Characteristics of vortices in equilibrium scour holes at interfering piers

Abstract

Flow behaviour around multiple piers is a classical problem in recent days. Prediction of magnitudes of scour near multiple piers is an important issue to the hydraulic engineers. Nowadays group of similar piers or complex piers are becoming much more significant in performing bridge drawing and design for several reasons related to achieve geophysical and economic advantages. The critical scour mechanism for pier groups is highly complex in nature and is obviously very difficult to estimate local scour depth, where the direct use of the outcomes acquired from single pier may be problematical. An estimate of maximum possible scour around any bridge pier group is essential for its safe design. Due to the mutual interference between piers at close proximity, the inline-eccentric-inline pier arrangements present a mutual interference on scouring and on hydraulic behaviour. Hence, the interference between the wake vortex of the inline-front pier and horseshoe vortex of the eccentric-middle pier in addition to the horseshoe vortex of the inline-rear pier and wake vortex of the eccentric-middle pier play an important role in the creation and formation of the greater scour depth around multiple identical piers. The combined strength of both vortices enhance scour hole at the eccentric-middle pier thereby increasing the rate of sediment transport, moving the sediment away from the upstream axis of symmetry as the flow approaches towards downward direction. Over the years, a large number of investigation on single pier, of various shapes, arrangement have been studied by many researchers using laboratory flume based experiments and incorporating the use of non-dimensional equations inferencing in some semi-empirical equations for simulating scour depths. A long-standing concern is the tendency of some of these equations to over predict the maximum scour depth for field or even for laboratory conditions. A lack of understanding of the flow structure around the multiple bridge piers and their interaction with the bed sediment seems to be at least partly responsible for such occurrence. Many researchers highlighted information on velocity and subsequent turbulent fields around single piers. There are a large number of studies around pier groups and complex piers that focus only on the prediction of the maximum scour depth around the pier. For a pier in a multi-pier arrangement, combined effects are notable, in addition with those parameters which influence scouring around single-pier arrangement. Most of the multi-pier experiments have been restricted to two pier arrangements, positioned either inline-eccentric, inline-inline or side by side that focused on the prediction of flow-field, turbulent field and the scour geometry and the effect of pier spacing on flow characteristics and on scour depth. In spite of the large number of investigations focusing on single pier and two piers experiments, a comprehensive understanding about atleast three pier inline-eccentric-inline arrangement, the effects of flow characteristics on bed sediments and vice versa are necessary. The wake vortex structures change behind the piers depending on the intermediate spacing between three piers. Previously published literatures reveal that, to date, there is no experimental progress on scour and flow fields around three circular and square piers arrangement. Hence, no comparison has been made comparing experimental results of turbulent horseshoe vortex flow within equilibrium scour holes at three circular piers and square piers. Hence, in the present research an attempt has been made to carry out experiments of clear water scour around three circular and square pier groups with inline-eccentric-inline arrangement and to delineate the scour geometry, the flow fields and turbulence fields (hydrodynamic flow formations) around the piers. Also it is essential to compare the above mentioned new findings between the experimental results obtained using circular and square piers. Throughout the entire research, same bed material, same experimental setup and same experimental conditions like approaching flow depth, depth-averaged velocity, pier width have been used. The first part focuses on the scour geometry, flow fields, turbulence fields and vortex (both horseshoe and wake) strengths around the three circular piers positioning in inline-eccentric-inline arrangement with varying longitudinal spacing between them with a constant eccentricity. Longitudinal spacing between inline front and eccentric middle piers were kept constant at 0.5 times the maximum equilibrium length of sediment transport of a single pier of identical shape and constant eccentricity. The detailed three dimensional velocities were captured instantaneously by using a velocimeter at five different vertical azimuth-planes (0°, ±45°, ±90°) positioned around the three piers and also at two horizontal-planes around the three piers at 4% (closest to bed) and 50% (mid of bed and water surface) of approaching flow-depth above bed level. The contour profiles and distribution of velocity-components, turbulence kinetic energy and intensities are computed and analysed at both vertical and horizontal planes around the piers. The velocity-vector plot and time-mean absolute-velocity acquired from velocity-profile shows detailed hydrodynamic flow characteristics. The vorticity and circulation generated at the upstream zone near the piers are determined by applying the techniques of computational hydro dynamics. In the last part of the research, a comparison has been made comparing the present findings of equilibrium scour geometry and hydrodynamic flow characteristics around circular and around square shaped piers’ experiments. The increased rate of sediment transport and strength of vortex are always found more for the square piers than circular piers and of larger scour depth just near the square eccentric pier and more transportation of sand along the left (eccentric pier) side wall. Therefore, the square pier causes to produce more scour than the circular pier and as well as shifts more sand towards the side walls of the flume.