main collaborators: janice coen, ncar morwenna griffiths,monash mary ann jenkins,york don latham,...
TRANSCRIPT
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Main Collaborators: Janice Coen, NCARMorwenna Griffiths,MonashMary Ann Jenkins,YorkDon Latham, USFSDon Middleton,NCARDavid Packham, BoMLarry Radke,NCAR/RIMichael Reeder,MonashRoland Stull, UBC
Coupled Fire-Atmosphere ResearchObservations and Modeling
by Terry L. Clark/UBC
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OUTLINEObservations using IR Imagery
-overview of IR camera and analysis techniques -some prescribed and wild fire field experiments
Model-Description of dynamic code-Description of early NCAR fire code
-some results-Description of current WFIS fire code
-some results
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OBSERVATIONSAND InfraRed Image ANALYSIS
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Onion sage brush fire in Owensvalley, Ca 1985 courtesy of C. George
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Street Patterns observed in Fires
Photo courtesy Brenner -observed in Florida
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OBSERVATIONS
A. Infrared Camera
- Inframetrics PM380
- 3 to 5 m
- 256 by 256 array
- Sterling cooler
- 16 deg lens gives 1.4 rad resolution per pixel
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Image Flow Analysis Applications• Understand fire behavior
• Calculate combustion zone winds
and their statistics
• Use derived data to validate
numerical models
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Image Flow Analysis
Assumptions
– IR camera sees incandescent soot particles– Motion is on a distorted two-dimensional surface – Local features can be followed for short periods– We can fit data to simple types of motions, i.e.
translation, rotation, dilation and shear
. .
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Image Registration1. Reduce image resolution (e.g. 7:1 in x and 5:1 in y)
2. Align image using IR intensity center of mass
3. Refine alignment using correlation analysis
n+1x +xy +yn(x,y))2
Minimize to estimate xandy
4. Extract linear trends in x t) andy(t)
5. Registered IR images:
- used in image flow analysis to estimate winds within the combustion zone
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Image Flow AnalysisGradient Approach (Helmholtz theorem): Two-dimensional motions can be represented as the sum of six components. Translation
. .
)( 210 fdtd
)()( 3022 yfxfdtd
Rotation
)(40 yfdtdyfxf
dtdxf
Uniform expansion
And two shear components which most researchers ignore.
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Robust StatisticsUsing the two components of translation,
0 fdtd
ztfxtf
w
u
zzfxzfxzfxxf
we obtain the matrix equation
which we solve at each pixel for u and w. If |u2 + w2| > S2 thenwe flag that pixel as an outlier and avoid considering it in future calculations. Typically, S= 20 to 40 m/s.
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Least Squares Minimization
ztf
xtf
w
u
zzfxzf
xzfxxf
After identifying outliers we sum over a patch of data as
We typically use 7 by 7 pixels.Outlier points are not included in any of the summations.
This simplest approach requires the inversion of a second order matrix to estimate u and w..
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International Crown Fire Modelling Exp
Cameras on 50fttower
•Canadian and US Forest Services •Near Fort Providence NWT Canada• Prescribed crown fires•150 by 150 m plots•June – July 1997•Tower based IR measurements•Plot 6–9 July 1997
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Workshop at the Site
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Plot 5 Fire Whirl
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Plot 6 Ignition
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Plot 6 at 2:08
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Plot 6 at 2:09
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Plot 6 at 2:10
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Plot 6 at 2:10 plus
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Plot 6 at 2:11
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Turbulent Burst Sequence
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Video: Plot 6 Visual
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Derived Winds for file=7004
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Fig 10a Clark et al. 1999, JAM
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Wild Fire Experiment
• NCAR• Sept 1998, Montana, Colorado and California • Infrared Camera mounted on NSF/NCAR C-130• Wildfires were the target of opportunity• First case was in Glacier National Park
-Challenge Fire Complex 4 Sept 98
-100 m long hairpin vortex observed with IR
-fire about 2 km away from camera
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IR Imagery from C-130FOD - Part II
Finger shot outabout 100 m in 1-2 sec
Indications of burning fuel after finger retreats
Hairpin or Turbulent Burst
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Video: IR Observations over Glacier National Park - 4 Sept 1998
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Northern Territory Grass Fire Experiment- Australia
• Spear grass burns 40 km South of Darwin
• Kerosene grass burns near Batchelor
• Used 19 m high cherry-picker as platform
• Platform motion requires apparent motion treatment
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Cherry Picker
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Fuel Type - Australian Spear Grass (Sorgum_Intrans)
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Litchfield Kerosene Grass (19 m up)
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Video: IR Data Hughes plot 3
Camera 200 m from fire
giving 30 cm pixels
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Video: Unprocessed Images Plot-3a Hughes Airfield
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Video: Fire Winds using least squareswith registration
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Velocity Statistics – Least Squares
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MODELING
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Numerical Model• 3D Non-hydrostatic 2nd order finite-differences
• Terrain following - geo-spherical coordinates
• Vertical and horizontal grid refinement
– 2-way interaction
• Vertically stretched grids with grid refinement• - Clark (1977,JCP), Clark-Farley(1984,JAS),
• Clark-Hall(1991,JCP; 1996,JAM)
• Boundary-Initial conditions from NWP
• Bulk parameterizations of rain/ice processes-Kessler (rain), Murray-Koenig (ice) and much more
recent approaches under development.
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Coordinates
.
),(
)()(
),()/),()((
gridsconstantontocastisModel
orography.theofheighttheis
and
00
where
1
Δζ
yxh
HHFF
yxhHyxhFz
incrementslatitudeconstantrepresents
incrementslongitudeconstantrepresents
yΔ
xΔ
Horizontal Coordinates
Vertically Stretched Coordinates
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Multi-Processing Approach
Message Passing Interface (MPI) software is usedfor multi-processing.
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Model Configuration for 3 LayersNVRT=0 (no tiling)
SingleProcessorframework
N=1
N=3
N=2
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Multi-Processing Configuration for 3 Layers
NVRT=1 (with tiling) Layer 1 details
Multi-processorFramework
Four sub-domainsper layerMCPU=4
N=3
N=1
N=4
N=2
Green=lmx1,lmx2lmy1,lmy2
Blue=mi2mo
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Grid Refinement Using 5 Domains
Example fromClark et al. 2000, JAS
Grid size ranges from26 km to 200 m(4:1 4:1 4:1 2:1)
CO
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DOMAIN 5 OROGRAPHY
Example fromClark et al. 2000, JAS
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Wildfire Modeling
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Rational for Wildfire Modeling
• Wildfire propagation physics is poorly understood• FS spread models use empirical fits from
– Low intensity small fires– Laboratory fire tunnels– Neither can hope to represent the vast parameter space of intense fires
• Understanding fire behavior involves– Combustion winds interacting with the fire and ambient flow– Fire-atmosphere heat exchange– Fire-fuel heat exchange – Chemical release and transport by the convection
• Some Applications of Coupled Fire-Atmosphere Models – Study burn paradigms – Understand fire related sources/sinks to atmospheric budgets– Develop suppression techniques– Like the NWP problem, fires are too non-linear to predict using empirically
derived rule based techniques, i.e. there is no empirical fit to a severe nonlinear event.
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NCAR FIRE CODE
-Physical treatment of fire at a very ‘first cut’ level, i.e. useful for preliminary evaluation.
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Fire Atmosphere Coupling
The sensible and latent heat fluxes were added to the vertical diffusion terms as:
)5()),((
)4()),((
txFvqvqKqvvqK
txsFKK
Where Fs and Fare the heat and moisture inputs from the fire. Fs can reach values up to 1-3 MW m-2. .
)/exp(,,),,(
)/exp(),,(),,(
lzhyxlFzyxlFszhyxsFzyxsF
Where s and l are extinction lengths for the sensible andlatent heat fluxes.
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Spread Rate Treatment
01 RwspS )(
)( nvww
s is the slope coefficient
is the wind coefficient
nv is the wind normal to the fire front
R0 depends on fuel type and moisture content.A BURNUP type curve is used to describethe rate of mass loss for each fuel cell.
BEHAVE formulation
• Strong need for improved spread rate parametrizations appropriate for coupled fire-atmosphere models
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Fire Line Propagation Scheme
•Contour advection scheme•Avoids assuming shape such as ellipse•We want the physics to determine shape
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Tracer Code
• Area inside tracers designates ignited fuel•Thex and y of fire model range from 10 to 300 m • z ranges from 5 to 10 m• Fuel cellxf and yf range from 5 to 60 m
•Grid point method used to track fuel and fire• Four particles (or tracers) for each fuel cell•Local contour advection scheme used to move tracers
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Fig3 Normal Vectors
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Fig 4 Spot Fire- Zero Wind
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Fig 5 Spot fire- West Wind of 3m/s
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Fig 6 Spot Fires at t=200 s
Kinematical test usingBEHAVE with fuel class-3 (tall grass)U= 3 m/s
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Fig 7. Line fire in oscillatory wind
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Fig. 8 Windmill time seqence
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Fig 9 Windmill -Converging fire lines
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Video: Animation of six tests
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Line fire propagating over a hill
• Slope about 60 percent
• Nh/U about 1 giving deep evanescent modes
• Without fire critical level amplifies wind speeds by about a factor of 3
• Fire winds dominate ‘ambient flow’
• Line spreads up hill with parabolic shape
• Bifurcation occurs at hill crest
• Vortex production along front
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Video: Buoyancy and Enstrophy
Red=2 deg Buoy
Purple= .1/s Enstropy
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Video – 4 deg K buoyancywith volume rendering of smoke
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Video: Isosurfaces of .07/s vertical vorticity
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Video: Plan View of Vertical Vorticity
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Weather Fire Integrated System(WFIS) FIRE CODE
-Physical parameterization of fire at the level ofsophistication where we can begin to compare with the natural event.
Following results fromClark, Griffiths,Reeder and Latham, 2003:Numerical Simulations of Grassland Fires in the Northern Territory, Australia: A New Sub-Grid-Scale Fire Parameterization, J. Geophysical Research, (in Press)
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Assumptions1. Assume a time scale for combustion b, e.g. 1-2 s.
2. Assume combusting material and air are two interacting fluids
3. The mixing ratio of the combustible material is Mf.
4. The burning portion of Mf has a volume mixing ratio f.
5. The temperature of the combusting material is Tb.
6. The effective grid cell area of radiation is determined so that Tb (maximum) = 1200k. This assumption provides us with an
effective volume/area (= lf) ratio.7. Exchange of heat between the combusting fluid and the air is
assumed to occur from:• Interfacial molecular diffusion using an ls length scale
(without/with cases treated)• H20 and CO2 absorption. (not considered as yet)
8. Above assumptions designed for models using horizontal and vertical grid sizes of about 1 to 3 m.
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Governing Equations
)( fMKbSfSzfM
dt
d
Sb Mf / b
Sf Soexp( z / s )
d
dtMb Sb Sv (KMb)
Sv 0.56Sb 0.56Mf / b
Conservation equation for Mb (smoke)
Conservation equation for the mixing ratio of combusting material, Mf
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Governing Equations
)(
)(42
*
bTKpc
bTfs
pcbSbCf
bTbT
dt
dpc
d
dt
sf(Tb )(K)
Thermodynamic equation for temperature of combusting material,Tb.
Conservation equation for showing the fire-atmosphere heat exchange terms.
Effective buoyancy that drives updrafts is now defined as
bTfffef )1(
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Figure 8.Horizontal and vertical cross sections from experiment CFA. (a) is at the surface, (b) is at x = 150 m, (c) is a section through
y = 150 m, and (d) is taken at a height of 1 m.
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Figure 4.Maximum and minimum u, v, w and maximum
versus time for experiment CFA.
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Figure 6.Plan view of fire line at t = 1.4 min showing the wind vectors
at z = 2m above ground and plotted every 2 m for experiment CFA. The bold line marks the position of the fire front. Only a section of
the domain is shown to highlight the variations along the fire.
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Figure 7.(a) Maximum combustion temperature, (K), (b) spread rate (ms-1), (c) heat flux (MWm-2) and (d) total sensible heat flux (GW) from
the fire versus time for experiment CFA.
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Figure 9.Horizontal cross section of heat flux at z = 3 m above the surface for experiment CFA
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Figure 2. at t = 24 s from a series of Australian grass fire experiments. The values of are 1., .5, .25 and .125 cm, respectively. The contour
interval of is 8 K.
ls =1 cm ls =.5 cm
ls =.25 cm ls =.125 cm
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AVI showing Isosurface of W (1 m/s)(yellow) with Tb (600K) in blue shown near surface
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Conclusions
1. Provided research funding is forthcoming we have a modeling framework that can be extended to the self-determining level by treating the fire-fuel heat exchange.
2. We can easily add new field variables to study the transport of various chemical species.
3. We can also treat additional physical effects through the addition of new field variables, e.g. break Mf into two variables to consider flashover effects.