Identification of transfer functions for wind-induced pressures on prismatic buildings
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Buildings in the atmospheric boundary layer experience highly fluctuating wind pressures which result from turbulence in the undisturbed upstream flow and building generated turbulence through the processes of buffeting and interaction. An aerodynamic admittance is usually introduced to relate the spectral characteristics of the pressures to the spectral properties of the impinging turbulence. A modified quasi-steady approach, currently in use for predicting fluctuating wind pressures on low buildings, accounts for the building-generated turbulence by means of a smgle admittance function(f). This approach implies that the longitudinal and lateral wind velocities are modified in the same manner by the presence of the building in the flow, which may not be true physically. A general linear-quadratic model, usmg the concept of multle transfer functions, is proposed to relate the spectra of the fluctuating longitudin (M) and lateral (v) components of the upstream wind velocity to that of pressures on building surfaces. Four transfer functions are defined: one each associated with the linear and second-order terms of the M and v velocity components. The proposed formulation enables a weighted decomposition of the measured output pressure spectrum into components representing the contributions from the linear and quadratic terms of the input velocity components and the remaining uncorrelated residual and/or noise effects. This approach provides physical insight into the mechanism which produces pressures on building surfaces. The four transfer functions are optimally identified from cross-spectra/cross-bispectra of the velocity and surface pressure fluctuations by minimizing the residual/noise spectrum. The proposed model is applied to gain an understanding of the form and behavior of the transfer functions associated with the incident wind in producing pressures on surfaces of buildings. The study is primarily based on full-scale wind velocity and pressure data collected on a 13.7mx9.1 m x 4m flat-roof metal test building, located on a flat open terrain, at the Texas Tech Wind Engineering Research Field Laboratory. Results are presented for typical flow regions on the building: windward wall, roof separation and reattachment zones, leeward wall, side wall, roof corner and area-averaged cases.