Acir Mércio Loredo-Souza

A novel approach for wind tunnel modelling of transmission lines

Abstract This paper is concerned with the behaviour of transmission line cables in severe boundary layer winds. It examines the effect of the scale of turbulence on the response of a line-like structure (cable model) through wind tunnel tests and the comparison of wind tunnel tests with theoretical predictions made through the statistical method using influence lines. Consistency with theory allowed the development of a new modelling approach to conductor systems using a distorted horizontal length scale (span wise) to accommodate these systems in the wind tunnel. Transverse and oblique wind incidences were tested. From the results obtained in the experimental work, it is apparent that the new modelling approach to conductor systems in wind tunnels is a valid technique. It is necessary, however, that a correction be made in the values of the variance of the response measured in the distorted model.

The effects of high winds on transmission lines

Abstract The behaviour of transmission lines under severe winds is examined. Possible reasons to explain transmission line failures in strong winds are investigated through aeroelastic wind tunnel tests, and the experimental results are successfully compared with theoretical predictions made through the statistical method using influence lines. Cables with different characteristics are simulated and tested at transverse wind incidence. The results obtained confirm the importance of turbulence in the dynamic response and demonstrate that the aerodynamic damping plays an important role in the dynamic behaviour of the cables. They also show that, although the background response may indeed be the biggest contributor for the total fluctuating response in most typical cases, the resonant response can be important depending on the characteristics of the structure and of the wind flow, and should not be neglected in the design procedures.

The influence of the design methodology in the response of transmission towers to wind loading

Abstract From a theoretical approach, the design procedure for the establishment of wind loading on transmission towers was reviewed and current procedures, such as Davenports gust response factor (GRF), were compared with the statistical method using influence lines (SIL), which is considered more realistic. This latter approach can account for unbalanced loading effects, shear and axial loads and the effects of higher modes of vibration in the calculation of the response factors. Several responses due to certain assumed transverse wind characteristics were calculated for some typical transmission towers. The main findings were: (a) Peak loads calculated using SIL were larger than peak loads given by the GRF. (b) The dynamic response of transmission structures is strongly dependent on the turbulence intensity level and its spectrum. (c) For members in which there is reversal in the forces on the load position, the resonant response in the second mode of vibration was bigger, even by four to five times, than the corresponding one in the first mode. Although this effect is not as severe in terms of resulting stresses when all the components are computed in the peak responses, it can lead to fatigue problems. From the current results it can be concluded that the incorporation of the dynamic properties of transmission structures in the design methodologies is needed and that the statistical method using influence lines is a more correct approach since it allows for the inclusion of a larger number of factors in the design methodology.

Wind loads on attached canopies and their effect on the pressure distribution over arch-roof industrial buildings

Abstract Arch-roof industrial buildings are very wind sensitive. The current aerodynamic coefficients in wind codes do not contemplate the possibility of existence of canopies attached to the buildings. This paper presents the results of an investigation on the influence exerted by canopies on the static wind actions on arch-roof industrial buildings. Six scale models of these arch-roof buildings were tested, with five types of canopies attached. Three of these canopies were instrumented and the static wind pressures were measured. The tests were done at the boundary layer wind tunnel of the Universidade Federal do Rio Grande do Sul. The results show that the aerodynamic coefficients for the roof are not affected by the canopies, in the case of axial incidence. However, the influence on the pressure distribution is noticeable for wind incidence perpendicular to the main axis of the arch roofs and for other incidences as well. This influence is discussed in the paper. The aerodynamic coefficients for the design of the arch-roofs, with and without the attached canopies are given. Aerodynamic coefficients for design of the canopies are also suggested. Furthermore, the paper discusses the relation between the magnitude of the canopy design forces and the canopy width, as well as the relation between the canopy height location and the height of the building wall. The results were compared with design recommendations from previous work of Jancauskas and Holmes (in: US National Conference on Wind Engineering, Proceedings, Texas Tech University, Lubbock, 1985) and Jancauskas and Eddleston (in: International Conference on Wind Engineering, Fotodruck J. Mainz, Aachen, 1987).

Wind tunnel aeroelastic studies on the behaviour of two parallel cables

Abstract The behaviour of two parallel transmission line cables under high winds is examined through aeroelastic wind tunnel testing of cable models. Cables with different characteristics and spacing between them are simulated and tested at transverse wind incidence. The results obtained demonstrate good agreement between measured values and theoretical prediction. Correlation coefficients of the drag forces of the two cables were obtained. They vary little with wind velocity and are bigger for the smallest separation between cables. The coherence between the forces was also found to be smaller for the larger separations between cables and decreases with velocity.

The Wind-Driven Rain and the Buildings: Directional Driving Rain, Experimental Simulation and Quantification of Wetness Areas

The theme involving the driving rain has been the subject of various studies that seek to contribute to the understanding of this phenomenon and its interaction with the buildings. In this sense, research discusses some aspects related to driving rain, such as the development and application of a methodology for quantifying wetting areas on models exposed to experimental driving rain, and the determination to the direction of the driving rain, in Porto Alegre city—Brazil, through the use of semi-empirical method, more specifically, a wind-driven rain relationship formula. The findings of this study demonstrate that the building facades oriented to the East/South quadrant have higher incidence level of driving rain, considering the Porto Alegre City-RS-Brazil. Furthermore, considering the application of the methodology, it was found, based on the images obtained in the experimental tests that both the building shape and the surroundings influence the degree of driving rain wetting on the models facades. In this way, the model with a square base and without surroundings showed the least amount of wet area when compared to the others.

FLOW IN THE WAKE OF WIND TURBINES: TURBULENCE SPECTRAL ANALYSIS BY WIND TUNNEL TESTS

The interaction between the incident wind and wind turbines in a wind farm causes mean velocity deficit and increased levels of turbulence in the wake. The turbulent flow is characterized by the superposition of the wind turbine wakes. In this work, the technique of turbulence spectral evaluation to reduce scale models in a boundary layer wind tunnel is presented, and different measurements of velocity fluctuations are analyzed. The results allow evaluating the spectrum configuration at different frequency ranges and the differences of the spectral behavior between the incident wind and the turbine wake flow.

A comparison study between seven procedures to predict vortex-induced vibrations on industrial chimneys

Structures like towers and industrial chimneys are quite vulnerable to the vortex shedding phenomenon, due to their slenderness and non-aerodynamic form. Furthermore, due to their low structural damping, these structures are also more likely to reach large displacement amplitudes, which is caused by the lock-in effect. Although these structures are considered simple from the structural and aerodynamic viewpoints, the study of crosswind vibrations in these structures is quite complicated, as it involves the interaction of complex topics of fluid and structural mechanics, turning a reliable determination of the structural response into one of the most complicated problems in Wind Engineering. Because of that, this study aimed to compare some methods for predicting the response due to the vortex shedding phenomenon using full scale data from industrial chimneys. The chosen methods, which are exposed in codes and standards like Eurocode, National Building Code of Canada and CICIND Model Code for Steel Chimneys, derive from the Ruscheweyh’s correlation length model and the Vickery & Basu’s spectral mathematical model. In addition, these methods are also compared to three proposals made for the Brazilian Wind Code. This study concludes that the methods based on the Vickery and Basu’s model work better for large displacement amplitudes.

Experimental study of the neighborhood effects on the mean wind loading over two equivalent high-rise buildings

This paper presents a series of results with respect to the mean values of shear, base moment and torsion acting in a building obtained through an experimental wind tunnel study using the standard building proposed by the Commonwealth Advisory Aeronautical Research Council (CAARC) as building reference. In the loading determination, the interference of a neighboring building with similar geometric characteristics to the CAARC was simulated, considering variations of positioning and spacing in relation to the reference building. It was concluded that the presence of the neighboring building increased the mean loads in the reference building for a significant number of directions considered. In the case of the considered deviations and the proposed provisions by this study, it was concluded that the vicinity factor that would contemplate the majority of the results obtained in the tests should increase the wind loads by at least 60% in relation to the values obtained for the building reference considered in isolation.

A numerical study on the aerodynamic performance of building cross-sections using corner modifications

A numerical investigation is performed in this work in order to evaluate the aerodynamic performance of building cross-section configurations by using corner modifications. The CAARC tall building model is utilized here as reference geometry, which is reshaped considering chamfered and recessed corners. The numerical scheme adopted in this work is presented and simulations are carried-out to obtain the wind loads on the building structures by means of aerodynamic coefficients as well as the flow field conditions near the model’s location. The explicit two-step Taylor-Galerkin scheme is employed in the context of the finite element method, where eight-node hexahedral finite elements with one-point quadrature are used for spatial discretization. Turbulence is described using the LES methodology, with a dynamic sub-grid scale model. Predictions obtained here are compared with experimental and numerical investigations performed previously. Results show that the use of corner modifications can reduce significantly the aerodynamic forces on the building structures, improve flow conditions near the building locations and increase the Strouhal number, which may have an important influence on aeroelastic effects.