Laboratory Study on the Effect of Second Perforated Sill Opening on Flow Pattern and Hydraulic Jump Characteristics in Horizontal Stilling Basin

One of the most extensively investigated phenomena in hydraulic engineering is the hydraulic jump that numerous investigators have studied it over the past century. There are different types of stilling basins, including Standard stilling basin USBR, Stilling basin SAF, Stilling basin with continuous sill and stilling basin with perforated sill noted. Each basin, depending on the intensity of the hydraulic jump, usually needs components to reduce the length of the hydraulic jump as much as possible while shaping it at a specific location. These components include the chute Block, Baffle Piers, and perforated sills that is the subject of this study. These types of dampers are the most common energy dampers in dams and irrigation and drainage networks and generally have high efficiency of over 60% in energy dissipation. The purpose of this study is evaluation of the effect of second perforated sill opening on flow pattern and hydraulic jump characteristics in horizontal stilling basin dissipation of energy and required tail water depth. Also determine the optimal distance of two perforated sills from the beginning of the stilling basin with a fixed height for perforated sill.

Experiments were carried out on one and two perforated sills in the Shahid Chamran University Lab in Ahwaz, in a Plexiglas flume and an iron tank. Experiments were carried out so that, given the minimum and maximum flow rates in the flume, the height of such discharges was marked on a deck mounted next to the storage tank at the beginning of the flume. After determining the optimal distance of a single perforated sill, the two perforated sills were tested in such a way that the second wall was located at distances 10, 20 and 30 centimeters from the first perforated sill. After investigating the results and determining the best perforated sill pair in increasing energy loss and decreasing basin length, the best perforated sill pair with constant distance between them were moved to the jump toe at three distances of 50, 60 and 70 centimeters from the first wall to give the best The distance for a pair of perforated sills are obtained. In all experiments to avoid the splashing conditions, a forced jump was created so that the jump would first be submerged and then a full (non-submerged) jump would be formed by opening the end gate of the flume. Experiments were carried out in the form of 32 tests for different discharges in range of 47.3 to 145.5 lit/s and Froude number in ranges of 3.6 to 11.2.

Maximum energy loss compared to free hydraulic jump increases 10%, i.e. perforated sill energy loss rate reached 83.2% at Froude number 11.2. The length of hydraulic jump in the stilling basin with the single perforated sill is reduced to 2.2 times the secondary depth of the hydraulic jump. In other words, the perforated sill reduces the jump length by 64.5% compared to the free jump. In this study experiments were conducted to evaluate the effects of two perforated sills with circular holes and fixed opening percentage to 50 for the first perforated sill and different opening percentage to (12.5%-25%-50%) for the second perforated sill on flow pattern and hydraulic jump characteristics in horizontal stilling basin. It should be noted that several tests were performed to confirm the perforated sill pair with relative distances 60 and 70 centimeters from the jump toe at the mentioned positions and showed that the perforated sill pair still with relative distances 16.7 and 23.3 had the greatest effect on decreasing the relative length of the jump. Based on the results of experiments, an analytical expression was developed for the prediction of the length of hydraulic jump in the case of two perforated sills. Results of experiments on two perforated sills showed that they can only reduce the length of hydraulic jump to an acceptable level that the distance between they provide the conditions for creating a stable jump and the length of jump do not decrease by reducing the distance between the sills.

Results of experiments show that two perforated sills with opening percentage (50% - 12.5%) increase relative energy loss up to 83.7%. Also the length of hydraulic jump in the stilling basin with two perforated sills with opening percentage 50 is reduced to 2.15 times the secondary depth of the free hydraulic jump. The results showed that two perforated sills with opening percentage of 50 for the first and second perforated sills were selected as the best twin of sills to create the perfect and sustainable jumps. In comparison with the other considered couples causes the less erosion potential in downstream of stilling basins due to transferring of the hydraulic rollers from the bed to the surface of flow.