فهرست مطالب احسان جعفری ندوشن

Numerical simulation has increasingly become a very important tool for solving complex phenomena in fluid mechanics. Today, computational fluid dynamics (CFD), which uses numerical simulation to solve fluid flow problems, has become a major tool in many scientific and engineering studies. So far, most simulations have been performed using network-based Euler methods. The main problem with using such methods is the presence of sharp and mobile joints as well as free surface cells that require complex behaviors. In recent years, a new generation of numerical methods called mesh-free particle methods (Lagrangian) has been developed to solve computational fluid dynamics (CFD) problems. In these methods, the domain of the fluid is represented by a set of moving particles in the Lagrangian system. Mesh-free Lagrangian methods, including SPH and MPS, allow numerical modeling of flow in the face of large deformations or Discontinuity of boundaries. Due to the importance of the subject, the purpose of this study is to simulate the hydrodynamics of flow on the overflow of Dahan Ghaleh dam using a mesh-free Lagrangian method based on weakly compressible moving particle semi-implicit formulation (WC-MPS).

In Lagrangian methods, unlike the Eulerian method, instead of networking the solution field and discrete the equations on the nodes, the solution field is divided into a number of particles and the discrete equations are solved on these particles. In fact, the governing equations are transformed into particle interaction equations using different operators. In the WC-MPS method, the system is considered as a system with weakly compressibility and calculates the pressure of each particle using the state equation. The MPS method uses particle density to track the free surface. Because there are no particles outside the free surface, the density of the particles on the free surface decreases severely. A particle is known as a free surface particle whose density is somewhat lower than the standard particle density. The value of this limit may be selected from 80% to 99% depending on the problem. Therefore, the pressure of this particle on the free surface will be set to zero in each time step and there is no need to apply any additional conditions for the free surface. For solid (impermeable) boundaries, such as walls or beds, In the vicinity of solid boundaries, the particle density decreases, which can lead to computational disturbances. Therefore, a number of ghost particles are located outside the boundaries to prevent this density reduction. In order to model the inlet and outlet flow, the particle recycling method at the inlet and outlet boundaries, which was developed by Jafari Nodoushan et al. (2016) was used.

In this study hydraulic parameters of the flow on the broad crested weirs, chute, and flip bucket of Dahan Ghaleh dam are investigated. At first, the free surface profile was compared at times t = 1, 5, 10, 15, 30 seconds for three flow rates of 461, 600, and 904 m3/s. when the discharge is 600 m3/s, the depth and velocity of the flow at the beginning of the channel will be 1.1 m and 9.1 m/s, respectively. The value of the depth and velocity of the flow at the beginning of the channel in WCMPS modeling is 1.191 m and 8.420 m/s, respectively, which shows the accuracy of the model in predicting the depth and velocity of the flow. In this research, the pressure is simulated in the form of pressure contour at different points in terms (pa) for three different flow rates. The highest pressure is near the bed in the approach channel and no part of the broad crested weirs, chute, and flip bucket is under negative pressure. Simulation of cavitation index has been done in broad crested weirs, chute, and flip bucket. the results showed that the cavitation index in the range of 110m to 140m has approached the critical value and the possibility of creating cavitation phenomenon in this area is more than in other places, but considering the cavitation index value in this area is greater than 1.8, according to the recommendation Falvey (1990) it does not need to be protected against cavitation. At the end, the height and length of the jump from the edge of the bucket for different discharges have been investigated.

In this study, the hydraulic parameters of the flow on the broad crested weirs, chute, and flip bucket of Dahan Ghaleh dam are investigated. The desired model has simulated the depth and velocity of the flow at the beginning of the channel after of the overflow, depth, velocity, and Froude number of the flow in the bucket floor with an error of less than 10%, and the jump length per discharge 600 (m3/s) with a maximum error of 12% has simulated. The comparison of the results indicates the accuracy and reliability of the results of the developed model.

Silos are widely used in many industrial and engineering processes, so discharge grain from the silos is considered as one of the most important issues. Due to the high cost of laboratory studies on different materials and conditions, computational methods are used as a substitute approach for much less cost. Because of the ability of nonlinear Lagrangian methods to model large deformations and discontinuities, this study develops and evaluates a mesh-free Lagrangian model based on a weakly-compressible MPS formulation to simulate of the discharge of the granular silo. In Lagrangian methods, unlike the Eulerian method, instead of networking the solution field and breaking the equations on the nodes, the solution field is divided into a number of particles and the broken equations are solved on these particles. In fact, the governing equations are transformed into particle interaction equations using different operators. In the meantime, the particles that are closer to the particle under study will have a greater effect on that particle. In such a way that the effect of relatively distant particles can be ignored in comparison with closer particles and the interaction between particles can be limited to a specific domain called the radius of effect. The effect of each particle on the calculated particle is measured by a weight function. In the WC-MPS method, the system is considered as a system with weakly compressibility and calculates the pressure of each particle using the state equation. In this study, the Tait's state equation is used, which is used for high pressure water flow. The MPS method uses particle density to track the free surface. Because there are no particles outside the free surface, the density of the particles on the free surface decreases sharply. A particle is known as a free surface particle whose density is somewhat lower than the standard particle density. The value of this limit may be selected from 80% to 99% depending on the problem Therefore, the pressure of this particle on the free surface will be set to zero in each time step and in the MPS method, and there is no need to apply any additional condition for the free surface. For solid (impermeable) boundaries, such as walls or beds, this boundary condition is applied. In the vicinity of solid boundaries, the particle density decreases, which can lead to computational disturbances. Therefore, a number of ghost particles are located outside the boundaries to prevent this density reduction.  In this method the granular material is considered as a non-Newtonian visco-plastic fluid and an exponentially Herschel-Balky (H-B) rheological model in combination with pressure-dependent yield criteria model is employed to model non-cohesive grain behavior. The ability of the developed numerical method to discharge grain from silos has been evaluated and compared with the experimental results and the DEM method. Comparison of the results of the developed numerical method for the discharge of grains from the silo with the available experimental measurements and the DEM method shows the capabilities of the proposed model to accurately predict the surface and velocity profiles in this sample problem.

Dams have played an important role in the development of human civilization, the simplest of which is the provision of water resources in agriculture, industry and drinking. The share of earthen and gravel dams, which often have tunnel overflows and shoots, is significant. In dams with this type of overflow, increasing the height of the dam increases the flow velocity on the shot and increases the probability of cavitation. To protect hydraulic structures such as overflows, shoots, and lower discharges from cavitation damage, some air is typically added to the flow in areas with a cavitation index below the critical value. By using aerators, erosions that occur due to cavitation on overflow surfaces can be prevented. Aerators are usually installed on the floor and sometimes on the side walls of the overflow, separating the high-velocity currents from the surface of the overflow and preventing erosion at the rigid boundaries by artificially introducing air into the flow.

Most air inlet and outlet experiments focus on the average concentration of air in the stream and require the measurement of the amount and manner of air out of the stream. Computational fluid dynamics is a relatively new method and a review of studies in numerical modeling of overflows shows that the use of this tool as a research tool in research institutes began and gradually accepted by the hydraulic engineering community. Using computational fluid dynamics alongside physical models is a good way to reduce costs and save time. Due to the high accuracy of this method in determining the jump length of the flow jet, its results can be used to determine the geometric parameters of aerators such as the width of the air distribution duct. As the slope of the shot increases, the entry of air into the stream increases and also the changes in air concentration decrease, so the need for the presence of aerator decreases. Therefore, in the present study, using Fisher (2007) laboratory data to numerically simulate the flow through the aerator, the changes in air concentration along the shoot bed have been investigated. For this purpose, FLUENT software was used to model the two-phase air-water flow and the length of the flow jet jump was used as an important and effective factor in the entry of air into the flow. . Although aerators have been proposed since 1970’s but today there is no any reliable design guideline for determinate aerator spacing.

By determining the trend of changes in bed air concentration, the distance between two aerators can be determined. The air in the stream causes the stream to condense and dampens the shocks caused by the explosion and bursting of the bubbles, thus reducing the damage caused by cavitation; On the other hand, if more than necessary to prevent cavitation, air enters the stream, causing the flow to become bulky, and higher walls should be considered for the shot, which is not economically appropriate. Therefore, it is important to determine the minimum air concentration required to prevent cavitation damage. Determining the location of the second aerator can be determined according to the minimum required air concentration and the length of the flow jet for the height and landing number upstream of the first aerator. And the ramp angle increases and decreases as the water level above the aerator increases. Determining how the air concentration changes downstream of the shot aerator is important for calculating the distance of the aerators from each other, and FLUENT models the process of these changes well. The comparison between numerical and laboratory results shows a very good agreement between the laboratory and numerical model. Finally, a relation for the distribution of air concentration in the substrate was presented, which has a good fit with laboratory data. Due to the importance of the point of impact of the current to the shooting bed (sudden outflow of air due to the impact), the point was used as a reference for calculations.

The comparison between numerical and laboratory results shows a very good agreement between the laboratory and numerical model. In general, the results showed that the air concentration of the lower bed of the aerators increases with increasing landing number, ramp height, step height and ramp angle, and decreases with increasing water height upstream of the aerator, and increases with increasing slope of the air flow and also changes. The air concentration decreases, so the need for aeration is reduced. Keywords Minimum air concentration, Cavitation, Computational Fluid Dynamics, Overflow, Aeration.

The Performance of shooting pool, in addition to the quality of the area in which the flow collides with it, depends to the height of the jet drop, the angle of the water flow, the depth of the jet and the concentration of the jet. By increasing the height of the jet drop, the fall velocity increases and subsequently the jetchr('39')s energy will be more intrusive. Different collision area from the scouring phenomenon that considered in the design step, may lead to damages to the structure of the dam. For this purpose, an Adaptive Neural Fuzzy Inference System (ANFIS) model and Differential Evolution Algorithm were used to optimize the geometry of the flip bucket spillway to reduce the scour depth of the jet. To construct a fuzzy inference system and calculate the scour depth based on available input/output patterns, the ANFIS fuzzy-inductive inference system was used. ANFIS model was used to train input and output data using laboratory data. Finally, using the differential evolution algorithm and also the ANFIS model output, the geometry of the flip bucket spillway has been optimized by defining the objective function for minimizing the depth of the scour hole in the downstream of the spillway.

The increase of population and demand for water, agriculture and energy cause a rapid increase in construction of the dam has been.  Moreover, the increase of dam height to increase storage and estimate the need for water cause increase the velocity of water over the spillway has been.  Spillways, chutes and bottom outlets are important hydraulic structures for dam safety. Due to high velocities combined with low pressures, cavitation damages may occur on chute bottom and cause major damages or endanger the dam stability. Damage experience for flows in spillway tunnels and chutes indicates that damage becomes significant when water velocities exceed 30m/s, this velocity or head can be considered as the borderline for high velocity or high head flows. Introducing air to high-speed flow is necessary to prevent pressure reduction and its events such as cavitation. It is possible to protect spillway surfaces from cavitation damages using aerator devices. Usually, the air entering the flow is not reached to the bed chute.  It is necessary to install the first aerated according to topographic conditions and cavitation index at the appropriate location. By determining the process of changing the air concentration of the bed, the distance between the two aerators can be determined. The air in the flow causes the compression and damping that cause Bursting bubbles. In result, the damage caused by cavitation is reduced.  So cavitation investigations will be necessary and need to reduce and prevent cavitation damages. The length of the flow jet has a fundamental role in determining the distance between two aerators. With increases, the length of the jump, the contact surface of the upper and lower layers of the jet is in more contact with the air and affects the amount of air entering the flow. The absorbed air is removed from the flow after the Jet collision. By determining the minimum concentration of air in the bed, an optimal distance between the two aerations can be selected to prevent cavitation damage. Select the minimum air concentration of bed is based on the relationship provided by Wood (1983). It means that the concentration of average air in the stream is higher, which causes the flow of bulking and requires taller walls for the sides of the shut Which is not economically feasible. The variation in the air concentration of the bed can only be considered as a function of the length of the jump flow and the upstream heights. Creating a suitable duct for providing A negative cavity pressure Lead to the better performance that causes increases the distance between aeration. So use of aerators in suitable places and the entrance of air to water flow is a most effective way to reduce this damage, therefore in this study, an equation has been derived to estimate the distance between two aerators base on 1200 data of 90 experiments with R2 more than 0.84.  results this study have been compared and investigated for aerator of Azad dam.

Rapid sediment motion, which is usually induced by highly erosive and transient flow (e.g. in rivers), is commonly encountered in hydro-environmental problems. Dam-break flow on erodible bed, landslide, failure of river banks and reservoir sediment flushing are few examples of these problems. Accurate prediction of the complexities involved in this water-sediment two-phase system (a multiphase granular flow) is challenging for many conventional mesh-based models. Due to their ability to handle the fragmentation and deformations of interfaces, the mesh-free particle/Lagrangian methods provide a unique opportunity to handle such complexities. The objective of this paper is to develop and implement a multiphase mesh-free model, based on Moving Particle Semi-Implicit (MPS) method, for simulation of rapid sediments transport. The model considers the sediment material as a non-Newtonian viscoplastic fluid whose behaviors is predicted using an exponentially-regularized Herschel-Bulkley (H-B) rheological model. MPS method has some fluctuations non-physical pressure associated. Such fluctuations (though small) can induce some non-physical vibrations and yield criteria for granular flows will be affected. In some recent SPH and MPS studies the use of hydrostatic pressure have been proposed for cases where vertical accelerations are negligible. In cases with significant vertical acceleration, the hydrostatic pressure is not applicable. In this study, by using a smoothed thermodynamic pressure instead of hydrostatic pressure, an improved granular flow simulation is achieved. Two test cases including: dry sediment collapse and mobile-bed dam-break are modeled and compared with experimental results. Results show that sedimentation processes are well-reproduced by the developed model and numerical results show good agreement with laboratory measurements.

Mesh-free particle (Lagrangian) methods، such as moving-particle semi-implicit (MPS) and smoothed particle hydrodynamics (SPH)، are the newest methods in computational fluid dynamics، which have been applied in flow problems with large deformations and inconsistency. The aim of ths research was to develop and improve the simulation of free surface flows، using the new method of weakly compressible MPS (WC-MPS). In the MPS method، pressure is determined by solving Poisson equation. This equation is solved implicitly، which needs too much computer time. In the present research، the WC-MPS method is used to calculate pressure. In this method، as in SPH method، the state equation is used. This equation is solved explicitly، which does not occupy too much computer time. To evaluate the proposed method، the famous applied flow problem of dam break is analyzed. The program is written in C language and validations are performed for this code. To compare the Lagrangian approach with Eulerian approach، dam break is modeled by using FLOW-3D software too. The results of modeling approaches and physical models showed that both approaches have acceptable accuracy in modeling the free surface flow، but the accuracy of Lagrangian approach، especially the WC-MPS، is more than Eulerian approach. The proposed methos had some pressure oscillations، which were analyzed thereafter. Simulation of free surface flows using Weakly compressible moving-particle semi-implicit methodMesh-free particle (Lagrangian) methods، such as moving-particle semi-implicit (MPS) and smoothed particle hydrodynamics (SPH)، are the newest methods in computational fluid dynamics، which have been applied in flow problems with large deformations and inconsistency. The aim of ths research was to develop and improve the simulation of free surface flows، using the new method of weakly compressible MPS (WC-MPS). In the MPS method، pressure is determined by solving Poisson equation. This equation is solved implicitly، which needs too much computer time. In the present research، the WC-MPS method is used to calculate pressure. In this method، as in SPH method، the state equation is used. This equation is solved explicitly، which does not occupy too much computer time. To evaluate the proposed method، the famous applied flow problem of dam break is analyzed. The program is written in C language and validations are performed for this code. To compare the Lagrangian approach with Eulerian approach، dam break is modeled by using FLOW-3D software too. The results of modeling approaches and physical models showed that both approaches have acceptable accuracy in modeling the free surface flow، but the accuracy of Lagrangian approach، especially the WC-MPS، is more than Eulerian approach. The proposed methos had some pressure oscillations، which were analyzed thereafter. Simulation of free surface flows using Weakly compressible moving-particle semi-implicit method

Mesh-free particle (Lagrangian) methods, such as moving-particle semi-implicit (MPS) and smoothed particle hydrodynamics (SPH), are the newest methods in computational fluid dynamics, which have been applied in flow problems with large deformations and inconsistency. The aim of this research was to develop and improve the simulation of open-boundary free-surface flows, using the new method of weakly compressible MPS (WC-MPS). Most studies in the field of Lagrangian models are concerned with closed boundary conditions. This study, introduces a comprehensive method for MPS modeling of cases with inflow and outflow boundaries. Inflow and outflow boundaries are considered and the boundary conditions and recycling of particles are prescribed. The algorithm not only improves the inlet and outlet boundary conditions, but also reduces the pressure fluctuations at boundaries. To evaluate the proposed method, the famous applied flow problem of ogee spillway and steady shallow flow over curved bed is analyzed. The program is written in C language and validations are performed for this code. Comparison of the results of proposed model and physical models showed acceptable accuracy in modeling the free surface flows with open boundary conditions.

Labyrinth spillway is an appropriate option to pass PMF discharge. The most advantages of this type of spillway are higher discharge capacity, easy aeration as well as low fluctuations of flow surface. It is essential to find the optimum geometry considering the maximum passing discharge under specific hydraulic conditions with minimum construction cost. In this study, fuzzy inference system (FIS) and genetic algorithm (GA) were used to optimize the spillway's geometry and satisfy the hydraulic conditions. Applying FIS to evaluate coefficient based on available input - output pattern, ANFIS was employed. Finally, based on GA and ANFIS model output a cost function was defined to minimize the expense under appropriate hydraulic condition.

Labyrinth spillways as appropriate hydraulic structure to pass PMF discharge have been considered. To replace existing spillways of reservoirs which in terms of capacity they are not hydraulically suitable، these types of spillways are recommended. In a certain width with similar head these spillways with nonlinear crest can pass greater discharges compare to spillways with frontal crest. The Labyrinth spillway is a type of hydraulic structure which can be used for the existing and new reservoirs. This structure is considered for hydraulic engineers as a good alternative to keep the water level of the reservoirs in an acceptable position when there is a limit on the maximum level of the water surface of the reservoir. The plan shape of the labyrinth spillway consists of broken lines in the form of triangular، trapezoidal، semi circular and rectangular with several repeating cycles. The main objective of using this type of spillway is to increase the flow capacity of the reservoir while keeping the crest and the water surface of the reservoirs in a reasonable level. The use of this type of reservoir goes back to the early 1920’s; however، the real studies and research in this field started form 1968. The majority of researches in this field are for those with triangular or trapezoidal in plan. The most advantages of using labyrinth spillway in the reservoirs and channels are as follows: 1- If the width of the spillway or channels cannot be chosen enough wide due to upstream or downstream site condition; 2- If the maximum level of the water in the reservoir during the flooding season is limited; 3- If it is desirable to provide larger water storage of the reservoir by making the crest level of the spillway higher without causing floodplain of upstream lands during high inflow to the reservoir; 4- If the capacity of the existing spillways are not large enough with respect to current standards of dams safety; 5- It is more economical due to the need of smaller discharge channel downstream of the spillway; and6- If the reservoir needs to have spillways with the gates to provide larger amount of water storage، in the case of labyrinth spillway the needs of the gates can be eliminated which is very advantageous both in terms of primary costs and maintenance costs. In this study by considering various angles of walls and lengths of nose different types of trapezoidal labyrinth spillways were designed. Applying Flow-3D software which is analytical software to solve equations of flow domain passed over spillway based on Eulerian viewpoint، numerical models were constructed. In order to model turbulence، the K-ε model in RNG state was used. Also to identify position of free surface profile، the VOF method was applied. To verify the results، experimental data of Tullis and Amanian (1995) was employed which the model results showed good agreement with the experimental data. Passing various discharges over the spillway variation of discharge coefficient was studied. It was showed with increase in length and angle of the spillway wall in flow direction the discharge coefficient would increase which is due to less interaction between the flow layers. This increase in small angles due to the reduction in over lapping layers is a considerable loss. At the end based on the obtained results a procedure to design a trapezoidal labyrinth spillway with choosing the angle of wall along the length of the nose was developed.

Stepped spillway's are one of hydraulic components of dams that made by straight step's near the spillway's crest to the toe to dissipate energy. In this study to simulate the flow over the stepped spillway, Flow3D software that is analytic flow field was used. The k-ε(RNG) model was the turbulence model which had been applied, and to determine the free surface flow profiles VOF model was used. The skimming flow regime was only considered. For this purpose 112 spillway models was designed that from which, 96 models were stepped models and 16 models were smooth models. Stepped models had six configurations, two step sizes and four different slopes (15degrees, 30degrees, 45degrees and 60degrees) below the contact points and also step edges were followed WES profile. Smooth spillways had also corresponding specifications for the slope and crest profiles. The goal of this study was investigating on the effects of discharge, spillway's slope, number of steps, configuration and steps roughness on energy dissipation. It was observed that discharge flowing over the spillway and slope of spillway were among the most effective parameters to achieve the highest energy dissipation on stepped spillways and in higher discharges, the number of steps and its configurations had less effects. Also a mentioned configuration considering the parameters to reach the maximum energy dissipation was introduced.

Labyrinth spillways were considered as one of the appropriate options to pass PMF discharge where the system had difficulties. In a certain width in similar head these spillways with nonlinear crest can pass greater discharges compare to spillways with frontal crest. Since they pass large discharge under low hydraulic heads and need to smaller place in plane compare to other types of spillways, they are considered as economical structures. Therefore it is essential to apply optimum geometry with maximum passing discharge under specific hydraulic conditions with minimum construction cost. For this purpose in this research fuzzy inference system and genetic algorithms to optimize the spillway's geometry which satisfy the hydraulic conditions were used. To apply fuzzy inference system and to evaluate the coefficient based on available input - output pattern, from it ANFIS was employed. In this section in ANFIS model using experimental data input data such as angle of spillway walls along the flow (α), ratio of total head to spillway height (Ht/p), and discharge coefficient (Cd) were trained. Finally, using genetic algorithm and ANFIS model output, optimum geometry was found with defining the minimum cost function to satisfy appropriate hydraulic conditions.