الفهرس 
Experimental and numerical investigation of collar effect on local scour and flow structure around single cylindrical pier
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Abstract
The scour phenomenon around bridge piers causes a lot of annually damages all over the world. In this regard, to understand how to control scours around a cylindrical pier, it is necessary to describe the flow structure and vortices system with and without protection. Therefore, in this study the collar effect on local scour and flow structure around single bridge pier investigated experimentally and numerically by using a commercial CFD solver (FLUENT) three dimensional model for circular collar diameter of (2D, 3D, and 4D) and elevations of (zero, -0.5D, -1D, -1.5D) from the bed level, where D is the pier diameter. The experimental results showed that, the best location and diameter of collar found to be at one half pier diameter below original bed level and 4 times of pier diameter respectively. In this arrangement the scour depth is reduced by 52.74%, scouring volume is decreased by 56% and scour hole surface area is decreased by 19% compared with unprotected pier. The effect of the collar located at the bed level on the scour depth reduction is insignificant. Scour hole volume and surface area are a function of collar diameter and collar elevation. The depth and location of maximum scour depth is a function of collar diameter and level. The scour depth reduction increases as collar diameter increases and collar elevation decreases till specific level. from the experimental results, five equations are proposed to predict the optimum collar elevation, maximum equilibrium scour depth, percentage of scour depth reduction and upstream and downstream pier equilibrium scour depth. from the numerical simulation results and analysis, the effect of the collar on the local scour and flow structure may be concluded as: Maximum longitudinal and transversal velocity at pier increases with increasing collar diameter