Particle Image Velocimetry for Flow Visualization around Generic Bridge Shapes

Abstract eng:
This paper examines the flow field around generic bridge shape models using the particle image velocimetry technique (PIV). The experiments were conducted in a small demonstration wind tunnel of “continuous circuit” design. Vanes in the corners of the tunnel turn the airflow and screens and honeycombs smoothen the flow. The tunnel has a quadratic working section (0.25m x 0.25m). Sets of different models “generic bridge shapes” are studied for flow visualization. Models are fabricated out of Plexi-glass to avoid shadows and reflections in the light sheet. The models are simplified versions of existing bridge decks excluding railing and other details. The scale of the section models is designed to be 1 : 225 resulting in an average width of 130 mm and a height of 18.4 mm. All models were mounted vertically in the tunnel. The angle of attack was varied between -15° and +15° using 5° increments. The PIV set-up used for these experiments was self-developed. The correlation algorithms were programmed in LabVIEW and IMAQ Vision. Nonlinear fit functions are used for calibration and image de-warping. A high-speed, high-resolution interline-transfer CCD digital camera that features a built in electronic shutter with exposure times as short as 127 microseconds is used for recording. A dual laser-head system designed to provide a highly stable green light source for PIV applications generated the light sheet. Detection and correction of false vectors and calculation of derived flow magnitudes are two features employed by the PIV method that enhance the quality of the delivered PIV vector maps. After the velocity flow fields are validated, standard differentiation schemes can be used to perform numerical vector field operations to compute streamlines and vorticity, even in the very complex flow patterns that occur around bridge decks. The vorticity field estimate is used to determine vortex-shedding frequencies. Drag forces are computed using averaged velocity flow fields.

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