Laboratory Study of Free and Submerged Flow in Inclines Weirs

Any structure that is in the flow path and establishes a simple, specific and definite flow relationship and depth around it is called a flow controller. Weirs are structures that raise the water level behind them and create control sections and are simple means of measuring discharge. Bazin & Schwalt (1898) were the first to conduct experiments on rectangular broad crested weirs. Since then, many researchers have done a great deal of research on a variety of weirs. Among them we can mention: Woodburn (1932), Tracy(1957), Smith(1959), Abou-seida & Quraishi (1976), Hager&Schwalt (1994), Sargison & Percy (2009),….Due to the importance of flow measurement in open channels, the purpose of this study was to investigate the hydraulic performance of inclined weir, to determine the discharge conveyancecoefficient in this kind of structures in free and submerged flow condition, and to provide relationships to predict the discharge conveyancecoefficient.

Based on Equation 1, we can determine the weir discharge. In this equation Q is the discharge (m3 / s), Cd is the discharge coefficient (no dimension), B is the weir length (m), g is the acceleration (m / s2), and H is the height of the water over the weir (m).Also rewrite from the equation 1 to form 2 by defining it   as the discharge conveyancecoefficient (Mohamed, 2010):In this study, the equation 2 is based on the calculation of the discharge throughput on weirs. The following two equations for free and submerged flow are presented using dimensional analysis.The experiments were conducted in a plexiglass flume (Fig. 1) made of the British ArmField Company with a length of 15 m, a width of 30 cm and a height of 50 cm. A total of 60 experiments (30 free-flow and 30 submerged-flow experiments) were performed on the inclined weir.

A) Free flow condition
In this series of experiments, with increasing H/Lw ratio, the discharge conveynce coefficient of all three sloping weirs increased, but the coefficient of conveynce discharge of rectangular broad crested weir decreased (Fig. 1). This figure shows that among inclined weirs are most sensitive to H/Lw changes.Also comparison of the inclined weir with the rectangular broad crested weir indicates that the rectangular broad crested weir later is less sensitive to H/Lw. Equation 5 was derived to calculate the discharge conveyancecoefficient (Cf) in inclined weirs under free-flow conditions. B) Submerged flow conditionMultiple regression was used to investigate the interaction of the extracted dimensionless parameters on the discharge conveyancecoefficient and to provide a mathematical relation to predict these values. Equation 6 was derived to calculate the Cs coefficient in inclined weirs under submerged flow conditions. Figure 2 shows the computational and observational Cf, Cs in free and submerged flow conditions. The scattering of these points relative to the 45 ° line shows that the correlation coefficient of the experimental and computational values for free and submerged flow are equal to 0.9 and 0.91 respectively.

The results show that:- Increasing in H/Lw ratio leads increases in discharge conveyance coefficient of all three sloping weirs and decreases in rectangular broad crested weir. - For a constant value of the H/Lw ratio, the highest discharge coefficient is related to the upstream and downstream slope weir model (SCW-UD-1) and the lowest discharge coefficient is to the upstream slope weir model (SCW-U-1).-The upstream and downstream slope weir are most sensitive to H/Lw changes and rectangular broad crested weir is less sensitive to changes in H/Lw ratio. - As the H/p < /em> ratio increases, the discharge conveyance coefficient of all three sloping weir models increases.