible flow past a flat plate of finite thickness and length is simulated using the discrete vortex method. Both a fixed plate and a plate undergoing a harmonic heave and pitch motion are studied. The Reynolds number is 104 and the record onset flow speed, Ufc is in the range 2–14. The fundamental kinematic relation between the velocity and the vorticity is used in a novel approach to determine the surface vorticity. An efficient influence matrix technique is used in a fast adaptive multipole algorithm context to obtain a mesh-free method. The numerical results are compared with the steady-state Blasius solution, and with the inviscid solution for the flow past an oscillating plate by Theodorsen.

Reynolds number effects in the flow around a bluff bridge deck cross section

A commom assumption in wind engineering is that the flow around bluff and sharp edged bodies is independent of Reynolds number, Re. An important practical implication of this assumption is that aerodynamic parameters such as Strouhal number, lift and drag coefficients may be obtained from low speed wind tunnel model tests and applied directly to the prototype structure.

Aerodynamics of the Tacoma Narrows Bridge - 60 Years Later

This article takes a new approach to understanding the aerodynamics failure mechanism responsible for the collapse of the Tacoma Narrows Bridge 60 years ago. It is demonstrated that the instability mechanism is associated with the formation and drift of large vortices from the upwind edge of the bridge girder cross section. An empirical model of the vortex formation and drift process is formulated that allows critical wind speed for onset of the instability to be estimated. The model is supported by numerical simulations and experimental results. It is shown that a structural modification of the Tacoma Narrows Bridge cross section would have suppressed the vortex formation or counteracted the aerodynamic actions and thus rendered the bridge more aerodynamically stable.

A generalized model for assessment of vortex-induced vibrations of flexible structures

The present paper proposes a one degree of freedom (1DOF) non-linear model of self limiting cyclic wind loads for application in finite element method analyses of light structures subjected to vortex shedding excitation under lock-in conditions. Being empirical by nature, the model includes three independent parameters to be determined from response tests with representative aero-elastic wind tunnel models or prototypes. The paper discusses methods for parameter identification from response data. The paper also evaluates the proposed load model versus other 1DOF empirical vortex shedding models which have found some acceptance in wind engineering.

Prediction of aeroelastic stability of suspension bridges during erection

Abstract During erection of long-span suspension bridges the stiffening girder lacks torsional continuity. Thus, it becomes important to consider flutter-type aeroelastic instabilities. During the early stages of deck erection, the critical wind speed for onset of flutter becomes particularly low and in this case it may prove interesting to consider flutter control by provision of eccentric ballast. The present paper extends standard bridge flutter routines based on measured aerodynamic derivatives to include the effect of eccentric deployment of ballast. The potential enhancement of the critical wind speed by this method is discussed and the importance of applying actual measured aerodynamic derivatives for the bridge section in question is emphasised as opposed to the use of theoretical “flat plate” aerodynamic derivatives.

Computer modelling of flow around bridges using LES and FEM

Abstract An efficient large eddy simulation model (LES) using finite element method (FEM) is presented. The Navier–Stokes equations are solved using an implicit procedure. The three-dimensional LES model could capture the drag crisis phenomena for a circular cylinder. Only 2% of the grid point used by Tamura is used to capture the drag crisis. This is possible because of the use of accurate approximation of convection term using FEM which reduces the numerical diffusion. This model is used to study the flow over Great Belt East Bridge. The computed drag and Strouhal number are in reasonable agreement with wind tunnel measurement.