Investigation of turbulence flow and sediment entrainment around a bridge pier

Abstract

Local scour around the obstacles such as bridge pier is the result of complex interaction between turbulent flow and sediment particles at the mobile bed. The entrainment of sediment particles from the bed is stochastic in nature and it is strongly influenced by instantaneous shear stresses of the bursting process. The focus of this study is the investigation of turbulent flow and analysis of contribution of each bursting events to the Reynolds shear stress to find the dominant bursting events and also the flow structure around the circular bridge pier. The velocities around the bridge pier were measured in three dimensions using an Acoustic Doppler Velocimeter. These velocities were measured at different positions around the bridge pier and at different depths. Quadrant analysis is used to recognize the susceptible regions for sediment entrainment and deposition. According to quadrant analysis sedimentation is the dominant effect in the scour hole whereas at higher levels the erosion force becomes more important. In downstream of the pier, sediment particles are put in suspension and transported downstream due to sweep. Our results indicate that the secondary currents are more dominant in downstream of the pier, as compared to the upstream of the pier. Consequently the maximum velocity in downstream of the pier takes place in a location closer to the bed. In upstream of the pier the streamwise component of velocity becomes positive for which the universal log-law turns out to be valid.

Spacetime dynamics of bed forms due to turbulence around submerged bridge piers

“…For s vw component, due to flow reversal and the dominance of the secondary current, negative shear stress was observed at the just downstream (Izadinia et al 2013) and around right side of cylinder base for circular and elliptical cases and adjacent to the cylinder downstream of square and triangular cylinders, indicating outward flux of momentum. Whereas, positive s vw was observed at the upstream (Izadinia et al 2013) and around the left side of the cylinder base, indicating inward flux of momentum. Again, at the upstream maximum scour region, greater shear stress values were witnessed for this s vw component.…”

“…Again, at the upstream maximum scour region, greater shear stress values were witnessed for this s vw component. However, for the s uv component, negative shear stress was observed upstream of all cylinders due to the return flow (Izadinia et al 2013) and the maximum magnitude of s uv among the four cases was detected for the square cylinder case. Again, just downstream (Tr-13) of the square cylinder, significant negative shear stress was detected which possessed maximum s uv magnitude, whereas for other three cylinders, very small positive s uv were observed at this location.…”

“…In the downstream scoured location (Tr-13), where the scour-depth for the elliptical cylinder was lowest than the other cases and bed was negatively sloped, near the bed, ejections (Q2) were the mostly occurring events (Izadinia et al 2013). The prevailing ejection (Q2) events cause the suspension of sediment particles in the flow (Termini and Sammartano 2012).…”

“…Hr u r v is used as a filter which cancels out all those data whose strength is less than H times r u r v . The space fraction or occurrence probability of each of the quadrant events is computed (Lu and Willmarth 1973;Wu and Christensen 2006;Izadinia et al 2013), which gives the fraction of total observations (n) contributing to the instantaneous Reynolds shear stress by each of the quadrant events for a given H, using…”

“…In spite of extensive experimental works on local scour around piers, only very few investigations have been made to understand the turbulent characteristics of flow and the associated coherent structures within the scour hole (Istiarto 2001;Hopfinger et al 2004;Bey et al 2007;Kirkil et al 2008;Termini and Sammartano 2012;Link et al 2012;Izadinia et al 2013;Maity and Mazumder 2014;Keshavarzi et al 2014). Termini (2005) and Termini and Sammartano (2008) investigated the influence of horizontal turbulence on the bed form creation.…”