Clarification of spatial and temporal coherent structure of pressure and velocity field around bluff body for evaluation of gust response of structures

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This research aims to clarify the spatial and temporal coherent structure of surface pressure filed on a bluff body and its relationship to the surrounding velocity fields with flow separation and reattachment. These information is to be contributed to develop the advanced aerodynamic admittance and the joint mode acceptance which are important for the evaluation of gust response of structures due to turbulent natural wind.The surface pressure on 2-D rectangular cylinders with side ratio B/D=5 and 1 (B : width of cross section, D : depth of cross section) were measured at various position on side surface, simultaneously in turbulent and in smooth flow. Velocity field surrounding the bluff body, which consists of separating shear layer, flow reattachment on body side surface, were also measured at two different points by hot-wire anemometers.In the first year, the similarity between the aerodynamic admittance (equivalent Sears function) and the frequency transfer function obtained by th … More e aerodynamic derivatives (equivalent Wagner function) was confirmed by measuring surface pressure of a B/D=5 rectangular prosm. The influence of body motion on the spanwise coherence of surface pressure was also investigated. Consequently, the coherence does not get any significant influence of body motion. And the aerodynamic gust force was confirmed to be treated independently from the self-excited force. In the second year, the spanwise extension of surface pressure fluctuation was investigated under wide range of reduced frequency of approaching vertical sinusoidal gust. The extending properties were found to be significantly depending on the reduced frequency. When the reduced frequency is nearly equal to the characteristic frequency of the shear layer instability, where the shear layer instability is enhanced most sensitively by the external stimulation, the extension towards spanwise direction was not clearly captured. On the other hand, when the reduced frequency is relatively lower, the pressure fluctuation seemed to be transported more smoothly to the spanwise direction. Therefore, it was concluded that the increase of spanwise coherence of a gust force on a 2-D cross section comparing to the coherence of approaching flow velocity is mainly realized in lower reduced frequency region. In the last year, surface pressure field and the velocity field near a rectangular cross section were decomposed to the POD modes in smooth and in turbulent flow. As a result, the contribution rate of the non-uniform modes (the POD mode shape indicates non-uniform along spanwise direction) of the surface pressure was increased more in turbulent flow because of the non-uniform velocity distribution. Whereas, the non-uniform POD modes of the velocity field showed more dominant in turbulent flow. Therefore, the contribution rate of the uniform modes indicates relatively larger value for the pressure field than for the velocity field. And the formation of almost uniform separating shear layer from 2-D body controls the increase of the spanwise coherence of the 2-D gust forces. Less