Title: ENVE5103 Lecture 3a
1ENVE5103 Lecture 3a
- Gaussian Dispersion Modelling for a Single Stack
- Downwind maximum ground level concentration
- Ground reflection
- Mixing height limitation
- Plume rise
- Stack tip downwash
- Effects Requiring Special Techniques
- Building downwash
- Cavity and wake effects
- Elevated terrain
- Fumigation
2Gaussian Dispersion Modelling for a Single Stack
3Gaussian Dispersion Modelling for a Single Stack
- Downwind maximum ground level concentration
- Ground reflection
- Mixing height limitation
- Plume rise
- Stack tip downwash
4 2-D STEADY DISPERSION MODEL
- Solution for windspeed of u m/s and continuous
release of Q g/s of pollutant at x y 0
(stack location) and z H (the effective
stack height) - H h ??h
- h physical stack height, ?
- ?h plume rise
5 GROUND LEVEL CONCENTRATION ALONG CENTER LINE
- We are most interested in ground level, z0,
concentrations (where humans and other life forms
reside), - On the center line, y0, (where concentrations
are at their maximum
6 2-D STEADY DISPERSION MODEL GROUND
REFLECTION
- From the release height of H above ground,
dispersion can progress upward towards the mixing
height. In the downward direction the ground acts
as a mirror unless the pollutant gets deposited. - The effect of the ground can be handled
mathematically by treating the reflection as
another point source located below ground (at - H)
7Figure 4-3 Wark, Warner Davis
- Use of an imaginary sourceto describe reflection
at the ground
8Figure 4-4 Wark, Warner Davis
- Effect of ground reflection on pollutant
concentration
9MAXIMUM GROUND LEVEL CONCENTRATION
- At z 0 (cwith reflection ) 2(cwithout
reflection ) - Not as simple at other z.
- C(x,0,0) first increases with x due to ground
reflection but horizontal dispersion (y
direction) eventually decreases it. (Fig 4-5) - The location and magnitude of the maximum
concentration can be determined from the
equations above. Fig 4-8 provides a convenient
tool. Other empirical methods are also available
10Figure 4-5 Wark, Warner Davis
- Concentration profiles along the center line of a
stack plume
11Figure 4-8 Wark, Warner Davis
- Maximum Cu/Q value as a function of stability
class and downwind distance
12MIXING HEIGHT LIMITATION
- As the ground represents a lower limit to the
vertical dispersion, the mixing height represents
an upper limit. Multiple reflections from the
ground and the stable layer above need to be
considered giving rise to - Approximation
- No effect of mixing height for xltxL
- Completely mixed in the z direction for xgt2 xL
- Interpolate (on log-log plot) in between xL
and 2 xL - xL corresponds to ?z0.47(L-H)
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14 ONE DIMENSIONAL SPREADING MIXING HEIGHT
LIMITATION
- After a sufficient distance downstream (say ?z
mixing height) ?the plume can only disperse
horizontally. - If we consider the plume well mixed in the
vertical direction, we can obtain -
- where L mixing height
15 PLUME RISE
- H h ??h
- h physical stack height, ?
- ?h plume rise due to thermal buoyancy and
momentum - Correlations of various complexity exist between
plume rise, stack temperature, stack velocity,
atmospheric conditions etc. (e.g. Hollands,
equation 6.35 de Nevers)
16PLUME RISE - HOLLANDS EQUATION
17PLUME RISE - BUOYANCY AND MOMENTUM FLUXES
18Table 4-6 Wark, Warner Davis
- Equations for calculating final plume rise
19THE U.S. EPA SCREEN(3) MODEL Buoyancy Induced
Dispersion (BID)
- Entrainment due to shear between plume and
outside air increases dispersion in the plume.
SCREEN model uses dispersion parameters that are
larger than those previously reported from
Prairie experiments. - More noticeable for concentrations near plume
level, than for ground-level concentrations
20STACK TIP DOWNWASH
- For Vs lt 1.5 us
- (Vs stack gas velocity, us wind velocity at stack
height) - hs physical stack height
- hs physical stack height, corrected for stack
downwash - Note that maximum downwash correction is 3 stack
diameters -
- As before, we have H hs ??h
21Effects Requiring Special Techniques
- SCREEN3 will automate all the basic Gaussian
dispersion calculations mentioned above for a
single source (point, area, flare, volume). - In addition, SCREEN3 incorporates special
techniques for dealing with - Building downwash
- Cavity and wake effects
- Elevated terrain
- Fumigation
22Building downwash
23BUILDING DOWNWASH AND WAKE EFFECTS
- Figs. 3-19 and 3-20 demonstrate these. Special
treatments are included in models. - BUILDING DOWNWASH - Simple rule of thumb
- downwash unlikely to be a problem if
- hs ? hb 1.5 Lb
- hs stack height
- hb building height
- Lb the lesser of either building height or
maximum projected building width. - Good Engineering Practice (GEP) rule for stack
design. -
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26BUILDING DOWNWASH AND WAKE EFFECTS
- Downwash procedures documented in ISC Users
Guide. - SCREEN also implements these procedures.
- CAVITY
- Calculations based on
- minimum
- maximum
- horizontal distances alongwind
-
- WAKE
- Near Wake downwind distance lt 10 Lb
- Far wake downwind distance gt 10 Lb
-
27SCREEN3 User Guide
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29- Structure Influence Zone (SIZ) For downwash
analyses with direction-specific building
dimensions, wake effects are assumed to occur if
the stack is within a rectangle composed of two
lines perpendicular to the wind direction, one at
5L downwind of the building and the other at 2L
upwind of the building, and by two lines parallel
to the wind direction, each at 0.5L away from
each side of the building, as shown below. L is
the lesser of the height or projected width. This
rectangular area has been termed a Structure
Influence Zone (SIZ). Any stack within the SIZ
for any wind direction is potentially affected by
GEP wake effects for some wind direction or range
of wind directions, and shall be included in the
modelling project. Please see Figure 4.5 and
Figure 4.6.
30Figure 4.5 GEP 5L and Structure Influence Zone
(SIZ) Areas of Influence (after U.S. EPA(19)).
31- Figure 4.6 GEP 360 5L and Structure Influence
Zone (SIZ) Areas of Influence (after U.S.
EPA(24)).
32- Building downwash for point sources that are
within the Area of Influence of a building shall
be considered. For US EPA regulatory
applications, a building is considered
sufficiently close to a stack to cause wake
effects when the distance between the stack and
the nearest part of the building is less than or
equal to five (5) times the lesser of the
building height or the projected width of the
building. - Distancestack-bldg lt 5L
33- For point sources within the Area of Influence,
building downwash information (direction-specific
building heights and widths) shall be included in
the modelling project. - The Building Profile Input Program (BPIP or
BPIP-PRIME) can compute the direction-specific
building heights and widths once the basic
building data is entered.
34Elevated Terrain Analysis
- Complex Terrain as illustrated in Figure 5.1,
where terrain elevations for the surrounding
area, are above the top of the stack being
evaluated in the air modelling analysis. - Simple Terrain where terrain elevations for the
surrounding area are not above the top of the
stack being evaluated in the air modelling
analysis. The Simple terrain can be divided
into two categories
35Simple Terrain
- Simple Flat Terrain is used where terrain
elevations are assumed not to exceed stack base
elevation. If this option is used, then terrain
height is considered to be 0.0 m. - Simple Elevated Terrain, as illustrated in Figure
5.2 is used where terrain elevations exceed stack
base but are below stack height.
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38Regulatory (MOE) wording for terrain
considerations
- Terrain data
- 16. (1) If an approved dispersion model is used
for the purposes of this Part with respect to any
point of impingement that has an elevation higher
than the lowest point from which the relevant
contaminant is discharged from a source of
contaminant, the model shall be used in a manner
that employs terrain data.
39Fumigation
- Fumigation occurs when a plume that was
originally emitted into a stable layer is mixed
rapidly to ground-level when unstable air below
the plume reaches plume level. Fumigation can
cause very high ground-level concentrations. - Typical situations in which fumigation occurs
are - 1. Breaking up of the nocturnal radiation
inversion by solar warming of the ground surface
(Fig 5.15 deNevers) - 2. Shoreline fumigation caused by advection of
pollutants from a stable marine environment to an
unstable inland environment and - Advection of pollutants from a stable rural
environment to a turbulent urban environment.
40Figure 5.15 de Nevers
41Shoreline fumigation
- A stable onshore breeze carries a shoreline
source inland - Breeze encounters air from solar heated soil,
mixing from below - The mixing from below pulls plume to ground
- The end result is the same as in fumigation due
to inversion break-up (Fig. 5.15 deNevers)
although the effects leading up to it are
different - SCREEN3 addresses both of these situations.