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

Numerical simulation of rivulet evolution on a circular cylinder in an airflow

On wet and windy days, the inclined cables of cable-stayed bridges may experience a large amplitude oscillation known as Rain-Wind-Induced Vibration (RWIV). It has previously been shown by 'in-situ' and wind-tunnel studies that the formation of rain-water accumulations or 'rivulets' at approximately the separation points of the external aerodynamic flow field and the resulting effect that these rivulets have on this field may be one of the primary mechanisms for RWIV. A numerical method has been developed to undertake simulations of certain aspects of RWIV, in particular, rivulet formation and evolution. Specifically a two-dimensional model for the evolution of a thin film of water on the outer surface of a horizontal circular cylinder subject to the pressure and shear forces that result from the external flow field is presented. Numerical simulations of the resulting evolution equation using a bespoke pseudo-spectral solver capture the formation of two-dimensional rivulets, the geometry, location andgrowth rate of which are all in good agreement with previous studies. Examinations of how the distribution and magnitude of aerodynamic loading and the Reynolds number influence the rivulet temporal evolution are undertaken, theresults of which indicate that while all three affect the temporal evolution, the distribution of the loading has the greatest effect

Development and Validation of a Discrete Vortex Method for the Prediction of Separated Incompressible Flows Around Bluff Bodies. G.U. Aero Report 9622

A vortex method has been developed at the Department of Aerospace Engineering, University of Glasgow, for predicting separated, incompressible flow around two dimensional bodies. The method is a Lagrangian technique, with the vorticity field discretised into a series of particles, that are then tracked in the flow through time. The Biot-Savart law is used to calculate the velocity of each particle and random walks are employed to model flow diffusion. The method has successfully been validated against numerous test cases for aerofoils, however, no significant work has been carried out on validating the method for bluff body calculations, especially bodies with sharp comers. This report presents the necessary modifications to the method, that enable reliable calculations to be made for flow fields around bluff bodies. Results of some calculations are presented against experimental data for simple bluff body geometries. Qualitatively, the results are quite encouraging, although these preliminary calculations have highlighted areas of modelling that need to be addressed in future work to improve the level of agreement with experimental data. Also presented in the report are details of a new speed up routine to improve the efficiency of the calculation

Practical application of CFD for wind loading on tall buildings

This paper is concerned with assessing the scope of appicabiity for computational fluid dynamics(CFD) in the field of structural engineering, with a particular reference to tall buildings. Modern design trends and advances in engineering materials have encouraged the demand for taller and more slender structures. This pattern induces inherent structural flexibility; these cases exceed the limitations of the quasi-static method offered by current codes of practice. Wind tunnel testing is the traditional solution for such dynamically sensitive structures. However, even this scaled modelling approach is clouded by some uncertainties, including scaling the Reynolds number and assuming damping values for the aeroelastic model. While CFD cannot be used as a replacement for wind tunnel testing, there are results within the literature to suggest it has the potential to act as a complimentary tool - provided it is used within its capabilities. The paper outlines the various turbulence models that are available and summarises the extent of their application in a practical structural engineering sense. It also details the user-defined criteria that must be satisfied and discusses the potential for simplified models in tall building CFD analyses, with a view to promoting more efficient and practical solutions

Numerical simulation of rivulet evolution on a horizontal cable subject to an external aerodynamic field

On wet and windy days, the inclined cables of cable-stayed bridges may experience a large amplitude oscillation known as rain-wind-induced vibration (RWIV). It has previously been shown by in situ and wind-tunnel studies that the formation of rain-water accumulations or ‘rivulets’ at approximately the separation points of the external aerodynamic flow field and the resulting effect that these rivulets have on this field may be one of the primary mechanisms for RWIV. A numerical method has been developed to undertake simulations of certain aspects of RWIV, in particular, rivulet formation and evolution. Specifically a two-dimensional model for the evolution of a thin film of water on the outer surface of a horizontal circular cylinder subject to the pressure and shear forces that result from the external flow field is presented. Numerical simulations of the resulting evolution equation using a bespoke pseudo-spectral solver capture the formation of two-dimensional rivulets, the geometry, location and growth rate of which are all in good agreement with previous studies. Examinations of how the distribution and magnitude of aerodynamic loading and the Reynolds number influence the rivulet temporal evolution are undertaken, the results of which indicate that while all three affect the temporal evolution, the distribution of the loading has the greatest effect

New developments in rain–wind-induced vibrations of cables

On wet and windy days, the inclined cables of cable stayed bridges can experience large amplitude, potentially damaging oscillations known as rain-wind-induced vibration (RWIV). RWIV is believed to be the result of a complicated non-linear interaction between rivulets of rain water that run down the cables and the wind loading on the cables from the unsteady aerodynamics; however, despite a considerable international research effort, the underlying physical mechanism that governs this oscillation is still not satisfactorily understood. An international workshop on RWIV was held in April 2008, hosted at the University of Strathclyde. The main outcomes of this workshop are summarised in the paper. A numerical method to investigate aspects of the RWIV phenomenon has recently been developed by the authors, which couples an unsteady aerodynamic solver to a thin-film model based on lubrication theory for the flow of the rain water to ascertain the motion of the rivulets owing to the unsteady aerodynamic field. This novel numerical technique, which is still in the relatively early stages of development, has already provided useful information on the coupling between the external aerodynamic flow and the rivulet, and a summary of some of the key results to date is presented

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

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

Characterization of the eddy dissipation model for the analysis of hydrogen-fueled scramjets

The eddy dissipation model (EDM) is analysed with respect to the ability to address the turbulence–combustion interaction process inside hydrogen-fuelled scramjet engines designed to operate at high Mach numbers (≈7–12). The aim is to identify the most appropriate strategy for the use of the model and the calibration of the modelling constants for future design purposes. To this end, three hydrogen-fuelled experimental scramjet configurations with different fuel injection approaches are studied numerically. The first case consists of parallel fuel injection and it is shown that relying on estimates of ignition delay from a 1D kinetics program can greatly improve the effectiveness of the EDM. This was achieved through a proposed zonal approach. The second case considers fuel injection behind a strut. Here the EDM predicts two reacting layers along the domain which is in agreement with experimental temperature profiles close to the point of injection but not the case any more at the downstream end of the test section. The first two scramjet test cases demonstrated that the kinetic limit, which can be applied to the EDM, does not improve the predictions in comparison to experimental data. The last case considered a transverse injection of hydrogen and the EDM approach provided overall good agreement with experimental pressure traces except in the vicinity of the injection location. The EDM appears to be a suitable tool for scramjet combustor analysis incorporating different fuel injection mechanisms with hydrogen. More specifically, the considered test cases demonstrate that the model provides reasonable predictions of pressure, velocity, temperature and composition