Craig ClementsSan José State University
Shaorn ZhongMichigan State University
Xindi Bian and Warren HeilmanNorthern Research Station, USDA
Scott GoodrickSouthern Research Station,USDA
Turbulence Kinetic Energy and Fire-Induced Turbulence Kinetic Energy and Fire-Induced WindsWinds Observed during FireFluxObserved during FireFlux
Seventh Symposium on Fire and Forest Meteorology23-25 October 2007Bar Harbor, Maine
Overview of Talk
Photo by M. Patel
1. Observations1. Observations
• Fire-Induced CirculationsFire-Induced Circulations
•Turbulence Kinetic EnergyTurbulence Kinetic Energy
• Other turbulent statisticsOther turbulent statistics
2. Summary and conclusions2. Summary and conclusions
Fire-Induced Surface Winds(Main Tower, 2-m level 1-sec)
Time (CST)
Thermocouple
Vertical Velocity
Wind Speed and Direction
Fire-Induced Surface Winds(Short Tower 2-m level 1-sec)
Wind Speed and Direction
Vertical Velocity
Thermocouple
Thermocouple Damage
Convergence Zone
Comparison of Surface Winds Outside of Burn Plot
0
1
2
3
4
5
6
7
8
9
10
0 5 10 15 20 25 30 35 40 45 50 55 60
Time (min)
North Tripod
Main Tower
South Tower
Model Comparison
Cunningham and Linn 2007
-10
-8
-6
-4
-2
0
2
4
6
810
u
v-10
-8
-6
-4
-2
0
2
4
6
8
10
-4
-3
-2
-1
0
1
2
3
4
-4
-3
-2
-1
0
1
2
3
4
150-5-10 5 10 150-5-10 5 10
150-5-10 5 10 150-5-10 5 10Time f rom Ignition (min)
Time f rom Ignition (min)
Time f rom Ignition (min)
Time f rom Ignition (min)
Main Tower, w
Main Tower Short Tower
Short Tower, w
(a) (b)
(c) (d)
FireFlux 2006
Upper Plume Structure(28 m Level)
Wind Speed and Direction
Vertical Velocity
Temperature
Water Vapor
period of downdrafts
Thermal Structure of Fire Plume
250
200
150
100
50
Tem
pera
ture
(C)
Downdraft ahead of Fire front
Turbulent Eddy Flux (Eddy-Covariance)
• Transport of a quantity by eddies or swirls.• The covariance of a velocity component and any quantity.
1
0 1
1 1cov( ) ( ) ( )N N
i ii i
w W W w wN N
Net upwardheat flux
0
EddyMixes some air
downAnd some air up
z
´= _w´= _ ´= +
w´= +
(adapted from R. Stull)
-5
0
5
10
15
20
25
30
35
2 m
10 m
28 m
43 m
Time (CST)
1230 1235 1240 1245 1250 1255 1300
H w Ts ' '
Main Tower
Turbulent Sensible Heat Fluxes
Instantaneous heat fluxes = ~0.8-1.0 MW m-2
• is a measure of the intensity of turbulence• simply the summed velocity variances
TKE e u v w 12
2 2 2
Turbulence Kinetic Energy (TKE) During Fire
Photo by Laura Hightower
0
5
10
15
-10 -5 0 5 10 15Time from ignition (min)
0
5
10
15
-10 -5 0 5 10 15Time from ignition (min)
0
5
10
15
-10 -5 0 5 10 15Time from ignition (min)
0
5
10
15
-20 -10 0 10 20 30 40 50Time from ignition (min)
0
5
10
15
-20 -10 0 10 20 30 40 50Time from ignition (min)
0
5
10
15
-20 -10 0 10 20 30 40 50Time from ignition (min)
FireFlux
Pilot Study Pilot Study Pilot Study
FireFluxFireFlux 43 m
2 m
28 m10 m
Wind Velocity Variances During Fire
u 2 v 2 w 2
u 2 v 2 w 2
I II III IV V VI VII
I. Time rate change of TKE, or local storage of TKE
II. Advection of TKE by the mean flow
III. Buoyancy production or destruction
IV. Mechanical or shear production
V. TKE transport or dispersion
VI. Pressure correlation or redistribution
VII. Viscous dissipation
i
j
j
j
jjii
jj x
puxeu
xUuuug
xeU
te ''1'
''''3
Turbulence Kinetic Energy Budget
-0.1
0
0.1
0.2
0.3
0.4
0.5
-20 -10 0 10 20 30 40 50Time from ignition (min)
Pilot Study
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
-10 -5 0 5 10 15Time from ignition (min)
FireFlux 43 m
2 m
28 m10 m
Buoyancy Flux During Fire
B gT
w Tfv
v ( )
-5-4-3-2-1012345
-10 -5 0 5 10 15Time from ignition (min)
-5-4-3-2-1012345
-10 -5 0 5 10 15Time from ignition (min)
-5-4-3-2-1012345
-20 -10 0 10 20 30 40 50Time from ignition (min)
-5-4-3-2-1012345
-20 -10 0 10 20 30 40 50Time from ignition (min)
FireFlux Pilot Study
Pilot Study(c) (d)FireFlux
43 m
2 m
28 m10 m
(a) (b)
Turbulent Momentum Fluxes During Fire
u w
v w
A Conceptual Modelfor Fire-Atmosphere Interaction
Weak convergencezone
isotropic
anisotropic
Downdraftsentrain background
air
Wind shearcauses tilted plume
and turbulence generation
Shear induced turbulence influences horizontal vortex
• Fire-induced surface winds were 2-3 times stronger than ambient winds.
• A convergence region formed downwind of the fire front, but was shorter in duration than expected.
• Inflow velocities were much weaker than expected.
• Observed instantaneous upward vertical velocities were on the order of 10 m s-1 and downward vertical velocities = 5 m s-1
• Directly measured sensible heat fluxes were ~28.5 kW m-2 occurred at higher levels in the plume rather than near the surface.
• However, estimated instantaneous heat fluxes at the surface were on the order of 0.8 - 1.0 MW m-2.
Summary and Conclusions
•The observed TKE during the grass fires increased due to the variance in the ambient wind component (fire direction) rather than the contribution from all three velocity components.
•The turbulence within the upper fire plume, is isotropic and equally driven by both buoyancy and wind shear.
• While near surface turbulence is anisotropic and driven by variance in the horizontal momentum rather than buoyancy.
•This suggests that although buoyancy is important, mechanically generated wind shear is responsible for the observed turbulence in grass fires.
Summary and Conclusions