DRAG CHARACTERISTICS OF CYLINDRICAL PIERS WITH SLOTS AND/OR COLLARS IN SUBCRITICAL FLOW

The flow field around a circular bridge pier is a complex three-dimensional phenomenon. Notwithstanding the considerable effort made in the study of local scour by many researchers, an understanding of the mechanics of local scour around a bridge pier remains far from complete. The drag force exerted on the structure is related to the wake. Larger the wake, higher is the drag force and scour potential. Efforts to reduce the drag by reducing/altering the wake structure are widely reported in literature. Use of slot-collar combination is one among them. In this study, an attempt has been made to establish the drag characteristics of cylindrical pier models, with different slot-collar combinations. This experimental study is conducted using a physical hydraulic model operated under clear-water and rigid bed conditions. All the experiments are conducted in a re-circulating tilting flume. Experiments are run for different combinations of slot and collar. Drag coefficient of the pier is computed using the momentum equation. The drag studies indicate that the drag coefficient in general is found to decrease with the provision of slot/collar. The value of drag coefficient is reduced by about 33 % for a bottom slot/collar combination.