STUDY ON PERFORMANCE OF SUBMERGED VANES WITH COLLAR

Abstract

Increasing capital costs, emerging environmental concerns and rising maintenance expenses of the conventional river training works around the world have led to the development of submerged vanes in practice. Submerged vanes are being favoured for control of sediment movement, scour and deposition. The submerged vane functions through generation of secondary circulation for bringing about desired sediment redistribution within the channel cross-section. The strength of secondary circulation is profoundly influenced by the magnitude of the incident angle of vane to the approach flow direction. Most of the studies on submerged vanes are restricted to the incident angle in the range of 150 to 200. In recent years, researchers have been experimenting with higher incident angles in (heir search for an optimal angle of a submerged vane for to extract optimal level of secondary circulation. However, the major impediment in the use of an optimal angle from the standpoint of secondary circulation hinges on occurrence of destabilizing local scour phenomenon especially around the leading edge of the vane. Research forays into the problem of scour reduction around a submerged vane for ensuring its stability have not yet been reported by the investigators so far. The main thrust of the present study is centered around the use of collar as an effective device to restrict the incidence of destabilizing scour phenomenon around a submerged vane. In this study, elaborate experimentations have been conducted to investigate the. problem in depth with different collar and submerged vane configurations to determine the optimal angle of attack. Most of the earlier investigators experimented with Froudc numbers falling between 0 to 0.25. Therefore, two Froude numbers were selected for the study, one was 0.25 and other was kept in the middle range of it as 0.13. The local scour around the different types of submerged vanes such as rectangular, trapezoidal, double curve type I, double curve type II, JI and J2 type were studied without collar for Froude numbers 0.13 and 0.25. The maximum scour depth was observed to have taken place in the case of rectangular vanes. Therefore, scour influencing variables were identified for rectangular vanes and also the model was developed for scour depth considering the rectangular vane. It was observed from the model that flow Froude number is more important than the densimetric Froude number. Cost analysis was carried out for the conventional method of riprap with filter and the collar for retardation of scour hole formation at the leading edge of rectangular vane for two Froude numbers of 0.13 and 0.25. The collar was found to be better option economically. The most effective performance of the collar device was found for rectangular and trapezoidal vanes at the two Froude numbers of 0.13 and 0.25 with two representative median size sediments considered in the study namely 0.225 mm and 0.405 mm. Field study in the Solani river near Roorkee was carried out and it was found that the collar is highly effective in reduction the local scour around the vane, without any material impairment of its hydraulic function. Optimal angle of attack for rectangular vane with collar considering three different degrees of submergence was investigated and its value lies close to 400 as reported by other researchers. It could be seen that the optimal angle of attack remains insensitive to degree of submergence and also remains unaffected by collar. Optimal angle of attack was also determined in case of curved vanes such as double curve type I, double curve type II, JI and J2 type and its value in these cases was found to lie close to 450. Tapering of leading edge of rectangular vane with collar (called trapezoidal vane here) is found to noticeably influence the strength of vane induced secondary circulation iii at the same angle of attack and similar hydraulic condition. Also the aspect ratio was seen to wield considerable influence on the rectangular vane induced secondary circulation at the optimal angle of attack, 400 and for the similar hydraulic condition. A dimensionless moment of momentum (MOM) concept has been introduced in this work to study the vane induced secondary circulation. Streamwise decay of vane induced secondary circulation for rectangular and trapezoidal vane is not found to exhibit definite pattern. Dike formation phenomenon triggered by vane close to bank-line at relatively higher Froudc number of flow such as 0.25 does not seen to follow a straight alignment but possesses a sort of curvilinear shape bended towards the suction side. Dike formation process at smaller Froude number of flow such as 0.13 is observed to be not appreciable.