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

8

10

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

Te

mp

era

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 1

22 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 50

Time from ignition (min)

FireFlux

Pilot Study Pilot Study Pilot Study

FireFluxFireFlux 43 m

2 m

28 m

10 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

pu

x

eu

x

Uuuu

g

x

eU

t

e ''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 50

Time 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

Bg

Tw Tf

vv ( )

-5

-4

-3

-2

-1

0

1

2

3

4

5

-10 -5 0 5 10 15Time from ignition (min)

-5

-4

-3

-2

-1

0

1

2

3

4

5

-10 -5 0 5 10 15Time from ignition (min)

-5

-4

-3

-2

-1

0

1

2

3

4

5

-20 -10 0 10 20 30 40 50Time from ignition (min)

-5

-4

-3

-2

-1

0

1

2

3

4

5

-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