Effect of bridge abutment length on turbulence structure and flow through the opening

The method of large eddy simulation (LES) was employed to investigate the flow and turbulence structure around bridge abutments of different lengths placed in a compound, asymmetric channel. The simulations were faithful representations of large-scale physical model experiments that were conducted in the hydraulics laboratory at the Georgia Institute of Technology. The experiments are considered idealized hydraulic models of the Towaliga River bridge at Macon, Georgia, consisting of flat horizontal floodplains on both sides of a parabolic main channel, two spill-through abutments with varying lengths [long-set back (LSB) and short-set back (SSB)], and a bridge spanning across the abutments. In the LES, a free flow scenario was simulated where the water surface was not perturbed by the bridge at any point. The Reynolds numbers, based on the bulk velocity and hydraulic radius, were 76,300 and 96,500 for LSB and SSB abutments, respectively. Validation of the simulation results using data from the
complementary experiment is presented and agreement is found to be reasonably good. A thorough comparison of various flow variables between LSB and SSB scenarios to highlight the effect of flow contraction was carried out in terms of flow separation and instantaneous secondary flow, streamwise velocity, streamlines, stream traces, and turbulence structures. Further flow instability and vortex shedding generated in the shear layer downstream of the abutments were quantified by analyzing time series of the instantaneous velocity in the form of the probability density function, quadrant analysis, and power density spectra.