Cross-sectional blockage effect on the drag force of a scaled round-ended pier model under directional flow

Abstract

Flume testing of a scaled model under steady flow is an essential reference for accessing the flooding-induced drag force on a prototype pier, but is highly susceptible to the cross-sectional blockage effect. Round-ended bridge piers have a non-axisymmetric cross-section, of which the drag force varies significantly with inflow direction. A flume testing program was carried out to illustrate the cross-sectional blockage effect under different inflow directions. CFD numerical simulations were then conducted to understand the blockage effect on the drag force, flow field and hydrodynamic pressure of the round-ended pier with different blockage ratios and inflow directions. The major findings include: 1) the blockage effect affects the drag force of the scaled model and changes with inflow direction; and 2) the limit of occurrence of the cross-sectional blockage effect is defined by the critical blockage ratio and given as a function of inflow direction. An empirical approach was finally developed to correct the cross-sectional blockage effect on the drag force of the round-ended pier model under directional flow. The influences of the blockage ratio and inflow direction were included in the empirical calculation of the drag force on the prototype from the scaled model test.