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

The complex interactions between streameise flow and curvature- induced secondary flow in bends, bed morphology cause erosion near the outer bank, and deposition near the inner bank, that it can endanger the outer bank’s stability and also reduce the navigable width of the rivers. There are several techniques to protect the outer bank, reduce adverse impacts and control bed erosion in bends. Methods used in most parts of the world include submerged groynes, spur dikes, and bandal-like structures. But, meny these Methods have the disadvantage of being fixed constructions on the bed river that caused a threat for navigation. This paper describes a new way that consists in changes the bed morphology with provoking changes in the flow pattern. Dugue, et al. (2011, 2012) studied morph dynamic bends rivers with the Air-bubble screen method. Their results reveal that the air bubble screen is able to modify the flow pattern by shifting the maximum scouring from the outer bank towards the center of the flume and does not endanger its stability anymore. Dugue et al, 2013 studied scour reduction at a 193° bend by an air-bubble screen method. Velocity pattern at 70° bend showed that the bubble-induced flow pattern overcame the secondary flow induced by the outer bank while decreasing the morphology slope. It also showed 50 percent reduction in the maximum scour depth. Shukry (1949) is one of the first researchers in the study of erosion of the outer bank of rivers in a canal with a 180- degree bend with various proportions of the rivers radius to its width. There have been very few investigations about air bubble screen in 90 bend, therefor  in this study the effect of the air bubble screen on flow pattern and Secondary flow strength  and bed shear stress in 90 degree bend under Froude numbers 0.45 is examine.

When building a bridge over a river section, to reduce the length of the bridge deck structure for economic and operational reasons, usually part of the river floodplain is blocked on both sides by access embankments. This causes narrowing of the river and changes in the hydraulic conditions. As the velocity gradient increases in the narrowed section, the shear stress on the riverbed also increases. Increased shear stress can cause erosion in the riverbed and at the base of foundations and backpacks, exposing the bridge to hydraulic damage. On the other hand, sometimes the direction of the axis of the bridge is not perpendicular to the direction of the flow, and the so-called bridge has an angle of inclination. In these bridges, access embankments can be called flow-guiding and embankment embankments according to their location in the water flow path and how they operate. In this paper, the effect of symmetric and asymmetric progress of Bihadar bridge access embankments in floodplain on hydraulic flow is investigated. To improve the hydraulic conditions of the flow through the rift embankment, the asymmetric advance of the access embankments has been proposed and investigated. For this purpose, three-dimensional flow modeling was put on the agenda. Initially, the numerical model was validated based on previous experimental studies. Then, the effects of symmetrical advancement of access embankments at different angles of the bridge were evaluated and then the asymmetric progression of access embankments to reduce shear stress in the river section was proposed and investigated. The results show that the 70% advance of the flow-conducting embankment at the critical peak angle, due to the constant flow cross section, reduces the maximum velocity and shear stress by 14 and 35% in the studied river section, respectively.

Hydrodynamically, there is a complex confrontation among the porous obstacles along the flow path and the fluid is significantly important Existence of obstacles in the flow path causes changes in hydraulics and hydrodynamic parameters of the flow. Among the hydrodynamic parameters that change due to the presence of obstacles in the flow path, we can mention the intensity of flow turbulence. Since turbulence is related to the energy dissipation of flow, it has always been important to study this phenomenon.one of the most important issues of river engineering is The construction of obstacles in the fluid path, especially when these obstacles are built at the river crossing. The results of studying the behavior of fluid around porous obstacles can be used in the design of gabion groins, as well as the construction of gabion obstacles in the flow path, to dissipate flow energy .In this study, the flow structure around porous groins on the side of the canal and porous obstacles in the middle of a straight channel with a fixed bed has been investigated in a laboratory. The ADV  was used to measure three-dimensional velocities and reynolds stresses around the gabion obstacles with different porosity on the side and middle of the channel The obstacles on the side of the canal act as groins and the obstacles in the middle of the canal act as obstacle consuming the energy of the stream. The velocity was measured at 1265 points for groins and it was measured at 1525  points for obstacles located in the middle of the channel .The results showed that the three-dimensional velocity components  decrease with increasing porosity in groins and obstacles. Also, the separation of flow, return flow, ..., is more severe when the obstacles is in the middle of the channel than when the groin is on the side of the channel wall.Also, the effect of porosity percentage on obstacles is much greater and clearer than on groins.And the intensity of turbulence and the extent of the area have the maximum intensity of turbulence in the obstacles in the middle of the canal is more severe than the groins in the side wall. The maximum amount of kinetic energy for obstacles is somewhat larger than for similar groins. However, the maximum turbulence intensity for the obstacles with porosity percentages of 0, 20, 40 and 60 is about 2.95, 2.4, 1.9 and 1.6 times the maximum turbulence intensity in the same groin, which is relatively large. Therefore, it can be understood that the presence of a obstacles in the middle of the channel, although it does not cause much change in flow energy, but the presence of obstacles in the middle causes the current energy dissipation up to about 2 times the current energy dissipation behind the groin.and, the process of reducing the intensity of turbulence is slower at higher porosities. Finally, The width of the zone with more turbulence intensity in the cross section for the obstacles with porosity of 0, 20, 40 and 60% is about 2.3, 2.1, 2 and 1.9 times the range in the same groin, respectively. Which in itself indicates a greater depreciation of the flow behind the obstacle.

In this research, the SSIIM numerical model is investigated to investigate the flow pattern, velocity meters, current strength and turbulence parameters around the pier of the vertical bridge with submerged vanes. The parameters of different overlap length, and the distance of the submerged vanes upstream from the pier of the bridge and from each other in sharp 180-degree bend in the steep-ratio hydraulic radius 2 with a height of 90 cm and a width of 100 cm and a length of straight direction upstream and downstream of the bend respectively 6.5 m and 5 m, were analyzed. SSIIM software was used to investigate the flow field around the around the cylindrical bridge pier and upstream submerged vanes. The K-ε turbulence model was also used to solve the Navier-Stokes equations. In order to validate, the results of the simulated model were compared with the available experimental data in the case of submerged vanes located upstream of the bridge pier. The match between the numerical and experimental data indicated the proper performance of the SSIIM numerical model in modeling the flow pattern in the problem under study. The results showed that the range of secondary flow power was measured in all models with the presence of upstream submerged vanes from 10.5 to 12%. In the case where the submerged vanes did not overlap with each other and the placement of the submerged vanes at a distance of 2.5 times the pier diameter from the pier and a distance equal to 2 times the pier diameter from each other and above the pier is the lowest value and in the overlapping state 100 Percentage of vanes, in the arrangement of submerged vanes at a distance of 5 times the pier diameter and 2 times the pier diameter of each other and above the pier has its highest value. Shear stress changes after local scouring were also calculated. The maximum shear stress from the beginning of the bend to near the bend exit was inclined towards the inner bank of the bend and in the bend output range the maximum shear stress was transferred to the middle of the bend and then the second half of the bend. The results also showed that the tangential velocity was increased as the immersion range of the submerged vanes and the pier approached, so that the maximum tangential velocity occurred in the passage through the pier. As the height from the initial bed increases, the maximum positive tangential velocities increase. The results also showed that at the level near the bed, the radial flow is towards the inner bank. The greatest difference in radial velocities, as opposed to tangential velocities, is observed near the bed. The highest changes in the maximum landing number are related to models with submerged vanes at a distance of 7.5 times the pier diameter, which by changing the distance between the vanes from 1 to 1.5 times the pier diameter from each other, an increase of 12.5% And by changing the distance of the submerged vanes to 2 times the pier diameter from each other, it decreases by 11%.

One of the phenomena that has been studied both theoretically and experimentally in hydraulic engineering is hydraulic jump. Hydraulic jump causes the flow to lose a considerable amount of energy due to the change in its regime from supercritical to subcritical. Since fluid flows in a conduit may be either of a free type (open-channels hydraulic) or of an under-pressure type (under pressure conduits hydraulic), the hydraulic jump can occur in both situations regarding the type and function of the system. Therefore, it can be said that hydraulic jump can occur in open channels as stilling basin. It can also occur in downward inclined pipes that contain a large air package. Additionally, since the present paper deals with pipelines, the following general statements can be mentioned about them. Pipelines are used to transfer fluids such as water, oil, gas, and wastewater offshore or onshore, either in the form of two-phase or multi-phase flows. The importance and function of the flows inside the pipeline must be studied. Export pipelines transfer fluids from platforms or FPSO (Floating Production, Storage, and Offloading) to the beach and they usually contain gas-condensate or oil with a little water. Infield pipelines transfer flows from the wells or manifolds to the platform or FPSO [Guo et al (2014)]. Offshore pipeline design includes structural, geometrical, and hydraulic designs. In structural design such matters as buckling and collapse during pipeline operation are considered, and several studies have been carried out regarding these matters. In geometrical design, diameter determination parameters (based on the flow capacity and precise analysis of the flow assurance in offshore pipelines), and wall thickness (according to the standards) are considered, each of which is somehow related to the flow hydraulics. Therefore, hydraulic design of pipeline is of utmost importance and problems related to this area must be examined precisely. However, it needs to be considered that most of the studies about hydraulic jump are carried out on channels, open conduits and stilling basin. Flows in open channels can shift from supercritical to subcritical. Such shifts happen very suddenly and appear due to hydraulic jump [Akan (2006), Lauchlan (2005) and Vasconcelos and Wright, (2009)]. Most of the studies about the hydraulic of pipelines that contain multi-phase liquid and gas flows concentrate on the function of flow regimes, pressure and sever slug. Therefore, it can be claimed that there is a lack in studying hydraulic jump in under pressure pipelines with multi-phase liquid-gas flows, because most of the studies are carried out on wastewater transfer lines and open conduits. The present paper, therefore, deals with the numerical analysis of hydraulic jump in pipelines with two phase water-air flows. To this aim, some experimental information has been taken from Pothof (2011) to verify the accuracy of findings and numerical modeling. It must be noted that Pothof has worked on wastewater pipelines that function gravitationally, while this study deals with under pressure pipelines in offshore conditions.

Regarding Pothof's experimental work (2011) which analyzes the occurrence of hydraulic jump, and using its data, the hydraulic jump is then numerically analyzed.In order to analyze the hydraulic jump numerically, roughness values, geometrical properties of the pipeline, and the angles are specified. General characteristics of the submerged pipe are also presented. Some other properties include: diameter=8in, water flowrate=128160kg/hr, air flowrate=32040kg/hr.Pressure loss in pipelines with multi-phase flows, includes frictional and hydrostatic pressure loss, is calculated based on mixture density and velocity. In hydrostatic pressure loss, it must be noted that since in horizontal pipelines ΔZ=0, the pressure loss istherefore zero, too. In upward inclined pipelines this value is positive (in line with increasing the total loss) and in downward inclined pipelines it is negative (in line with decreasing the total loss). In order to determine flow regimes semi-theoretically, Taitel and Dukler (1976) first modeled a stratified flow in a pipe, presupposing that the flow has been stable. Then, they determined how the flow regime was transferred from stratified to other flow regimes. The results of their analyses that are presented in a map for two-phase gas-liquid flow is used in this paper to determine flow regimes in two phase flow.

Hydraulic jump occurs in open water transfer structures and channels and its behavior and occurrence in such situations is studied precisely. Similarly, it is important to study it in pipelines, because it affects the behavior of the fluid inside the pipeline. When there is a pipeline containing a two-phase water-air flow, it can be assumed that the fluid inside the pipeline might face a hydraulic jump. Predicting such a phenomenon and its effects on pipeline during its working lifetime is a very important issue in the industries that deal with pipelines which transfer multi-phase fluids. Therefore, the present paper studies the occurrence of hydraulic jump in pipelines with two-phase water-air flows.

It can be concluded, based on the hydraulic gradient and the resulted losses, that hydraulic jump occurs when there is some air in the pipeline; and as the experimental researches showed, as the angle increases, the jump height increases, too. In all of the above- mentioned analyses except for the mode in which the angle is 30 degrees, flow regimes, according to Taitel and Dukler are annular mist for A-B, annular mist for B-C, and annular mist for B-D. Regarding what has been said, it can be stated that when total pressure loss in the negative direction (i.e., when the hydrostatic loss overcomes frictional loss) approaches zero, the flow regime might change.

Flow structure around bridge pier is a very complicated phenomenon. Due to the special geometric and structural conditions in some cases, it is required that the piers be placed in pairs next to each other with special arrangements, which leads to a more complex flow structure around the piers. In this study, the variations of the flow velocity and turbulent kinetic energy around single and the twin bridge piers with a circular cross section is simulated using the Fluent model. The twin piers are placed in three configurations including tandem, side-by-side, and at inclined with the flow direction. The three-dimensional components of flow velocity, streamlines, and velocity contours have been investigated for both single and twin piers. By comparing the longitudinal velocity between measured and simulated conditions in two selected cross sections, the average error for the single tandem piers was 7.3% and 3.54%, respectively. Also, the longitudinal velocity in the tandem, side-by-side and inclined piers has decreased by 2.34% and 9.27% and increased by 87.8%, respectively, compared to the single pier conditions. In general, due to the minimum values of turbulent velocity and kinetic energy, the side-by-side model is recommended as the most appropriate arrangement of the piers with respect to the flow direction.

Rivers can be classified as straight, meandering and braiding, Meandering is the most common plan-form acquired by natural rivers. The meandering channel flow is considerably more complex than the straight channel. Compound meandering channels are abundant in nature that including the main channel for the flow in normal times and one or two floodplain (usually covered with plants) at the sides during floods. Vegetation is an important property of many rivers, enhancing amenity values for people and providing habitat for other organisms. Vegetation stabilizes stream banks, provides shade that performs an essential role in nutrient cycling and water quality and supports wildlife. Therefore, it is necessary to study this issue in order to better understand the flow during floods. This study is focused on the influence of vegetation on overbank flow characteristics. In this research, the effect of non-submerged rigid artificial vegetation in the floodplain on two relative depths of 0.35 and 0.55 has been studied in the laboratory.

The experimental research was carried out in a non-mobile bed meandering channel constructed in a 10 m long and 0.78 m wide flume which included the main channel and two floodplains on its sides. The channel wavelength and meander belt width were one meter and 0.58 m, respectively with the sinuosity of 1.3. The geometrical parameters for the main channel were: width, Bmc=0.2 m and depth, Hmc= 0.1 m. Plastic cylinders of 9mm diameter are used to simulate the emergent floodplain vegetation. A movable weir located at downstream of flume controlled water level. Velocity data were extracted and analyzed using Acoustic Doppler Velocimetry. The minimum recording time for each point velocity was 60s. ADV measures the 3D velocities of water particles located 5 cm below its probe. The measurement sections located 6 m downstream of channel inlet, with the names of S1 to S5.

The results showed that the presence of vegetation in the plain flood for a constant relative depth has reduced the flow capacity. The decrease in discharge for depths of 0.35 and 0.55 is equal to 23 and 12 percent, respectively, for a density of 0.77 percent per unit area of one square meter. The pattern of contour lines of the longitudinal velocity in the main channel in the presence of vegetation changes at both relative flow depths relative to the uncovered state. The absolute values of velocity in the main channel in the uncovered state are greater than in the covered state. Also, the values in the transverse and vertical velocity components in plain floods with vegetation are much higher than in uncovered conditions. The directional secondary vectors of the flow in section S1 indicate a counter-clockwise flow in the main channel. The presence of vegetation at a relative depth of 0.55 has reduced the size and values of these vectors in both the main channel and the floodplain. It seems that the presence of coverage, as observed during the experiments, has changed the patterns and directions of vectors on the floodplain. These changes are also observed at the relative depth of 0.35, but specifically the presence of vegetation at this relative depth has caused the flow transfer from the right floodplain to the left floodplain. For all sections, the average values of longitudinal velocity on both sides of the floodplain are greater than the uncovered state and increase by moving away from the main channel to the glass wall of the channel. Although the capacity of covered flow is less than the uncovered one, flow velocities in and around the main channel seem to be close to those measured in uncovered channel. This indicates the high impact of floodplain vegetation on the hydraulics of the flow in the compound meandering channels. Also, the presence of cylinders has increased turbulence and consequently increased shear stress. The values of shear stress at the bottom of the main channel and the convex coast of the floodplain are higher than other areas. In addition, the cover has increased shear stress in the floodplain and reduced the kinetic energy of turbulence (TKE) in the floodplain.

In this study, using a laboratory model, the effect of non-submerged rigid artificial vegetation on the floodplain of a compound meandering channel was investigated. The following is a summary of the results of this study. The presence of vegetation reduced the water transfer capacity, due to the increased resistance to flow. The average longitudinal velocity of the flow in the cross section of the channel in the uncovered state is higher than in the case with the cover. Due to the non-submerged rigid artificial vegetation used, the shear stress in the floodplain has increased. Keywords: Flood, Flow Pattern, Meander Channel, Relative Depth, Vegetation

Investigation of the scour phenomenon is very important. If it is not possible to control the scour around structures such as bridges or bridge supports. Includes irreparable damages. The scour around the bridge piers causes to instability of them, and without applying an appropriate solution, it eventually leads to demolition of the structure. Therefore, study on the mechanism of the occurrence of the scour and the effective parameters on amount of scour are important. So, already various studies have been done on the mechanism of scour around hydraulic structures especially bridges. In the field of scour around bridges, researches are more focused on scour of piers, but no effective results were obtained on the scour phenomenon around the piers. Bed erosion and transport of sand material from its location by a flow called scour. Local scour is a special type of scour that may occur around the bridge piers or bridge abutments. This type of scour is main reason of many bridge failures in the word. Because of this, supply of methods to control and reduce these phenomena is important. One of the methods of scour reduction around the bridge pier or abutment is installation a thin flat rigid plate (collar) on the pier or abutment. There is no comprehensive study to use this method for protecting the pier against scour so far. Therefore, this topic was considered for this research.

This study was performed in the flume with a length of 6 m, a width of 0.72 m, a height of 0.6 m and constant bed slope equal to 0 m in Hydraulic Laboratory of Shahid Chamran University. The bed materials were non cohesive sediment with an average diameter equal to 0.73 mm and geometric standard deviation of 1.22. As well as Plexiglas plates with a thickness of 3 mm was used to build the collar. In this study, 27 tests were performed to measured, sediment movement and determine the two-dimensional velocity components. In each section, a series of tests were performed as a control experiment. The tests of sediment were done in the 3 of flow rate equal to 25, 30 and 35 liters per second. In this condition, Froude Number was equal to 0.26, 0.32 and 0.37 respectively. The 3 unsymmetrical collars with different dimensions in four three Numbers were tested. To do so first general non dimensional relationship was developed. Then series of experimental tests were conducted in a physical model using three different Zc (0, 0.25 and 0.5 high).

Dimensionless plots were obtained regarding effect of dimension of both collars on scour reduction around bridge pier. Different positions of installation of collar were investigated. Dimensionless plots were obtained for determination of collar performance in various heights. Different positions of installation of collar were investigated. Dimensionless plots were obtained for determination of collar performance in various heights. In this section, one of the collars had the best performance, and it was called optimum collar. Local scour around a bridge pier results from the flow and pier interaction and separation of the flow at the sides of the pier. One of the methods used as a local scour countermeasure at bridge piers is collar. Then, we have been check about netted unsymmetrical collar. Also two tests were performed to determine three-dimensional components of velocity in different depth. The results show that the performance of collar in unsymmetrical collar improve by increasing its dimension. The collars located on the bed performance are better than the one located above the bed.

The results show that the collars have important role in retardation of scour development. In section of determination of three-dimensional components of velocity, the results show that the collar act as a shield against down flow. It can control the horseshoe scour around the pier. Literature review shows that cylinders forms, the maximum scour depth occurs in the case of cylindrical pier. Therefore, in this research cylindrical shape for circular bridge pier was selected and effects of Froude number were investigated on scour development. Result shows, that increasing froude numbers will increase the amount scour. Also, as the height of the collar mounting height is increased, the scour depth and the width of the scour hole increase. Following this research, three-dimensional components of velocities were determined with adv. velocimetry and then use for drawing flow pattern. Also result confirms that, maximum velocity occurs near the bed, because the down flow developed the significant secondary currents, and down flow and generated vortex are effective parameters on bridge scour.

Bridge scour is one of the most important challenges in river engineering. Research into local scour has primarily focused on investigating the impact of different hydrodynamic conditions on scour in a bed of uniformly graded material. However, local scour investigations in a bed of uniformly well-graded material provided knowledge of the underlying processes, field sediment beds are much more complex consisting of non-uniform sediment mixtures ("σ" _"g" ">1.4" ). In the case of complex sediment beds, selective transport of the finer particles due to unequal mobility can make the bed surface to be armored. There have been relatively few studies reported in the literature relating to scour in complex sediment beds, and most of these relate to quite specific situations. With regards to natural river materials (non-uniform sediments) and its great effects on the dimension and the time evolution of scour hole, the interaction of flow-structures with non-uniform sediments is very crucial due to armor layer development. The aim of this study is to improve understanding of scour development and armoring evolution in non-uniform sediment beds for estimating the scour depth in more realistic field conditions. Therefore, the rate of variation of erosion over time around single cylindrical pier is investigated in different bed sediment types. The armored layer due to selective transport of the finer particles in non-uniform sediments causes complexity for predicting equilibrium scour depth. The present experiments on local bridge scour were conducted in hydraulic laboratory of the Bu-Ali Sina University (Hamadan, Iran). The pier model with a diameter of 4 cm was put inside a 0.5 m wide, 10.5 m long and 0.5 m depth rectangular tilting flume. In this study, the number of 15 experiments were organized at five different sediment beds, uniform and non-uniform in two steady flow condition (20 and 35 l/s with the same flow intensity of u/uc~0.9) along with an unsteady flow. The duration of tests was fixed at 8 hours in all runs based on the empirical method given by Ettema (1980). The experimental results revealed that with increasing flow rate from 20 to 35 l/s (increasing follow shallowness, h/b) at the same sediment bed, not only larger scour depths were recorded, but also the armor layer became coarser. The comparison between the bed configurations with uniform and non-uniform sediments represented dramatical reduction of the scour depth regards with increasing sediment non-uniformity. The effect of non-uniform sediments on scour in a current clear water conditions showed that maximum scour depth was less than scour depth in a uniform sand with the same d50 value. The comparison between these two mentioned bed configurations showed that the change in geometric standard deviation ("σ" _"g" ) from 1.4 to 2 (altering the uniform bed to non-uniform), decreased the maximum depth of scour by 70% and 60% in two corresponding experiments. As the armor layer coarser grains remains at upstream flow bed and at the vicinity of scour hole in the same flow intensity, the scour depth was decreased. Otherwise, there was not remarkable decrease on the scour depth by increasing non-uniformity index, since two sediment beds types were non-uniform. Also, a slight increase on scour depth has been observed by reduction of median grain size in the beds with non-uniform sediments at the same geometric standard deviation. By taking into account of the grain size of the armor layer and ice cover roughness, Wu et al. (2014) analyzed the dimensionless maximum scour depth and they found out that with an increase in grain size of the armor layer, the dimensionless maximum scour depth decreases. Singh et al. (2018) investigated the incipient motion for gravel particles in cohesionless sediment mixtures having silt and sand. The visual observations of the channel bed after the end of incipient motion indicated appearance of gravel particles at the top surface of the sediment bed. The critical shear stress for the gravel particles was found to be lower in the presence of silt. Presence of silt in the mixture affects the critical shear stress for gravel particles. They concluded from the present study that high silt content in the mixture leads to the higher deviation of critical shear stress from the revised Shields curve. They proposed an equation for the determination of critical shear stress of gravel particles in the non-uniform sediment mixture. The results showed that scour depths were reduced dramatically as sediment non-uniformity index ("σ" _"g" ) increase in clear water conditions. The larger particles form an armor layer protecting the bed from eroding. Also, the observation indicated that with increasing flow depth, the armor layer coarsens, and larger scour depths were recorded. However, scour depth increase rate was very different for the various bed sediment types.

One of the usual methods for river banks protection is using spur dike structures that if properly designed and executed, in addition to controlling erosion, It leads to the rehabilitation of rivers margin valuable lands. Spur dikes affect the streamlines and it make changes in the velocity and direction of the flow, leading to major changes in the bed topography around the spur dike as well as the beaches. Recognizing and directing these changes will lead to the River Affordable Planning in the desired areas.  In the present study, the effect of the Deflecting open gabion spur dike, attracting and repelling on the bed topography of the flow path and flow pattern has been investigated. ADV was used to measure flow velocity in different directions. This velocity meter is submitted by transmitting waves of 10 or 16 MHz frequency from a transmitter to a sample size of 6 mm in diameter and 3 mm in diameter at a distance of 5 cm from the transmitter and receiving waves by receiver antennas measures the velocity of particles within the sampling volume. The device has the ability to measure the distance from the floor inside the water. Therefore, taking into account the baseline level, the measured distance at each point was deducted from the base value and the scouring of that point was obtained. In these experiments, the Froude number was fixed at 0.26. Also, the depth of flow in the set of experiments is 14.6 cm, which is extracted according to the discharge rate and the displacement threshold formula. The experiments were carried out in such a way that after the equilibrium of the bedding and scouring harvest, the flow pattern was started using the ADV device.
 In this review, the performance of the spur dikes will be compared with the impervious spur dike. The results show that by decreasing the porosity of the spur dikes, the mainstream deviation and the intensity of the secondary flows around the spur dike have increased, which increases the topographic changes of the bed and creates larger cavities around the spur dike. As the erosion rate increases around the spur dike, sedimentation on the edges increases.In all three types of spur dikes, with increasing porosity, the dimensions of the scour hole are reduced. By increasing the porosity of the spur dike, the flow velocity from the pores of the spur dike increases, which reduces the difference in the flow rate from the headland and the flow through the pores of the spur dike and reduces the ability to carry flow sedimentation. In a spur dike with 50% porosity, bed topography changes occurred in a very small area around the spur dike and focused on the nose, while for a spur dike with zero porosity, the topography of the bed, depending on the type of spur dike, is several times the length of the spur dike, in Channel length and width occurred. The attracting spur dike has created much less variation in the flow pattern due to the way it is placed in the path of flow, and therefore the bed topography is less influenced by the presence of the spur dike.

An air-bubble screen is an innovative technique for riverbank protection along the bends which improve the navigation conditions in the alluvial rivers. In this research, the effects of this structure on a 90° mild and long bend on the bed morphology and flow pattern have been investigated. To this end, the experiments were carried out with three angles 0, 45, 90 degree and four Froude numbers of 0.37, 0.41, 0.45, 0.47 combined with three air discharge. Experiments performed in a mild bend using clearwater showed that the bubble screen was able to redistribute the velocity patterns and bed morphology in the bend. In addition, the results showed that maximum bend scouring is reduced about 47% andoccurs further away from the outer bank and high velocity zone shifted toward the center of the channel. The results showed that the maximum and minimum scour depths in outer bank with 0 and 90 degrees angles respectively. Also, by increase of air discharge ratio the maximum scour depth was decreased.

In this study, the impacts of skew angle of two bridge piers with respect to the flow direction on the equilibrium scour depth in front of the piers under clear water condition is investigated. For this purpose, the piers are aligned with four different skew angles with respect to the flow direction. Additionally, to obtain the equilibrium scour depth in front of the piers, the experiments are performed until reaching the equilibrium time. The results show that increasing the skew-angle increases the equilibrium scour depth and time. The minimum and maximum equilibrium scour depths at both piers occur at the skew-angles of 0 and 60 degree, respectively. Regarding the analysis of the results, the maximum equilibrium scour depths in front of the piers, for the skew angles less than 28 degree occur at the upstream pier while for the skew angles greater than 28 degree is shifted to the downstream pier. In the following, formula are provided to estimate the scour depths in front of the piers, utilizing the Semi-logarithmic linear regression method and observed data.

Spur dikes are known as the most popular hydraulic structures which are utilized to reduce shores and river banks erosion. These structures change the flow and sediment transport pattern and so they can control hydraulic conditions and prevent shores erosion and cause sediment trapping. In this paper effects of changes in angles and various arrangement of spur dikes (small to large or vice versa) for group of parallel, unequal, non-submerged and impermeable spur dikes are numerically investigated on flow pattern, bed erosion and sedimentation. Validation of results of Flow-3D numerical model with experimental data, shows the high precision of the numerical model. Also the results show that scour depth in large to small arrangement of the spur dikes and 45 degrees orientation has 55% reduction and in small to large arrangement and 135 degrees orientation has 72% reduction in comparison with group of equal length spur dikes perpendicular to the shore.

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.

Spur dikes are hydraulic structures that are used to protect river banks. In this paper, parameters influencing flow pattern around non-submerged attracting series of T- head spur dikes in straight channel are investigated. Evaluating of the flow pattern indicates that using spur dikes causes reversed flow and increasing the horizontal and vertical components of velocity at upstream of spur dikes. Longitudinal component of velocities at the front of the first spur dike wing is more than longitudinal velocities in the front of the second and third spur dike wing and in front of the second spur dike wing is more than third spur dike wing. The horizontal component of velocity are maximum the surface near to bed in reversed flow region and the negative transvers velocities of reversed flow are maximum near the free surface between spur dikes and in the front of the second and third spur dike wing. The deep positive velocities are seen in the edge of wing upstream of first spur dike and between spur dikes. Investigation of turbulence kinetic energy in flow pattern shows that it is maximum in the edge of wing upstream of first spur dike and has increased in wing of spur dike's shear layer. Investigation of Reynolds stresses in flow pattern shows that component $-{rho u_{i}^{'}}{nu_{i}^{'}}$ is maximum compared to two other components and the maximum value of that is occurred in the downstream and among of spur dikes. The maximum value of $-{rho u_{i}^{'}}{nu_{i}^{'}}$ component is respectively about 5 and 6 times the values of $-{rho u_{i}^{'}}{w_{i}^{'}}$ and $-{rho nu_{i}^{'}}{w_{i}^{'}}$ components. Component $-{rho u_{i}^{'}}{w_{i}^{'}}$is at minimum compared to two other components and component $-{rho nu_{i}^{'}}{w_{i}^{'}}$ at the edge of wing upstream of spur dikes and the minimum value of that occurs in the downstream and among of spur dikes. Investigation of bed shear stress shows that the maximum of bed shear has occurred at the edge of wing upstream of the first spur dike.

The scour around the pier leads to bridge demolition. Therefore, the perception of the factors’ behavior affecting the scour and the capability to predict this phenomenon can offer important information to bridge designers. The development of a hole inside the pier is one the proposed solutions for reducing pier scour. In order to a better perception of the mechanism of hollow performance and its effect on the flow hydrodynamic around the pier, the numerical modeling of flow around the pier having a circular section with and without hollow is accomplished in rigid bed. The characteristics of the flow around the pier with and without the hollow are compared by the available laboratory information and using the 3 dimensional Flow3D model. The results showed that the development of hollow in pier leads to wider vortices after the pier and it is expected that the scour depth decreases around the pier. According to the results of this study, the turbulence energy and the bed shear stress can be reduced to 20% and 56%. Based on results of shear stress and comparing it with the critical shear stress, it is predicted that the amount of scouring around the pier when a hollow is developed can be reduced. Also, holes cause the less flow separation and returns flow after pier.

Flow in river bends is three-dimensional, the combination of secondary flow and longitudinal flow in rivers leads to the formation of helical flow which is the main cause of erosion in the outer bend and sedimentation in the inner bend. So far, many numerical models regarding flow patterns in rivers have been introduced and available to river engineering scholars. Moges et al. (2010) simulated the hydraulic and sediment flow in Meander Rivers using SRH-2D two-dimensional model and eventually determined the best equation for predicting the water surface profile and bed level changes in sediment situations. Naji Abhari et al. (2010) studied the flow pattern in a 90 degree bend. The results of their study results showed that from the beginning of the bend till the 30 degrees angle the maximum velocity is close to the inner wall and from the 30 degree angle till the end of the bend the maximum velocity is diverted to the outer bend. Maghrebi (2012) studied the turbulent flow pattern in a longitudinal range of Karoon river, containing two 180 degree steep bends using MIKE21FM and CCHE2D models, the results showed that that both models simulate the flow pattern accurately. The aim of the present study is simulation of the flow pattern in a 90 degree mild bend using CCHE2D and SRH2D models.