Wind engineering is a part of Aerodynamic engineering, Structural engineering, Meteorology, and Applied physics to analyze the effects of wind in the natural and the built environment and studies the possible damage, inconvenience or benefits which may result from wind. In the field of engineering it includes strong winds, which may cause discomfort, as well as extreme winds, such as in a tornado, hurricane or heavy storm, which may cause widespread destruction.
Wind engineering draws upon meteorology, fluid dynamics, mechanics, geographic information systems and a number of specialist engineering disciplines including aerodynamics and structural dynamics. The tools used include atmospheric models, atmospheric boundary layer wind tunnels, computational fluid dynamics models, and other wind load test facilities.
Wind engineering involves, among other topics:
- Wind impact on structures (buildings, bridges, towers).
- Wind load on Façade.
- Wind comfort near buildings (microclimate).
- Effects of wind on the ventilation system in a building.
- Wind climate for wind energy.
- Air pollution near buildings.
Wind load on Building
Wind load on building is a complex phenomenon because of the many flow situations arising from the interaction of wind and structures. Wind is composed of a varying sizes of eddies and rotational characteristics carried along in a general stream of air moving relative to earth’s surface. These eddies give wind its gusty or turbulent character. The gustiness of strong winds in the lower levels of the atmosphere largely arises from interaction with surface features. Near the earth surface, the motion is opposed, and the wind speed reduce by the surface friction.
At the surface, the wind speed reduce to zero and then begins to increase with height and at some height , So it could be at a certain altitude conditions, seismic forces acting on the tall building does not become dominant compared to the influence of the wind. Wind load regulation in Indonesia adopted a regulation of ASCE 7-02 (American Society of Civil Engineers Minimum Design Loads for Buildings and Other Structures). ASCE 7-02 provides three procedures for calculating wind loads for buildings and other structures, including the main wind-force-resisting systems and all components.
Wind tunnel studies is the good way to analysis wind load on building compare to analytical study. Wind tunnel studies are recommended for situations when wake buffeting may exist due to significant upwind obstructions such as hills or significant aerodynamic term that describes a turbulent fluid region on the downstream side of a body, where strong eddies are generated which may impose critical fluctuating wind loads on structures downstream especially if the frequency content of turbulence excites the resonant frequency of the downstream structure.
This wake buffeting can be very important for slender towers and other dynamically sensitive structures where overall structural wind loads can be increased dramatically. Wind tunnel studies are also recommended for structure whose site location makes them subject to channeling effect caused by topographical or neighboring tall buiding such as in the city center or central business. In that situation the wind velocity from certain wind directions will be locally accelerated as the flow is squeezed between the upwind obstructions causing increased wind loading on the nearby structures.
The objective of wind tunnel test for building are:
- Cladding Wind Pressure Study
Wind loads on Cladding will be calculated from measured pressure distribution on both external surface and internal surface of cladding, with 36 wind directions at 10 degrees’ increment. Several pressure transducers will be used to measure multi wind pressure on cladding. Pressure measurement points determined by the owner and wind tunnel engineer. Wind pressure study give us the important data about distribution of pressure at the cladding which consisting of positive and negative pressur and the location and the value of maximum positive and negative pressure at 36 wind directions.
- Structural Loads
Wind for Structural Loads Study will be calculated for measured all aerodynamically important features, such us base shear force and moments. The balance will be installed at the bottom of model to measure aerodynamic force and moment with 36 wind directions at 10 degrees’ increment. Structural load test give us the important data about the distribution of equivalent static wind loads ,shear forces, and bending moment along the height of the building with combinations wind load (cross and along wind load) in all critical directions.
- Aero-elastic Test
Vortex induced vibration can arise in the smooth and turbulent flow for buildings. the resonant velocity of vortex induced vibration or the onset velocity of galloping can be within the design wind velocity. The prediction of this response requires wind tunnel test. The aeroelastic wind tunnel model should be able to meet the dynamic similarity between the model and the building prototype. give us the important data about the prediction of serviceability wind loads that can be used to derive the displacements of the buildings in all critical directions and Assessment of the predicted acceleration of the building’s top occupied floor with respect to occupant comfort criteria. The other important data is the critical speed when the vortex induced vibration occur and compare to the Wind speed of 1 year, 10 years, 50 years, 100 years and 1,000 year return periods.
- Pedestrian Comfort Wind Study
The objective of the study is to simulate and assess the comfort level of the wind environment at grade around pedestrian important locations such as building entrances, and also on the upper level terraces. In this case, pedestrian level wind of several locations as well as surrounding the building for 36 wind direction will be measured. The parameters include mean wind speed and peak wind speed. The peak wind speed is also an important index of assessment on comfort and safety for the pedestrian wind environment. We combined the local long term micrometeorological condition with the wind tunnel measurements to analyze the pedestrian level wind environment.
Aerodynamic and Aeroelastic Testing of Long Span Bridges
Wind load is one of the structural load which has influence on bridge design, especially for structural and aerodynamic design. The important role of wind loads is more significant after it caused several of bridge structures to either collapse completely with catastrophic failure such as Tacoma Narrows Bridge at 1940 or experience serviceability discomforts at Volgograd bridge where the authorities closed the bridge to all motor traffic due to strong oscillations caused by windy conditions. Many researches and studies have been carried out all over the world in order to analyze and model the wind behavior and its relative static and dynamic interactions with the structural responses to the turbulent wind load by using computational fluid dynamic or wind tunnel test. A long span bridge can be considered as a flexible structure, because it has a very small slenderness-ratio, the ratio between width over span. Under certain wind conditions, the structure could have aerodynamic and aeroelastic instability.
Aerodynamic instability occurs if the generated wake vortex excites the bridge structure developing into a resonance. The phenomenon is called vortex induced vibration. In addition, a situation of discomfort might appear if the separated flow reattaches on the bridge deck.
Flutter instability is caused by dynamic interactions between the aerodynamic forces/moments with structural inertia of the deck. The phenomenon usually involves a frequency coupling between two or more vibration modes. It can destroy the whole deck structure.
To avoid both instabilities during or after the construction, the bridge designer should be able to predict the critical velocity (Vf) of flutter as well as the aforementioned vortex induced vibration.
The most practical and accurate method to predict the critical velocity is wind tunnel simulation. In addition, CFD (Computational Fluid Dynamics) software could also used to aerodynamic analysis.
Computational Fluid Dynamics (CFD) Method
Analysis of long-span bridges aerodynamic load and stability are important to know. The initial step is to design and choose the appropriate bridge girder geometry in two dimensions (2D) that will be obtained bridges that meet the criteria of strength and stability The method is mainly used to analyze the flow field around the bridge deck section and the prediction of aerodynamic forces/moments.
Static Test of A Bridge Section Model
The test objective is to measure the aerodynamic forces/moments based on pressure distributions over the model or measured directly using a balance system. The model is two dimensional (a section of the bridge deck), rigidly mounted and its flow is also two dimensional.
Dynamic test of A Bridge Section Model
The test objective is to measure the vibration of the bridge because of wind load and to predict the critical velocity such as vortex induced vibration, flutter, or another aeroealstic phenomenon. The model is only two dimensional and rigid, suspended on a spring and its flow is also two dimensional. Sectional model dynamic testing is important to understand the interaction between wind load and structure of bridge deck with the railing without the pylon and cable.
Dynamic Test of a Full Aeroelastic Model
In order to carry out final verification of the wind resistance safety of the long span bridge design at service and construction period, full aeroelastic model wind tunnel tests should be conducted. Full aeroelastic testing is important to describe some design considerations for long span bridge and some parts of test results such as deflection, flutter instability and gust responses. The test objective is to predict the critical wind speed for flutter, based on a model that replicates the structural dynamics of all bridge components.