Transcript Document

Transient simulation of a microburst outflow:
Review & proposed new approach
W.E. LIN
C. NOVACCO
Dr. E. SAVORY
PhD Candidate
MESc Candidate
Associate Professor
Department of Mechanical and Materials Engineering
May 2006
What is a microburst?
Image ID: nssl1120, National Severe Storms Laboratory Collection
Photographer: Moller AR, NOAA, National Weather Service.
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)
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].
Previous approaches to physical modelling
Impinging jet
experiments:
 Letchford &
Illidge [1999]
 Wood et al [2001]
 Chay &
Letchford [2002]
 Letchford &
Chay [2002]
 Xu [2004]
Released fluid
experiments:
 Mason et al
[2005]
 Lundgren et al [1992]
 Alahyari & Longmire [1995]
 Alahyari [1995]
 Yao & Lundgren [1996]
Literature review
Stationary
microburst
Translating
microburst
Scale
Lundgren et al [1992]
Alahyari & Longmire [1995]
Released
fluid
1:9000
Alahyari [1995]
Yao & Lundgren [1996]
 Letchford & Illidge [1999]
Impinging
jet
 Wood et al [2001]
 Chay & Letchford [2002]
 Letchford & Chay [2002]
 Xu [2004]
 Mason et al [2005]
Steady flow
Transient flow
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 1982.
Mason et al
[2005]
CFD simulation [Kim et al, 2005]
Vector colour: velocity magnitude.
Red vectors are largest values.
Contours: pressure.
 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]
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.
Gated slot
Preliminary facility
To stepper motor
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
Slot jet flow
2-D wall jet
x/b = 0
x
Uj
b
Transient slot jet time history
Andrews AFB downburst
1 Aug 1983 [Fujita, 1985]
time
 Filter out poor
actuations
 Ensemble
average
remaining
time histories
 Shape
depends on
tgate (0.30 s)
 Sharp rise to
Umax
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
Developing burst front
Image ID: nssl0106, National Severe
Storms Laboratory
[1970]
Collection
Photographer: Waranauskas BR, NOAA, National Weather
Service. Taken during JAWS project on 15 July 1982.
z [mm]
HWA measurements: transient, x/b=30
280
270
260
250
240
230
220
210
200
190
180
170
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
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 profile
55 z-pts at x/b=30, y=0
0
5
10
15
20
25
30
<U> (ensemble averaged) [m/s]
 Build up a composite vertical profile from 10 actuations at each z
 Comparison of profiles at 3 spanwise locations (at the same time)
35
HWA measurements: transient, x/b=20, y=0
250
t = 0.02 s
200
t = 0.075 s
z [mm]
t = 0.13 s
150
t = 0.18 s
t = 0.28 s
100
t =1.00 s
50
0
0
5
10
15
20
25
30
35
40
<U> (ensemble averaged) [m/s]
 t histories of U at 55 z-locations ~> evolution of <U> profiles with time
Alternate gate design
Alternate gate design
Temporal development of <U> profiles at x/b = 10, y=0
tgate = 0.1 s, 180° actuation
25
20
0.025
0.065
0.11
0.12
0.13
0.16
z [mm]
15
10
5
0
0
5
10
15
20
25
U [m/s]
<U>
[m/s]
30
35
40
45
Simulation scale summary
Study
Geometric scale
Velocity scale
1:22000
(1:9000 - 1:45000)
1:85
1:25000
1:300
1:26000
(1:10500 - 1:52500)
1:6.7
1:3000
(1:2400 - 1:6100 )
1:3
Comments
Buoyancy-driven flow
Lundgren et al [6],
experimental
Alahyari & Longmire [7],
experimental
Release of fluid from a
stationary cylinder vessel
into a tank of ambient fluid of
lesser density
Impinging jet
Kim et al [13],
computational
Mason et al [14],
experimental
Slot jet (present results with preliminary facility)
Quasi-steady simulation
1:800 - 1:4000
Transient simulation
1:700
Slot jet (anticipated results with full-size facility)
Quasi-steady simulation
1:200 - 1:1000
Transient simulation
1:700
1:2
Impulsive start of a stationary
continuous jet
Actuated stationary
continuous jet
2-D slot jet
t
1:1 - 1:2
2-D slot jet, 6.75 times larger
than small facility
U
j
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
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
Questions & comments are welcome!
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, 89:535552.
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.