Presentation on theme: "Transient simulation of a microburst outflow: Review & proposed new approach May 2006 W.E. LIN PhD Candidate C. NOVACCO MESc Candidate Dr. E. SAVORY Associate."— Presentation transcript:
Transient simulation of a microburst outflow: Review & proposed new approach May 2006 W.E. LIN PhD Candidate C. NOVACCO MESc Candidate Dr. E. SAVORY Associate Professor Department of Mechanical and Materials Engineering
What is a microburst? Sequence of events: updraft precipitation downdraft evaporation acceleration Impingement at ground leads to radially expanding burst front Travelling / stationary Brief event: NIMROD/JAWS avg duration (3.1 & 2.9 min) Image ID: nssl1120, National Severe Storms Laboratory Collection Photographer: Moller AR, NOAA, National Weather Service.
Evidence of downburst damage Transmission lines Damaged tower in central Victoria, Australia in 1993 [Holmes, 2001]. Damaged tower in Ontario, Canada in April 1996 [Loredo Souza, 1996].
Released fluid experiments: Lundgren et al  Alahyari & Longmire  Alahyari  Yao & Lundgren  Impinging jet experiments: Letchford & Illidge  Wood et al  Chay & Letchford  Letchford & Chay  Xu  Mason et al  Previous approaches to physical modelling
Translating microburst Literature review Released fluid Stationary microburst Lundgren et al  Alahyari & Longmire  Alahyari  Yao & Lundgren  Impinging jet Letchford & Illidge  Wood et al  Chay & Letchford  Xu  Mason et al  Letchford & Chay  Steady flow Transient flow Scale 1:9000 1:2400
Transient nature of the flow Developing burst front Image ID: nssl0106, NSSL Collection Photographer: Waranauskas BR, NOAA, National Weather Service. Taken during JAWS project on 15 July Mason et al 
CFD simulation [Kim et al, 2005] FLUENT Small impinging jet experiment D j = m z/D j = 4 U j = 7.5 m/s Initial vortex formation → largest velocities at small heights D vortex /D j is ~3.4 times smaller than in released fluid experiment [Alahyari, 1995] Vector colour: velocity magnitude. Red vectors are largest values. Contours: pressure.
Present approach Current state of BLWT1 [annotations added to original drawing by UWO BLWTL]. Proposed modification for downburst simulation. Focus on just the outflow region to maximize z m 2-D jet from a rectangular slot instead of 3-D impinging jet from a round nozzle Large-scale implementation as a modular addition to an existing facility Gated slot
Preliminary facility U J = 45 m/s U D = 4 m/s Fully developed region U M = 8-13 m/s Gate assembly for transient flow experiments Preliminary facility is a 1:6.75 model of planned large facility To stepper motor
Slot jet flow UjUj b x/b = 0 x 2-D wall jet
Filter out poor actuations Ensemble average remaining time histories Shape depends on t gate (0.30 s) Sharp rise to U max Transient slot jet time history time Andrews AFB downburst 1 Aug 1983 [Fujita, 1985]
Flow visualization Fog fluid illuminated by a laser sheet b = m x/b = U j ~ 4 m/s Manual gate actuation Δt open < 1 s for vortex agrees with Verhoff  Developing burst front Image ID: nssl0106, National Severe Storms Laboratory Collection Photographer: Waranauskas BR, NOAA, National Weather Service. T aken during JAWS project on 15 July 1982.
Build up a composite vertical profile from 10 actuations at each z Comparison of profiles at 3 spanwise locations (at the same time) HWA measurements: transient, x/b=30 vertical profile 55 z-pts at x/b=30, y=0
t histories of U at 55 z-locations ~> evolution of profiles with time HWA measurements: transient, x/b=20, y=0
Alternate gate design
Simulation scale summary t UjUj
Summary & conclusions Review of previous physical simulations: - small-scale only - few transient studies Design and implementation of a preliminary microburst simulator Proof of concept with flow visualization / HWA measurements Can create a large-scale transient burst similar to a microburst outflow
Recommendations for future work Refinement of design using CFD PIV in preliminary facility Importance of gate actuation parameters, track gate position Large-scale facility: modular assembly, tighter tolerances, co-flow Design and testing of aeroelastic transmission line tower models
Questions & comments are welcome! Acknowledgements: Advanced Fluid Mechanics Research Group C Vandelaar & B Stuart University Machine Services R Struke & G Aartsen Western Engineering Electronics Shop W Altahan & M Gaylard Western Engineering technicians GA Kopp UWO BLWTL RJ Martinuzzi University of Calgary
Primary references: Alahyari AA, December Dynamics of laboratory simulated microbursts. University of Minnesota; PhD thesis, 166 pages. Fujita TT, Tornadoes and downbursts in the context of generalized planetary scales. Journal of Atmospheric Sciences, 38(8): Fujita TT, The downburst: microburst and macroburst. University of Chicago, Dept. of Geophysical Sciences; Satellite and Mesometeorology Research Project, Research Paper #210. Kim J, Ho TCE and Hangan H, Downburst induced dynamic responses of a tall building. 10 th Americas Conference on Wind Engineering, Baton Rouge, Louisiana. Letchford CW and Chay MT, Pressure distributions on a cube in a simulated thunderstorm downburst. Part B: moving downburst observations. Journal of Wind Engineering and Industrial Aerodynamics, 90: Letchford CW and Illidge G, Turbulence and topographic effects in simulated thunderstorm downdrafts by wind tunnel jet. Wind Engineering into the 21 st Century, Proceedings of the 10 th International Conference on Wind Engineering, June, Copenhagen, Balkema, Netherlands; Lundgren TS, Yao J and Mansour NN, Microburst modelling and scaling. Journal of Fluid Mechanics, 239: Mason MS, Letchford CW and James DL, Pulsed wall jet simulation of a stationary thunderstorm downburst, Part A: Physical structure and flow field characterization. Journal of Wind Engineering and Industrial Aerodynamics, 93: Wood GS, Kwok KCS, Motteram NA and Fletcher DF, Physical and numerical modelling of thunderstorm downbursts. Journal of Wind Engineering and Industrial Aerodynamics, 89: Xu Z, December Experimental and analytical modeling of high intensity winds. University of Western Ontario; PhD thesis, 184 pages. Yao J and Lundgren TS, Experimental investigation of microbursts. Experiments in Fluids, 21:17-25.