| Pier which is widely used in the structure such as high-pile wharf and offshore platforms is a common structure in harbor engineering and ocean engineering. When a pier stands in the flow, the presence of it will change the flow field around it. The changes will break the original balance state of the sediment, leading to local scour. The local scour around the pile foundation will influence the bearing capacity and stability of the structure with an decrease in the embedded depth. The local scour does great harm to the structure and should be take into consideration in engineering.Due to the limitation of the traditional protective measures for local scour, in recent years several new protective measures have been proposed through changing the flow field factors which cause local scour. Horseshoe vortex plays an important role in the problem of local scour around the foundation of a pier. Studying methods that weaken the horseshoe vortex has important engineering significance. Based on the current scour protection methods, a new method is presented that a controlling ring is attached to the lower part of a pier to weaken the horseshoe vortex so thus to reduce the local scour around the pier in this paper. The controlling ring weaken the horseshoe vortex by changing the flow situation of the pier foundation. The flow changed when the protruding horseshoe vortices wrap around the controlling ring. The effects of the ring are investigated through laboratory experiments. The experiments include two parts:under the initial stage of the scour and under the local scour equilibrium stage. Uniform flow is used in experiments and the ring’s vertical positions are varied to adjust its distance from the sea bed. The main variable parameters are current velocity, the cross-section shape of the control ring, the location that a ring is attached to the lower part of the pier. The parameters under the local scour equilibrium stage that shows better results at the initial stage is focused research.The experimental data, shape of the local scour hole,scour limit depth and the scour depth-time curve at different observation point, are monitored. Finally, this paper presents an analysis under different conditions which is compared based on the analysis of measured data and corresponding charts. The experimental data shows that the controlling ring can weaken the horseshoe vortex, and reduce the seabed material removal up to40%or more. This scour protection measure is simple and easily installed, and is adaptable to uncertain flows direction. This can provide useful reference basis for scour protection around pier. |
