laboratory modeling of atmospheric dispersion at the fluid modeling facility of the u.s....

Laboratory Modeling of Atmospheric Dispersionat the

Fluid Modeling Facilityof the

U.S. Environmental Protection Agency

byWilliam H. Snyder

MiniTech Presentation15 March 2006

Meteorological Wind Tunnel

Test section: 3.7m wide, 2.1m high, 18.3 m longFree-stream speeds: 0.5 to 10 m/s.

Water Channel / Towing Tank

Test section: 2.4m wide, 1.2m deep, 25m longTowing speeds: 1 to 50 cm/s.

Convection Tank

Block roughness

Honeycomb

Side view

Spires cut off

Perspective View – Spires & Block Roughness

Boundary-layer Generation Scheme

0 .6 1.2 1.8 2.4 3 3.6 4.20

300

600

900

1200

1500

U, m/s

z, m

m

Cube Height

x, mm 8002 983111660153180.16 Power law

Mean Velocity Profiles

Conc

Wind-tunnel demonstration of the influence of building width and height on the plume distribution in the wake.

1

2

4

10

Variable:

Building Width in Crosswind Direction

Centerplane Streamlines

Observations

•Cavity length increases from 1.5h to 5.5h•Horseshoe vortex more prominent with wider buildings – reverse horseshoe vortex also observed downstream

•Most prominent effect is streamline lifting, but cavity height increases slowly with building width•2D Bldg should show closed streamlines in cavity

Video-image, pseudo-color representation of concentration in building wakes. Instantaneous concentrations on left and long-term averages on right.

Cubical Building Array

Two-Dimensional Building Array

WORLD TRADE CENTERSITE

FLOW

N

1:600 SCALE MODEL OF LOWER MANHATTAN

Three Decades of Building Studies

• Contributions to the Scientific Understanding

• Rules of Thumb (“1 + 1½ times” rule for building-downwash prevention)

• Resulted in several Agency guidelines and regulations, including the guideline for performing Good Engineering Practice stack height analyses

• Provided the basis for the downwash algorithms in the ISCST and AERMOD models and the flow distortion algorithms in other applied models such as QUIC.

Wind-tunnel study of plume downwash in complex terrain -- buoyant stack emissions from the Waste Technologies Industries municipal incinerator in East Liverpool, OH.

Russian Hill Study: Streamline Patterns Derived from Wind-Tunnel Measurements over Three Idealized Hills with Maximum Slopes of 26o, 16o and 10o.

Other Studies in Wind Tunnel

• Stack-tip downwash• Emissions from open-pit coal mines• Area sources• Roadways• Dense-gas studies• These studies have all resulted in improved algorithms in the Agency’s arsenal of applied dispersion models

Tow Direction

Neutral Layer (fresh water)

Stratified Layer (w/ saltwater)

Static Density Gradient

Dye Plumes

Sampling Rake

∆zi

zi

Typical Setup in Towing Tank

Plumes Released Above and Below the Dividing-Streamline Height

Wind direction sensitivity of concentration pattern over Cinder Cone Butte in stable stratification in the towing tank.

Wave Patterns & Ground-based Rotor

Elevated Rotor

Complex Terrain

• Wind-tunnel and towing-tank studies provided a strong foundation for the development and evaluation of the next generation of regulatory complex terrain models

• The concepts of the dividing streamline and stable plume impaction were demonstrated and refined

• Deflection of the height of the mixing layer by terrain

Convection Tank

Convection Tank

Fluorescent Dyes

PLAN VIEW OF CONVECTION TANK WITH LASER SHEET LIGHTING

VIDEO CAMERA

LASER TABLE

SOURCE

Laser-sheet Lighting System

Scanner mirror

Parabolic

Mirror

Argon-ion Laser Beam

22 24 26 28 30 32 34 360

4

8

12

16

20

24

28

Temperature, oC

z, c

m

Before surface heating

6.8 min

14.8 min

Typical temperature profiles obtained during plume experiment. Timesare between commencement of heating and midpoints of traverses.

Laser-illuminated buoyant plume in the convection tank

and downwind distancesfor various plume buoyanciesPseudo-color images - mean concentration cross sections

Puff Release Mechanism

Average Concentration of Medium-Buoyancy Puff Release

t = 0.1 to 4 t*

Dispersion in CBL

• Many features of original tank upgraded• Big advantage: ability to duplicate conditions• Buoyant plumes & puffs

• Penetrate into inversion• Gravity spreading in inversion layer• Eventually mixed down to the ground• Extreme "spottiness" in instantaneous views