Experimental Study On Hydraulic Characteristics Of Oblique V-shaped Broad-crested Weir

With the development of hydraulic engineering,in the process of renovation or new construction of hydraulic structures,due to the constraints of terrain,topographic conditions,riverbed evolution and discharge requirements,some new types of weirs have emerged in terms of weir layout.Oblique weir is one of them,and most of the diversion weirs are oblique weirs.Broad-crested weir is widely used in river course,but at present,the study of oblique weir is mostly sharp-crested weir,rarely broad-crested weir,so it is necessary to study the hydraulic characteristics of oblique broad-crested weir.In order to improve the self-regulating performance of broad-crested weir,V-shaped weir surface is proposed to adjust the distribution of flow through the weir.By means of physical model test,the flow characteristics and overflow capacity of normal broad-crested weir,normal V-shaped broad-crested weir,oblique broad-crested weir and oblique V-shaped broad-crested weir are compared and analyzed,which enriches the flow movement theory of new weir overflow.At the same time,the discharge coefficient formula of the new weir type is fitted,which provides a certain reference value for related projects.In this study,the physical model test method was used,the flow patterns of weirs were observed,PIV flow measurement technology,velocity meter and water level measurement were used to measure the flow velocity and water level in front,on and behind the weir.The distribution of water level and flow field and flow capacity of each weir type are compared and analyzed.The test is divided into four groups according to weir type,each group is divided into20 working conditions according to weir height,discharge and downstream water depth,a total of 80.The results are as follows:(1)Under the condition of free outflow,when the discharge is the same,the hydraulic jump submergence coefficients of the downstream are oblique V-shaped broad-crested weir > oblique broad-crested weir;oblique V-shaped broad-crested weir < normal broad-crested weir < normal V-shaped broad-crested weir.Compared with the normal weir,the downstream hydraulic jump submergence coefficient of oblique weir is smaller,while the design of V-shaped weir surface improves the downstream hydraulic jump submergence coefficient.(2)When the oblique V-shaped broad-crested weir is overflowing,the main flow line in front of the weir deviates to the left side of the flume,and the horizontal water level difference exists on the vertical sections.The fluctuation head from upstream to the crest of the weir on the left bank is larger than that on the right bank,and the water level on the right bank behind the weir is higher than that on the left bank,and the fluctuation is larger.When the discharge is constant,the downstream water level of oblique V-shaped broad-crested weir decreases with the increase of weir height.(3)The velocity distribution of oblique V-shaped broad-crested weir is not uniform,the velocity in front of the weir decreases gradually from left to right,and the maximum velocity appears on the left side;the smaller the height of the weir,the larger the discharge,the more uniform the velocity distribution of vertical sections in the front triangle area of the weir;Compared with oblique broad-crested weir,the flow patternbehind the weir is more stable,and only when the discharge per width is larger,the flow behind the right jump is in a rapid stream,and it is a threat to the bank and riverbed.(4)The upstream streamline of the oblique V-shaped broad-crested weir is perpendicular to the width of the flume.In the triangular area in front of the weir,the streamline begins to deflect.The higher the weir height is,the smaller the discharge is,and the larger the deflection angle is.The deflection angle of streamline is less than the weir angle under each working condition,and it is not perpendicular to the axis of the weir.(5)Within the range of submergence degree from 0 to 0.8,the overflow capacity of each weir type basically remains unchanged with the increase of submergence degree,and the overflow capacity : oblique V-shaped broad-crested weir > normal broad-crested weir.When the submergence degree exceeds 0.8,the flow capacity of each weir type decreases,and the reduction of oblique V-shaped broad-crested weir is less than that of normal broad-crested weir.When the submergence degree is 0 and the discharge is small,the overflow capacity of oblique broad-crested weir is less than that of normal broad-crested weir,which causes failure.In this test flow range,oblique V-shaped broad-crested weir has no failure compared with normal V-shaped broad-crested weir,and the overflow capacity is oblique V-shaped broad-crested weir > normal V-shaped broad-crested weir > normal broad-crested weir.(6)With the increase of relative weir height,the flow coefficient of oblique V-shaped broad-crested weir decreases in general,and the variation amplitude decreases accordingly.(7)The flow coefficient formulas of the normal V-shaped broad-crested weir and the oblique V-shaped broad-crested weir are fitted by mathematical method.The calculated values are within the error range of ±5% and the fitting effect is better.