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CFD simulation based evaluation of the effects of bridge pier streamlining on alleviating local scour

Local scour is known as one of the most critical causes leading to bridge failure. Scour countermeasures suggested by HEC-23 (2009) are either ‘passively’ armoring the riverbed (e.g. riprap and gabion), or ‘actively’ reducing the turbulence intensity in the vicinity of hydraulic structure (e.g. guide bank and submerged vane). Streamlining of the pier is proposed in this study as a potentially feasible alternative to reduce the scour potential by ‘actively’ decreasing the vortex intensity. Computational Fluid Dynamics simulation were applied: 1) to investigate the feasibility of pier streamlining as a scour countermeasure; 2) to identify the optimal pier cross-section by a parametric study; and 3) to study the effect of pier’s vertical profile on scour potential and identify the relatively optimal pier geometry. As a compromise between the computation cost and efficiency, the Reynolds-Averaged Navier-Stokes (RANS) method is adopted since our research involves a large amount of simulation cases. In the feasibility study, results from five test cases with different streamlining features are systematically analyzed and compared, both qualitatively and quantitatively. It is found that streamlined pier nose and sidewalls help reduce the downward flow in front of the piers, the vortex intensity around the pier and the maximum bed shear stress. In the following cross-section optimization study, the cross section is parameterized using Bézier curves so that a set of simulation cases can be performed systematically. The cross-section that results in the smallest value of the maximum bed shear stress is selected as the optimal cross-section based on the excess shear stress theory. The optimal cross-section is employed in the final three-dimensional (3D) study of the effect of pier’s vertical profile on scour extent. The vertical curvatures are parameterized using Bézier curves and a total of 51 3D simulation cases with different curvatures for the pier’s vertical profile are constructed by varying the Bézier parameters. RANS simulations are conducted to model the flow and bed shear stress patterns for all test cases, which maintain the same block ratio to be comparable. It is found that concaved (or sloped) nose and concaved sidewall both help reduce the maximum shear stress around piers and increasing the concave curvature of the sidewall helps enhance such reduction effect. Increasing the concave curvature of the nose, however, tends to diminish such reduction effect.

Author(s):

Junliang Tao    
University of Akron
United States

Junhong Li    
University of Akron
United States