Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Engineering Science

Program

Mechanical and Materials Engineering

Supervisor

Savory, Eric

Abstract

The outflow velocity vector fields of simulated stationary thunderstorm downbursts and downburst lines were resolved in two plane orientations, vertical and horizontal, using a dense fluid release system and Particle Image Velocimetry. Single event releases were scaled to show agreement with past studies based on radial velocity magnitude and location and radial and vertical front propagation. The vortex trajectory of a single event using the vortex aspect ratio supported density's role in the outflow after the time and location of the maximum radial velocity. The interaction between two release events varied temporally and spatially and showed that the interaction region's lateral outflow produces the highest velocities of up to 1.5 times that of a single event while velocities in the vertical plane remained unaffected. Scaled separation distances greater than 3 km produce weak outflow interactions with no increased velocities beyond those of a single event.

Summary for Lay Audience

Thunderstorms can produce high wind velocities from strong downdrafts know as downbursts. Downbursts produce strong near-ground wind flows that pose a structural aviation engineering hazard that must be designed for. These hazardous winds were modelled at a reduced scale using a dense fluid release cylinder. An imaging technique called Particle Image Velocimetry was used to capture the outflow's velocity from the release cylinder. Single release events were found to compare to past studies based on the location and value of the highest near-ground velocities and the descent and spread of the outflow. The system was expanded to include a second cylinder to explore the velocities produced in the collision region of two release events with the second event released at different times and over different separation distances. Outflow collisions spaced far apart were found to exist as individual events where the near-ground velocities are no higher than a single event, and the vortices meet to lift each other upwards to create elevated vertical velocities. Outflow collisions from events spaced close together and released close in time produced high ground velocities that, with values up to 1.5 times that of a single event and vortices that are forced to remain close to the ground.

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