2019, Farnaz Meshkinfar Graduated

Flow patterns of scour around the cone bridge piers

Abstract

Scour is a phenomenon which occurs as a result of bed erosion. Local scouring is special type of the scour which is considered as one of the main reasons of bridges failure. Identifying different features of flow patterns around the pier is a necessary condition to understand the scour mechanism. Although there are many studies which have been conducted to explore the depth of scouring around cylindrical and cone piers, the flow patterns around the cone piers have not been studied adequately.

The present study explores the flow patterns around the cylindrical and cone piers. Experiments have been carried out for two cases of the flat bed (before scouring) and the equilibrium scour. Cone and cylindrical piers with diameters of 60 and 70 mm were used with discharges of 43 and 29 L/s. The side angle of the cone pier was set at 2.5 and 5 degrees. The experiments were done in a channel with the length of 9 m, width of 0.6 m and height of 0.7 m. Sand particles of diameter 0.74 mm were placed at channel bed. Measurement of instantaneous velocity was done in the central axis of the channel, upstream and downstream of the pier with 3D Acoustic Doppler Velocimeter (ADV). The channel bed was fixed by cement and stone powder to facilitate the measurement of the velocity field.

Results of the experiments indicate that the cone pier decreases vertical and longitudinal velocities at the upstream of the pier. As the side angle of the cone pier increases, the extent of scour hole is reduced and the values of longitudinal velocity decrease as well. The upstream longitudinal velocity profile of the cone pier is more uniform than the cylindrical pier. As a result, the downward pressure gradient reduces and consequently, the downflow strength in the vertical plane reduces. The maximum values of the vertical velocity near the pier upstream decrease as the side angle of the cone pier increases. The values of longitudinal and vertical velocity are not significantly different for both cone and cylindrical piers at downstream. The maximum values of vertical velocity slightly decrease as the side angle increases.