Transient simulation of a microburst outflow: Review & proposed new approach

1. Transient simulation of a microburst outflow: Review & proposed new approach W.E. LIN PhD Candidate C. NOVACCO MESc Candidate Dr. E. SAVORY Associate Professor Department of Mechanical and Materials Engineering May 2006

2. Image ID: nssl1120, National Severe Storms Laboratory CollectionPhotographer: Moller AR, NOAA, National Weather Service. 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)

3. Evidence of downburst damage • Transmission lines Damaged tower in Ontario, Canada in April 1996 [Loredo Souza, 1996]. Damaged tower in central Victoria, Australia in 1993 [Holmes, 2001].

4. Impinging jet experiments: • Letchford & Illidge [1999] • Wood et al [2001] • Chay & Letchford [2002] • Letchford & Chay [2002] • Xu [2004] • Mason et al [2005] Released fluid experiments: • Lundgren et al [1992] • Alahyari & Longmire [1995] • Alahyari [1995] • Yao & Lundgren [1996] Previous approaches to physical modelling

5. Scale 1:9000 1:2400 Steady flow Transient flow Literature review Stationary microburst Translating microburst • Lundgren et al [1992] • Alahyari & Longmire [1995] • Alahyari [1995] • Yao & Lundgren [1996] Released fluid • Letchford & Illidge [1999] • Wood et al [2001] • Chay & Letchford [2002] • Xu [2004] • Mason et al [2005] • Letchford & Chay [2002] Impinging jet

6. 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 1982. Mason et al [2005]

7. CFD simulation [Kim et al, 2005] • FLUENT • Small impinging jet experiment Dj = 0.0381 m z/Dj = 4 Uj = 7.5 m/s • Initial vortex formation →largest velocities at small heights • Dvortex/Dj is ~3.4 times smaller than in released fluid experiment [Alahyari, 1995] Vector colour: velocity magnitude. Red vectors are largest values. Contours: pressure.

8. Gated slot Present approach • Focus on just the outflow region to maximize zm • 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 Current state of BLWT1 [annotations added to original drawing by UWO BLWTL]. Proposed modification for downburst simulation.

9. To stepper motor Preliminary facility UJ = 45 m/s Fully developed region UM = 8-13 m/s UD = 4 m/s • Gate assembly for transient flow experiments • Preliminary facility is a 1:6.75 model of planned large facility

10. 2-D wall jet Slot jet flow x/b = 0 x Uj b

11. Andrews AFB downburst 1 Aug 1983 [Fujita, 1985] time Transient slot jet time history • Filter out poor actuations • Ensemble average remaining time histories • Shape depends on tgate (0.30 s) • Sharp rise to Umax

12. Developing burst front Image ID: nssl0106, National Severe Storms Laboratory Collection Photographer: Waranauskas BR, NOAA, National Weather Service. Taken during JAWS project on 15 July 1982. Flow visualization • Fog fluid illuminated by a laser sheet • b = 0.013 m • x/b = 10 -15 • Uj ~ 4 m/s • Manual gate actuation • Δtopen < 1 s for vortex agrees with Verhoff [1970]

13. HWA measurements: transient, x/b=30 vertical profile 55 z-pts at x/b=30, y=0 • Build up a composite vertical profile from 10 actuations at each z • Comparison of profiles at 3 spanwise locations (at the same time)

14. HWA measurements: transient, x/b=20, y=0 • t histories of U at 55 z-locations ~> evolution of <U> profiles with time

15. Alternate gate design

16. Alternate gate design <U> [m/s]

17. t Uj Simulation scale summary

18. 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

19. 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

20. Acknowledgements: Advanced Fluid Mechanics Research Groupwww.eng.uwo.ca/research/afm 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 Questions & comments are welcome!

25. Primary references: Alahyari AA, December 1995. Dynamics of laboratory simulated microbursts. University of Minnesota; PhD thesis, 166 pages. Fujita TT, 1981. Tornadoes and downbursts in the context of generalized planetary scales. Journal of Atmospheric Sciences, 38(8):1511-1534. Fujita TT, 1985. 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, 2005. Downburst induced dynamic responses of a tall building. 10th Americas Conference on Wind Engineering, Baton Rouge, Louisiana. Letchford CW and Chay MT, 2002. Pressure distributions on a cube in a simulated thunderstorm downburst. Part B: moving downburst observations. Journal of Wind Engineering and Industrial Aerodynamics, 90:733-753. Letchford CW and Illidge G, 1999. Turbulence and topographic effects in simulated thunderstorm downdrafts by wind tunnel jet. Wind Engineering into the 21st Century, Proceedings of the 10th International Conference on Wind Engineering, 21-25 June, Copenhagen, Balkema, Netherlands; 1907-1912. Lundgren TS, Yao J and Mansour NN, 1992. Microburst modelling and scaling. Journal of Fluid Mechanics, 239:461-488. Mason MS, Letchford CW and James DL, 2005. Pulsed wall jet simulation of a stationary thunderstorm downburst, Part A: Physical structure and flow field characterization. Journal of Wind Engineering and Industrial Aerodynamics, 93:557-580. Wood GS, Kwok KCS, Motteram NA and Fletcher DF, 2001. Physical and numerical modelling of thunderstorm downbursts. Journal of Wind Engineering and Industrial Aerodynamics, 89:535- 552. Xu Z, December 2004. Experimental and analytical modeling of high intensity winds. University of Western Ontario; PhD thesis, 184 pages. Yao J and Lundgren TS, 1996. Experimental investigation of microbursts. Experiments in Fluids, 21:17-25.