جستجوی مقالات مرتبط با کلیدواژه "شدت آشفتگی" در نشریات گروه "عمران"

The various methods have been extensively studied by different researchers to reduce scouring around bridge piers, such as riprap, concrete blocks (CAU), collars, sacrificial piers, creating slot and roughness on the pier, and flow guide vanes, and their results generally relate to determining the size, location, and scope of installation and other geometric characteristics of the devices. Many countermeasures to prevent or reduce local scour around bridge piers, did not have the desired effective, and the concrete armor units (CAUs), which are made to protect shores from erosion caused by waves, have received very little attention in terms of bed armoring around the bridge piers.Therefore, the aim of the present research is to experimentally investigate the hydrodynamics of flow around a new element designed to protect the bed around bridge piers from local scour. The F-jacks, a concrete armor unit (CAU), is introduced and its role on flow characteristics is evaluated for the first time in this study. This concrete element is a new design of the A-jacks concrete model, with one leg and five branches on top, and the angle between the branches surrounding the leg and the central branch is 30 degrees to ensure minimum contact between the legs of the element and the sediment surface. The selection of a 30-degree angle for the branches of the F-jacks element is due to its similarity to the diameter of the bridge pier, to provide complete coverage around the pier.

The experiments of this research were performed in a 7-meter long, 50-centimeter wide, and 0.0001 slope flume with a rigid bed. A wooden cylinder with a diameter of D = 45mm and the same height as the flume was used as a model for a bridge pier and installed at a distance of 4.5 meters from the beginning of the flume (to develop the flow). The water depth (h) in the experiments was constant and equal to 15 centimeters, the flow rate (Q) was 0.021 m3/s, and the flow regime was fully turbulent and subcritical.In order to understand the physics of flow in relation to three-dimensional velocity variations, a Nortek Acoustic Doppler velocimetry (ADV) with a frequency of 25 Hz and a sampling duration of 120 seconds was used. In order to evaluate the hydrodynamics of the flow around the protected pier with F-jacks units, three different placement patterns around the pier were considered: 1) a non-dense arrangement (P1), in which 24 F-jacks elements were placed next to each other around the pier, 2) a dense arrangement (P2), in which 22 F-jacks elements are interlocked around the pier, and 3) an SP arrangement, which refers to a situation where the single pier is placed in the central axis of the flume.In addition to the measurement grid on the vertical XZ plane, a set of measurements was taken on the horizontal XY plane at Z/h=0.47 for each of the three selected patterns. To ensure the development of the flow in the test area, normalized meanvelocity component profiles were shown to follow a similar trend for two 4 and 4.3 meter-long sections from the beginning of the flume, and the velocity data conforms to the logarithmic law of velocity distribution that confirms the validity of velocity data for developed flow conditions. Also, to verify the accuracy and sufficiency of measurements in the developed flow region under investigation, the power spectral density (PSD) of time series for all three velocity components shows that the slope of the power spectrum agrees well with the Kolmogorov -5/3 law in the inertial sub range.

Contour and vector plots of the time-average streamwise velocity component (u ̅) indicated that when the F-jacks elements were placed according to the P2 pattern around the pier, the flow pattern around the pier changes completely. Where, in the upstream of the pier, the average velocity significantly decreased from the water surface to the bottom, indicating the growth of the minimum and weakening of the downflow and horseshoe vortices. In the downstream of the pier, the high-velocity flow region at rear the pier disappeared, and the flow turbulence was significantly reduced, and the region of flow recirculation in the wake of the pier completely disappeared.With the placement of F-jacks units around the pier, a strong upward vertical velocity (w ̅ ) is obvious around the pier compared to the single pier (SP pattern), which is stronger in the dense arrangement of the elements (P2 pattern). This factor refers to the positive effect of the presence of elements in reducing the growth of downflow (negative vertical velocity), reducing bed disturbances, and turbulence transfer away from the bed region in the wake area around the bridge pier.The streamwise and vertical components of flow turbulence intensity (u_rms 〖,w〗_rms ) significantly decreased with the placement of F-jacks units around the pier according to the P2 pattern. Where, in the near-bed region around the pier, the turbulence intensity decreased by an average of about 93% compared to the SP pattern, indicating the high ability of F-jacks elements in controlling and reducing flow fluctuations and turbulence in this region and diverting flow fluctuations towards the water surface and away from the bed.Comparison of Reynolds shear stress on XY plane at Z/h=0.47 for the three mentioned patterns revealed that -ρ(u^' w^' ) ̅ in the SP pattern is approximately 95% higher than P1 and P2 patterns, at the vicinity of the bridge pier. Furthermore, the magnitude of -ρ(u^' w^' ) ̅ has significantly decreased with the placement of F-jacks units as the P2 pattern around the pier.

The laboratory results presented in this paper provide a new understanding of flow behavior details around the bridge pier model with a new design of F-jacks armor units surrounding it on rigid bed conditions. The overall conclusion of this study showed that when F-jacks units are placed densely (P2 pattern) around the pier, the flow turbulence in this area is significantly reduced.

Laboratory experiments were carried out to investigate the hydraulics, turbulence structure, and sediment removal capacity of bridge pier with different adjusted roughness. Four experimental models of the roughness were employed to measure the effect of the geometry of the roughness on the scour hole formation and depth. The effects of roughness geometry on the turbulence structure and local erosion through the bridges were studied for one sand sediment size. The three-dimensional velocity profiles and turbulence characteristics of flow downstream and upstream of the bridge pier were measured by Acoustic Doppler velocimetry (ADV) technique due to different adjusted roughnesses. After filtration process by WinADV software, the contour plots of the turbulence kinetic energy were depicted due to triangular roughness. The maximum longitudinal eroded section, which is commonly located by bridge pier corners, was measured through the experiments. The results of the scour hole measurements indicate that due to increasing the length of the roughness, the scour hole depth values decreased; however, increasing the vertical distance has the same effect on the scour hole formation. Furthermore, different geometries of the employed roughness illustrate that the triangular roughness has less scour hole depth than other experimental models with the same hydraulic condition. The detailed information of the magnitude, distribution, and probability of turbulence structures was extracted from time-series data using power spectra. The turbulence data were compared with the topography of upstream erosion. Analysis of the power spectrum density function indicated that the discontinuous adjusted roughness remarkably reduced the downward energy level of vortexes at the upstream of bridge pier, which is supposed that the gap opening through the roughness can increase downward jets impact. It appears that this condition reduces the turbulence kinetic energy near the bed elevation at the upstream face of squire bridge pier.

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.

Due to the simplicity of construction, vertical drops are widely used to reduce the steep slope of the canal and the volume of earthworks in irrigation and drainage canals. The upstream regime of flow in structures can be subcritical or supercritical. Stilling basins are usually used to dissipate the energy and prevent the bed erosion. Due to the fact that concrete materials are used in the construction of the stilling basin, Hydraulic engineers are always looking for a way to minimize the construction cost of downstream stilling basin and increase downstream energy loss of these structures. The dimensions of the downstream stilling basin depend on the geometry and hydraulic parameters of vertical drop. So In the present study, the effect of serrated drop edge on energy dissipation is investigated numerically using Flow3D software. Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses computers to analyze and simulate complex fluid problems. Flow-3D software is one of the most widely used software in the field of computational fluid dynamics.One of the prominent features of this software is the ability to simulate free-surface flow by VOF method. The governing equations of fluid flow are continuity and momentum equations. In Flow 3D software, several turbulence models are implemented. In the present study, k-ε and RNG turbulence models were used to perform the simulation. An experimental vertical drop set up with a height of 25 cm, width of 46 cm and a relative critical depth ranging from 0.2 to 0.35 was used for simulation. Total relative energy loss was used to validate the numerical results. Afterwards, different arrangements of dented (serrated) edge were used to simulate the flow on a vertical drop. The squared shapes in plan were used. The dimensions of dented edges which distributed symmetrically along the width were 6.9 and 4.6 cm (0.15 and 0.1 times the width of the flume) and their thicknesses were 2 cm. So, the number of dented edges was 3 and 4, respectively. The total number of meshes was considered to be 1237500. According to the dimensional analysis, the relative energy loss can be expressed as equation (1): ΔE/Eu=f(yc/h, n, α) (1) where, yc/h is the relative critical depth, n is the number of serrated edge and α is the relative dimensions of the serrated edge. The RNG turbulence model showed better agreement with laboratory values compared to the k-ε turbulence model. The results showed that the use of dented vertical drop increases the energy loss for the same relative depth in downstream, length of falling jet and the turbulence intensity compared to the simple vertical drop. In the dented model, irregularities in the streamlines of downstream increased significantly. Increasing in dimensions of the dented edges and decreasing their number caused more irregularity in streamline and augmentation of the turbulence. So, the model with 3 dented edges (relative dimension of 0.15) performed the most turbulence and irregularity in the downstream streamlines. Energy losses in vertical drop with 3 and 4 dented edges and ordinary vertical drop are compared. The average energy losses were 26, 38, 15 and 25 percent, respectively. Although the use of dented edges increases the length of falling jet, but the stilling basin length for energy loss in models with dented edges is less than the ordinary model. According to the results of the present study, the vertical drop with 3 dented edges and relative dimension of 0.15 performs the highest energy loss as compared to the ordinary vertical drop and other models of the present study. In this study, the Froude number ranged from 3.7 to 4.5 in the ordinary vertical drop to 2.7 to 2.9. Since a stilling basin is usually constructed at downstream of the vertical drop to dissipate the destructive kinetic energy of the flow and the dimensions of the stilling basin depends on the Froude number, so the use of dented edges in the vertical drop has such advantages as reduction in basin dimensions, augmentation in energy loss and lower depth for tail-water to form the hydraulic jump. Therefore, considering the hydraulic and economic conditions of the design, it is possible to use dented edges in practice to reduce the dimensions of the stilling basins and increasing the energy loss of flow in downstream of vertical drops. Some other features and conditions are not considered in this study. So, it is suggested that the effect of angle of dented edges on energy loss and other hydraulic parameters could be investigated.

It is important in Hydraulic and river engineering to estimate the mean velocity and turbulence intensity to identify the presence of secondary currents, its shape and position. The flow of channels consists of three components of velocity: one in the direction of flow ,one in the transverse and the orther in the vertical directions of the channel. Due to the heterogeneity of the velocity fluctuations, a series of vortex vortices in the channel section to be formed which is called secondary currents cells. The secondary currents are dependent on factors such as bed roughness, channel slope and bed shear stress. The present study investigates the effect of bed roughness form on the pattern of secondary currents with numerical modeling in Flow-3D software by using RNG turbulent model. In order to successfully model and reduce the cost of simulating the near wall region, mesh sensitivity analysis has been done and numerical domain has been divided into some subdomains. This research has been carried out according to the data of the Negara laboratory model (for triangular roughness) carried out at the Hydraulic Laboratory of Singapore National University. In the results obtained from the mean velocity profile, the mean error for the roughness trough was 9.94% and for roughness crest was 3.71%. in the case of shear velocity, the error was obtained at three cross sections x=4m, x=5m and x=6m respectively 6.58%, 6.86% and 5.67% which demonstrated the good fit of the numerical model results with the reference laboratory model. The flow conditions in the channel were designed and studied for three geometries of roughness (with same height) ,i.e., rectangular roughness, trapezoidal roughness with an internal angle of 80 degrees and trapezoidal roughness with an internal angle of 55 degrees that are most used in hydraulic structures. The results of the study on the turbulence intensity, secondary currents and turbulence kinetic energy showed the effect of trapezoidal roughness with an internal angle of 55 degrees relative to the other two forms of roughness. The difference in the turbulence intensity in trapezoidal roughness with an internal angle of 55 degrees relative to a triangular roughness was obtained about 4.54%. The location of the center of the secondary current cell was in the depth of about 0.2 meters. The tendency of cells to the side walls of the channel is also affected by roughness geometry. In trapezoidal roughness with an internal angle of 55 degrees, there is a tendency to both walls. The turbulence kinetic energy contour with the center of the nucleus begins at the roughness crest with an approximate value of 0.0002 for all roughness, but its location was different in relation to the roughness geometry. In fact, the turbulence kinetic energy is the effect of fluctuating components of the velocity that the existence of external rotations is the effect of the redistribution of energy by velocity tensor, which is responsible for the formation of secondary current cells. As has been pointed out in the literature, results showed the role of roughness shape in the appearance of the secondary currents.

In the present research, USBR stilling basin Types I, II and III were simulated numerically after validation by physical model data. Regarding position of forming hydraulic jump in each type, hydrodynamic parameters such as pressure, velocity vorticity and turbulence intensity were compared and analyzed in three standard stilling basins. RNG, VOF and FAVOR models were used to simulate turbulent, free surface and solid faces, respectively. Results showed that the performance of USBR Type III and II are greater than Type I, due to significant influence of chute blocks in Type III and end sill in Type II stilling basin. Presence of obstacles during jump had little effect on the turbulence intensity but reduced turbulence in the tail water. Therefore, turbulence in the tail water of stilling basin Type II and Type III was reduced about 11.76% and 26.72%, respectively in comparison with Type I stilling basin which caused to reduce the damage in the downstream. Vorticity in Type II was about 98% and 66.6% lower than Types I and III, respectively. In tail water of this stilling basis this parameter was about zero. Moreover, stilling basin Type II reduced the energy of flow more than Type I and Type III.

In the present paper flow pattern at a constriction in an open channel is studied experimentally. A constriction as a model of bridge site is constructed in a flume 73.5 cm wide and with erodible bed. Measurements were carried out after the scour hole was reached to its equilibrium condition. Tests were conducted with one flow discharge and two tail waters. Flow velocity was measured in 3 directions simultaneously with an ADV. Based on instantaneous velocities, turbulence intensity was also calculated and analyzed. The effect of constriction on flow pattern was then studied based on velocity and turbulence measurements in x, y and z directions.