Numerical investigation of flow behaviour and scour around offshore piles and bridge piers with different cross-sectional shapes

The main aim of this study was to gain a deeper knowledge of flow behaviour and scour formation adjacent to bridge piers and offshore wind turbine foundations in shallow water using numerical modelling. Based on the literature, the capability of the previous models in the prediction of scour around the piers was attached to the pier shapes and its orientation angle to the flow. Hence, in this study, a coupled approach between a hydrodynamic and a morphodynamic model was implemented. The model was validated against eight different test cases for flow and sediment transport modelling. Overall, the results confirmed that the developed model has an acceptable capability in predicting local scour independent of the pier cross-section shapes. Furthermore, the model was applied to predict the flow field, bed shear stress and the development of local scour around a square-shaped pier with 45˚ (diamond) and 90˚ (square) orientation angle to the flow, a hexagon-shaped pier with 30˚ and 60˚ orientation angle to the flow and a group of two identical side-by-side circular cylinders with different spacing ratios G⁄D= 1, 2 and 3 under current flow and live-bed condition, where D represents the pile diameter and G is the distance between the piles. In the case of modelling a single pier, it was found that the maximum and minimum normalized quasi-equilibrium local scour depth occur for the square-shaped piers with 90^° and 45^° orientation angles to the flow, respectively. The normalized scour depth in quasi-equilibrium condition for the square pier was found to be almost twice (1.94) of that for the diamond pier. This ratio in the case of the hexagon-shaped pier for 60^° and 30^° orientation angles to the flow decreased to a value of 1.09. From these results, it was realized that an increase in the number of sides of the piers cross-section decrease the impact of the change in orientation angle to the flow in terms of normalized scour depth. It was also inferred that with an increase in the number of sides of the pier's cross-section the resultant normalized scour depth in equilibrium condition tend to get closer to that of the circular pier regardless of pier orientation angle to the flow. In the case of the local scour simulation around a group of two identical side-by-side piles, it was noticed that a deeper scour hole was observed in the area between the piles in all cases. However, due to the stronger turbulent structures and higher contraction of streamlines in the case of G⁄D=1, a deeper hole was observed adjacent to the piles in comparison to that of two other cases. The effect mitigated with an increase in spacing between the piles. The results showed that for G⁄D≥ 3 the pile group effect gradually vanishes, while it is assumed that the pile group effects are completely negligible for G⁄D≥ 5. An equation was fitted for the computed equilibrium scour depths for different spacing ratios which can be used for the prediction of the maximum scour depth adjacent to two side-by-side piles under steady current flow and live-bed condition for a specific range (G⁄D≥ 1).