On the dynamics of coherent structures near straight and T-head spur dikes

This study employs improved delayed detached eddy simulation to investigate coherent flow structures and bed shear stresses around unsubmerged straight and T-head spur dikes. Vortex topology identified using the λ2 criterion reveals a strong horseshoe-like vortex (HSV) for the straight spur dike, whereas the T-head spur dike produces a weakened HSV and a dominant upstream circulation that suppresses downflow and near-bed vorticity production. Probability density function (PDF) and joint PDFs of velocity fluctuations reveal three distinct instantaneous flow states for the HSV: back-flow, zero-flow, and intermediate-flow, for both configurations. Proper orthogonal decomposition (POD) shows that for the straight dike, the first mode captures the mean-flow upwash and downwash induced by the HSV, while higher modes represent oscillations between the three flow states. In contrast, for the T-head dike, all dominant POD modes are associated with HSV oscillations. Analysis of the POD temporal coefficients shows that the zero-flow condition is more prevalent for the straight dike, whereas the back-flow condition dominates for the T-head dike. In the wake, both configurations generate shear-layer vortices that shed, merge, and form rib-like multiscale structures. Reduced vortex arching in the T-head dike keeps shear layer vortices more vertically oriented, suppressing near-bed horizontal vorticity and sediment entrainment. The bed shear stress distribution shows that the straight dike produces an order-of-magnitude greater probability of exceeding a critical sediment-mobility threshold than the T-head dike.