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EXTREME WINDS Thunderstorm, Gust Front, Microburst

EXTREME WINDS Thunderstorm, Gust Front, Microburst. Hossein Ghaednia Instructor Dr. S. Cheng Civil and Environmental Engineering Department University of Windsor March 29th 2011. Outline. History Review Physical Nature Simulation Experimental study Numerical study Analytical study

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EXTREME WINDS Thunderstorm, Gust Front, Microburst

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  1. EXTREME WINDSThunderstorm, Gust Front, Microburst • HosseinGhaednia • Instructor Dr. S. Cheng • Civil and Environmental Engineering Department • University of Windsor • March 29th 2011

  2. Outline • History Review • Physical Nature • Simulation • Experimental study • Numerical study • Analytical study • Concluding remarks • Future studies Wind Engineering Project

  3. History Review • importance of thunderstorms was recognized by Whittingham in 1960s • Gomes and Vickery, specifically analyzed Sydney thunderstorms for extreme wind speeds in 1975 • In 1975, For first time Fujita defined a downburst as a column of descending air impacting the ground and violently bursting out. • Terms of Microburst and Macroburst for first time presented by Fujita in 1985 • After That time, Lots of effort have been done to study thire mechanisms and a way to simulate them. Wind Engineering Project

  4. Physical Nature • Thunderstorm in three stages (Proposed by Battan) 1. Cumulus* stage 2. Mature stage 3. Dissipating stage * Cumulus clouds typically form when warm air rises and reaches a level of cool air Wind Engineering Project

  5. Physical Nature 1. Cumulus* stage Convection is the primary mechanism that produces updrafts. 2. Mature stage Evaporation and water loading are two of the factors responsible for the formation of a strong downdraft 3. Dissipating stage At this point up drafting has stopped and the downdraft has spread over the entire cell. Wind Engineering Project

  6. Physical Nature • Four stages of Thunderstorm outflow (Proposed by Wakimoto): 1. Formative stage 2. Early Mature stage 3. Late Mature stage 4. Dissipating stage Wind Engineering Project

  7. Physical Nature 1. Formative stage Rain and cold air first penetrate the base of the cell during the formative stage of the outflow 2. Early Mature stage precipitation roll, forms during the early mature stage of the outflow 3. Late Mature stage The descending air source is almost depleted at this stage 4. Dissipating stage The depth of the gust front shrinks and its structure weakens Wind Engineering Project

  8. Physical Nature 1. Formative stage Rain and cold air first penetrate the base of the cell during the formative stage of the outflow 2. Early Mature stage precipitation roll, forms during the early mature stage of the outflow 3. Late Mature stage The descending air source is almost depleted at this stage 4. Dissipating stage The depth of the gust front shrinks and its structure weakens Wind Engineering Project

  9. Physical Nature Downbursts and outflow gust fronts (Fujita 1974) downdrafts could be sustained, and even strengthen, such that they impact the ground in a devastating manner very strong winds at ground level were associated with this ring vortex expanding and stretching during the life of a downburst Wind Engineering Project

  10. Physical Nature Ring Vortex: Wind Engineering Project

  11. Physical Nature Classification of downburst: 1. Microburst Horizontal extent of damaging wind :less than 4 km Wind speed: 75 m/s Duration:3~5 min 2. Macroburst Horizontal extent of damaging wind :greater than 4 km Wind speed: 60 m/s Duration: 30 ~ 40 min * Macroburstor sever gust front Wind Engineering Project

  12. Simulation 1. Experimental (wind tunnel) 2. Numerical 3. Analytical Whether dealing with the aerodynamics of buildings, bridges or towers many issues remain to be fully resolved including the role of non-stationary gust interactions, Reynolds number effects, and the significance of small-scale turbulence Wind Engineering Project

  13. Simulation 1. Experimental (wind tunnel) 2. Numerical 3. Analytical Whether dealing with the aerodynamics of buildings, bridges or towers many issues remain to be fully resolved including the role of non-stationary gust interactions, Reynolds number effects, and the significance of small-scale turbulence Wind Engineering Project

  14. Simulation Experimental Study (wind tunnel) : 1. Wall jet 2. Using Valve 3. The Moving Jet Wind Tunnel 4. Outlet-Situated Aperture 5. Flat Plate with High Incidence to the Oncoming Flow 6. Multiple Fan Wind Tunnel System 7. Proposed large-scale modeling of the transient features of a downburst outflow Wind Engineering Project

  15. Vertical board Blower Working section Contraction Diffusing section Jet Experimental Method 1. Impinging Wall jet Drawbacks: a) Scale is approximately 1: 25000 when compared with full scale data b) Couldn’t reproduce transient wind velocity 2. Using Valve Wind Engineering Project

  16. Experimental Method 3. The Moving Jet Wind Tunnel (Letchford, 2002) Downburst could be replicated when the translation speed of the jet exceeded 20% of the downdraft speed of the jet Drawbacks: a) Scale is approximately 1: 3000 when compared with full scale data b) outflow from the jet was very similar to a stationary wall jet Wind Engineering Project

  17. Experimental Method 4. Outlet-Situated Aperture(Mason, 2005) reproduce a downburst’s ring vortex by using a wall jet fitted with an outlet-situated aperture. Wind Engineering Project

  18. Experimental Method Aperture • The aperture used for pulsing the flow • The aperture comprised of 16 sheet metal blades each less than 1mm thick. • A lever arm attached to the top plate allowed full opening and closing of the aperture. • Open : 500 mm • Close : 5 mm diameter hole left • The average time taken to open the aperture was 0.2 s. Wind Engineering Project

  19. Experimental Method Visualize the downburst flow The aperture used for pulsing the flow • The aperture comprised of 16 sheet metal blades each less than 1mm thick. • A lever arm attached to the top plate allowed full opening and closing of the aperture. • Open : 500 mm • Closed : 5 mm diameter hole left • The average time taken to open the aperture was 0.2 s. Wind Engineering Project

  20. Experimental Method 4. Outlet-Situated Aperture(Mason, 2005) Drawbacks: a) The pulsing, opening and closing of the outflow orifice is described as unwieldy b) any shape effect produced by the opening of the aperture would be more pronounced in the resulting flow Wind Engineering Project

  21. Experimental Method 5. Flat Plate with High Incidence to the Oncoming Flow Limitation: a) Couldn’t used to simulate downburst b) Ring Vortex is different from the real one Wind Engineering Project

  22. Experimental Method 6. Multiple Fan Wind Tunnel System (Miyazaki University, Japan) • The current version of this wind tunnel facility comprises a bank of 99 fans • useful for investigating gust affecting tall buildings. • multiple fan wind tunnel that is capable of reproducing many of the statistical parameters of atmospheric boundary layer flows, such as turbulence intensity, power spectrum, and velocity time histories. Wind Engineering Project

  23. Experimental Method 4. Multiple Fan Wind Tunnel System (Miyazaki University, Japan, 2009) • Each fan is driven by AC servo motors, which can be controlled at different frequencies up to 25Hz to generate a fluctuating flow field, and each are independently controlled. • This independent control allows for phase shifting among the fans, allowing for traverse and vertical turbulence to be introduced • Based on the dynamic nature of this system, and its inherent capability to tailor flow fields, it was an ideal choice to aid in the modeling of gust front type flows Wind Engineering Project

  24. Experimental Method 4. Multiple Fan Wind Tunnel System (Miyazaki University, Japan) Drawbacks: a) It cannot realize the entire low-level jet type velocity profile of the gust front Wind Engineering Project

  25. Experimental Method 7. Proposed large-scale modeling of the transient features of a downburst outflow (Western Ontario Un. ) • The objective is to reproduce a velocity history rise and decay that matches field observations Wind Engineering Project

  26. Experimental Method 7. Proposed large-scale modeling of the transient features of a downburst outflow • Rotating Gate 2. Translating Gate Wind Engineering Project

  27. Simulation Numerical Study : 1. Complex Computational Domains 2. Modeled one point statistics 3. Stochastic methods 4. Computational Fluid Dynamics (CFD) Wind Engineering Project

  28. Numerical Study 2. Modeled one point statistics • have additionally modeled one point statistics, capturing the transient dynamics of a single point in the flow field in order to provide inputs for structural dynamic models 1. Complex Computational Domains • Initial numerical simulations (e.g. Proctor (1988, 1989)) utilized complex computational domains to simulate the large scale meteorological flows for use in aircraft simulations • Many of the studies mentioned previously in relation to the civil engineering applications, have sought to simulate the physical testing environment Wind Engineering Project

  29. Numerical Study 3. Stochastic methods • Attempted to simulate downburst wind time histories through a series of decompositional methods, i.e. discrete wavelet transforms and empirical mode decomposition, that eliminate the need for the stationary assumption used in traditional approaches 4. Computational Fluid Dynamics (CFD) • This method widely used by scientist today • Use of computational fluid dynamics (CFD) in modeling downbursts is potentially valid, but will continue to need refinement Wind Engineering Project

  30. Numerical Study(Famous Studies) Influence of topography on simulated downburst (Sydney University, 2009) • It was generally found that storm maximum wind speeds could be increased by up to 30% because of the presence of a topographic feature at the location of maximum wind speeds. Comparing predicted velocity profile amplification with that of a steady flow impinging jet, similar results were found despite the simplifications made in the impinging jet model. Downburst line near-surface outflows (University of Western Ontario, 2011) • It is apparent from the current study that complex downdraft interactions, driven by buoyancy, are responsible for the formation of unique small scale outflow features, high speed burst swaths, increased damaging surface footprints, and large amplification factors. All of these increase the hazard presented by a downburst line when compared to that of an isolated downburst with the same forcing as the individual events in the downburst line Wind Engineering Project

  31. Simulation Analytical Study : 1. Wavelet Analysis Wind Engineering Project

  32. Concluding remarks Severe wind events, particularly those emanating from thunderstorms, are well understood in some respects and difficult to define in others. This primarily stems from the lack of full scale measurements and data from which to construct fully developed models. Damage, however, is easily seen in the aftermath of the storms, but questions still remain as to the exact mechanisms of the damage Lots of innovative methods are proposed by scientists in order to simulate thunderstorms and downbursts but in the most cases they were not successful in modeling the secondary behavior of downburst (Vortex ring) and transient velocity of wind. Each of these numerical and experimental frameworks replicates specific features of gust fronts and downbursts, but have drawbacks when simulating other features or dealing with issues of scale Wind Engineering Project

  33. Future • Some investigates should be done on finding a way to select the real data from Microburst and Gust front. • Finding a way to reproduce the ideal Ring Vortex in wind tunnel. • Work on a new analytical method to simulate the Downburst that will lead us to improve and modify current numerical methods Wind Engineering Project

  34. Question Thanks Wind Engineering Project

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