Welcome back to Sports and Building
Aerodynamics and the week on wind-tunnel testing.
We start again with a module question.
Which type of wind tunnel has the longest test-section
length compared to the other test-section dimensions width and height?
Is that the atmospheric boundary layer wind tunnel, the aeronautical
wind tunnel, the climatic wind tunnel, or the smoke wind tunnel.
Hang on to your answer and we'll come back to this question later in this module.
At the end of this module, you
will understand the characteristics of an atmospheric boundary
layer wind tunnel, the differences between this
type of wind tunnel and other wind tunnels.
The requirements for successful atmospheric boundary layer testing.
You'll understand the definition of Wind
Engineering and the importance of this field.
And the many applications of atmospheric boundary layer wind-tunnel testing.
An atmospheric boundary layer wind tunnel is actually
a wind tunnel that is used to simulate
the lower part of the atmospheric boundary layer,
usually in neutral conditions, this means for strong winds.
So there are no significant temperature effects.
In aeronautical wind tunnels, automobile wind
tunnels and climatic wind tunnels usually,
the approaching wind flow is uniform and of low turbulence as illustrated here.
However, in an atmospheric boundary layer wind
tunnel we have to reproduce the increase
of wind speed with height and also
the appropriate turbulence intensity profiles and turbulence spectra.
So we have a flow here that is definitely non-uniform in space and time
and of high turbulence, which is completely
opposite to the case with aeronautical wind tunnels.
However, I have to make a comment here with these two figures.
Of course close to the walls of the wind tunnel,
at the walls of the wind tunnel, the velocity is zero.
So in reality also here there are boundary layers at the walls of the wind tunnel.
And this is actually nicely indicated in this graph here, and you see also
that this boundary layer thickness increases as
the flow traverses through the wind tunnel.
So it is important in atmospheric boundary layer wind tunnels
to reproduce this atmospheric boundary layer to the best possible extent.
And you see here an illustration of mean wind speed profiles.
This is a drawing by Davenport, where you see
the increase of wind speed over different types of terrain.
In an atmospheric boundary layer wind tunnel, we are going to
use specific features to generate these atmospheric boundary layer profiles.
There are different types: spires,
barriers, carpets, distributed roughness elements.
And this is a nice schematic that illustrates
the position of these features in the tunnel.
So you first have the spires, and they will start
producing the gradient in the atmospheric boundary layer profiles.
Then there is a trip or the barrier.
After that, there are roughness elements.
And after that, there can be a carpet and then you arrive in the test
section, where the turntable and the building
model or any other model is placed.
Let's look at these items,
these features in a photograph.
So we are looking here towards the entrance of the wind tunnel.
And here you have the spires.
Here is the barrier, here are the distributed roughness elements.
And you see here a stadium model on the wind-tunnel turntable.
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Often you need a long fetch in order to establish these boundary layer profiles.
And the length of such a test section is
often 10 to 15 times the test section width.
So indeed, for a given test section width and height,
these wind tunnels are generally much longer than other wind tunnels.
This is an example.
This is the closed-circuit atmospheric boundary
wind tunnel at the Von Karman Institute.
Where we indeed you see a very long test section.
This is a non-atmospheric boundary layer wind tunnel at the
same institute and you see actually indeed a very short test section.
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But there's also different terrain roughnesses in reality.
And different terrain roughnesses means that you will have to apply different
combinations of spires, barriers and roughness
elements to generate different atmospheric boundary layer profiles.
So that's an additional complexity
in atmospheric boundary layer wind-tunnel testing.
So use these different combinations and then
a wide variety of profiles can be generated.
Initially, quite some years ago, wind-tunnel tests of pressures and wind
loads on buildings were performed in
aeronautical wind tunnels because there were
no atmospheric boundary layer wind tunnels, but also because it was not fully
realized how important these atmospheric boundary
layer profiles are for the final results.
So, indeed, quite some tests were made with a uniform velocity and very low
turbulence, and this way, a catalogue was made
of pressure coefficients on buildings and later on,
it was was indeed stated that the results that were obtained then, when these were
compared to the results from atmospheric boundary layer wind
tunnel that these were very different, completely different.
And this is an example here of a graph illustrating
the pressure coefficient contours on a building surface for an
aeronautical wind tunnel with a smooth approach flow profile and an
atmospheric boundary wind tunnel, and you indeed see very large differences.
So all these results have to be thrown away and new
tests had to be done in appropriate atmospheric boundary layer wind tunnels.
But because at the beginning, there were not
many of those wind tunnels around, this testing was
done in aeronautics or general-purpose wind tunnels,
in which a lot of effort was being spent
on generating these atmospheric boundary layer profiles.
However it was always a limitation that these wind
tunnels had a very short test-section length.
So it was very difficult to get
appropriate boundary layer profiles in this tunnel.
And that's why later on specific-purpose atmospheric boundary layer wind
tunnels were devised and now they are present in many countries worldwide.
And it's a very specific character of these wind tunnels and also the very
different results obtained in such wind tunnels
which clearly proves the importance of their existence.
That actually gave birth to a new research field called Wind Engineering.
So what is Wind Engineering?
This is one definition where it is stated that Wind
Engineering is best defined as the rational treatment of
interactions between wind and the atmospheric boundary layer and
man and his works on the surface of earth.
It's a very clear definition.
This is another one which stresses the
combination of different fields into this discipline.
Wind Engineering combines the fields of
meteorology, fluid dynamics, structural mechanics and statistical
analysis to minimize the unfavorable effects of
the wind and maximize the favorable ones.
And there's a wide range of Wind Engineering topics.
Sports and Building Aerodynamics are two.
But there's also more specific building applications
such as wind-structure interaction, pedestrian wind comfort,
wind-driven rain on building facades and pollutant
dispersion in buildings, around buildings and urban areas.
Wind Engineering is internationally grouped in the International Association
of Wind Engineering, which has national members from all over
the world, high-quality conference series, and even an official journal,
which is the Journal of Wind Engineering and Industrial Aerodynamics.
In the next week of this MOOC
Computational Fluid Dynamics plays a major role.
And Computational Fluid Dynamics, when part
of Wind Engineering, is called Computational
Wind Engineering, and we'll address that in more detail in that week.
Let's focus on some requirements for ABL wind-tunnel testing.
These are the onces by Plate and Cermak
saying that you need proper scaling
of buildings and topographic features, you need to match
Reynolds numbers, Rossby numbers and give appropriate
kinematic simulation of air flow, boundary layer velocity
distribution, and turbulence and finally the zero
pressure gradient that occurs in the real world
has to also be reproduced in the
wind tunnel. Let's first look at matching Reynolds numbers.
The Reynolds number as discussed in the first week is the
product of a velocity scale and a length scale divided by the kinematic viscosity.
Let's assume now that we do an atmospheric boundary
layer wind-tunnel test at model scale; 1 to 50.
So, this is how the length scale is scaled.
If we do a wind-tunnel test usually we use air,
so the kinematic viscosity here given without units remains the same.
And if we then have to match the Reynolds number this means that our speed has to be
increased from 10 meters per second, which it could
be in full scale, to 500 meters per second.
Which is not subsonic anymore.
So this will not allow us to match Mach
numbers, so this matching of Reynolds numbers is not possible.
And often in general, matching Reynolds numbers in wind-tunnel testing
is not possible when you do it at a reduced scale.
But, it is not always extremely important,
because sometimes the separation points are fixed.
Let's look for example at these animations, on the left side,
you see the circular cylinder that we have discussed in the first
week and indeed there, we investigated and we analyzed the change
of position of the separation point as a function of the Reynolds number.
And indeed because this is a body without
sharp edges the separation points are not fixed.
However, for buildings with sharp edges, the separation points are at those edges.
And that means that this kind of flow is easier to scale down
in the wind tunnel without having severe
consequences from not matching the Reynolds numbers.
So the sharp edges are important.
We should at least exceed a certain minimum Reynolds
threshold value, and often this is taken as 10,000.
And it's always important also to make some tests at other Reynolds numbers, for
example, by increasing the speed in the
wind tunnel to investigate potential Reynolds number independence.
Then the Rossby number.
The Rossby number is related to the rotation of the
earth, and the effect that that has on wind flow.
Often, this is not very significant
in atmospheric boundary layer wind-tunnel testing.
And, if it would be, it would be very hard to simulate this.
Then the kinematic simulation of air
flow, boundary layer distribution and so on. Well we
use the appropriate features in the wind tunnel
to generate the roughness features to generate
the right boundary layer profiles and well it's stated that at least up
to a height of 130% of the building height these profiles should be established.
But preferably they should go all the way up to the ceiling of the wind tunnel.
And by that, indeed we use combinations of spirals and often cubical roughness elements.
where finally then the model itself is placed on the wind tunnel turntable
and that is rotated to investigate different wind directions.
Finally, there is matching the zero pressure gradient found in the real world.
Often this is not so important in aeronautical
and climatic wind tunnels, but in an atmospheric boundary
layer wind tunnel where we build a very thick
boundary layer, this is really an issue of importance.
So this can be cancelled by using
an adjustable test-section roof, as illustrated here.
And this way, by careful adjustment of the roof, depending on the type of
boundary layer profile that we are generating,
we can establish a near-zero pressure gradient.
So, finally a question, what about temperatures?
What about non-neutral stratification in the ABL?
Well, usually, wind engineers get a break here.
Because when wind speed is strong, the atmospheric boundary layer is so well
mixed, that the effect of temperature gradients on the flow is negligible.
And this applies for many studies; for pedestrian level wind
conditions where you want to focus on high wind speed.
The wind loads on buildings and structures, again the focus on high
wind speed, wind energy focus on high wind speed, and snow drift.
However it does not apply for air pollution and
for example, the urban heat island effect which are
episodes that are often most severe at low
wind speed and then thermal stratification can become very important.
But it is extremely difficult to appropriately
model stable or unstable stratification in wind tunnels.
And that could be indeed, a task for CFD.
Back to the module question.
Which type of wind tunnel has the longest test section length?
Well, from the slides we saw before, this
is definitely the atmospheric boundary layer wind tunnel
because we need a very long fetch, test section
length to establish the appropriate atmospheric boundary layer profiles.
In this module, we learned about the characteristics of an atmospheric boundary
layer wind tunnel, the differences between ABL
wind tunnels and other tunnels, the requirements
for successful ABL testing, the definition of Wind Engineering and the importance of
this field and about quite some applications of ABL wind-tunnel testing.
In the next module we are going to focus on the
position of the different wind-tunnel components, and on their function and purpose.
Thank you for watching, and we
hope to see you again in the next module.
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