Characterization of thunderstorm downburst winds through CFD techniques

The characteristic wind field of a certain region is mostly governed by the climatology of its larger scale area. In the case of mid-latitude regions (e.g. Europe), their climatology is determined by the extra-tropical cyclones at the larger synoptic scale. Atmospheric boundary layer (ABL) winds based on synoptic-scale structures are hence considered as the foundation for codes and standards used to assess the wind loading of structures and to design structures to prevent wind-related damage accordingly. In addition to the ABL winds, the mid-latitude regions are also prone to winds of a non-synoptic origin at the mesoscale level, with thunderstorm outflows or downbursts being the representative of such non-synoptic wind action. Since they are determined by a set of features that makes them fundamentally different from the ABL winds, downbursts can produce the corresponding wind action that is often fatal to low-rise and mid-rise structures. On these grounds, a comprehensive initiative to enable a better understanding of fundamental downburst flow features relevant for the structural loading was framed under the umbrella of the ERC THUNDERR Project. The present thesis, as the numerical modeling part of the THUNDERR Project framework, aims to address the physical characteristics of thunderstorm downbursts through the application of Computational Fluid Dynamics (CFD) technique. The focus of this work is placed on the CFD reconstruction of experimental tests of the reduced-scale thunderstorm downbursts carried out in the WindEEE Dome Research Institute (University of Western Ontario, Canada). Although they recreate the downburst flow field, the experimental analysis is restricted to the limited number of probe points. In that perspective, CFD allows expanding the analysis of experimental tests to the entire flow field, which can reveal phenomenological aspects that are either challenging or impossible to retrieve from experimental tests only. Two fundamental downburst scenarios were analyzed: (i) an isolated vertical downburst, and (ii) a downburst embedded inside the approaching ABL flow. For that purpose, three CFD approaches of a ranging complexity level were adopted. The unsteady Reynolds-Averaged Navier-Stokes (URANS), hybrid Scale-Adaptive Simulations (SAS), and Large-Eddy Simulations were used, and their overall reliability was examined. Theimplications of the WindEEE Dome specific geometrical features (i.e. bell-mouth inflow nozzle) on the downburst flow reconstruction by the facility were further discussed. The bulk of the thesis discusses the dominant flow features of the downburst with the particular emphasis on the dynamics of dominant vortex structures (i.e. primary vortex, secondary vortex, trailing ring vortices) and their spatio-temporal influence on the vertical profiles of radial velocity component. The non-dimensional flow characteristics of interest were evaluated such as the trajectory of the primary vortex and the spatial dependence of the velocity of primary vortex propagation. Analyses were further extended for the case of a joint downburst and ABL wind interaction to address the dynamics between two different wind fields, and the genesis of the worst condition in terms of the maximum radial velocity due to the ABL wind entrainment was discussed. The flow field was analyzed across various azimuth angles with respect to the ABL flow to report on the flow asymmetry, and general implications of such downburst configuration on spatio-temporal evolution of wind velocity profiles which can produce severe conditions for low-rise and mid-rise structures.