Wind Pressure

Wind pressures on the building exterior are calculated at each simulation time step from data read from the weather file. Wind speed and direction data is combined with information on opening orientations and wind exposures to generate wind pressures on each external opening. The calculation involves wind pressure coefficients derived from wind tunnel experiments, combined with an adjustment for wind turbulence.

 

The pressure exerted by the wind on a building is a complex function of wind speed, wind direction and building geometry. The surrounding terrain and nearby obstructions can also be important factors. For practical purposes, wind pressures are often estimated using wind pressure coefficients. These coefficients relate the wind pressure on a building surface to the wind speed, using a relationship of the form

                                                                                                            (1)

where

             is wind pressure (Pa)

             is wind pressure coefficient

is air density (kg/m ³ )

  is a reference wind speed (m/s)

 

Wind pressure coefficients may be obtained by a variety of means, including in situ measurements, CFD studies and wind tunnel experiments. Those used in MacroFlo are derived from wind tunnel experiments on simple building models. These experiments provide wind pressure coefficients for various types of surface (referred to as Exposure Types) for a range of relative wind directions (angles of attack). Exposure Types characterise both the geometrical aspects of the surface (such as roof pitch) and the degree of sheltering by nearby obstructions.

In line with the convention adopted in the data used by MacroFlo, the reference wind speed (v) appearing in the pressure formula is normally the free stream wind speed at the building height. (For an exception to this rule, see below.) The variable v is estimated from the meteorological wind speed, u, using an empirical expression for the variation of wind speed with height and terrain type:

                                                                                                       (2)

where

  is meteorological wind speed measured at height 10m in open country (m/s)

  is height above the ground (m)

and a and K are coefficients taking the following values for different terrain types (as set in APlocate):

 

 

Data in this table is taken from ASHRAE Handbook of Fundamentals (2001) [6]. ASHRAE provide the following formula for wind speed at height h, based on the assumption of a power law velocity profile (with exponent a) applying up to a height δ, where δ is the thickness of the atmospheric boundary layer:

                                                                                                 (3)

where

            = 270 m is the Layer Thickness for the meteorological site (assumed to be of type ‘Country’)

            = 0.14 is the Exponent for the meteorological site (assumed to be of type ‘Country’)

            = is the measurement height for the meteorological site (assumed to be 10 m)

 

In accordance with equations 2 and 3, the values of K appearing in the above table are calculated from

                                                                                                (4)

 

An alternative reference wind speed is used in cases not strictly covered by the experimental data, namely buildings of more than 12.5m in height where the specified exposure type applies to low-rise buildings. In such cases the reference wind speed depends on the height of the opening. For openings less than 12.5m off the ground, v is taken to be the free stream wind speed at 12.5m, or at the height of the building (whichever is the smaller). For openings above 12.5m, v is taken to be the free stream wind speed at the height of the opening. To avoid this compromise, it is recommended that where the building is more than 12.5m in height, the ‘High-rise’ exposure types should be used.

 

MacroFlo uses wind pressure coefficients taken from the Air Infiltration and Ventilation Centre’s publication Air Infiltration Calculation Techniques – An Applications Guide [2]. These coefficients, which are derived from wind tunnel experiments, fall into two categories:

Those applicable to ‘low-rise’ buildings of up to 3 storeys (or about 12.5m).

Those applicable to ‘high-rise’ buildings of more than 3 storeys. These are identified by exposure types with names beginning ‘high-rise’.

 

Wind pressure coefficients are provided for a range of exposure types within each category – ‘exposed wall’, ‘sheltered roof < 10 deg’, etc – and for 16 azimuthal angles of attack (reduced to 9 by symmetry). The values used are shown in Table 1 .

An exposure type must be set for all Opening Types. When selecting Exposure Types in the MacroFlo Opening Types Manager the following considerations should be borne in mind.

The terms ‘exposed’, ‘semi-exposed’ and ‘sheltered’ in the Exposure Type name refer to the degree of sheltering of the building by surrounding buildings and other obstructions. ‘Exposed’ denotes a building standing in open ground with no obstructions nearby, ‘semi-exposed’ denotes a building with nearby obstructions lower in height than the building itself, and ‘sheltered’ is appropriate when the surrounding obstructions are of similar height to the building.

 

In the case of the low-rise buildings, the terms ‘long’ and ‘short’ relate to the shape of the building viewed in plan. The experiments on which the wind pressure coefficients are based dealt with a square building and a rectangular building with a plan aspect ratio of 2:1. ‘Short wall’ and ‘long wall’ refer respectively to the walls on the short and long sides of the rectangular building, and ‘wall’ refers to the walls of the square building. ‘Long roof’ and ‘roof’ refer respectively to the pitched roofs on the rectangular and square buildings.

In the case of the high-rise buildings, the coefficients cover vertical walls and flat roofs at a range of elevations. The elevation is expressed using the ration h/H, where h is the height of the opening and H is the height of the building. Thus the exposure type

high-rise semi-exp wall h/H=0.6

would be appropriate for an opening in a wall about 60% of the way up a building surrounded by other buildings of lesser height.

 

When selecting appropriate exposure types for MacroFlo Opening Types, you should make a judgement as to which of the available exposure types most accurately describes the exposure of the surfaces to which the Opening Types are to be assigned.

To cover flat roofs, which were not treated in the experimental study, additional Exposure Types have been added: ‘exposed flat roof’, ‘semi-exposed flat roof’, ‘sheltered flat roof’ and so on. Wind pressure coefficients for these Exposure Types have been estimated by extrapolating the available experimental data.

An exposure type appropriate for internal openings, ‘internal’, is also provided. If applied to external openings, this exposure type sets all wind pressure coefficients to zero, allowing you to investigate flow patterns in the absence of wind pressures.

 

Adjustments are applied to wind pressures to allow for the effects of wind turbulence, as described in a later section.

 

Table 1 a Wind pressure coefficients (low-rise buildings)

 

Table 2 b Wind pressure coefficients (high-rise buildings)