Building Downwash

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Building Downwash
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Building Downwash
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Cavity and Wakes
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Model of flow around a sharp edged 3-D building
in a deep boundary layer
Reattachment lines on sides and roof
Incident wind profile
Lateral edge and elevated vortex pair
Cavity zone
Turbulent wake
Mean cavity reattachment line
Separation lines and Horseshoe vortex system
Length L, Width W, Height H
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Building Downwash
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Cavity and Wakes
ZR
ZR
XR
2YR
2YR
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Aerodynamic Wake

• Region where local velocities are different from
  the free stream values
• Streamline Separation at an object
• Eddy Recirculation (generally lower flow region)
• Turbulent Shear Region (generally higher flow)
• Reattachment of Streamlines
• Near Wake (Cavity)
• Usually on the lee side of the object.
• Far Wake
• Effect of another object on the separated
  streamlines
• Estimation of Wake/Cavity Boundary
• Effect of Building geometry

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Two Points of Concern

• Dispersion Plume in the presence of Buildings
• What happens to the plume from an existing stack
  and existing buildings
• Design of Stacks in the Presence of Building
• What are the design guidelines if a new emission
  source is being proposed in an area full of
  buildings.

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Extent of the Wake Boundary for short Buildings
L/H lt 2
XR Extent of the cavity past the downwind edge
of the building Measured from the upwind edge of
the building. (From Hosker, 1979)
Hosker, R. P. (1979) Empirical Estimation of
Wake cavity Size behind Block-type structures-
Fourth Symposium on Turbulence, Diffusion and Air
Pollution, Reno Nevada, Americam Meteorological
Society
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Extent of the Wake Boundary for Tall Buildings
L/H gt 2
XR Extent of the cavity past the downwind edge
of the building Measured from the lee edge of the
building. (From Hosker, 1979)
YR maximum half-width Crosswind cavity Extent
XMY Distance at which maximum crosswind cavity
occurs
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Effective stack height with buildings
Step 1 To prevent stack-tip downwash
Recommended Stack height to prevent stack tip
downwash
The above rule was limited in use
Aesthetics Economics Too Conservative in
preventing high ground level C
For Vertical Stacks
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Effective stack height with buildings
Step 2 Building induced downwash

• stack induced downwash is first determined,
• then building effect is appended

Let ?b be the smaller of H or W
If hs gt H 1.5 ?b, ? hs hs
If hs lt H ? hs hs 1.5 ?b
If H lt hs ltH 1.5 ?b ? hs 2hs (H 1.5 ?b)
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Effective stack height with buildings
Ste 3 Entrained plumes
If hs gt ?b, plume remains aloft. Go Step 4 If
hs lt ?b, plume trapped in cavity and treat as
ground level source with area ?b2. Go Step 5.
If hs ltH 1.5 ?b, And if release point is on
the roof Or within ?b /4 of the building, plume
is within the zone of the building and should be
treated as a ground source with area ?b2. Go to
Step 5.
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Effective stack height with buildings
Step 4 - Plume buoyancy effect
If plume is air (mostly) and Tplume same as Tamb
? hs hs

• If not calculate density difference
• ? (Me/Ma)(Ta/Te) 1
• Where a is air and e is effluent.
• lt 0 ? standard procedure for hs
• gt 0 ? other procedures used.

Step 5. Downwind concentration far from the stack
Use usual formula from dispersion model.
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Concentration in Cavity Wake.
More appropriate (takes bldg. dim. into account)
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Concentration immediately downwind of wake cavity
For trapped plumes consider source as ground
level
For other cases
where
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Good Engineering Practice Stack Height
1985 Regulations -
GEP Stack height greater of the following
- a) 65 m from the base of the stack -b) HG
2.5 H for stacks in existence before Jan, 1979.
For All Other Stacks HG H 1.5 L Where H
is the height of the nearest building and L is
the distance to the nearest building
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Multiple Stacks
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EPA Air Quality and Dispersion Models
http//www.epa.gov/scram001/tt22.htm

• Preferred and Recommended Models
• AERMOD (AMS/EPA Regulatory Model)
• CALINE (California Line Model)
• CALPUFF (Long term Transport)
• CDTMPLUS
• ISC (Industrial Source Complex)
• ISC PRIME (Plume Rise Model Enhancement)

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ISC

• Most widely used Currently
• Features
• Steady State Gaussian Dispersion Model
• Short term and Long term models (ISCST and ISCLT)
• Multiple sources point, area, volume sources
• Receptor grid can be assigned
• Uses Pasquill Stability classes to estimate
  dispersion coefficients
• Accounts for dry and wet deposition
• Multiple averaging time options
• Limited Building downwash effects
• Limitations
• Cannot account for reactive components
• Best used only for fairly uniform terrain
• Assumes homogeneous (or uniform) parameters
• Limited Building downwash effects (Can be
  improved by adding a PRIME module)

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AERMOD http//www.epa.gov/scram001/tt26.htm

• Replacement for ISC
• Features
• Similar to ISC - Steady State Gaussian Dispersion
  Model
• Does not use Pasquill Stability classes to
  estimate dispersion coefficients
• Improves estimates for dispersion in the PBL by
  accounting for varying dispersion rates with
  height
• Refined turbulence based on current PBL theory
• Advanced treatment of mixing height, plume rise
  and complex terrain
• Improved Building downwash effects
• Limitations
• Does not yet account for reactive components, wet
  and dry deposition
• Requires more extensive meteorological data