An experimental study to characterize unsteady vortex and flow structures around a twin-box-girder bridge deck model

In the present study, an experimental study is performed to characterize the unsteady vortex and flow structures around a twin-box-girder bridge deck model. A digital particle image velocimetry (PIV) system was used to conduct detailed flow field measurements under different gap ratio conditions; and a digital pressure measurement system based on the model DSA3217 pressure scanner (Scanivalve Corporation) was simultaneously used to measure the pressure distribution on the test models. Four gap ratios (i.e., the ratio of the gap width to the deck height) are employed in the wind tunnel tests. The two-dimensional flow structures around the twin-box-girder models with different gap ratios are obtained based on a PIV measurement. The swirling strength and turbulent kinetic energy are then analyzed to study the influence of gap ratio to the unsteady vortex and flow structures around the twin-box-girder models. The measurement results clearly indicate that the vortex shedding will move from the trailing edge of the downwind box to the rear edges of the upwind box when the gap ratio increases, as a result of increasing the turbulent kinetic energy in the gap region. Combined with the measured fluctuating pressure coefficient distributions on the twin-box-girder models, it is found that the twin-box-girder bridges with large gap ratios will be prone to excite vortex-induced vibrations under the wind compared with those bridges with small gap ratios.