Flow fields around tandem and staggered piers on a mobile bed

Abstract

An experimental investigation on flow fields within the scour holes upstream and downstream of circular piers positioned in tandem and staggered arrangements is reported and compared with isolated piers on mobile beds with uniform sediment. The instantaneous bed elevations and instantaneous three dimensional (3D) velocities were measured using a 5 MHz Ultrasonic Ranging system and 16 MHz micro down looking acoustic Doppler velocimeter, respectively. The velocity and flow depth were measured at different locations under near equilibrium bed scour conditions. The measured 3D velocities were processed for the computation of flow parameters, such as velocity fields, streamline patterns, vorticity fields, and circulation. Furthermore, turbulence intensities, turbulent kinetic energy, Reynolds shear stresses, and bed shear stresses around the piers for all three pier configurations were computed from the detrended velocity signals to identify significant differences in the flow parameters and turbulence in the tandem and staggered pier arrangements as compared to those for an isolated pier. A recirculation zone was found near the bed in front of the rear pier in the tandem case from the streamline patterns. The vortices in the bi-vortex system were observed to be opposite to each other in the gap between the three piers in the staggered case. A strong secondary vortex also was observed apart from the primary vortex at the foot of the pier (θ = 0°) in all the three configurations. The strength of the horseshoe vortex (combination of primary and secondary vortices) was found to be higher at the front piers of the staggered arrangement as compared to those of the tandem piers, followed by the isolated pier. The bed shear stresses were found to be higher for the staggered piers than for the tandem piers in the direction of flow (θ = 0°). However, a 50% reduction in the bed shear stresses was observed behind the tandem piers at θ = 180°. The study reported in this paper provides the foundation for further investigation of countermeasures against local scour around tandem and staggered bridge piers on a mobile bed with non-uniform sediment.