ENVIRONMENTAL WIND FLOWS AROUND BUILDINGS OUTLINING FLOW MECHANISMS – FLOW STRUCTURE AROUND ISOLATED BUILDING. Why architects need the knowledge about the effect of a building on the local wind? 1. Such knowledge enables us to estimate of how much people will be buffeted by the wind outside the building, on the ground or on aerial walkways round the building. 2. The external wind can considerably influence on heating and ventilating systems and thus affect their performance. 3. The wind flow round a building should be known if the dispersal of pollutants in its vicinity is to be evaluated (e.g. the concentrations produced by a source of pollution upwind). 4. The noise generated in a building surrounding by the wind flowing around by the wind flowing around it, e.g. the edge – tones, are extremally irritating. 5. Aeroplanes and helicopters are very sensitive to cross winds and vertical gusts when landing. The wake behind hangars at airports or the wind round tall buildings near heliports can present problems to aircraft in landing.

ENVIRONMENTAL WIND FLOWS AROUND BUILDINGS OUTLINING FLOW MECHANISMS – FLOW STRUCTURE AROUND ISOLATED BUILDING There are two quite separate mechanisms, which cause highinduced wind speeds at ground level: The first type of flow is caused by the pressure distribution on the windward face of a building. Upstream face pressure related to local wind dynamic pressure which increases with height, induces flow vertically down the face below the stagnation point. This flow rolls up into a standing vortex system at the base of the building, causing high wind speeds in this region. The second type of flow is caused by the pressure difference between the low pressure wake regions (lee-ward and side faces) and the relatively high pressure regions at the base on the windward face. Flow directly between these two regions through arcades or around corners can cause very high local wind speeds.

ENVIRONMENTAL WIND FLOWS AROUND BUILDINGS OUTLINING FLOW MECHANISMS – FLOW STRUCTURE AROUND ISOLATED BUILDING Flow field around a building

ENVIRONMENTAL WIND FLOWS AROUND BUILDINGS Model of flow near a sharp-edge building in a deep boundary layer (Hosker, 1979). Flow field around a bluff body appears highly complicated, since it is defined by impingement, separation, reattachment, circulation, vortices etc. (phenomena which are considered difficult to resolve in the word of fluid dynamics)

FLOW STRUCTURE AROUND AN ISOLATED BUILDING ENVIRONMENTAL WIND FLOWS AROUND BUILDINGS The descriptions of the flow near structures are mainly based upon laboratory observations of smoke patterns, neutrally-buoyant bubble-streak photographs, and oil-film analysis. Most of such data have been obtained in low speed aerodynamic or meteorological wind or water tunnels. 1. Flow over a building creates a positive pressure zone on the upstream side and negative zones on the roof, lee, and lateral sides. Although the pressure magnitudes change within these zones depending on wind speed for a given orientation, their relative magnitudes are unaffected. 2. The zones of pressure induce bending of mean streamlines, secondary flow motions and separation. Because the wind is turbulent, the contour regions of displacements or cavity will not be constant in size or shape. 3. For an isolated building the approach wind profile may induce a set of “horse-shoe” shaped vortices which warp around the buildings which generally disrupt these eddies. Hence they do not persist much beyond the cavity itself. 4. The cavity region is considered as a closed recirculation zone bounded by the separation streamline rising from the upwind edge of the building roof. 5. Beyond the cavity region, aerodynamic downwash may persist for an extended distance. Separation effects and secondary motions induced by the buildings bring about velocity deficits and turbulence excesses in the wake region which persists from 5 to 30 building heights downwind. Penthouses, towers, air conditioning equipment, auxiliary buildings, and similar irregularities will change the flow conditions described above. Rounding a building or use of set – backs will decrease the heights of cavity, perturb reattachment locations, modify dispersion patterns downwind of a building complex.

ENVIRONMENTAL WIND FLOWS AROUND BUILDINGS Influence of the object geometry on the circulation zone extent

BUILDINGS INTERACTION ENVIRONMENTAL WIND FLOWS AROUND BUILDINGS Wind velocity profiles for different distances B between two buildings

ENVIRONMENTAL WIND FLOWS AROUND BUILDINGS Surface pressure distributions on the walls of the buildings in different distances B/H.

ENVIRONMENTAL WIND FLOWS AROUND BUILDINGS Summary of wind effects on people based on the Beaufort scale (after Penwarden)

ENVIRONMENTAL WIND FLOWS AROUND BUILDINGS COMFORT PARAMETER Numerator: sum of the mean wind speed and RMS at the site being examined Denominator: sum of like quantities at a reference site.

ENVIRONMENTAL WIND FLOWS AROUND BUILDINGS

COMFORT PARAMETER

Downstream effect Under certain conditions the vortex flow behind a lower upstream building can augment the vortex in front of the larger building, further increasing the high wind speeds at the base. ENVIRONMENTAL WIND FLOWS AROUND BUILDINGS Ground level wind speed measurements in front of a large building being affected by a lower upstream building.

GUIDE LINES FOR AVOIDING HIGH INDUCED WIND SPEEDS AT GROUND LEVEL ENVIRONMENTAL WIND FLOWS AROUND BUILDINGS 1.Building will only induce high ground level wind speeds if a significant part of the building height is exposed to direct wind flows. 2. Building should be regarded as potentially exposed for any particular direction if half of the building height is clearly above upstream buildings, which can provide shielding. N o t e: The shielding effect of upstream buildings is one of the most significant factors to be taken into account when an assessment of environmental wind conditions is being made.

ENVIRONMENTAL WIND FLOWS AROUND BUILDINGS GUIDE LINES FOR AVOIDING HIGH INDUCED WIND SPEEDS AT GROUND LEVEL 3. If a building is regarded as being exposed for some or all wind directions, there are some fundamental configuration factors which can be taken into account. a) Rectangular buildings with a main face rising clearly from ground level, normal to any of the unshielded prevailing wind directions, should be avoided as the most common reason of environmental wind problems. b) Rectangular buildings placed on a podium or lower stage building complex which deflects the downward wind flow before it reaches ground level, can be satisfactorily used in many cases. Canopies can be used to deflect this flow and lift a standing vortex above pedestrian level.

ENVIRONMENTAL WIND FLOWS AROUND BUILDINGS GUIDE LINES FOR AVOIDING HIGH INDUCED WIND SPEEDS AT GROUND LEVEL 4. Arcades and colonnades at the base of exposed buildings can provide openings between the high upstream face pressures and low base pressures through which high wind speeds can be induced. 5. Gardens and architectural features can be placed at a corner to prevent pedestrian access close to the corner and to provide a local wind break for areas immediately downstream of the corner. 6. In most cases, even when buildings are exposed, it is usually possible to provide canopies, local wind breaks, sealed arcades etc to achieve satisfactory wind conditions in adjacent areas. It has became common practice now to submit building design proposals to wind tunnel testing. It enables us to determine the wind environment around the buildings and to modify the air flow by adding protection to achieve acceptable conditions.

Surface oil visualisation of the flow around isolated ground-mounted cylinder and sample configuration of the model buildings. ENVIRONMENTAL WIND FLOWS AROUND BUILDINGS