Abstract
A new type of collar, the hooked-collar, was studied through experiments and numerical methods. Tests were conducted using a hooked collar of a width of 1.25b and a height of 0.25b, where b is the bridge-pier width. The hooked-collar efficiency was evaluated by testing different hooked-collar placements within the bridge-pier, which were compared to the bridge-pier without any collar. A double hooked-collar configuration, one placed at the bed level and the other buried 0.25b, was the most efficient at reducing the scour hole. In other cases, a hooked-collar positioned 0.25b above the bed slightly reduced the scour hole and had similar scour patterns when compared to the pier without the hooked-collar. The flow fields along the vertical symmetrical plane in the experiments are also presented. Laboratory experiments and numerical tests show that maximal downflow is highly reduced along with a corresponding decrease in horseshoe vortex strength for the experiments with the hooked-collar, compared to cases without the hooked-collar. The flow fields reveal that the maximum turbulent kinetic energy decreases with the installation of the hooked-collar.
Highlights
Local scour around a hydraulic structure such as a bridge pier or abutment is caused by the enhanced transport capacity of sediment around a structure and by the formation of vortices.Excessive local scouring compromises the stability of these structures and has been attributed to be the main cause of bridge pier failure [1,2]; reducing the maximum local scour depth at bridge piers has been a topic of safe design
For the piers with a hooked-collar placed at the bed level (Supplementary Information, Figure S3), the scour hole was first observed in a region ±90 of the pier
It is well documented that the horseshoe vortex is one of the key processes by which scour around piers develops [11,23,30]
Summary
Local scour around a hydraulic structure such as a bridge pier or abutment is caused by the enhanced transport capacity of sediment around a structure and by the formation of vortices.Excessive local scouring compromises the stability of these structures and has been attributed to be the main cause of bridge pier failure [1,2]; reducing the maximum local scour depth at bridge piers has been a topic of safe design. The flow field at a pier is coupled with a complex horseshoe vortex system initiated from the downflow upstream of the pier, which is thought to be the major impelling process behind the growth of the scour depth, and wake vortices downstream of the pier [3,4]. These vortices create scour holes around the pier when the shear stress on the stream bed becomes greater than the critical shear stress. The first consists of increasing stream bed resistance to withstand erosion
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