Experimental study on flow field and scour around bell mouth groin

Abstract

A study was undertaken to investigate the flow field and scour around bell mouth groin in a large mobile physical model facility (46m x 11 m) of BVET. For this purposc a total number of 24 test runs were performed. Three difTerent discharge and four different angle and two groin condition (submerged and non-submerged) was considered in the present study. Twelve tests were conducted for non-submerged conditions of which 6 (six) tests were with 14 cm depth and 6 (six) tests were with 18 cm depth. Similarly, another 12 (twelve) tests were conducted with submerged condition of which 6 (six) had a water depth of 22 em and the rest six had a flow depth of 26 cm. Water depth was controlled by adjusting the tailgate on down stream. The actual numbers of test conducted are more than the number mentioned in the programme because few trial tests were necessary to determine the zone of influence. The groin model used in the test runs was constructed as bell mouth type. The top width of bell mouth groin was 12.5 cm and the bottom width was 32.5 cm. The height of groin above the bed surface was 20 cm and below the bed level was 40 cm. The top diameter at head of the groin was 15 cm and at base on the bed level was 35 cm. All tests were conducted for 8 hours duration in clear water condition. In order to avoid the ripple formation, the coarse sand with dso = 0.75 mm was selected as bed material. Scour depth for 90° angle was observed to be the maximum and for 135° it was the minimum. The time to reach maximum scour depth was found to vary with discharge and angle of attack, The study showed that maximum scour depth and deposition pattern changed for the case of different test runs. In general for different groin arrangements the near bed velocity around the vicinity of bell mouth groin was reduced. In most of the cases, shifting away of maximum velocity was observed that caused shifting of maximum discharge away from the bell mouth groin. Analysis of velocity contour also revealed this trend of shifting maximum velocity away from the bell mouth groin. Scour depth varied with velocity variation of flow and an increasing tendency of scour depth has been observed with increasing flow intensity. Maximum scour variation shows an increasing trend with increasing Froude number. Empirical relationships were developed using experimental data to predict maximum scour depth. Proposed equations have been compared with available scour equations.