Influence of bridge pier shape on flow field and scour geometry

摘要

The objective of the present paper is to investigate experimentally the flow field and the local scour around bridge pier of different shapes on a sediment bed. Experiments were carried out using individual piers of rectangular, oblong, trapezoidal, triangular and lenticular shapes with a common aspect ratio to compare the flow field and scour geometry among the shapes at identical flow conditions. The novelty of this study is to identify the scour structures due to different pier shapes and associated mean flows and turbulence, which are responsible for generation of scour. A 3D Micro-acoustic Doppler velocimeter (ADV) was used to record the instantaneous velocity data for five different discharges. It is observed that upstream scour depth was maximum at the leading edge of the rectangular pier, and minimum at the same location for the lenticular. Here, the horseshoe vortex is the main agent of the scour development for any kind of pier shapes, even for the sharp edges, though there is an immediate bifurcation of flow in the triangular and lenticular-nosed piers, where the scour formation is less than the others. The changes in mean velocities and Reynolds stresses due to the pier shapes are discussed and compared to that of flat-bed surface. Since the shape of the pier alters the flow and turbulence, one can expect large variations in flow behavior, and hence different scour formations are observed. The results can be used to identify scour structures around the piers of different shapes for designing and suggesting appropriate protection measures.