Application of Wind Lidar Technology in Bridge Engineering

Abstract

A wind Doppler LiDAR (Light Detection And Ranging) is a highly effective tool for investigating key phenomena in the atmospheric boundary layer and remotely measuring wind and turbulence, both of which are crucial in wind engineering. Over the past several decades, Doppler lidars have been widely used for evaluating wind resources, testing the performance of wind turbines, and monitoring turbine wakes. This PhD project focuses specifically on continuous-wave (CW) lidar measurements, a type of wind Doppler lidar. CW Doppler lidars offer high spatial and temporal resolution over distances of several hundred meters. A single CW lidar captures the line-of-sight wind velocity as a weighted average across a domain represented by a Lorentzian function, which becomes wider with the increase in measurement distance. Complementary to that, long-range, pulsed lidars are able to capture wind components along the laser beam over distances of a few kilometers, with a fixed sampling volume but at a lower sampling than the CW lidars. Several long-span bridges spanning several kilometers wide fjords are either under construction or planned in Norway as part of the Coastal Highway Route E39 project. These highly slender structures are particularly sensitive to wind loads caused by atmospheric turbulence, which is significantly affected by the surrounding terrain in complex ways. Additionally, some innovative bridge designs require wind and turbulence data that have not been previously explored in the context of bridge aerodynamics. Wind lidar technology holds great promise for collecting wind data in previously unreachable areas across deep fjords and inside wind tunnels, which can enhance the foundation for bridge design. This PhD project centers on utilizing lidar technology to aid in the planning and design of long-span bridges. It involves characterizing atmospheric turbulence conditions around existing long-span bridges in the complex fjord environment, as well as studying wind flow around bridge deck models in wind tunnels. These efforts contribute to validating numerical models for wind-induced bridge responses, helping to ensure structural safety and reduce costs. Additionally, wind lidar measurements from a planned bridge site are analyzed to introduce a novel approach for reconstructing wind velocity fluctuations using image processing techniques. The project focuses on the following:  Application of short-range lidars to study the smaller scale flow field around a bridge deck.  Lidar application to investigate the wind flow interaction with a bridge deck at model scale, in a wind tunnel.  Analysis of the available long-range lidar measurement data from a planned fjord crossing towards a possible standardization of lidarbased assessment of local wind conditions in the bridge design process. The second part of this thesis introduces the application of the WindScanner system, which uses dual and triple CW Doppler lidars to capture wind vectors around a suspension bridge. The aim is to evaluate their effectiveness in measuring turbulence characteristics crucial for bridge design, both the incoming flow and the flow affected by the bridge girder. The findings are compared to those obtained from sonic anemometers mounted on the bridge deck. This study is the first to employ three CW lidars to investigate the turbulent three-dimensional wind flow around a bridge. Subsequently, the use of lidars in a wind tunnel is explored by testing a bridge deck section model of the Lysefjord Bridge within the boundary layer test section at the Politecnico di Milano's closed-circuit wind tunnel. To simulate boundary layer turbulence similar to real-life conditions, spires and floor roughness elements are deployed. A continuous-wave lidar with two 2” telescopes is used alongside conventional instruments like Cobra probes to measure wake flow, including mean profiles and turbulence, along a vertical line behind the deck. The data were also used to study the intensity and frequency content of the vortex-shedding process. Another measurement configuration was adopted to observe the lateral coherence of wind Components both in the wake and the undisturbed flow. Finally, the available wind data from long-range lidars at a planned bridge site is used to investigate and identify the wind flow pattern in the fjord, and then the potential of the image processing approach to reconstruct wind time series from two lidar units with parallel beams is assessed. This project demonstrates the promising potential of lidars for measuring turbulent wind both at full scale in a large fjord and at model scale in a wind tunnel around a suspension bridge superstructure.