Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Engineering Science

Program

Mechanical and Materials Engineering

Supervisor

Savory, Eric

Abstract

A downburst is a natural phenomenon that occurs during thunderstorms, creating hazards and damage to infrastructure due to the strong winds produced. This research contributes to the current literature by applying the Lundgren et al. scaling parameters (R0, T0, V0) to scale cooling source (CS) downburst simulations. The research also investigates the effects of the environmental lapse rate (ELR) on full-scale downburst outflows and compares the full-cloud model with the subcloud model simulations. The results showed that the scaling parameters preserve the temporal and vertical radial wind speed profile shape, as well as the wind speed locations. The primary and the secondary vortices are also preserved. Lower ELR resulted in downbursts with lower peak wind speed. A peak radial wind speed of 52.0 m/s is observed when ELR = 9.80 K/km, which decreases to 39.6 m/s when ELR = 7.76 K/km. Lower ELR also delayed the downdraft evolution, which is reflected in the outflow development. This research also found that the CS model can be employed to simulate the more realistic simulations of downbursts produced by full-cloud models. This aids in simulating realistic downbursts without relying on the expensive full-cloud models.

Summary for Lay Audience

Downbursts are a natural phenomenon characterized by outward and straight-line strong winds near the ground. A downburst phenomenon can be identified when several trees in a forest are pushed in one direction. A downburst is modelled experimentally using the liquid release method, which consists of releasing a fluid that is slightly denser than the ambient fluid contained in a tank or flume, creating a downdraft that descends to the ground surface. Previous researchers proposed length (R0), time (T0), and velocity (V0) scaling parameters that allowed comparisons of liquid release experiments with real-world events. In addition, these scaling parameters allow comparison with the numerical simulation results. This research implements these parameters to scale numerical model data obtained from a full-scale downburst simulation using a cooling source (CS) model, which is a numerical model used to simulate downbursts. The resulting scaled velocity profiles are similar to the scaled profiles observed in full-scale downburst measurements and preserve the vertical location of the near-ground peak wind speed, which is of great interest to the wind and the structural engineers. The research also examines the effects of environmental conditions on downburst events. The results showed that downbursts are affected by the temperature lapse rate of the ambient, which is defined as the decrease of temperature with altitude. Weak wind speeds are produced when the temperature lapse rate is less than 9.80 K/km. However, the structure of the downflow and the radial outflow does not change as long as the downdraft intensity is high enough so that it reaches the surface before losing its driving force. Finally, the thesis investigates the use of the CS model to replicate downburst events produced by the more realistic numerical models that simulate thunderstorm clouds (full-cloud models). This lays the groundwork for the use of CS models to simulate more realistic downburst events without relying on the expensive full-cloud model simulations.

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