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