Experimental and numerical investigation of pressure-flow clear-water scour in submerged regime around bridge piers Elham Nasiri-Dehsorkhi elham.nasiri@cv.iut.ac.ir October, 02, 2020 Department of Civil Engineering Isfahan University of Technology, Isfahan 84156-83111, Iran Degree: Phd Language: Farsi Supervisors: M.R. Chamani (mchamani@cc.iut.ac.ir) and A.R. Kabiri-Samani (akabiri@cc.iut.ac.ir) Safety and economic issues are key factors in designing bridges. Predicting the extent and depth of scour is critical to bridge foundation design to ensure structural stability. However, bridges on waterways are vulnerable to damage due to extreme hydrologic events that cause severe floods. In pressure flow, the vertical contraction leads to partial deflection of the upstream flow into the bed, increasing the flow velocity and a change in flow distribution at the bridge section. The curvature of the flow under a submerged bridge deck deviates the pressure distribution from the hydrostatic profile. The magnitude of scour at bridge piers amplifies in comparison with the free surface flow. The complexity of pressure flow around a cylindrical pier under the bridge deck increases due to vortices' formation and the three-dimensional flow separation. The horseshoe vortex around the pier compresses vertically, causing much more turbulence and shear stress over the bed. The present dissertation investigated vertical contraction in bridge decks known as “pressure flow”. The scouring subjected to the pressure flow around a cylindrical pier under clear water conditions were studied based on model experimentation and numerical simulations. The experiments comprised of three cases of the pressure flow scour without the pier (PF), pier scour in free surface flow (FSFP), and pressure flow (PFP) conditions. The model bridge deck was a rectangular block. The effects of different hydraulic flow aspects and the bridge's geometric features, including the approach flow depth and the pier diameter, on the maximum depth of scour were studied. The three-dimensional velocity measurements were performed at different positions along the centerline and a transversal central cross-section on a fixed flat bed, after the equilibrium of scour using an Acoustic Doppler Velocimeter (ADV). The numerical simulations of the scour pattern, and flow field have been done using SSIIM 1 numerical code and OpenFOAM software. The longitudinal scour profiles in PF were described by two similarity equations. The results indicated that the maximum scour depth of the pier in PFP increased by 1.7-2.4 times compared to FSFP. With a higher submergence ratio, the approach flow depth and the pier diameter are critical parameters in increasing the scour depth. Increasing the submergence ratio pushes the down-flow toward the pier base. Besides, as the bridge submergence increases, the scour hole becomes wider both up-and downstream of the pier. An extensive set of measured data was included to derive a relationship predicting the maximum scour depth for PF and PFP conditions. Furthermore, the temporal development of the pressure scour depth with and/or without the pier was studied. Results showed that the pier scour rate during the initial phase is very large especially for the high submergence ratios. Results show that the longitudinal and vertical velocity components of the pressure-flow increase in the vicinity of the pier compared to those of the free-surface flow. Large values of the turbulence intensities, turbulent kinetic energy, and Reynolds shear stress are observed at the bridge's deck upstream edge within the shear layer and downstream of the pier under the deck due to the generation of wake vortices. Quadrant analysis was used to recognize the susceptible regions for sediment entrainment and deposition. It is found that sweep, ejection, and inward interaction are the most effective sediment entrainment mechanisms around a single circular bridge pier under a pressure flow. A comparison between the flow fields for flat-bed and scoured-bed cases indicated the maximum longitudinal velocity in the scoured-bed case becomes smaller than that in the flat-bed case. The extension of the wake region in the scoured-bed case is smaller than in the flat-bed case. KEY WORDS Bridges, Cylindrical pier, Pressure-flow, Turbulence, Reynolds stress, Scour