The objective of this project is to conduct a fundamental study to quantify the characteristics of violent microburst winds and their loading effects on ground-based civil structures. A synergistic approach is used in this study that involves laboratory simulations using a translating microburst simulator and scaled models of civil structures, numerical simulations using CFD, and field data for validation of the simulated flow field. Particle Image Velocimetry is used to provide detailed flow-field measurements while existing CFD models are extended to include buoyancy effects to verify the similarities between a natural microburst generated by descending cold air with those generated in laboratory simulations. The characteristics of unsteady flow structures of microburst winds and resultant wind loads on civil structures are investigated in great detail. One of the outcomes of this study will be knowledge of wind-speed profiles of microburst winds in various terrains, and a catalog of aerodynamic loads for typical civil structures that will be useful for future design of such structures.
The goal of this project is to elucidate the underlying physics for a better understanding of microburst wind hazard in order to provide more accurate prediction of wind damage potential to built structures with the ultimate goal of reducing the loss of life or injury and economic loss. Society at large will benefit significantly from improved predictions of damage potential of violent near-ground winds and improved building or design codes that can help in mitigating this type of wind hazard. This project will also benefit graduate, undergraduate and K-12 students as well as public in general through improved curricula, laboratory demonstrations, Internet-based disseminations and news media outlets.
The primary objective of this project was to conduct a funadamental study to quantify the characteristics of violent microburst winds and their loading effects on civil structures, particularly buildings. A synergistic approach was used in this study that involved laboratory simulations using a microburst simulator and small-scaled models of buildings, numerical simulations using Computational Fluid Dynamics (CFD), and field data for validation of the simulated flow field. A laser-based technique known as Particle Image Velocimetry (PIV) was used to map the flow field near the ground while an existing CFD model was employed to verify the similarities between a natural microburst generated by a descending cold air with those generated in laboratory simulations. The characteristics of unsteady flow structures of microburst winds and resultant wind loads on buildings were investigated in great details. Wind resulting from a microburst and its effects are quite distinct from those of the ABL (Atmospheric Boundary Layer) or straight-line wind. Wind in a microburst flows radially outwards from its center and is transient in nature. The mean wind speed, mean turbulence intensity and bounday-layer height vary with radial distance (r) from the microburst center and are time dependent. The mean wind speed shows an increase in magnitude up to a certain elevation and then a steady decrease. The maximum wind speed occurs at a normalized radial distance of around r/D of 1 and at an elevation that depends on the diameter (D) of the microburst (D < = 4000 m). The maximum turbulence intensity occurs around r/D of 1.5. Both external and internal pressures acting on a low-rise building are dependent on the radial location (r) of the microburst with respect to the building. Low-rise buildings experience high positive external pressure and large downward force if a microburst occurs near the building (0< = r/D < = 0.5 or core region). In the outburst region (r/D > 1), the distribution of pressure is similar to those obtained in the ABL wind. Comparison of the overall mean wind loads reveal that a gable-roofed building with a low-roof angle (< 17 deg.) and a building with circular cross-section with a conical roof experience reduced aerodynamic drag when they are located in the outburst region. The normalized pressure distribution along the centerline of a gable-roofed building shows that it is different from the ASCE 7, the standard used for calculating minimum design loads based on straight-line winds, for radial locations within r/D of 0.5 but similar for radial locations from r/D = 0.5 to 2. For example, significant positive pressure occurs on the building roof in the core region as opposed to suction or negative pressures that buildings are designed for. Generally, the maximum mean pressure (normalized) on buildings occurs at around r/D=1.0 while the maximum pressure fluctuation (normalized) occurs at r/D=1.5. Since the micrburst winds can be higher in magnitude than design ABL winds in the continental USA, outside its hurricane region, the mean and fluctuating loads on the buildings are expected to be higher than the design loads for buildngs located in this region should a microburst occurs. Like the low-rise buildings, mean pressure distribution on a high-rise building depends on the radial location within the microburst flow field. In the outburst region (r/D > 1), an "upside-down" along-wind force distribution pattern along the height of the high-rise building was observed contrary to that in the ABL wind. The commonly used technique for laboratory simulation, the steady impinging jet, provides an average flow field with radial-velocity profile at the critical location (max velocity location), but it lacks time-dependent information which can prove to be critical for building design. An alternate laboratory model, named as transient impinging jet model, was developed for the purpose of improving the laboratory simulation technique. Comparison of time-dependent flow fileds of a simulated microburst from a computational "cooling-source model" with those from the new laboratory model and those from the field has shown that the flow field evolution predicted by the transient impinging jet model is similar to that of the cooling source model except with some differences. Thus, the transient impinging jet model provides an alternative model for future laboratory studies to explore transient loads on civil structures in a microburst. Microburst like tornadoes result from thunderstorms. It can produce winds that can exceed 150 mph (67 m/s) causing immense property damage. This project resulted in a better understanding of underlying physics of the dynamic flow field of microburst wind and its interaction with buildings that will lead to a more accurate prediction of wind damage to buildings and improved building code in the future. This project has raised public awareness of wind hazards and benefitted graduate, undergraduate and K-12 students, research scholars as well as public in general through improved curricula, laboratory demonstrations, publications and news media outlets.
