Effect of using single square pile at upstream of the orifice on pressurized flushing

1- Introduction
In pressurized flushing, the removing of sediments is performed under a constant head above the intake. Given the low pressurized flushing efficiency, it is necessary to provide solutions to increase its efficiency. Few studies have been carried out on increasing the efficiency, so far. By applying a group of cylindrical columns in the upstream of the outlet, Madadi et al. (2016) increased the pressurized flushing up to 250 percent, comparing to the control condition. Also, by installing a semi-cylindrical structure in the upstream of the outlet, Madadi et al. (2017) were able to increase the pressurized flushing up to percent, comparing to the control condition. By proposing new methods, this research tries to study the effect of using a single square pile at the upstream of the outlet on the dimensions and volume of the flushing cone in pressurized flushing.

2- Materials and Methods
To perform the experiments, three flow rates (Q) of 4.17, 6.39 and 8.34 l/s were considered. In all the experiments, the sediments level (Hs) was considered constant and leveled at the lower edge of the outlet. The water level in the flume to the center of the outlet (Hw) was considered to be 52 centimeters in all the experiments. Also, the output outlet diameter (Do) was considered to be 7 centimeters. The sediment grading used in all the experiments was constant (d50 = 0.5 mm). The experiment duration was considered to be 150 minutes in all the experiments. In order to study the effect of the square pile size (Dp) on the dimensions and efficiency of the pressurized flushing cone, four side sizes of 1.4, 2.1, 2.8 and 3.5 cm were used. The pile installation distance from the outlet (Lp) was considered to be 4.9 cm, through conducting consecutive. In order to study the effect of the pile installation distance (Lp), the pile that had the highest influence of the flushing was installed in different distances from the outlet upstream. These distances were defined as a ratio of the outlet diameter and equal to Lp/Do = 0.7, 1.2, 1.7, 2.2. Subsequently, the desired pile was installed in the determined distances from the upstream of the outlet and experiment stages were carried out.

3- Results and Discussion
The results from the experiments related to the installation of the piles suggested that the presence of square pile has a considerable influence in the increasing of the flushing cone volume, so that for the flow rate of 4.17 l/s and installation of the pile with DP =3.5 cm and LP=4.9, the flushing cone volume had a relative increase of 362 percent, comparing to the control condition. Also, in the same condition, comparing to the control condition, the depth, length and width of the flushing cone had a 120, 57 and 42 percent decrease, respectively. Due to the collision of the current lines with the upstream face of the pile when a pile is installed in In front of the flow with sedimentary bed, a series of downward currents are formed, that are called horseshoe vortices, that led to scour phenomenon around the pile. Also, due to the separation of the stream lines in the downstream of the pile, a low-pressure region is formed that leads to the rise and movement of the sediments. Installing the column in the upstream of the outlet causes the eroded sediments due to the presence of a pile exit the outlet in addition to the sediments due to the flushing and the efficiency of the flushing increases. The main influence of installing the pile in all the flow rates is related to the pile relative size of Dp/Do = 0.5. This could be attributed to the fact that by the increase in the size of the pile, more downwards vortices are formed in the upstream of the pile. Also, separation of current lines in this condition is higher and low-pressure region is formed with a larger area in the downstream. Hence, more sediments rise from the upstream of the outlet and exit it. In order to study the influence of pile installation distance on the flushing volume, sediment with the size of Dp/Do = 0.5 which had the highest influence on the abovementioned parameters, was installed at four relative distances of Lp/Do = 0.7, 1.2, 1.7, 2.2 from the upstream of the outlet. By the increasing in the pile installation distance from the outlet upstream, the flushing cone volume decrease an at Lp/Do = 2.2, the influence of pile installation is almost diminished and the cone volume is almost equal to the control condition. It could be said that by the increasing in the distance of the pile from the outlet and the decreasing in the velocity of current collision with the pile, the downward currents power in the upstream of the pile and vortices rise in the downstream decrease and as a result, the influence of the presence of the pile on the increase of the flushing cone decreases.

4- Conclusion
Studying the influence of installing square pile on the upstream of the outlet on the pressurized flushing efficiency showed that presence of one pile leads to an increasing in the flushing efficiency. The highest influence of pile installation was related to the largest pile (Dp/Do = 0.5), so that in this condition, the flushing cone volume had 362 percent increasing, comparing to the control condition. In other words, with the same rate of water discharge from the outlet in the condition without installing the pile, the sediment discharge volume could be considerably increased by installing a pile.

5- Keyword
Flushing, pile, Flushing cone, Orifice.