Experimental investigation of local scour and morphology around bottom vane

Abstract

Bottom vanes are small foil type flow training structures mounted vertically on the riverbed with an angle to the prevailing flow. The effectiveness of vane is dependant on the height, length, shape, water depth and angle of attack of the approach flow. The development and magnitude of local scour for different angle of attacks are still undefined, though local scour is of utmost importance to the design of a vane. The foundation should be able to sustain the maximum scour depth around the vane. In Bangladesh, failure of submerged vanes often occurred due to excessive scouring. Regarding the complexity mentioned above, the present study was performed under mobile bed conditions with the aim of investigating the general pattern of local scour around bottom vanes with variable vane height and angle of attack and an attempt was made to develop an expression using dimensional analysis for predicting local scour around vanes. Dimensional analysis was done using Reyleigh method. The expression developed from the approach was compared with available scour predictors for rectangular bridge pier and performance of the expression was evaluated by computing the discrepancy factor. For this purpose a total number of 20 test runs were performed with four different vane heights (6cm, 9cm, 12cm and 18cm) and five different angle of attack (10°,15°,20°,30° and 40\ In evaluating the general behavior of local scour around bottom vane, sand with a grain size of dso of 0.18mm was used which is similar to the major rivers of Bangladesh. The length of the vane was chosen to be 40cm. For the total experiment the discharge and water level were kept constant to a value 200 liter/sec and 30 em respectively. Under this fixed boundary conditions the equilibrium scour depth for each test run was achieved after a minimum of 16 hI'S running depending on the angle of attack. The flow field and bed level measurements were then carried out to analyze the scour pattern around bottom vane.Scour contour maps were constructed and analyzed to investigate the extent and location of maximum scour hole. Longitudinal and cross sectional profiles of bed topography were also prepared to critically analyze the scour pattern in this regard. Velocity contour maps and vector plotting were also prepared to observe the change in flow velocity due to formation of scour around bottom vanes. On completion of these experiments and analysis, it appeared that the maximum scour depth occurred at the leading edge of vane at pressure side. The scour depth was found to be increasing with increasing vane height and angle of attack. At the pressure side of vane, the presence of horseshoe vortex was found which is responsible for the formation of local scour around the vane. At the suction side near the vane tip the vortex core was found. This core indicates the presence of tip vortex that travels downstream of the vane was responsible for the formation of erosion gully at the downstream. At further downstream the vortex core was found to be decayed. Moreover it was also found that vane has no impact on the upstream velocity.