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Experimental and Numerical Investigation of the Flow Field around Complex Bridge Piers with and without the Scour Hole

Beheshti, Ali Asghar | 2010

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 40493 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Ataie Ashtiani, Behzad
  7. Abstract:
  8. Local scour around bridge piers is one of the main causes of bridge failures Application of existing equations to complex flow patterns is problematic and often leads to questionable results in the field applications. A lack of understanding of the complex flow field and erosion mechanisms seems to be at least partly responsible for this state. Despite the rapid expansion of the literature on the flow around cylindrical piers, there are still challenges for flow field comprehension and estimating design scour depths around complex bridge piers. Due to geotechnical and economical reasons, bridge designs often leads to a complex foundation. Laboratory studies of local scour around complex foundations are time consuming and expensive as a large number of variables must be considered. A more satisfactory approach for further applications in field situations is to simulate the flow field and scouring processes using a three-dimensional (3D) numerical model. In this study, three-dimensional turbulent flow field around a complex bridge pier placed on a rough fixed bed and in the equilibrium scour hole is experimentally investigated. The complex pier foundation consists of a column, a pile cap, and a 2×4 pile group. All of the elements are exposed to the approaching flow. An acoustic-Doppler velocimeter was used to measure instantaneously the three components of the velocities at different horizontal and vertical planes. Profiles and contours of timeaveraged velocity components, turbulent intensity components, turbulent kinetic energy, and Reynolds stresses, as well as velocity vectors are presented and discussed at different vertical and horizontal planes. The approaching boundary layer at the upstream of the pile cap separated in two vertical directions and induced an upward flow toward the column and a contracted downward flow below the pile cap and toward the piles. The contracted upward flow on the pile cap interacts with downflow in the front of the column and deflects toward the side of the pier, which in return produces a strong downflow along the side of the pile cap. The flow at the rear of the pile cap is very complex. The strong downward flow at the downstream and near the top of the pile cap in interaction with the reverse flow behind the column and upward flow near the bed produce two vortices close to the upper and lower corners of the pile cap with opposite direction of rotation. On the other hand, the back-flow from the wake of the pile cap is forced into the top region resulting in a secondary recirculation at the wake of the column. From these results it is shown that the main feature of the flow responsible for the entrainment of bed sediments are a contracted (pressurized) flow below the pile cap and toward the piles. Also deflected flow around the pile cap and a strong down-flow along the sides of the pile cap at the upstream region have been considered as other causes of sediment entrainment. Moreover, the vortex flow behind the pile cap and an amplification of turbulence intensity along the sides of the pile cap at the downstream region can be other reason for sediment entrainment. The mean velocity contours and velocity profiles obtained from above measurements are used to evaluate a 3D numerical model (SSIIM) and this model is applied to study the details of flow past complex bridge foundations. Numerical simulations are used to enhance the knowledge of the flow and to reveal near field flow structures around complex piers in details, as the measurement of these details were impracticable in laboratory experiments using existing devices. Some numerical simulations were conducted on different shapes of complex piers. The results show the complexity of flow around different elements of complex piers including the flow between piles, around the pile cap, and around the column. The interaction of flow around different elements together with different patterns of flow and bed shear stresses around different shapes of complex piers including the different pile cap positions are investigated and discussed. Finally, the experimental data from different researchers on local scour, including the data obtained from experiments conducted in the hydraulic laboratory of Civil Engineering Department of Sharif University of Technology, are used to evaluate existing methods and obtaining new prediction equations for local scour around complex piers.
  9. Keywords:
  10. Scour Hole ; Bridge Pier ; Turbulent Flow ; Complex Pier ; Numerical Modeling

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