جستجوی مقالات مرتبط با کلیدواژه "steady flow" در نشریات گروه "عمران"

Coarse-grained gravel (rockfill material) has numerous applications in engineering including filtration, gabion construction, channel lining, stilling basins, ponds, and cobble stone dams as well as flood control. In the fine-grained media, there is a laminar flow with a linear relation between hydraulic gradient and flow velocity so that the flow follows Darcy law (Eq. 1) (McWhorter and Sunada, 1977). However, in the coarse-grained media, due to the presence of voids, flow velocity is high with a tendency to the turbulent flow formation (Hansen et al. 1995), and there is a nonlinear relation between hydraulic gradient and flow velocity and flow follows non-Darcy law. Hydraulic gradient equations in the non-Darcy media considering steady flow condition are classified into two groups of power and binomial equations, according to Eqs. 2 and 3 (Forchheimer, 1901; Leps, 1973; Stephenson, 1979).

IntroductionNon-darcy flows into two categories: parallel flows (such as gravel dams, gabions, etc.) and radial flows (such as flows near wells drilled in coarse-grained alluvial beds, etc.) are divided. In the first category, streamlines are almost parallel so that there is no curvature or contraction of streamlines in the plan view. This type of flow is found in both pressurized and free-surface modes. Radial non-darcy flow analysis has many applications in the fields of civil engineering, geology, oil, and gas. The equations governing the radial non-darcy flow are solved using numerical methods of finite differences, finite elements and finite volumes. Solving these equations requires boundary conditions and a lot of data and is almost bulky, time consuming and costly. While, gradually varied flow theory, requires much less data and is easier and less expensive. For this reason, in the present study, for the first time, using experimental data recorded in a large-scale (almost real) device, the application of the gradually varied flow theory in radial non-darcy flows with free surface has been investigated. In other words, since the calculation of water surface profiles in a radial rockfill is of great importance. In the present study, using large-scale (almost real) experimental data and the gradually varied flow theory, the water surface profile in radial non-darcy flow with free surface and in steady state has been investigated.MethodologyIn the present study, due to the compatibility of cylindrical coordinates and its adaptation to the physics of problems related to radial flows, a device has been constructed in the laboratory of Bu Ali Sina University in the form of a semi-cylinder with a diameter of 6 meters and a height of 3 meters. The dimensions of this device are made on a large scale and the effects limitations have practically no effect on the testing process. To measure piezometric pressure, piezometric grids have been used. The device has a volume of 14,000 liters and a capacity of materials weighing approximately 40 tons. Four pumps are installed in parallel at the top of the device to generate the required flow. Coarse-grained river materials with a diameter between 2 to 10 cm, a porosity of 40%, a Cu of 2.13, and a Cc of 1.016 have been used. To perform the tests, the model is first filled to a certain height (53, 60, 70, 85, 95, 110, 120, 140, 150, and 160 cm) by pumping operations. The flow rate created in these experiments is in the range of 49.94 to 53.16 L/s.Results and DiscussionOne-dimensional analysis of steady-non-darcy flow using gradually varied flow theory and two-dimensional analysis using Parkin equation solution. Most research has been done in parallel flow rockfills. Also, solving the Parkin equation in both parallel and radial flows requires a lot of data such as boundary conditions upstream and downstream, as well as the boundary condition of the water surface profile, and the calculation process is complex and time-consuming. The gradually varied flow theory requires much less data than solving the Parkin equation, and the water surface profile obtained from it is also used as the main boundary condition in solving the Parkin equation. In other words, calculating the water surface profile in a radial rockfill is very important to studying the movement of water. Also, the water surface profile is the main boundary condition in the two-dimensional analysis of steady flow (solving the Parkin equation), and with it, upstream and downstream boundary conditions will be practically available. For this reason, in the present study, using large-scale (almost real) experimental data and the gradually varied flow theory, the water surface profile in the case of radial non-darcy flow has been calculated. To calculate the flow depth at different points (water surface profile) using the gradually varied flow theory, the amount of flow depth at one point and the coefficients m and n must be available. Since the flow depth measurement in the well (downstream of the desired interval) can be measured, in the present study, the calculations started from the downstream (depth of flow in the well).ConclusionIf the gradually varied flow theory is used to calculate the water surface profile in the case of radial non-darcy flow with a free surface, the mean relative error in the case of pumped heights is 53, 60, 70, 85, 95, 110, 120, 140, 150 and 160 cm are equal to 1.56, 0.96, 0.61, 0.45, 0.28, 0.19, 0.13, 0.16, 0.11 and 0.05 are calculated, respectively. In other words, the average mean relative error (MRE) of calculating the water surface profile for different heights of pumped water is equal to 0.45%. Also, according to the obtained results, the greater the depth of water pumped upstream, the higher accuracy of the gradually varied flow theory.KeywordsRadial Non-Darcy Flow, Steady Flow, One-Dimensional Analysis, Gradually Varied Flow Theory.

Solving the problem with the meshless method is based on selecting a series of points from inside the computational area and boundaries without meshing. In the present study, the phenomenon of seepage below the dam under steady and unsteady flow conditions has been investigated by combining the Meshless method and the Finite Difference Method. Problem solving and calibrating operations were done by coding in MATLAB software. The Meshless method was used for spatial sentences and the Finite Difference Method was used for the discretization of temporal sentences. The results showed that the shape factor (α) for low points is 0.85 and for high points is 0.52, which indicates the proximity of the initial approximations to the main answer. Considering that, the shape factor depends on the geometry and the governing equation, so the same shape factor was obtained for the steady and unsteady conditions equal to 0.52. In the unsteady condition, with the water level behind the dam remaining constant, the water head below the dam also reaches a constant value over time. Also, examination of the results showed that in numerical problem solving, a low error is not a criterion and among the various basic functions, only the MQ function has the better hydraulic performance to draw equipotential lines, so that for 133 points, the shape factor and root mean square error index are 0.52 and 0.0108, respectively.

Pipelines are used all around the world to transport fluids from one location to another. When these pipelines meet rivers, seas and oceans pipes are laid on the solid bed and it causes changes in the flow pattern around the pipes. In result of these changes, the shear stress of bed under pipelines and turbulence of current will be increased, and scour will occur under pipelines and the scour hole will form and develop. These holes cause damage and failure to the pipe due to the pipe weight. In case of failure of the pipe, irreparable damages will incur to the environment and there will be huge financial costs. Therefore, it is very important to study the scour depth and effective variables to reduce scour and prevent damages. Researchers have conducted experimental and numerical studies on scour phenomenon and have provided relations over the years.
In this research the effect of various factors on this phenomenon in steady current are investigated using Gaussian process regression (GPR) and support vector machine (SVM) and it is compared with the previous presented relations. To this end several laboratory data were used and after defining several non-dimensional parameters the performance of these methods was evaluated. The result of this research demonstrated that these methods are better than experimental relations and have promising outcomes. This study have shown that an SVM model with ℎ/D, D/d, Re and S0 variables in steady current have the best results. It is worth mentioning that ℎ and variables in steady current have the most significant effect on the scour below pipelines.

By constructing a pipe containing fluids such as water and oil crosswise with the direction of the river, the pattern of river flow around the pipe changes. . These changes in the flow pattern around the pipe and an increase in shear stress on the substrate, which results in a scouring hole under the pipeline. Local scouring around pipelines across the river is one of the most important causes of their failure and destruction. Therefore, it is very important to study the mechanism of occurrence of this phenomenon around the pipelines and to evaluate the amount of scouring and the characteristics of the local scour hole around them. Wu and Chiew (2013) investigated the scour hole and the flow field around a pipeline under steady flow. The flow field in these experiments was measured by an acoustic Doppler velocimeter. The results of this study showed that the presence of vortices due to the pressure difference created upstream and downstream of the pipe causes the formation of a force for the movement and displacement of sediments. Also, the flow from under the pipe into the scour hole causes it to expand further. Zhao et al. (2015) performed laboratory and numerical study of scouring under two consecutive pipelines with different distances from each other. In moving bed conditions, it was observed that the depth of the scour hole under the upstream pipe is slightly greater than the scour hole under the single pipe, while the depth of the scour hole under the downstream pipe is much greater than the scour depth compared to the single pipe. Yan et al. (2020) numerically examined the local scour around the pipeline across the river under steady flow conditions. In their study, the CFD method and variable mesh technique were used to model the sediments transport and the results were compared with the results of the laboratory model. The results showed that the method used to model scour and sediment problems respond satisfactorily. The aim of this study was to investigate the effect of the installation depth of pipe across the river in steady flow on temporal changes in scour pattern and sedimentation around the pipeline were processed by recording video information during each experiment.

The present study was performed in the hydraulic laboratory of the Faculty of Water and Environmental Engineering, Shahid Chamran University of Ahvaz in a rectangular flume 10 m long, 0.74 m wide, and 0.6 m high. The walls of the flume were made of glass and the floor was made of steel. The flume had reservoirs at the beginning and end and a section to calm the flow. To investigate the scouring phenomenon around the pipe crossing the waterway, in the middle of the flume, in an area, 1.5 m long and 15 cm thick, uniform sediments with Medium size (d50) 0.7 mm, relative density (Sg) 2.65, and standard deviation (σg) 1.4 were poured. In this study, pipes with a diameter of 20, 40, and 60 mm at a quarter of the pipe diameter under the bed (e/D= -0.25), bed installation depth (e/D= 0), a quarter of the pipe diameter above the bed (e/D= 0.25), and half of the pipe diameter above the bed (e/D= 0.5) were used. The pipes were made of PVC and were installed perpendicular to the flow across the flume. The experiments were performed at a flow rate of 33 liters per second and a flow depth of 14 cm. The duration of all experiments was 120 minutes. The total number of experiments was 12. In these experiments, clear water conditions prevailed.

In most of the researches in this field, a comprehensive study has not been done on the temporal changes of the scour hole parameters and their focus has been mainly on the scour depth parameter when the pipe is placed on the bed. Comparing the present laboratory study with other studies related to the study of diameter and depth of installation, one of the most relevant studies is related to the laboratory estimation of scour under the pipeline by Ataieyan (2012). In which the scour under the pipeline is investigated with emphasis on the effect of installation depth. The maximum amount of scouring was observed at a depth of one-fourth of the pipe diameter at the top of the bed. The result of the experiments performed in the present study also confirms that at the depth of installation, e/D= 0.25, due to the narrowing of a certain distance between the sub-pipe and the surface of the sedimentary bed and The formation of vertical and horizontal vortices showed the highest maximum scour depth compared to other installation depths. The results related to the effect of installation depth for different modes are as follows, e/D= 0.25, e/D= 0, e/D= -0.25, e/D= 0.5 from maximum to minimum, respectively, they had the highest amount of scour depth. Another parameter studied for scouring is the distance between the maximum scouring depth and the center of the pipe. This parameter is indicated by Xds. the location of the maximum scouring depth at the beginning of the experiment was moving upstream of the pipe. At installation depth, e/D= 0.5, the highest rate of maximum scouring depth was observed downstream compared to all cases. In all experiments, about 80 to 90% of the height of the deposition ridge occurred in the first 10 to 20 minutes of each experiment.

The results of this study showed that at all installation depths, 80 to 90% of the scouring depth was performed in the first 40 minutes of each experiment. The depth of pipe installation was one of the most influential factors on the dimensions of the scour hole. In all experiments, sediment from erosion was deposited downstream of the pipe and formed a sediment ridge. The maximum and minimum deposition heights occurred at the installation depths of e/D= 0.5 and e/D= 0.25, respectively.

In the present research, the formation and development of the delta and the interaction of the flow pattern and sediment in the reservoir under steady and unsteady flow condition of water and sediment is experimentally investigated. Laboratory observations showed that in spite of the perfect symmetry of the geometry and hydraulic conditions of the model, the flow at the reservoir entrance is randomly diverted to one side and an asymmetric but stable flow is created at the reservoir entrance. The sediment entry and its deposition in the reservoir leads to an unstable flow and a change in flow direction. The instability in unsteady flow is more severe than under steady flow condition. The results showed that with the growth of the delta, the deviation of delta decreases and approaches the symmetry. Delta development takes place in longitudinal and transvers directions and the maximum elongation of delta is about 0.8 at the initial stages, and decreases with delta development. An equation is developed for time of change in direction of flow in terms of the specific parameter of the hydrograph. In order to study the sedimentation pattern, non-dimensional parameters such as the length of delta (Xt*), deviation ratio (ψ) and delta elongation (η) are introduced. An equation is developed to estimate the length of delta using non-dimensional parameters.

Study on the physics of sediment particle movement at grain scale is essential for better understanding sediment transport phenomenon and estimating the rate of sediment transport in rivers and marine environment. Sediment particles basically transport in two modes of bed and suspended load. Bed load takes place through sliding، rolling and saltation، from which the latter is dominant. Many parameters influence on saltation phenomenon، which their effects are not fully understood. These influencing parameters make the saltation a stochastic phenomenon. In the present article the influence of the affecting parameters on movement of sediment particles at saltation mode of transport under unidirectional steady flow are investigated. A numerical model is developed to simulate the particle motion in bed load saltation with considering the main contributor forces. Then the influencing parameters that effect on the jump length and average velocity of the particles are studied. Among them are the initial condition، the particle position between other particles and the shape of particles. The influence of the velocity profile on the jump length and average velocity of the particles are also studied. In summary، the change in the initial condition including the initial velocity and angle produces less than 10% variation on the particle jump length and velocity. On the other hand the position of the grain between the other particles is considerably influential with 40% change in the jump length and average velocity. The particle shape is most important parameter in term of the influence on the jump length and average velocity; there is a 50% difference between the jump length of spherical particles and flake-shape particles، for average velocity it is about 10%. The result of the study improves our understanding of particle motion at grain scale and ultimately results in the better estimation of sediment transport rate.

Study on the physics of sediment particle movement at grain scale is essential for better understanding sediment transport phenomenon and estimating the rate of sediment transport in rivers and marine environment. Sediment particles basically transport in two modes of bed and suspended load. Bed load takes place through sliding، rolling and saltation، from which the latter is dominant. Many parameters influence on saltation phenomenon، which their effects are not fully understood. These influencing parameters make the saltation a stochastic phenomenon. In the present article the influence of the affecting parameters on movement of sediment particles at saltation mode of transport under unidirectional steady flow are investigated. A numerical model is developed to simulate the particle motion in bed load saltation with considering the main contributor forces. Then the influencing parameters that effect on the jump length and average velocity of the particles are studied. Among them are the initial condition، the particle position between other particles and the shape of particles. The influence of the velocity profile on the jump length and average velocity of the particles are also studied. In summary، the change in the initial condition including the initial velocity and angle produces less than 10% variation on the particle jump length and velocity. On the other hand the position of the grain between the other particles is considerably influential with 40% change in the jump length and average velocity. The particle shape is most important parameter in term of the influence on the jump length and average velocity; there is a 50% difference between the jump length of spherical particles and flake-shape particles، for average velocity it is about 10%. The result of the study improves our understanding of particle motion at grain scale and ultimately results in the better estimation of sediment transport rate.