Vortex induced at High Rise Building

Wind load on high rise building are very complex, because of many aspects can effects the phenomenon. For instance: shape of the building, terrain around the building, vortex shedding, pressure fluctuation, etc. Wind flow around the building causes the building to oscillate, along wind, cross wind direction, and torsion. Then, vibration on high rise building might reveals the occupant discomfort and fatigue in structural members and their linkage. Isyumov suggested acceptance criteria for the    limitation of wind-induced accelerations to ensure occupant comfort for return periods of both 1 and 10 years. Wind tunnel testing is now common practice for design of high rise buildings. The objective of wind tunnel testing is to determine the wind-induced loads and building top floor accelerations with which occupant comfort can be assessed against international building vibration acceptability and ocupant comfort criteria. Wind tunnel testing can produce reliable information for  designers and minimized the initial capital costs, and more significantly avoiding expensive maintenance costs associated with malfunctions due to structural failure. This paper examines a wind tunnel testing for analyzing vortex induced vibration and lock-in phenomenon on squared shape high rise building which can cause discomfort for the occupant.

Wind load on high rise building classified on two types: along wind load and cross wind load. The phenomenon in the along wind load is buffeting. It is cause by fluctuating wind around the building. That is, when turbulence intensity is high or turbulence flow dominate the wind load. In high turbulence intensity, the structural response is relatively high magnitude but in random fluctuation. In low turbulence intensity, the structural response is relatively low magnitude. Only at certain wind speed or natural frequency of structure the oscillation magnitude is high.

Vortex induced vibration is structural vibration induce phenomenon cause by vortex shedding around the building which its frequency is coincide with natural frequency of the structure. Another phenomenon in cross wind direction is lock-in, lock-in occur, not only in one critical wind velocity but also in several wind velocities. The occurrences show specific properties of a structure.

Wind flow over the surface of building will have pressure changes. Negative pressure which go to the upstream region will disturb boundary layer at the surface body. Disturbed flow generates turbulence flow around the surface, which have fluctuating velocity. Turbulence flow have many type of eddies depends on eddies size. Energy cascade theory by Richardson said that turbulence is consist of many size of eddies, where occur energy transfer from large size eddies to small size eddies and to the other smallest eddies until the smallest eddies dissipate energy to thermal energy. Some point in turbulence flow is call separation point where eddies or vortex start release from surface, that phenomenon commonly call vortex shedding. Vortex
shedding have some frequency depend on wind velocity, wind velocity direction, and the shape of building. When the frequency of vortex coincides with natural frequency of the building then the building will vibrate with amplitude higher than before. When the vortices are shed, that is break away from the surface of the building, an impulse is applied in the transverse direction. The vortex shedding frequency around the building follows the Strouhal relationship, and the building vibrates with small amplitude. When the flow velocity is sufficiently high, the vortex shedding frequency comes close to the first natural frequency, ω1, and locks on to this. Large amplitude vibrations in mode 1 will then quickly develop.

 

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