Master of Engineering Science
Mechanical and Materials Engineering
Downburst outflows emerging from thunderstorm producing clouds are contained within the atmosphere where the outflow interacts with the preexisting atmospheric boundary layer (ABL). This novel study employs a realistic approach for experimental simulation of downbursts by translating the downburst source within a scaled ABL within a hydraulic flume system that produces open channel flow. The density-driven model approach is used, involving an iris operated cylinder release mechanism translating inside the ABL generated over a restrictive fetch using passive turbulence generating devices at model scales of 1:5500 and 1:10,000. The velocity vector fields across a vertical plane revealed asymmetrical outflows generated from the complex interaction of the downburst outflow with the ABL. Also, peak velocities as high as 26.2 m/s at full-scale were observed at the downstream side of the outflow after touchdown. A lower cylinder release height and higher ABL flow speed generated larger magnitudes of peak velocity at the downstream end of the outflow.
Summary for Lay Audience
A downburst is a natural phenomenon occurring within the atmosphere wherein a heavy mass of air column descends from clouds during a thunderstorm which after reaching ground causes destructive near ground winds. The wind flow observed in the atmosphere and near ground is a part of a stratified(layered) boundary layer occurring over Earth’s surface where the wind speed increases starting from earth’s surface to the atmosphere, and since the downburst producing cloud is surrounded by winds the cloud translates with nearly the same speed as the atmospheric winds around it. Hence, it can be conceptualized that as the downbursts starts falling from its parent cloud the wind flowing around it will force the downflow to move in one prominent direction causing the downburst outflow to grow in the direction same as that of the surrounding winds. This realistic concept of downburst is applied in the present study by generating this event at a smaller scale experimentally by first creating the wind profile observed in the atmosphere known as atmospheric boundary layer (ABL) in an open-channel flow system. The wind speeds are computed using Particle image velocimetry (PIV) technique which involves tracking of suspended particles in the flow by illuminating them using lasers. The downburst is modelled using two fluids of different densities, where the dense fluid resembles the dense air mass from the cloud and the lighter fluid resembles the atmospheric air and is used to generate the ABL. The dense fluid is filled inside a cylinder mechanism with iris gates at the top and bottom walls which are opened to initiate the downburst event and translated inside the generated ABL at a certain height. The outcomes of this study are 1) the downburst outflow moving in the same direction as the background wind generated stronger wind speeds 2) the interaction of the downburst outflow with ABL created asymmetrical outflows 3) increasing the background wind speed maximized the peak winds observed in the downburst near ground 4) comparison of two release heights revealed the smaller release height generated stronger winds.
Jariwala, Shivani Ashitkumar, "Experimental simulation of a density-driven downburst translating within a turbulent boundary layer" (2021). Electronic Thesis and Dissertation Repository. 7656.