جستجوی مقالات مرتبط با کلیدواژه "حوضچه استغراق" در نشریات گروه "آب و خاک"

Accurate knowledge of where jets hit downstream of dams helps designers a lot in locating plunging pools. The purpose of this study is to investigate experimentally and numerically the trajectory of pressurized jets through the gates and to determine their location downstream of the dam. Ansys - Fluent software was used for numerical simulation. The results showed that there is a significant difference between the experimental values and the values extracted from the jet projectile equations for predicting the path of the pressurized jets, and this discrepancy may be due to the effect of air resistance on flow. The existing jet trajectory equations don’t consider air resistance. To minimize this difference, the projectile equation was modified. Also, the effect of changing the diameter of the dam gate and the discharge on the point of impact of the pressurized jet on the ground surface was examined. By increasing the diameter of the dam outlet at a constant discharge, the location of impact of the impinging jet to the ground from the dam toe decreases, and with increasing discharge at a constant diameter of outlet, the place of impact of the pressurized jet to the ground from the toe of the dam increases. In addition, the numerical simulation results showed that the dynamic pressure as well as the velocity at the point of impingement of the jets on the river bed have the maximum value that should be considered in designs. The dynamic pressure as well as the velocity of the jet coming out of the gate hit the ground with an average increase of 145% and 242% relative to the end edge of the gate, respectively. In addition, the breaking length of the pressurized jet was investigated in this study, and it was found that the breaking length of the pressurized jet increases with the increase in the initial velocity of the water jet.

The effect of dynamic vertical impinging jet on the static and dynamic scour depth is investigated in plunge pool bed, concerning the laboratory model. Experiments are conducted by changing jet installation height and a variety of grain size distribution. The basic scenarios are implemented with different nozzle elevation i.e. 24, 35.5, and 44 cm from the surface of alluvial bed. Then, the sub scenarios are developed by changing the grain size of alluvial bed with average diameters of 4.06, 7.14, and 8.73 mm and the output velocity of flow, with a variation range of 2.04 to 10.36 m s-1. Toward this end, concerning laboratory observations, various types of scour hole are classified based on the formation of the hole and the pressure factor of the jet. The results in adition to present an innovative classification of dynamic scour holes led to introduce an equation for estimating the relative difference of dynamic and static scour depths. A non-dimensional equation was presented as a simple and precise equation which would be recommended for estimation of the relative difference of dynamic and static scour depths (dd-ds)/hj. The relation had the correlation coefficient and root mean square error values of 0.72 and 0.096, respectively.

Plunging pools are constructed at the downstream of dam spillways to dissipate the excess energy. The turbulent flow velocity converts to the dynamic pressure due to impact of flow with the pool’s bed. The aim of this research is to determine impact pressure of a vertical jet on plunging pool bed. Hence, the experiments were carried out in four different discharges (ranging from 6 to 27.5 lit/s according to the nozzle diameters), three jet diameters including 4.3, 5.2 and 8.2 cm and four falling heights of 37, 60, 90 and 120 cm. Also the experiments are conducted in similar flow conditions for smooth and rough surfaces, and results are compared. Dynamic pressures were measured with a pressure transducer. The analysis of data showed that increase of jet diameter causes decrement in the dynamic pressure coefficient. Results revealed an increase in dynamic pressures due to roughness of contacting surface. Moreover, variations of jet Froude number and fall length have significant effects on the dynamic pressure coefficient. The impact pressure was the highest in the surface center and was gradually reduced in outer zones.

The security and stability of dams for flood passing through the spillway should be provided. So, kinetic energy of flow over large spillway must be dissipated. One of the energy dissipation structures at downstream dams, are plunging pools. The aim of this paper is to investigate dynamic pressure that is created by the impact of a series of rounded non-submerged jets on a flat plate with the roughness height of 0.8 cm in the angles of the impact of 30, 60 and 90 degrees. This research uses sensors to measure the instantaneous pressure (Pressure transducer) with the ability to record and store the dynamic pressure oscillation of water jet. The results showed that increasing the drop height, average coefficient of dynamic pressure decreases. The extreme dynamic pressures coefficient increased with increase fall height. The mean coefficient of dynamic pressure increases with increasing discharge. As the angle of the impact jet decreases, the dynamic pressures reduce. Also, the roughness increases the dynamic pressure up to 70% in the test interval.

Stilling basins are constructed at the downstream of dam spillways to dissipate the excess energy of the spilled flows. The turbulent flow velocity converts to the dynamic pressure due to impact of flow pockets with the basin’s walls and bed. Experiments were conducted using a pressure transmitter system to measure the resulted dynamic pressures from the impact of a series of rounded non-submerged jet on a flat plate. The analysis of data showed that a 18% moderate decrement in nozzle diameter caused a 14% moderate increment in the dynamic pressure coefficient and a 34% moderate decrement in jet drop length caused an approximately 13% increment in the dynamic pressure coefficient. Nevertheless, variations of the dynamic pressure coefficient with the increment of nozzle discharge had no regular trend and it began to decrease after reaching a maximum point.