Tutorial 2
The Driving Forces for Natural Ventilation
[See
Disclaimer and Important Notes]
This Section covers:
-
Estimating wind
Pressure [see important
notes];
-
Estimating Stack
Pressure;
-
Combining wind and
Stack Pressure.
1.
The Wind Pressure Equation
Wind striking an object induces, on that object, a
spatially distributed pressure pattern (Figure 2.1).
Figure 2.1. Wind striking a building induces a wind induced pressure
distribution.
The value of the induced
pressure at any particular point can be described by the following
equation:
Equation 2.1. Wind Pressure Equation.
A typical pattern of
pressure distribution is illustrated for a simple, essentially cubed
shaped building, in Figure 2.2 below.
Figure 2.2. Example Wind Pressure Distribution.
With respect to
atmospheric pressure, wind induces a positive pressure on the upwind face
of the object and a negative pressure on its sides and in the wake region
at the rear of the building. The windward roof face may also be at a
negative pressure unless the pitch angle exceeds about 30°.
To evaluate the
wind-induced pressure, the following data are needed:
- Building dimensions and shape;
- Information about surrounding terrain and
obstructions (both upwind and downwind);
- Location (e.g. city, urban, rural);
- Wind speed (and, for improved precision, direction);
- Spatial distribution of wind pressure coefficient.
Building Dimensions
and Shape: The
pressure pattern is highly dependent on the shape of the building.
Elementary and ‘pre-design’ analysis is only really possible by
approximating the plan-shape of the building by a rectangle or a series of
rectangles. Anything more complex may need to be analysed on an individual
basis in a wind tunnel. computational fluid dynamic (cfd) methods are
emerging as a potential means of analysing the pressure distribution
around buildings but require substantial computational effort, especially
if several wind speeds and directions are to be analysed.
Surrounding Terrain
and Obstructions: This again influences the pressure distribution
considerably. The simple approach is to describe the building as being
surrounded by buildings of equal height, half the height of completely
exposed. Again, this is suitable for basic pre-design analysis. The only
practical alternative is to consider a wind tunnel study in which a scale
model of the building and the surrounding buildings is incorporated.
Similarly this is an area where cfd analysis is emerging and will
probably, eventually present a practical alternative.
Location:
Apart from influencing the type of surrounding obstructions, location
influences the climatic driving forces of wind and temperature. Wind
strength in an urban location, for example, may be considerably lower than
that measured at a nearby ‘open-site’ weather station. Similarly, air
temperature may be higher in a city environment (‘heat island’
effect). A wind correction equation is presented below. At present a
suitable temperature conversion equation is not presented but it may be
worth increasing the open-site value, taken from the nearest
meteorological station, by a degree or so.
Wind Speed and
Direction: Wind
induced pressure increases with the square of the wind speed, while the
upwind and downwind faces of a structure clearly depends on the direction
of the wind. A key problem is
that the strength of the wind varies both with height above ground and the
intervening terrain, between the nearest meteorological station (usually
located in open countryside) and the site of the building, as illustrated
in Figure 2.3.
Figure 2.3.
Impact on wind speed of terrain and height above ground level.
In many cases, within an
urban environment, the wind speed at building height can be less than half
of that measured at a meteorological station. Since this value is squared,
failure to make the necessary correction to wind speed can lead to a
substantial overestimate in the wind induced pressure. A height and
terrain correction approach, described in British Standard 5925 (1991) is
presented in Table 2.1.
Spatial Distribution of
Wind Pressure Coefficient: The wind coefficient is assumed to be
independent of wind speed but varies according to location on the building
surface, the shape of the building and the nature of obstructions
surrounding the building. Face averaged values [see
Important Notes] for simple calculations
have been tabulated (e.g. BS5925:1990
[expose buildings], Guide to
Energy Efficient Ventilation [various degrees of
shielding and two plan area aspect ratios]. Bowen
and Wiren have both published comprehensive, spatially distributed
data for various building shapes and shielding, based on wind tunnel
studies Much of the information published in the Guide to
Ventilation are derived from these latter sources.
Table 2.1.
Wind speed correction approach as used in BS5925.
These datasets are
suitable for more complex calculations in which faced average values do
not provide a sufficient level of detail but where the cost of wind tunnel
studies is prohibitive. Some illustrative, simplistic example data is
presented in Figure 2.4.
Figure 2.4.
Example ‘faced averaged’ wind pressure coefficient data. This
data was based on averaging and amalgamating measurement data of Bowen and
Wiren. See The Guide to Energy Efficient Ventilation for more data.
A checklist summary of,
and the approximations needed, for basic calculations is presented in
Table 2.2.
Table
2.2. Checklist for simple ‘pre design’ or elementary
calculations.
Parameter
|
Simple
Design Analysis
(e.g.
methods as described in the AIVC Guide to Energy Efficient
Ventilation)
|
Detailed
Design Methods and Measurements
(e.g.
wind tunnel, cfd, specific local environmental measurements etc.)
|
|
Building
Shape
|
Represent
as a rectangle or a series of rectangles.
|
Complex:
Circular building, sections at different heights, courtyards etc.
|
|
Surrounding
Obstructions
|
Represent
as uniformly distributed equal to the height of the surrounding
structure, half the height or no obstructions
|
Complex:
Not uniformly distributed
|
|
Location
|
Specify
as ‘city’, ‘urban’, ‘rural’, ‘open country’. Use
simple correction methods for wind speed and air temperature.
|
Specific
detail needed (e.g. about degree of urbanisation, heat islands,
street canyons etc.).
|
|
Weather
Data
|
Taken
from nearest weather station (should be based on hourly records).
(Wind
speed corrected for terrain and building height)
|
Specific
on-site data needed.
|
|
Wind
Pressure Coefficient Data
|
Tabular
data for simple building shapes or, for slightly more complex
structures, spatially
distributed data as published by Bowen and Wiren.
|
Wind
tunnel data from individual scale model of the building and its
surroundings.
|
2.
The Stack Pressure Equation
Stack flow is driven by
the difference between the inside and outdoor air temperature. Assuming
that the indoor temperature is above the outdoor value, the inside air is
less dense and therefore lighter than the outside air. As a consequence,
the vertical pressure gradient, exerted by the indoor air, is steeper,
resulting in a pressure imbalance. If the enclosed space is penetrated by
openings at different heights, air flows through openings at the lowest
level and escapes through the upper openings (Figure 2.5).
This flow process is reversed if the indoor temperature is less
than the outdoor temperature. The location at which the indoor and outdoor
pressures are in balance is called the neutral pressure plane. If an
opening were present at this opening, there would be no airflow through
it.
Figure 2.5. The principle of stack flow.
Stack pressure is
calculated directly by the application of the Ideal Gas Laws. The
temperature difference or stack induced pressure at an opening with
respect any arbitrary datum height (e.g. surface level) is given by
Equation 2.2, where ‘h’ is the ‘height’ of the opening above (or
below) the chosen datum. Usually, the datum is taken as the surface or
floor level of the building. With basic ‘hand’ calculations, or very
elementary networks, however, there can be an advantage in taking the
datum as the level of the lowest open. This is because the relative stack
pressure at this level will be zero, thus saving the effort of a
calculation for each opening assigned to this level. In applying the stack
flow equation, it must be remembered that the temperature is given in
Kelvins (degrees absolute) and not in Centigrade.
Equation 2.2. The stack flow equation.
3.
Combining Wind Pressure with Stack Pressure
The pressures calculated
using Equations 2.1 and 2.2 are additive (Figure 2.6) i.e. they can be
summed directly together. This is not the same, however, as adding
together the airflow rates calculated individually for wind and stack
pressure.
Figure 2.6. Combining wind and stack
pressure.
This completes Tutorial
2
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