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

May 2006

W.E. LINPhD Candidate

C. NOVACCOMESc Candidate

Dr. E. SAVORYAssociate Professor

Department of Mechanical and Materials Engineering

What is a microburst? 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 CollectionPhotographer: Moller AR, NOAA, National Weather Service.

Evidence of downburst damageEvidence 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 fluidexperiments:

Lundgren et al [1992]

Alahyari & Longmire [1995]

Alahyari [1995]

Yao & Lundgren [1996]

Impinging jet experiments:

Letchford & Illidge [1999]

Wood et al [2001]

Chay & Letchford

[2002]

Letchford & Chay [2002]

Xu [2004]

Mason et al [2005]

Previous approaches to physical modellingPrevious approaches to physical modelling

Translating microburst

Literature reviewLiterature review

Released fluid

Stationary microburst

Lundgren et al [1992]

Alahyari & Longmire [1995]

Alahyari [1995]

Yao & Lundgren [1996]

Impinging jet

Letchford & Illidge [1999]

Wood et al [2001]

Chay & Letchford [2002]

Xu [2004]

Mason et al [2005]

Letchford & Chay [2002]

Steady flow

Transient flow

Scale

1:9000

1:2400

Transient nature of the flowTransient nature of the flow

Developing burst frontImage ID: nssl0106, NSSL CollectionPhotographer: Waranauskas BR, NOAA, National Weather

Service. Taken during JAWS project on 15 July 1982.

Mason et al [2005]

CFD simulation [Kim et al, 2005] CFD simulation [Kim et al, 2005]

FLUENT

Small impinging jet experimentDj = 0.0381 mz/Dj = 4Uj = 7.5 m/s

Initial vortex formation → largest velocities atsmall 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.

Present approachPresent 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 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

Gated slot

Preliminary facility Preliminary facility

UJ = 45 m/s

UD = 4 m/s

Fully developed region UM = 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 flowSlot jet flow

Uj

b

x/b = 0

x

2-D wall jet

Filter out poor actuations

Ensemble average remaining time histories

Shape depends on tgate (0.30 s)

Sharp rise to Umax

Transient slot jet time historyTransient slot jet time history

time

Andrews AFB downburst

1 Aug 1983 [Fujita, 1985]

Flow visualizationFlow 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 sfor vortex agrees with Verhoff [1970]

Developing burst frontImage ID: nssl0106, National Severe Storms Laboratory

CollectionPhotographer: Waranauskas BR, NOAA, National Weather

Service. Taken 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=30HWA measurements: transient, x/b=30

0102030405060708090

100110120130140150160170180190200210220230240250260270280

0 5 10 15 20 25 30 35<U> (ensemble averaged) [m/s]

z [m

m]

t = 0.07 s, y/Y = -0.22

t = 0.07 s, y/Y = 0

t = 0.07 s, y/Y = +0.22

t = 0.10 s, y/Y = -0.22

t = 0.10 s, y/Y = 0

t = 0.10 s, y/Y = +0.22

t = 0.1247 s, y/Y = -0.22

t = 0.1247 s, y/Y = 0

t = 0.1247 s, y/Y = +0.22

vertical profile55 z-pts at x/b=30, y=0

t histories of U at 55 z-locations ~> evolution of <U> profiles with time

0

50

100

150

200

250

0 5 10 15 20 25 30 35 40

<U> (ensemble averaged) [m/s]

z [m

m]

t = 0.02 s

t = 0.075 s

t = 0.13 s

t = 0.18 s

t = 0.28 s

t =1.00 s

HWA measurements: transient, x/b=20, y=0HWA measurements: transient, x/b=20, y=0

Alternate gate designAlternate gate design

Alternate gate designAlternate gate design

Temporal development of <U> profiles at x/b = 10, y=0tgate = 0.1 s, 180° actuation

0

5

10

15

20

25

0 5 10 15 20 25 30 35 40 45

U [m/s]

z [m

m]

0.025

0.065

0.11

0.12

0.13

0.16

<U> [m/s]

Simulation scale summarySimulation scale summary

tU

j

Study Geometric scale Velocity scale Comments

Buoyancy-driven flow

Lundgren et al [6], experimental

1:22000 (1:9000 - 1:45000)

1:85

Alahyari & Longmire [7], experimental

1:25000 1:300

Impinging jet

Kim et al [13], computational

1:26000 (1:10500 - 1:52500)

1:6.7Impulsive start of a stationary continuous jet

Mason et al [14], experimental

1:3000 (1:2400 - 1:6100 )

1:3Actuated stationary continuous jet

Slot jet (present results with preliminary facility)

Quasi-steady simulation 1:800 - 1:4000 -

Transient simulation 1:700 1:2

Slot jet (anticipated results with full-size facility)

Quasi-steady simulation 1:200 - 1:1000 -

Transient simulation 1:700 1:1 - 1:2

Release of fluid from a stationary cylinder vessel into a tank of ambient fluid of lesser density

2-D slot jet

2-D slot jet, 6.75 times larger than small facility

Summary & conclusionsSummary & 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 workRecommendations 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:Acknowledgements:

Advanced Fluid Mechanics Research Group www.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

Primary references: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.