An Experimental Study For The Scour Around A Vertical Cylinder In Tidal Flow

It is very common to meet the local flow around vertical structures, such as the piers in the rivers, the piles of the buildings in the sea, the vertical supporting pillar of the plateforms. One great engineering problem is the local scour, which around the pile's foundation is dangerous and will reduce the base structure stability bringing destruction to the engineering structure. When local flow around pillar begins, scour will develop due to the balance breaking. During the study of the local scour around piers, if the hydrodynamic characters are understood, there would be a possibility of the work on explaining the flow phenomena and finding the way to analyze scour quantitatively. Beside the physical characters, such as average particle size, viscosity of the sand, the flow field condition will affect the scour in a directly way. The research for the scour in steady flow field has been studied since decades ago, in which a lot of theories and empiric formulas were summarized. The research for the scour in wave field has also been developed for many years. However, there are still numbers of unknown questions about the scour in tidal flow field. Based on the above background, the experimental work of scour around a vertical cylinder in tidal flow field was carried out to study the differences between scour in tidal flow and scour in steady flow.Recently, there has been a great progress in technology of the underwater topographic measurement. The methods of measurement can be divided into two parts: contact type and non-contact type. The contact type was commonly applied in early years. Like steel rule and point gauge, the method has many flaws such as low accuracy, topography destroy, and limited use in real time measurement of flow field. Non-contact type, appearing in recent years, with representatives like sonar survey and laser distance sensor, is excellent in precise measurement, preserving the topography, and testing during experiment under clear-water, peace-surface circumstances contrast with the contact type. The laser distance measurement system was applied to obtain topographic data for local scour studies.The work would be planed two steps: the steady flow condition and the tidal flow condition. The velocity and the water-level would be the key factors, which also are the corresponding parameters in both two cases. As outcome being obtained, the topographic pictures will be printed, and some differences of two Cases in scour field pattern, max-depth of field, the field range and equilibrium duration will be taken into consideration. Finally, there would be an answer to the question that is it possible to use the theories and empiric equations of steady-flow scour to predict the scour in tidal flow. It turns out that under the same max-velocity, the field patterns in tidal flow differ remarkably from the ones in steady flow; however, the equilibrium duration and the equilibrium scour depth are similar in both cases.