computational fluid dynamics

Cavitation is a phenomenon during which, with the movement of the flow and the reduction of the liquid pressure to the saturated vapor pressure of the liquid in susceptible areas, bubbles of air are formed during the flow. The microjet becomes full of energy with the fluid, which when these micro jets collide with the walls, causing vibration and noise and destructive effects such as structural erosion of ship propellers, pump blades, and dams, as well as reducing efficiency, and malfunction of hydraulic devices. This research aims to study different simulation models of the cavitation phenomenon and compare them in the way of cavitation cloud creation and expansion and the effect of this phenomenon on the flow. The present research has been analyzed numerically using ANSYS FLUENT software. In this research, an attempt has been made to study and compare different cavitation models in various geometries. Also, in one example, the results of the numerical model have been compared with the results of the laboratory model. The results showed that the formation of cavitation phenomenon in 10 cm throat compared to 5 cm by 5.88%, in 10 cm throat compared to without throat by 64.71%, and in 5 cm throat compared to without throat by 5.62% has changed.

Floods can cause significant damage to goods and people, particularly in densely populated urban areas with high asset values. Flood risk is typically assessed using flow depth, flow velocity, and water level parameters (de Moel et al., 2009). Meja-Morales et al., (2021) investigated the impact of flow exchanges between a porous urban block and surrounding streets and found that porosity significantly affects urban flood flow characteristics. In another study, Meja-Morales et al., (2023) examined the effect of flow instability and open areas in urban blocks on key flood characteristics and reported that the instability level of incoming hydrographs greatly affects the volume of flood water stored in urban blocks. This research aims to evaluate the distribution of flow depth, velocity, and flow patterns in non-porous urban block streets by considering changes in stable inflow. The study seeks to understand multidirectional flow paths caused by the street network and develop a flood risk map for humans using Flow3D software. The validation results of the numerical model showed that the turbulence model had the highest correlation with the laboratory model, with a relative error of 3% and 6.8% for the velocity profile near the water surface and averaged velocity at depth, respectively. In all models, the right and upstream streets had the highest and lowest depth, speed, and human stability number, while the downstream street had the largest range of flood parameters, with 2 to 3 times the average speed and 3 to 4 danger zones for pedestrians. Increasing the flow rate at Inlet 1 for a constant flow rate at inlet 2 increased the flooding characteristics of the right and downstream streets while decreasing the speed in the left street. Conversely, increasing the flow rate at Inlet 2 for a constant flow rate at Inlet 1 increased the flooding characteristics of the left street, decreased the speed in the right and downstream streets, and had minimal effect on the flood characteristics of the upstream street.

Structures with large opening roofs are vulnerable to wind, in these structures due to the low dead load, the wind load has a greater effect on these types of structures. In the calculation of wind force, one of the coefficients related to the geometry of the structure is the pressure coefficient (Cp), which is provided for some common structures in the loading regulations. Arch opening of 0.1, 0.2 and 0.3 was done using wind tunnel test and numerical modeling based on Computational Fluid Dynamics (CFD) method using ANSYS software and wind pressure coefficients on these structures are presented, it can be seen that with increasing The ratio of the height to the opening of the arch of the maximum coefficients of negative wind pressure (suction), increased in such a way that on the middle axis of the structure in the state of α = 90o, the maximum negative pressure in the structure S-1, S-2, 3-S, is equal to -2 and It is -1.7 and -1.6. The pressure coefficients on the sides facing the wind show a positive number, which indicates the pressure on these surfaces, the maximum positive pressure occurs at α=90o (the state where the direction of the wind is perpendicular to the shed) and in this case the pressure coefficient is equal to +1. In structure S-1 (height-to-opening ratio 0.3), the maximum deformation created for the wind application angle is α=40o, which is 20% more than α=90 o.

Wind force is one of the lateral loads in the design of structures. One of the parameters for calculating wind force on structures is called pressure coefficient (CP), which is related to the geometry of the building. The design codes provide the pressure coefficient of conventional buildings. In the absence of these coefficients in the design codes, wind tunnel testing or numerical modeling should be used. Numerical method based on computational fluid dynamics (CFD) has been used in modeling to simulate wind flow.The model used in computational fluid dynamics in this research is the K-ε (Standard) model. The sphere is modeled with a diameter of 50 cm and the results of numerical modeling are compared with the results of the wind tunnel test presented in reference [2]. The dome is made with different height to span ratio, with increasing height to span ratio to maximum The negative pressure (suction) obtained at an angle of 90o increases as this value reaches –2.24 in dome 122, and it is also observed that at an angle of 150o to 180 o the pressure coefficient is constant for all domes. Finally, the equation the wind pressure coefficients of this type of domes is presented.

In this research work, the maximized transmitted torque due to the impulse from flowing water in an open channel has been studied for five types of cylindrical turbines to find the best water turbine in terms of maximum produced electrical energy. For this purpose, using numerical finite volume method, a set of turbine and blades, consisting of a 3-dimensional cylindrical water turbine of equal diameter and length (1m), with five different blade configurations, has been simulated in a 10 m long and 3 m wide rectangular open channel with no inclination, subjected to a water flow of 2 m/s velocity. Considering the weight of various elements, the set of turbine and blades has been designed so that it remains floating in the channel at various immersion depths. Furthermore, with change in flow depth, the immersion depth remains constant. Considering the magnitude of the flowing water impulse in the channel, the corresponding torque transmitted from the water to the blades of the five types of turbines was determined and the maximum torque value was obtained.

In the present research, five types of blades, attached to a hollow cylindrical turbine of 1 m length and 1 m diameter, have been used. The turbine floats on water at a particular depth in an open channel. The water speed in the open channel determines the torque due to the impulse from the flowing water. Considering the various blades, the resultant torque has been studied numerically using two-phase flow finite volume method. The material for the construction of the cylindrical turbine and the blades is dense polyethylene having a density of with 950 kg/m3. Moreover, the fluids considered in the finite volume numerical computations are water with a density of 998 kg/m3, and air with a density of 1 kg/m3 at a constant temperature of 20° Celsius. The hollow turbine cylinder has a volume of 0.78 m3 and is filled with air. The three-dimensional flow channel, in which the turbine is placed, is a rectangular concrete channel of 10 m length and 3 m width, through which water flow at a depth of 35 cm. To avoid the effect of surrounding walls on the transfer of the flow impulse to the turbine, width of the channel has been considered slightly oversized. The roughness values for the bottom surface of the concrete channel and the turbine walls and the blade set is 1 mm and 0.01 mm, respectively. The depth of the channel is constant at 1 m, which is equivalent to the average depth of most city open channels.

The difference between type 1 and type 2 turbines is in the blade’s angle along the turbine rotational axis. As a result, the produced torque by type 1 turbine is more than that of type 2. On one hand, increase in contact area between the blades and the flowing water in the channel results in higher torques. On the other hand, the angular shape of the increases the slip between the flow and the blades, which reduces the conversion of kinetic energy into static energy. Ultimately, the result of the above two phenomena in type 2 turbine is a reduction in the produced torque to about 15 N/m. In type 3 turbine, an increase in the produced torque was achieved through the increase in the number and shape of the turbine blades. Hence, the implementation of the aforementioned changes relative to type 2 turbine resulted in an increase in the produced torque to about 56 N/m. Therefore, increase in the number of blades and change in the blade shape in this type of turbine compensated for the effects of blade angle elongation in type 2 turbine. Furthermore, in type 4 turbine, the internal diameter of the blades was reduced, while the number of blades was increased, blades distribution angle was changed from 45° to a straight configuration, and the contact area also decreased. Consequently, the amount of flow slip along the blades also decreased. Specifically, the result of all the above mentioned changes in type 4 turbine was to reduce the produced torque to about 14 N/m compared to type 3 turbine. Therefore, the combined effect of reduction in the contact area and reduced internal diameter of the turbine blades is more dominant than the combined effect of increased number of blades and reduced flow slip on the blades. In type 5 turbine, number of blades was reduced by 10 and the blades internal diameter was tripled relative to type 4 turbine, which resulted in a significant increase in the produced torque. Therefore, type 5 turbine, as a floating turbine, may be recommended for production of electric energy in open channels.

Considering the results of the calculations, type 5 turbine with 21 semicircular shape blades, 11 cm in external diameter and 10.5 cm internal diameter, has a higher capacity to produce more torque compared to other types of turbine studied. The factors affecting the final produced torque include the contact area between the blades and the flowing water in the channel, the length of the blades along the turbine axis, the extent of slip of flow when facing the turbine blades, and the number of blades. The produced torque by type 5 turbine is 458.96 N/m, which is the highest among the turbine types studied in this research.

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.

Spillways are used in dam projects to convey flood flows and prevent destructive damages of downstream hydraulic systems, while the construction of other types of spillways is restricted, or there is a need to pass great values of flood discharge with a limited head. Siphon spillway is a kind of dam spillways employed in a number of dam reservoirs. This spillway is applied while the space for constructing other types of spillways is restricted, or when there is a need to pass great values of flood discharge with a limited head. One of the most important defects of this spillway is a complicated fluid-structure interaction due to the establishment of different flow regimes inside the spillway. This situation becomes more complicated when the spillway is not connected to the dam, working individually. Despite the importance of the operation of these hydraulic structures, there is a lack of comprehensive research to investigate the hydraulic behavior of these structures and the flow field around and inside them. In the present study, different unsteady flow regimes including sub-atmospheric flow, two-phase flow, and black-water flow and their effects on the hydraulic characteristics of siphon spillways were investigated. For this purpose, ANSYS CFX software was applied to simulate the flow field in such spillways. To validate the obtained flow regimes, via numerical modeling, experimental results of the former studies were used. Results indicated that the pressure fluctuations were mostly greater beneath the inlet and throat of the spillway compared to the other sections.

In this research, the behavior of the morning glory crest and throat were investigated in the event of flood. Spillway structure and dam reservoir fluid were respectively simulated using the finite element method and finite volume method in ANSYS software. The morning glory structure's behavior was performed in three scenarios: a) the static analysis scenario of the dam empty reservoir, b) the static analysis scenario of the full reservoir while there is no flow over the spillway crest, and c) the dynamic analysis scenario of structure-fluid interaction in two parts: 1- starting of the flood flow on the morning glory spillway, and 2- the steady state flood flow on the morning glory spillway. The concrete structure analysis was linearly done in all three scenarios. Based on the results, the most critical condition for the structure in terms of maximum tensile stress was 5.3 MPa in the first part of the third scenario. The maximum compressive stress was 12.3 MPa in the second part of the third scenario. Spillways are one of the most important and vital structures in the life of a dam. The efficiency and proper functioning of such structures requires a precise and responsible design. The morning glory spillways have a great deal of importance due to the standing structure inside the dam reservoir. The weight and hydrodynamic loading on the body of spillway concrete structure during a flood complicates the interaction analysis of the structure and the fluid. By using numerical methods in computational fluid dynamics and structural analysis, we will be able to predict the behavior of structure and fluid hydrodynamics parameters. In model's fluid domain, choosing the type of Navier---Stokes equations proportional to the efficient RANS mathematical model of turbulence, is very sensitive. Mainly one equation turbulence models such as the Spalart-Allmaras or two-equation models, such as K-Epsilon or K-Omega, are based on Boussinesq hypothesis. Boussinesq hypothesis explains how to estimate the components of the fluid stress tensor for the above-mentioned turbulence equations. This assumption, by simplifying the computation process, only a in some parts of the fluid domain, analyzes the turbulence as anisotropic.

Investigation of multi-physics problems such as flow-structure interaction (FSI) in free surface is very important in mechanical engineering, whereas numerical simulations of such problems have been widely conducted by researchers. The implementations of CFD in engineering applications are most of the time based on the Eulerian description. In this method, one can focus on flows at a fixed spatial point x at time t and any flow variable Φ is expressed as Φ (x, t). This description has been studied for over fifty years and is clearly understood. Most of commercial codes have been developed by using finite difference, finite element and finite volume approaches. Simulating free surface flow with most Eulerian CFD methods is potentially very difficult as explicit treatment of the free surface is required. Moreover, The problems of most Eulerian and mesh-based numerical methods for complex free surface deformations involves difficulties and complexities of various boundaries remeshing as well as moving boundaries and exact determination of free- surface fluid. Another description of study of CFD is the Lagrangian method where one can follow the history of an individual fluid parameter through the time. In the Lagrangian methods, any flow variable is expressed as Φ (x0, t), where the point vector x0 of the particle at the reference time t = 0. Smoothed Particle Hydrodynamics (SPH) is a meshless and fully Lagrangian method which is able to simulate the FSI problems due to its simplicity and capability, as there is no special treatment needed for the free surface. The current problem in hydrodynamic science and fluid engineering is studied as a complex phenomenon in free-surface flow. Smoothed Particle Hydrodynamics (SPH) is a flexible Lagrangian and meshless technique for CFD simulations initially developed by Lucy (1977) and Gingold and Monaghan (1977) to simulate the nonaxisymmetric phenomena in astrophysics. In recent years, the SPH method has been very popular in fluid mechanics, e.g. multiphase flows,3 heat conduction,4 underwater explosions, free-surface flows, etc. In this method, each particle carries an individual mass, position, velocity, internal energy and any other physical quantity. The Lagrangian nature of SPH would lead this method to be well suited to problems with large deformations and distorted free surfaces. Simplicity, robustness and relative accuracy in comparison with other numerical methods are the main advantages of using SPH.10 Moreover, the SPH method can handle fully nonlinear, multiply-connected free-surface problems and extend computations beyond wave breaking, which need complex treatments in other grid-based methods, e.g. Volume of Fluid (VoF).In this approach the computational domain is formed by a set of particles. Each particle represents macroscopic volume of fluid conveying information about the mass, density, pressure, speed, position and the other parameters related to the nature of the flow. However, the computational cost is a disadvantage of SPH because the time step is small because of the explicit integration scheme in a weakly compressible formulation. This method has been successfully applied to a range of free-surface problems which involve breaking and splashing up There is a choice of SPH formulation in the literature mostly expressed in weakly compressible forms where pressure is obtained from the equation of state In this research, SPH is used to investigate the flow-structure interaction in free surface. First, the simulation of dam break problem on a dry and infinite bed is shown and compared with the experimental data. Then, and after implementing the governing equations, the vibration of a beam is studied. Finally, the dam break problem on an elastic gate is shown. Comparison between the SPH results and available numerical and experimental data shows that the SPH method is useful method for simulating the FSI problem.

Tyrolean weir has a hydraulic trash rack that is installed through the river stream for water-intake. In this paper, hydraulic flow of this structure is simulated three-dimensionally, and its perfor-mance is studied through a change of the amount of the opening between trash racks. The modeling and analysis is done with AN-SYS CFX 16.0. The accuracy of numerical model and the results are confirmed by comparing the parameters such as discharge of the weir output and wetted trash rack length with models and re-sults of the prior researchers. To generalize the results, Tyrolean weir is investigated in cases of three different input discharge and three different inclinations of the trash rack from the horizon (0%, 10% and 20%). The results show that increasing the inflow in a given inclination of the weir surface, less part of the water-intake is as a slip of water over the network and pouring of that in the weir. For instance, 35% opening is required to water-intake of 50% of maximum input discharge of 12 lit/ (s.m).

Three-dimensional submerged jet at a sudden expansion includes chaotic hydrodynamics. At a sudden expansion, secondary flows developed adjacent to the potential core of the jet generate turbulence, and the formed eddies cause energy transfer and dissipation and decline of fluid momentum in the zone of established flow. By utilizing an efficient mathematical model of turbulence, hydrodynamic flow parameters can be predicted with a good accuracy in various locations. This paper studies the three-equation mathematical models of turbulence, namely the Walters and Cokljat (k-kl-ω), and the seven-equation Reynolds Stress mathematical model of turbulence. Comparison between the results of computational fluid dynamics using Ansys Fluent software and experimental results shows that Reynolds Stress model of turbulence predicts the results with a higher accuracy. It can be concluded that this higher accuracy is due to the use of individual transport equations for each component of the stress tensor in the normal conditions of inhomogeneous and anisotropic turbulence.
Kinetic energy, very high fluid momentum and pressure fluctuations are among characteristic of a submerged jets at a sudden expansion. How the energy is dissipated by the flow and how the secondary flow structures are generated need an extensive research. In the submerged jets, because secondary flows are developed in the vicinity of jet potential nuclear and eddies are generated in various sizes, the energy is received from the mean flow and will be being dissipated while being transferred. The dissipation process can be observed during the interaction between stress and strain fields of fluid elements (second-order tensor interaction). Formation of eddies with different sizes and decay of them into smaller structures prompt the process of turbulence diffusion. The energy-bearing eddies formed in the vicinity of the jet potential core are displaced by convection terms. After these eddies are displaced, they experience decay and reduction in size (Kolmogorov microscale) and finally disappear. Rotational dynamics around the jet potential core is of a great importance in terms of flow kinetic energy dissipation; it is why the sudden expansion ratio is a number that represents the range of rotation. Therefore, understanding the flow behavior as well as how the resulting energy is generated and dissipated requires the flow parameters to be known. In order to predict the most accurate (closest to reality) values of the hydrodynamic parameters of a submerged jet, it is necessary to utilize an efficient mathematical model. Among the proposed models of turbulence, only the multi-equation Reynolds stress mathematical model has included anisotropy. Based on what have been stated so far, it seems that the existence of discrete transport equations for each component of stress tensor for a fluid and turbulence kinetic energy dissipation as well as comparison with experimental results provide the possibility of acceptable accuracy in predicting the flow hydrodynamic parameters. In this model, the term of turbulence kinetic energy generation from the mean flow, energy dissipation term, and pressure-strain term transferring the turbulence kinetic energy toward different directions of the coordinate axes are among the very important elements of the transport equation.