A numerical investigation into the aerodynamic characteristics and aeroelastic stability of a footbridge

The results of a numerical investigation into the aerodynamic characteristics and aeroelastic stability of a proposed footbridge across a highway in the north of England are presented. The longer than usual span, along with the unusual nature of the pedestrian barriers, indicated that the deck configuration was likely to be beyond the reliable limits of the British design code BD 49/01. The calculations were performed using the discrete vortex method, DIVEX, developed at the Universities of Glasgow and Strathclyde. DIVEX has been successfully validated on a wide range of problems, including the aeroelastic response of bridge deck sections. In particular, the investigation focussed on the effects of non-standard pedestrian barriers on the structural integrity of the bridge. The proposed deck configuration incorporated a barrier comprised of angled flat plates, and the bridge was found to be unstable at low wind speeds with the plates having a strong turning effect on the flow at the leading edge of the deck. These effects are highlighted in both a static and dynamic analysis of the bridge deck, along with modifications to the design that aim to improve the aeroelastic stability of the deck. Proper orthogonal decomposition (POD) was also used to investigate the unsteady pressure field on the upper surface of the static bridge deck. The results of the flutter investigation and the POD analysis highlight the strong influence of the pedestrian barriers on the overall aerodynamic characteristics and aeroelastic stability of the bridge

Chicago Beach Resort Development - an Assessment of the Transverse Galloping Stability of the Tower Hotel Mast. G.U. Aero Report 9626.

This report forms the second part of a study, commissioned by W.S. Atkins and Partners Overseas, into the galloping stability of the prismatic elements of the Tower Hotel, which is part of the Chicago Beach Resort Development. The particular subject of this study is the mast which extends above the top of the Hotel, which was not investigated previously due to a lack of relevant aerodynamic data. The report presents aerodynamic data for a number of shapes which have been identified as possessing some of the important features of the mast cross-section. Along with the structural and modal data supplied by W.S. Atkins, an assessment of the critical galloping speeds of these shapes is made after extending the quasi-steady theory originally described in the first report. There is strong evidence to suggest that the mast will not experience an instability due to transverse galloping, however the absence of sharp comers increases significantly the sensitivity of the transverse force coefficient to body geometry and Reynolds number. Increased confidence in the assessment requires more accurate aerodynamic data, obtainable from a series of wind tunnel tests on representative scale models of the mast cross-section

A New Vortex Method for Modelling Two-Dimensional, Unsteady, Incompressible, Viscous Flows. G.U. Aero Report 9245

This report presents details of a new mathematical model of viscous, incompressible, unsteady flow around multiple closed bodies. A system of vortex particles is employed in the model, on all solid boundaries and in the wake, to represent quantities of vorticity. The method is a Lagrangian technique and does not require the generation of a flow mesh. A review of recent advances in vortex modelling is provided in the Introduction. Many of these ideas are incorporated into the model or are planned for future inclusion. Section 2 is the main core of the report where the theoretical development of the model is presented. The extensive numerical details have been omitted and will be presented in a future report. Conclusions are made regarding the development of the model and the verification procedure required to validate the algorithm

Chicago Beach Resort Development - A Review of Galloping of Rectangular Prisms Pertinent to the Stability of the Tower Hotel External Frame. G.U. Aero Report 9619

In this report a review, commissioned by W.S. Atkins and Partners Overseas, is presented of the factors which determine the high speed galloping stability characteristics of rectangular prisms. The information is subsequently used to make an assessment of the galloping stability of the external frame of the Tower Hotel, a structure which forms part of the Chicago Beach Resort Development. The predictions of linear theory are explained, and the effects of aspect ratio, freestream turbulence and reduced damping discussed in detail. The variety of oscillatory response is explained with reference to both non-linear theory and results from wind tunnel experiments. A number of prevention methods are proposed, based on results from model and full-scale tests. It is concluded that the factors discussed can have a marked effect on the galloping tendency and response of rectangular prisms. Notwithstanding the limitations of the sectional aerodynamic theory employed, an assessment of the transverse galloping stability of the Tower Hotel external frame is made. The added dampers, originally designed to suppress vortex excitation, also play an important role in the suppression of galloping in the upper two bays of the exoskeleton. With these dampers the calculated threshold wind speeds are substantially in excess of those likely to occur in the top two bays, even in extreme conditions. Without the dampers galloping excitation would be possible, especially in low turbulence flows. The lower two bays have higher natural frequencies and mass, and experience lower wind speeds. The corresponding level of damping required may be achievable without dampers. No published data has been found to enable an assessment to be made of the aeroelastic stability of the Tower Hotel mast. Although circular sections do not gallop, very slight deviations from circularity may result in an unstable section. The situation is exacerbated by the lower turbulence intensities at this height, as well as the given slenderness, exposure and fixing arrangements. It is recommended, therefore, that a series of aerodynamic tests be performed to investigate the stability of the mast

Numerical investigation of the effects of pedestrian barriers on aeroelastic stability of a proposed footbridge

A numerical investigation into the aerodynamic characteristics and aeroelastic stability of a proposed footbridge across a motorway in the north of England has been undertaken. The longer than usual span, along with the unusual nature of the pedestrian barriers, indicated that the deck configuration was likely to be beyond the reliable limits of the British design code BD 49/01. In particular, the investigation focussed on the susceptibility of the bridge due to flutter, and to assess if the design wind speeds could be met satisfactorily. The calculations were performed using the discrete vortex method, DIVEX, developed at the Universities of Glasgow and Strathclyde. DIVEX has been successfully validated on a wide range of problems, including the aeroelastic response of bridge deck sections. The proposed deck configuration, which incorporated a pedestrian barrier comprised of angled flat plates, was found to be unstable at low wind speeds with the plates having a strong turning effect on the flow at the leading edge of the deck. DIVEX was used to assess a number of alternative design options, investigating the stability with respect to flutter for each configuration. Reducing the number of flat plates and their angle to the deck lessened the effect of the barrier on the overall aerodynamic characteristics and increased the stability of the bridge to an acceptable level, with the critical flutter speed in excess of the specified design speed

A New Vortex Method for Modelling Two-Dimensional, Unsteady, Incompressible, Viscous Flows. G.U. Aero Report 9245

This report presents details of a new mathematical model of viscous, incompressible, unsteady flow around multiple closed bodies. A system of vortex particles is employed in the model, on all solid boundaries and in the wake, to represent quantities of vorticity. The method is a Lagrangian technique and does not require the generation of a flow mesh. A review of recent advances in vortex modelling is provided in the Introduction. Many of these ideas are incorporated into the model or are planned for future inclusion. Section 2 is the main core of the report where the theoretical development of the model is presented. The extensive numerical details have been omitted and will be presented in a future report. Conclusions are made regarding the development of the model and the verification procedure required to validate the algorithm

Application of a Zonal Decomposition Algorithm, to Improve the Computational Operation Count of the Discrete Vortex Method Calculation. G.U. Aero Report 9711.

The vortex method has proved a very useful tool for analysing separated, incompressible flow around two dimensional bodies. The method utilises a grid free, Lagrangian approach, to discretise the vorticity field into a series of vortex particles. These particles are then tracked in time, using the Biot-Savart law to calculate the velocity field. This calculation requires the velocity of each vortex to be found as a sum over all other particles in the flow field. A Discrete Vortex Method (DVM) has been developed at the Department of Aerospace Engineering, University of Glasgow. Currently, this vortex method uses a direct summation technique, which although relatively simple, leads to a computational operation count proportional to the square of the number of particles. In calculations that use a large number of particles, such as bluff body models, the direct summation technique becomes prohibitively expensive. A new algorithm for the velocity calculation has now been included in the DVM and is presented in this report. The procedure uses a zonal decomposition algorithm for the velocity summation. This allows the effect of groups of particles on the velocity to be calculated using a single series expansion, thus significantly reducing the operation count of the calculation. The algorithm utilises a hierarchical technique, so that the largest possible group of particles is used for each series expansion. The resulting operation count is 0(N+NlogN), and therefore offers a significant improvement over the direct summation method

Prediction of Unsteady Flow around Square and Rectangular Section Cylinders using a Discrete Vortex Method. G. U. Aero Report no. 9801

A Discrete Vortex Method has been developed at the Department of Aerospace Engineering, University of Glasgow to predict unsteady, incompressible, separated flows around closed bodies. The basis of the method is the discretisation of the vorticity field, rather than the velocity field, into a series of vortex particles which are free to move in the flow. The grid free nature of the method allows analysis of a wide range of problems for both stationary and moving bodies. This report presents a brief description of the numerical implementation, and presents the results of an extensive validation of the method on bluff body flow fields. Results are presented for the mean force coefficients, surface pressure coefficients and Strouhal numbers on a square section cylinder at varying angle of incidence. Also presented are the mean force coefficients and Strouhal numbers on rectangular cylinders. The results from the vortex method show good agreement, both qualitative and quantitative, with results taken from various experimental data

Simulation of Parallel Blade-Vortex Interaction using a Discrete Vortex Method. G.U. Aero Report no. 9832

Numerical results are presented for two-dimensional vortex-aerofoil interaction using a grid-free discrete vortex method. The effects of the passing vortex on the surface pressure distribution and hence the aerodynamic force and moment of the aerofoil are examined in detail for a variety of interaction geometries. For some head-on interaction cases, vortex-induced local flow separation is also predicted on the aft part of the aerofoil surfaces. Extensive comparisons are made with other numerical results and the results from the Glasgow University BVI windtunnel test, which show good agreement

A Brief Review of Unsteady Aerodynamic and Aeroelastic Phenomena of a Fan Installation. G. U. Aero Report no. 9719

This report contains a brief review of the unsteady aerodynamic and aeroelastic phenomena of relevance to the operation of the Inco fan installation. The review addresses issues associated with unsteady inlet flow phenomena due to an upstream vortical wake and the aeroelastic flutter characteristics of the rotor assembly. Based on the available information on inlet flow and blade structural dynamic characteristics a number of conclusions are made. Firstly the vortical wake will be three dimensional in nature, although the von Karman vortices generated from the motor housing will predominate. These have the potential to induce vibration in the fan blading, and should be investigated further. Secondly, it is recommended that the potential for stall flutter and single mode bending-torsion flutter be further investigated