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Discussion on session 18 — topographic modelling, dispersion of gaseous pollutants
Journal of Wind Engineering and Industrial Aerodynamics, 15 (1983) 115--119
115
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
DISCUSSION
ON S E S S I O N
18
TOPOGRAPHIC
-
MODELLING,
DISPERSION
OF G A S E O U S
POLLUTANTS
B.T.
Rapporteur:
Hawkins
CSIRO D i v i s i o n Highett,
DISCUSSION COMMENT
of Building
Australia
ON P A P E R BY DR R.G.J.
BY DR A.J.
Did you notice
FLAY AND DR H.W.
any s i g n i f i c a n t
change
in w i n d speed due to the change
tunnel
roughness
to the typical
model due to this effect,
as the m o d e l
REPLY
- Dr R.G.J.
expected
because
the r o u g h n e s s
changes
in flow c h a r a c t e r i s t i c s
in e f f e c t i v e
roughness
imposed on the local w i n d conditions
Significant
TEUNISSEN
BOWEN
change
AUTHOR'S
Research
roughness
over the m o d e l ?
is r e l a t i v e l y
long,
that w e r e being
such as a
from the u p s t r e a m
Any changes
over
the
would be super-
investigated.
Flay
in these flow c h a r a c t e r i s t i c s
the r o u g h n e s s
of the model,
immediately
upstream
were neither
of the d e t a i l e d
apart f r o m the a i r p o r t region,
observed model
nor
and
were e s s e n t i a l l y
the
same.
COMMENT
BY DR J. W I E R I N G A
In the field e x p e r i m e n t apartment
buiding.
measurement studies.
will
10 m above influence
as, among others,
T e u n i s s e n has shown in o t h e r
for the a p a r t m e n t
building
been made before
REPLY
- Dr. R.G.J.
velocity
The r e f e r e n c e
located
was located
26 m above
on the r e f e r e n c e
results?
the e f f e c t of the b u i l d i n g and full-scale
on a 10 m m a s t which structure
itself
was atop two
of the building.
10%.
'overspeed' Comparison
© 1983 Elsevier Science Publishers B.V.
Hence,
and wind
above the b u i l d i n g - g e n e r a t e d
The r e s u l t i n g
l o c a t i o n was o n l y about
on the
experiments.
the m a i n p a r t of the building
that this was well
for all w i n d directions.
0167-6105/83/$03.00
because
on the roof of the m a i n
showed
influence
an the
Flay
was m o u n t e d
the a n e m o m e t e r
at the a n e m o m e t e r
w i t h the m o d e l
was the same in both the m o d e l
anemometer
tunnel e x p e r i m e n t s
comparison
was n e c e s s a r y
8 m penthouses
layers
was m e a s u r e d building
significantly
No c o r r e c t i o n reference
velocity
the a p a r t m e n t
Has a c o r r e c t i o n
velocities
AUTHOR'S
a reference
At that height,
shear
caused by the b u i l d i n g of full-scale
116
h e l i c o p t e r m e a s u r e d profiles w i t h speeds o b t a i n e d from the a p a r t m e n t a n e m o m e t e r showed s i m i l a r o v e r s p e e d i n g of a b o u t 10%.
C O M M E N T B Y DR A.J. DUTT I w o u l d like to thank Dr Flay for his interesting paper. My only comments are: I.
W h a t is the size of m o d e l in r e l a t i o n to the transverse d i m e n s i o n of wind the tunnel test section and also in r e l a t i o n to the v e r t i c a l dimension, k e e p i n g in m i n d the block e f f e c t ?
2.
W h a t w i n d v e l o c i t y was used in the w i n d tunnel?
AUTHOR'S
I.
REPLY
-
Dr. R.G.J.
Flay
The m o d e l was fairly flat and its thickness was only about 2% of the tunnel h e i g h t so that b l o c k a g e e f f e c t s c o u l d be e x p e c t e d to be negligible. Its total w i d t h was nearly the full w i d t h of the test section, m e a s u r e m e n t s w e r e m a d e only in the centre regions,
but
so wall b o u n d a r y - l a y e r
effects w e r e not of concern.. 2.
In general,
the wind tunnel v e l o c i t y at the top of the b o u n d a r y - l a y e r was
about 14.0 m/s and was selected so that the v e l o c i t y scale factor b e t w e e n the m o d e l and f u l l - s c a l e results was unity.
C O M M E N T F R O M THE C H A I R The C h a i r m a n c o m m e n t e d that it was essential for m o r e work c o m p a r i n g m o d e l s to the real w o r l d such as r e p o r t e d in the p a p e r by Drs Flay and T e u n i s s e n to give m o r e a c c e p t a n c e to this type of work. He also suggested this type of work d e s c r i b e d in this s e s s i o n m a y well be useful for t r a n s l a t i n g wind d a t a from a d i s t a n t m e a s u r i n g p o i n t to a l o c a t i o n u n d e r investigation.
Dr J.-A. Hertig f i n i s h e d the session by showing a 35 m m film of work r e l e v a n t to that d e s c r i b e d in his paper.
D I S C U S S I O N ON P A P E R B Y D R J.A. H E R T I G AND DR P. L I S K A
COMMENT BY DISCUSSIONS EDITOR The f o l l o w i n g w r i t t e n comment was r e c e i v e d after the Conference.
C O M M E N T BY DR N.O. J E N S E N The m o d e l uses a surface t e m p e r a t u r e d i s t r i b u t i o n w h i c h is imposed.
This
is b a s i c a l l y a p p l y i n g the solution w h i c h is b e i n g sought, as t e m p e r a t u r e is a dynamic variable.
W h e n it is said in the p a p e r that:
"It did not seem p o s s i b l e
to use m e a s u r e m e n t s t a k e n from nature in order to a c c o m p l i s h all n e c e s s a r y
117
corrections",
it is a philosophical inconsistency;
modelling is done in order
to gain insight into problems of nature in cases where direct observations would be difficult.
But here it is said that knowledge is required beforehand
(e.g.
of surface temperature distributions), thus, it seems, making the modelling effort obsolete, as it is only reproducing what is known anyway. Despite all the prose about the complexity of the ABL the range of variability of K m (which may be many orders of magnitude)
is not appreciated.
Further there is considerable discussion about the requirements for similarity, although in the event they are violated.
As the flow is not kept turbulent no
features of boundary layer similarity (including points of separation, and the properties of wakes) are retained. scale separation
Further, problems specifically dependent on
(especially diffusion problems)
scope of this approach.
are completely outside the
Regarding distortion of the vertical scale, this is
even more serious in that similarity is not only neglected but contradicted. In addition to these general comments there are a few specific ones.
The
connection between Equation (7) and the Rossby number (which has the same construction but inverted) cannot be taken seriously when the time scale involved is not related to some sort of rotation. meaning of the Fr and Re numbers.
There is confusion in the
It is bluntly stated that:
"Knowing that
these numbers express the relationship between kinetic energy and viscous energy dissipation
...".
In fact, Fr is a measure of inertial forces relative to
buoyancy forces, and the turbulence Re number is merely a reciprocal drag coefficient, neither quantity expressing the relationship suggested. Last but not least I fail to see how a study of equations for laminar slope flow can indicate when similarity with atmospheric conditions is fulfilled.
The
equations studied are easily seen to be the shallow water equation
v
~
/g ~ h
,
h
=
xsin8
,
times a profile-fudge-function,
depending on the thickness Z of a laminar
boundary layer after fetch x: --
4
- - ~ x
AT s i n e g -~-
v
where D is the relevant analogous to laminar turbulent cases
entrainment
diffusivity.
flows. across
the flow becomes thinner
substitution
of turbulent
However, t u r b u l e n t
slope flows are not
I n some c a s e s m o s t o f t h e d r a g the upper boundary of the as it
accelerates
eddy viscosities
slope
down t h e
in place
is produced by flow.
slope.
of their
In other Eve n i f
laminar
counterparts
118
were valid (and of course this is nonsense) the above cited equations would by no chance come close to reality.
AUTHOR'S REPLY - Dr Jo-A. Hertig While we thank Dr Jensen for his questions and remarks, the scope of the paper is the simulation of flow patterns in complex terrain and not short distance diffusion in flat terrain.
Consequently the surface temperature ~
considered to be an approximately independent variable and we study its influence on the flow fields.
In fact the simulation techniques used allow us a
certain adaptation of the temperature field to the flow created because of the physical properties of the surface heat transfer. On the other hand the Earth's surface temperature is well known from satellite measurement or air plane teledetection.
These data are easily avail-
able in Switzerland and are always taken into consideration.
It is true that
the problem of the variability of K [] is the central question in such simulations. The influence of this factor is taken into consideration through temperature distortion and absolute temperature changes as presented in figure 2 of the paper.
Wooldridge and Lehmann have computed mean values of the Eulerian K
in the ABL from soundings carried out in the Rhine river valley.
m As indicated
in paragraph 2 under 4) in the paper K 15 m2/s.
has been found to vary between i and m It is precisely the appreciation of the influence of such variation
which entrains the apparently contradicted similarity conditions and the use of distorted geometry and temperature - the similarity conditions are not v[olated but adapted to these very complicated conditions.
Note that similarity ~riteri~
adaptation has been applied and tested in a great number of experiments described in the Climod project technical reports
[6].
Concerning the particular remarks, we make use of both the Rossby number and Homochronous number (H), 7).
The former (Ro) is related indeed to the Earth's
rotation and dictates the rotation speed of the platform.
The Homochronous
number (H) provides the time scale of temperature variation during the day-night cycles simulated on the models. It is explicitly stated that the Reynolds numbers express the relationship between kinetic energy and viscous dissipation. the following definition
This can be readily deduced from
: =
Re
P
V 3
(W grad V)V
for a unit volume of fluid. As far as the last remark is concerned the answer can be found in the original papers of Prandtl and the further paper of Defant on the actual slope
119
wind.
They were the first to establish the formula used in our paper.
The works of Lehmaan in the Climod project show that the influence of the variations of K
with height is not significant in the drainage flow simulations m on inclined surfaces. The principal contribution comes from the nature of the
threedimensional aspect of the real topography.
115
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
DISCUSSION
ON S E S S I O N
18
TOPOGRAPHIC
-
MODELLING,
DISPERSION
OF G A S E O U S
POLLUTANTS
B.T.
Rapporteur:
Hawkins
CSIRO D i v i s i o n Highett,
DISCUSSION COMMENT
of Building
Australia
ON P A P E R BY DR R.G.J.
BY DR A.J.
Did you notice
FLAY AND DR H.W.
any s i g n i f i c a n t
change
in w i n d speed due to the change
tunnel
roughness
to the typical
model due to this effect,
as the m o d e l
REPLY
- Dr R.G.J.
expected
because
the r o u g h n e s s
changes
in flow c h a r a c t e r i s t i c s
in e f f e c t i v e
roughness
imposed on the local w i n d conditions
Significant
TEUNISSEN
BOWEN
change
AUTHOR'S
Research
roughness
over the m o d e l ?
is r e l a t i v e l y
long,
that w e r e being
such as a
from the u p s t r e a m
Any changes
over
the
would be super-
investigated.
Flay
in these flow c h a r a c t e r i s t i c s
the r o u g h n e s s
of the model,
immediately
upstream
were neither
of the d e t a i l e d
apart f r o m the a i r p o r t region,
observed model
nor
and
were e s s e n t i a l l y
the
same.
COMMENT
BY DR J. W I E R I N G A
In the field e x p e r i m e n t apartment
buiding.
measurement studies.
will
10 m above influence
as, among others,
T e u n i s s e n has shown in o t h e r
for the a p a r t m e n t
building
been made before
REPLY
- Dr. R.G.J.
velocity
The r e f e r e n c e
located
was located
26 m above
on the r e f e r e n c e
results?
the e f f e c t of the b u i l d i n g and full-scale
on a 10 m m a s t which structure
itself
was atop two
of the building.
10%.
'overspeed' Comparison
© 1983 Elsevier Science Publishers B.V.
Hence,
and wind
above the b u i l d i n g - g e n e r a t e d
The r e s u l t i n g
l o c a t i o n was o n l y about
on the
experiments.
the m a i n p a r t of the building
that this was well
for all w i n d directions.
0167-6105/83/$03.00
because
on the roof of the m a i n
showed
influence
an the
Flay
was m o u n t e d
the a n e m o m e t e r
at the a n e m o m e t e r
w i t h the m o d e l
was the same in both the m o d e l
anemometer
tunnel e x p e r i m e n t s
comparison
was n e c e s s a r y
8 m penthouses
layers
was m e a s u r e d building
significantly
No c o r r e c t i o n reference
velocity
the a p a r t m e n t
Has a c o r r e c t i o n
velocities
AUTHOR'S
a reference
At that height,
shear
caused by the b u i l d i n g of full-scale
116
h e l i c o p t e r m e a s u r e d profiles w i t h speeds o b t a i n e d from the a p a r t m e n t a n e m o m e t e r showed s i m i l a r o v e r s p e e d i n g of a b o u t 10%.
C O M M E N T B Y DR A.J. DUTT I w o u l d like to thank Dr Flay for his interesting paper. My only comments are: I.
W h a t is the size of m o d e l in r e l a t i o n to the transverse d i m e n s i o n of wind the tunnel test section and also in r e l a t i o n to the v e r t i c a l dimension, k e e p i n g in m i n d the block e f f e c t ?
2.
W h a t w i n d v e l o c i t y was used in the w i n d tunnel?
AUTHOR'S
I.
REPLY
-
Dr. R.G.J.
Flay
The m o d e l was fairly flat and its thickness was only about 2% of the tunnel h e i g h t so that b l o c k a g e e f f e c t s c o u l d be e x p e c t e d to be negligible. Its total w i d t h was nearly the full w i d t h of the test section, m e a s u r e m e n t s w e r e m a d e only in the centre regions,
but
so wall b o u n d a r y - l a y e r
effects w e r e not of concern.. 2.
In general,
the wind tunnel v e l o c i t y at the top of the b o u n d a r y - l a y e r was
about 14.0 m/s and was selected so that the v e l o c i t y scale factor b e t w e e n the m o d e l and f u l l - s c a l e results was unity.
C O M M E N T F R O M THE C H A I R The C h a i r m a n c o m m e n t e d that it was essential for m o r e work c o m p a r i n g m o d e l s to the real w o r l d such as r e p o r t e d in the p a p e r by Drs Flay and T e u n i s s e n to give m o r e a c c e p t a n c e to this type of work. He also suggested this type of work d e s c r i b e d in this s e s s i o n m a y well be useful for t r a n s l a t i n g wind d a t a from a d i s t a n t m e a s u r i n g p o i n t to a l o c a t i o n u n d e r investigation.
Dr J.-A. Hertig f i n i s h e d the session by showing a 35 m m film of work r e l e v a n t to that d e s c r i b e d in his paper.
D I S C U S S I O N ON P A P E R B Y D R J.A. H E R T I G AND DR P. L I S K A
COMMENT BY DISCUSSIONS EDITOR The f o l l o w i n g w r i t t e n comment was r e c e i v e d after the Conference.
C O M M E N T BY DR N.O. J E N S E N The m o d e l uses a surface t e m p e r a t u r e d i s t r i b u t i o n w h i c h is imposed.
This
is b a s i c a l l y a p p l y i n g the solution w h i c h is b e i n g sought, as t e m p e r a t u r e is a dynamic variable.
W h e n it is said in the p a p e r that:
"It did not seem p o s s i b l e
to use m e a s u r e m e n t s t a k e n from nature in order to a c c o m p l i s h all n e c e s s a r y
117
corrections",
it is a philosophical inconsistency;
modelling is done in order
to gain insight into problems of nature in cases where direct observations would be difficult.
But here it is said that knowledge is required beforehand
(e.g.
of surface temperature distributions), thus, it seems, making the modelling effort obsolete, as it is only reproducing what is known anyway. Despite all the prose about the complexity of the ABL the range of variability of K m (which may be many orders of magnitude)
is not appreciated.
Further there is considerable discussion about the requirements for similarity, although in the event they are violated.
As the flow is not kept turbulent no
features of boundary layer similarity (including points of separation, and the properties of wakes) are retained. scale separation
Further, problems specifically dependent on
(especially diffusion problems)
scope of this approach.
are completely outside the
Regarding distortion of the vertical scale, this is
even more serious in that similarity is not only neglected but contradicted. In addition to these general comments there are a few specific ones.
The
connection between Equation (7) and the Rossby number (which has the same construction but inverted) cannot be taken seriously when the time scale involved is not related to some sort of rotation. meaning of the Fr and Re numbers.
There is confusion in the
It is bluntly stated that:
"Knowing that
these numbers express the relationship between kinetic energy and viscous energy dissipation
...".
In fact, Fr is a measure of inertial forces relative to
buoyancy forces, and the turbulence Re number is merely a reciprocal drag coefficient, neither quantity expressing the relationship suggested. Last but not least I fail to see how a study of equations for laminar slope flow can indicate when similarity with atmospheric conditions is fulfilled.
The
equations studied are easily seen to be the shallow water equation
v
~
/g ~ h
,
h
=
xsin8
,
times a profile-fudge-function,
depending on the thickness Z of a laminar
boundary layer after fetch x: --
4
- - ~ x
AT s i n e g -~-
v
where D is the relevant analogous to laminar turbulent cases
entrainment
diffusivity.
flows. across
the flow becomes thinner
substitution
of turbulent
However, t u r b u l e n t
slope flows are not
I n some c a s e s m o s t o f t h e d r a g the upper boundary of the as it
accelerates
eddy viscosities
slope
down t h e
in place
is produced by flow.
slope.
of their
In other Eve n i f
laminar
counterparts
118
were valid (and of course this is nonsense) the above cited equations would by no chance come close to reality.
AUTHOR'S REPLY - Dr Jo-A. Hertig While we thank Dr Jensen for his questions and remarks, the scope of the paper is the simulation of flow patterns in complex terrain and not short distance diffusion in flat terrain.
Consequently the surface temperature ~
considered to be an approximately independent variable and we study its influence on the flow fields.
In fact the simulation techniques used allow us a
certain adaptation of the temperature field to the flow created because of the physical properties of the surface heat transfer. On the other hand the Earth's surface temperature is well known from satellite measurement or air plane teledetection.
These data are easily avail-
able in Switzerland and are always taken into consideration.
It is true that
the problem of the variability of K [] is the central question in such simulations. The influence of this factor is taken into consideration through temperature distortion and absolute temperature changes as presented in figure 2 of the paper.
Wooldridge and Lehmann have computed mean values of the Eulerian K
in the ABL from soundings carried out in the Rhine river valley.
m As indicated
in paragraph 2 under 4) in the paper K 15 m2/s.
has been found to vary between i and m It is precisely the appreciation of the influence of such variation
which entrains the apparently contradicted similarity conditions and the use of distorted geometry and temperature - the similarity conditions are not v[olated but adapted to these very complicated conditions.
Note that similarity ~riteri~
adaptation has been applied and tested in a great number of experiments described in the Climod project technical reports
[6].
Concerning the particular remarks, we make use of both the Rossby number and Homochronous number (H), 7).
The former (Ro) is related indeed to the Earth's
rotation and dictates the rotation speed of the platform.
The Homochronous
number (H) provides the time scale of temperature variation during the day-night cycles simulated on the models. It is explicitly stated that the Reynolds numbers express the relationship between kinetic energy and viscous dissipation. the following definition
This can be readily deduced from
: =
Re
P
V 3
(W grad V)V
for a unit volume of fluid. As far as the last remark is concerned the answer can be found in the original papers of Prandtl and the further paper of Defant on the actual slope
119
wind.
They were the first to establish the formula used in our paper.
The works of Lehmaan in the Climod project show that the influence of the variations of K
with height is not significant in the drainage flow simulations m on inclined surfaces. The principal contribution comes from the nature of the
threedimensional aspect of the real topography.