Identification of transition flow zone in rectangular side orifice, experimental study

Introduction
Side sluice gates, side weirs and side orifices are commonly used flow diversion structures provided in the side of a channel to spill or divert water from the main channel. They are widely used in irrigation engineering, wastewater treatment plants, flocculation basins, sedimentation tanks, aeration basins, etc. Flow through a side orifice is similar to the flow through side weirs and side sluice gates. When water level is lower than upper limb of a slot, the slot behaves as side weir. When water level in the main channel is above the upper limb of the slot, the flow behaves like an orifice. For orifice spillway Hussain et al., 2014 reported orifice flow condition required head over the crest 1.5-1.7 times the height of the orifice opening. In the literature no study was found about the transition flow condition requirement of side orifice. In this paper, an experimental study has been carried out related to transition flow zone of side orifice which have not been taken up earlier.
Methodology
In this study, first, the variables affecting the transition flow zone are listed and then, non-dimensional parameters extracted using dimensional analysis. The experiment was carried out in a rectangular channel of 8 m length, 0.8 m width and 0.6 m depth at the Hydraulic and water structure Laboratory of department of water engineering, Shahid Bahonar university of Kerman. The channel was fitted with a sluice gate at the end of the channel to regulate the depth of flow, and a rectangular side orifice in the right side of the channel. Experiments were performed for four size of side orifice length (L) equal to 0.15, 0.2, 0.25 and 0.3 m and width (b) 0.05, 0.1 and 0.15 m. The channel consists of a smooth horizontal well painted steel bed with a vertical glass sidewall. The collection channel is 1.2 m wide and 2 m deep, and situated parallel to the main channel. Each size of orifice installed in two different height equal to 0.09 and 0.14 m above of channel bed. For each experiment used nine flow in main channel with different Fr number in such a way that water level changes from lower to upper than limb of side orifice. For each run, the output discharge of side orifice, water surface profile along main channel were measured. The non-dimensional stage discharge of side orifice calculated and for each run output discharge from orifice calculated by Hussain et al.,(2011) and Embroiled et al., (2009) equations for side orifice and side wire respectively. To evaluate the accuracy of the equations, the average percent error (APE) were used.
Results and Discussion
The non-dimensional stage-discharge curve and water surface profile along the main channel are plotted in order to estimate the transition flow zone in side orifice. The results of non-dimensional stage-discharge curves (figs 3-6) reveals that the length to width ratio of orifice b/L is, indeed, the predominant parameters which affect the transition zone. For values b/L smaller than 0.2 the transition zone not clearly detected. The results show that the distance of lower limb of orifice to the bottom of channel, W, is other effective parameter in forming of transition zone. When the lower limb of orifice is closer to the channel bottom the ratio b/L becomes insignificant parameter. Based on water surface levels along the channel and the side orifice, figures 7 and 8, for values b/L smaller than 0.2 the transition zone happen only in very small area below upper limb of a side orifice, but for b/L greater than 0.2 the transition zone occurs above and below of the upper limb of a side orifice. The calculated and observed output discharge in different situations were used to check the accuracy of discharge equations of side weir and side orifice when the transition flow occurred. The results (figure 9) shows that in transition zone the accuracy of exiting side and orifice discharge equations is well.
Conclusion
The present study provides an experimental examination of the transition flow of a rectangular, sharp crested side orifice. As a result of dimensional analysis, the results indicate that the dimensionless parameter b/ L is the predominant parameters which affect the transition zone. The transition flow zone occurred when the b/L is greater than 0.2, but in these situations transition flow zone depends mainly on the distance of lower limp of side orifice to the bottom of channel. The average percentage error of exiting discharge equations showed that these equation can be used to estimate the discharge of side orifice in transition flow zone.