جستجوی مقالات مرتبط با کلیدواژه "عمق پایاب" در نشریات گروه "عمران"

The downstream scour of the vertical drop can be one of the causes of instability and failure of this structure. In the present study, the downstream scour depth of this structure predicted using the support vector machine (SVM) method. For this purpose, 104 experimental data used to estimate the scour depth. hese data are a function of the two dimensionless parameters of dansimetric Froude number (Frj) and tailwater depth (yt / yj) that have been entered into the SVM in three different models. To evaluate the results, the evaluation criteria of R2, NRMSE, DC, and MARE used. The results showed that model number (1) with the input combination (Frj and yt / yj) with R2 = 0.9777, DC = 0.929, NRMSE = 0.0775, and MARE = 11.89% for the test stage leads to the best result. The SVM method also has appropriate accuracy, acceptable results, and desirable performance in estimating the scour depth. Also, it was found that the densimetric froude number has a greater effect on estimating the relative scour depth compared to the tailwater depth.

Hydraulic jump is a fast and irreversible variable flow that occurs downstream of hydraulic structures and the result of which is the rapid transformation of supercritical flow into subcritical flow, which increases the depth of the flow and causes a significant loss of energy. By examining the previous studies, it is clear that the roughness bed or the divergence plays an important role in hydraulic jump characteristics.  FLOW-3D is one of the suitable software in hydraulic Phenomenon modeling. In order to ensure the appropriate capability of FLOW-3D software in simulating hydraulic jump, first, a research related to this issue which has been investigated in the past in a laboratory, is simulated in the software. Then, by comparing the results of the numerical simulations with the laboratory data and ensuring the proper functioning of the software, new simulations are made. The experiments related to the physical model used were carried out in a laboratory flume with walls and floors made of transparent plexiglass, 5 meters long, 0.3 meters wide, 0.45 meters high, and zero longitudinal slope. To create a supercritical flow, a steel valve with a height of 0.65 meters and a thickness of 3 mm and an opening height of 1.7 cm was used for a non-prismatic channel with a divergence ratio of 0.33. In order to create symmetrical opening ratios of 0.33, glass boxes with a length of 0.5, a height of 0.2 and a width of 0.1 meters were placed on both sides of the flume. After the simulation and by checking the R2, MAE and RMSE parameters, the k-ɛ model and the mesh with the number of 687600 cells were selected as the optimal mesh. In this research, according to the selection of four types of bed (smooth bed, rough bed with hemispherical roughness and diameter of 3, 4 and 5 cm) and three divergence angles (7, 14 and 90 degrees) and five Froude numbers (Froude number: 4.34, 5.71, 6.95, 8.17, and 9.37) in total, Simulated for 60 different experiments.  The results showed that for the maximum discharge for the sudden divergent channel, the roughness of the bed with a diameter of 5 cm causes the amount of the flow depth to decrease by 19.77% compared to the smooth bed. Also, the sudden divergence of the bed reduces the flow level by 23.75%. In all cases, the value of y2/y1 increases with increasing Froude number. With the increase of the Froude number from 4.34 to 9.37, the value of y2/y1 on the flat bed increases by 15.54%. With the increase of the Froude number from 4.34 to 9.37, the value of y2/y1 on the rough bed with a diameter of 3 cm decreases by 5.67% on average compared to the smooth bed. As the roughness diameter increases from 3 to 5 cm, the value of (Lj/y1) decreases by 15.58% on average. The results show that the (EL/E1) ratio increases with the application of bed roughness. So that by applying a roughness with a diameter of 5 cm, the value of (EL/E1) compared to a smooth bed increases by about 5% on average, and by applying a sudden divergence, the value of (EL/E1) compared to a bed diverging under an angle of 7 degrees The average increases by 4.58%. FLOW-3D is a good software to prediction of hydraulic jump characteristics in divergent channel with smooth bed and rough bed and The k-ɛ turbulence model was selected as the optimal turbulence model. Increasing the bed roughness size decreases the secondary depth for all values ​​of 𝐹𝑟1. The sudden expanding of the rough bed with a roughness of 4 cm reduces the y2/y1 by 12.98% compared to the bed with a divergence under 7 degrees. Increasing the size of the roughness decreases the length of the hydraulic jump. According to the results, the length of the hydraulic jump in the rough bed with a diameter of 5 cm compared to the smooth bed decreased by 26.12%. Also, by increasing the roughness diameter from 3 to 5 cm, the value of (Lj/y1) decreases by 15.58% on average. By applying roughness with a diameter of 5 cm, the value of (EL/E1) increases by about 5% on average compared to the smooth bed. Results show that the divergence angle of the bed is effective on streamlines. But, increasing the roughness of the bed, there is no noticeable change in the streamlines. However, increasing the bed roughness size, increases the amount of disturbance energy in the section.

A spillway is a very important structure usually used to provide the controlled release of water from a dam or sometimes levee downstream, typically into the riverbed of the dammed river itself. Spillways ensure that water does not destroy parts of the structures not designed to convey water. many Energy-dissipating structures (such as ., hydraulic jump stilling basins, roller buckets, ski jump buckets) are usually located at the end of spillways discharge channels in order to dissipate the extra energy which comes from the water . Specifically, flip buckets are mainly placed at the end of f high dams in order to dissipate the energy with high-velocity flows . having said that these structures ( Flip buckets) are used when velocity of the water is larger than about 15-20 m/s. Energy dissipation controlling has always been one of the most significant and vital matter and concern of hydraulic scientists and researchers especially in tall dams. one of the most common method to dissipate energy is to discharge flow away from hydraulic structures and downstream by using Flip bucket spill way .The sky-jump spillway is an economical and effective solution to return water to a river . many years ago in the past various models of flip bucket spillways , like simple, splitter, and deflector, and so on have been built,but information about the ways of increasing energy dissipation and controlling the bad effects of the extra energy of the water are still limited and more studies are needed .in order to increase the efficiency of this dissipater structure some wedge shape deflectors were designed to decrease the effect of this destructive energy on down stream.in this regard the effect of deflector installation position on energy dissipation was experimentally investigated. The main result indicate that the increase of linear distance of deflectors from the bucket remarkably result in an increase in energy dissipation and a decrease in jet length..

To investigate the effect of deflector installation position on energy dissipation and the length of the jet a new experimental study was conducted in the Hydraulic laboratory of Shahid Chamran university of Ahvaz.3 wedged-shape structures with Hight of 10 cm ,,length of 6 cm and angle of 47 degree were made .40 experiments were performed at 4 different linear distance from the edge of the bucket and one deflector-free experiment was carried out as a compression to other experiments. It is worth mentioning that all experiments including deflector-free one ,were performed at 8 dimensionless parameter Yc/H . At each experiment the linear distance from the edge of bucket was increased and the depth of tailwater was read .In the end relative energy dissipation was concluded by measuring the total energy in the upstream and downstream. in addition to the energy dissipation , Jet length ( for each experiment )was determined by using Get data software and the photos taken during the study .

In general Data analysis demonstrated that the waged-shape structures resulted in a remarkable increase in the amount of energy dissipation and a major decrease in jet length ,due to the increase of water and air mixture compare to the deflector - free experiments .Indeed deflectors divide the incoming jet to two small different jets ,therefore the combination of these two small jets leads to the increase in water and air mixture and consequently the increase of relative energy dissipation .Moreover it has been shown that the energy dissipation increased by increasing the linear distance from the bucket. Furthermore the experimental study on jet length indicated a remarkable decrease in the jet length by the increasing of the linear distance.in other words the different installation position of deflector increase the Hight of jet trajectory .it directly increases the contact surface of jet and air and leads to increase of energy dissipation and consequently decrease of jet length.

In general the increase of linear distance from the edge of bucket ( in flip bucket spillway ) resulted in the increase in energy dissipation and the decrease in jet length .Maximum observed relative energy dissipation was 65.87 which occurred in the Yc/H = 0.027 and Lx/L = 7.3 and minimum observed relative energy dissipation was 57.54 ,which occurred in the Yc/H = 0.061 and Lx/L = 4.3. furthermore maximum observed jet length was105 cm, which occurred in Yc/H = 0.061 and its minimum was 35 cm, which occurred in Yc/H = 0.027. therefor according to the results of the all experiments using deflector in different possessions as a new way of controlling and increasing energy dissipation is highly recommended .

Introduction :

Non-linear Piano Key Weirs (PKW) enjoy not only a higher discharge but also a relatively simple and economic structure compared to linear weirs. Other advantages of the PKWs include the fact that they increase the discharge per unit width over the weir up to 100 𝑚3⁄𝑠, the discharge over this type of weir is at least four times that over ordinary (linear) weirs, it increases the capacity of the reservoirs, and it is cost effective with lower maintenance expenses. This type of weir is utilized mainly with the aim of increasing its capacity over the available spillways and also as controlling structures on newly constructed spillways. Despite the research studies conducted in this regard, the scour downstream of the trapezoidal Piano key weirs has not yet been investigated, and there has been no comprehensive information on the properties of the scour profile downstream of these weirs. Hence, given the several advantages of the Piano key weir and lack of a comprehensive research on the amount of scour downstream of this type of weir, the need for investigation on the scour downstream of this weir under different hydraulic conditions is obvious.

Experiments were conducted in a rectangular channel with a width of 75 cm, metal bed, glass walls and a height of 80 cm in the hydraulic laboratory of water and hydraulic structural engineering of Tarbiat Modares University, Tehran. Water flow was supplied from an underground storage tank using a pump with a maximum discharge of 85 Lit/sec. A valve installed at the end of the channel was used to adjust the flow depth in the channel. The PKW was set up and sealed at a distance of 1 m from the channel end. All experiments on the weir were conducted under free flow conditions. A layer of uniform sediments with an average diameter of 1.64 mm, a geometric standard deviation of 1.24, a height of 42.5 cm, and a length of 2 m was placed downstream of the weir. The type-A trapezoidal piano key weir made of thermoplastic (common PLA Filament) with a thickness of 1.2 cm was utilized in this study. Weirs with 6 keys (3 inlet keys and 3 outlet keys), a width of 75 cm (the same as the channel), and the crest length (B) and a height (P) of respectively 50 and 20 cm were used. The experiments were conducted with three values of discharge 0.03, 0.04 and 0.05 𝑚3⁄𝑠 and five tailwater depths varied from 0.08 to 0.18 m.

It is observed during the experiment that with the impact of the outlet flow from the weir with a downstream bed, the scour began and the process of erosion and sedimentation continuously occurred with development of the scour hole. Observations indicated that during the erosion stage, after the impact of the water jet with the bed and upon its dispersion, the scour of the bed material begins immediately and considerably. The material washed away from the bed is mostly transported downstream along with the flow and is deposited there. Generation of the scour hole entails circulation of flow inside the scour hole. Also, a portion of circulating flow inside the hole is deviated upward. A large body of sediments washed from the bed is transported downstream with the flow leaving the hole and a part of it rotates with the circulating flow inside the hole. A part of these sediments along with the circulating flow is deposited inside the scour hole and the other part is transported downstream by the flow leaving the scour hole. After the scour hole is generated, the jet entering the hole is divided into two parts at the deepest point of the scour hole. Such division is observable from the movements of sediments inside the hole so that some of the sediments are separated and transported downstream, leaving the scour hole, and the remaining sediments move and reside in the same zone as the circulating flow due to the backflow towards the weir foot. A sedimentary ridge is created downstream of the hole as a result of the eroded sediments concentration which have left the scour hole. As the scour hole is deepened, the sedimentary ridge is also enlarged. This trend continues until the flow is able to carry the particles out of the scour hole. These variations mainly occurred at the initial 20% of the test duration. It is also observable that the scour hole downstream of the weir is not symmetrical with the longitudinal axis of the channel, which is due to the effect of inlet and outlet keys on the direction of the outlet flow over the weir. However, the overall scour pattern is nearly the same under different conditions. Variations in the maximum scour depth downstream of the weir are approximately 5.8 to 8.8 times the water depth over the weir. The location of the maximum scour depth is also a function of discharge and tailwater depth, which was measured at a distance of 15 to 27 cm from the weir foot.

Conclusions :

This study has investigated the dimensions of the scour hole as well as variations of bed topography downstream of a trapezoidal piano key wer. It was found that with a 112% increase in the tailwater depth under a constant discharge of 30 L/s, the maximum scour depth is reduced by 37%. Increasing the discharge results in an increase in the dimensions of the scour hole, in such a way that a 66.6% increase in discharge with a constant tailwater depth has increased the scour hole by 18.5%. With an increase of the tailwater depth, the development of the scour hole downstream of the piano key weir is reduced. The minimum scour hole development in downstream direction was measured in tests with a discharge of 30 L/s and tailwater depths of 15 and 17 cm. 112 and 89.5% increase in the tailwater depth for discharge values of 30 and 40 L/s reduces the area of the scour hole by approximately 46 and 25% respectively.

Weirs are hydraulic structure commonly used for controlling flow characteristics and water level (Vischer, 1998). Also, in dams, weirs are responsible for the controlled release of flood flows from the dam reservoir to the downstream channels. One of the types of weir is piano key weir (PKW). Piano key weirs are also very cost-effective and cheap to maintain, increase reservoir storage capacity, and offer better flood control (Ortel, 2018). The main problem at the downstream of hydraulic structures, such as weir, is the scour and movement of bed materials. Scouring in the downstream of weirs is an important issue for weir stability and has been extensively researched. In this study, the geometry of scour holes in the downstream of piano key weirs was investigated by the use of experimental models. According to previous researches, trapezoidal piano key weirs (TPKW) are more efficient than rectangular piano key weir (RPKW) (Mehboudi, 2016). While there are limited studies on scour downstream of RPKWs, the scouring downstream of the trapezoidal piano key have not yet been researched according to the authors' knowledge(Jüstrich, 2016). So it is important to study their scour, and it is necessary to compare the performance of these two types of PKWs in terms of scouring issues. Due to the fact that the geometric shape of the weir affects the downstream scour condition, in this study the downstream scour of the piano key weir with trapezoidal geometric shape has been considered and the characteristics of the scour and its rate of was compared to a rectangular geometric shape. Accordingly, the impact of discharge and the tail water depth on the characteristics of the scour hole at the downstream of the rectangular and trapezoidal piano key weirs and the comparison of these changes in the two models have been considered. Measurements were made to predict the scour specifications of the rectangular and trapezoidal piano key weirs, including the maximum depth of the scour hole, the distance between maximum scour depth and weir foundation, and the length of the scour hole (Figure 2).

In this study, two experimental models of PKWs with rectangular and trapezoidal geometry were made and tested in a flume with a length of 6.0 m, a width of 1.0 m and a height of 0.6 m. 2.0 m length with an average thickness of 25 cm was formed from sandy material with median grain size d_50=7.8 m (Figure 3). Three hydraulic conditions in upstream and three different tail water depth considered in downstream and totally 18 experimental runs were conducted. A summary of the initial conditions, numbers, and codes of the experiments is given in Table 2. The range of changes in discharge in this research is between 19 to 33 liters per second. The reason for choosing this range for discharge is to examine the conditions of the scour profile in a wide range of flow rate changes. In addition, the dimensions of the channel and the pump used do not allow the discharge to exceed 33 liters per second.

Effect of discharge on the scour hole profile downstream of the rectangular and trapezoidal PKW models are shown in Figure (8). It can be seen that in all models, as discharge and upstream head increase, so do the hole depth and hole length and the distance of maximum scour depth from the weir toe. Previous studies have reported similar findings for linear and nonlinear weirs (Jüstrich, 2016). As shown in Figure (4),In this study, the arrangement of inlet and outlet keys of weirs was considered different from previous researches, so it was observed that the maximum depth of downstream scour occurs below the output keys, the reason for this is the external spill jets from the output keys, which cause the scour hole to fall into the downstream through vertex. Also, comparison the scour hole characteristics of downstream rectangular and trapezoidal piano key weirs in Figure (9) shows that the maximum scour hole depth downstream of the rectangular model is higher than the trapezoidal model. The higher score hole depth in the downstream of the rectangular model than the trapezoidal model in similar hydraulic conditions can be attributed to the fact that, for any given flow rate, upstream-downstream total head difference is greater in the rectangular weir comparison to the trapezoidal weir . As shown in Figure (11), For both rectangular and trapezoidal models, at a constant flow rate, the depth and length of the scour hole have decreased with the increase of tail water depth. It is due to a decrease in the height of the drop jet and increase in jet speed during impact to downstream flow. To determine the effect of the geometric shape of PKW and the tailwater depth, the scour characteristics in rectangular and trapezoidal models in the range of conducted experiments were examined and compared based on the configuration of Equation ∅_s/H=a〖F_rd〗^b (H/h)^c. Then, the nonlinear regression method was used to determine the coefficients a, b, and c and formulate a number of equations for predicting the maximum scour depth, its location, and the scour hole length for rectangular and trapezoidal PKWs. These equations are presented in Table (3) .

The measurement results of the cascade characteristics showed that with increasing the flow rate and decreasing the tail water depth, the geometric characteristics of the score hole increase in both models. Also, the depth of scour in the rectangular model is more than the trapezoidal model. in all discharges, on averagely, get decreases %7 the ratio of dimentionless maximum scour depth of trapezoidal piano key to the rectangular model. but this difference decreases with increasing flow and total head on the weir, so that when in F_rd>3.9 this difference becomes insignificant and there is not a significant difference in the shape of the hole for both models. Using the regression method, several equations with appropriate accuracy were formulated for predicting the maximum scour hole depth, its location, and the scour hole length downstream of the models.