جستجوی مقالات مرتبط با کلیدواژه « تنش برشی بستر » در نشریات گروه « آب و خاک »

Bed shear stress is one of the most important hydraulic parameters to determine the amount of bed and suspended load and the bed and bank scouring in rivers. Bed shear stress depends on bedforms (ripples, dunes, and anti-dunes) in alluvial rivers. In this study, the effect of artificial ripple bedforms on bed shear stress has been investigated. Two types of uniform granulation with average sizes (d50) of 0.51 and 2.18 mm were used to roughen the surface of the artificial ripples. The bedform length and height were 20 and 4 cm, respectively. The angles of its upstream and downstream to the horizon were selected equal to 16.4 and 32 degrees, respectively. Different flow rates (Q= 10, 15, 20, 25, and 30 l/s) and different bed slopes (S= 0, 0.0001, 0.0005, 0.001, and 0.0015) were examined. The results showed that by increasing the particle size on the bed surface, total shear stress (tb ), grain-related bed-shear stress (t¢b ), and form-related bed-shear stress ( t²b )  increase. The value of tb , t¢b , and t²b in bed form roughened by sediment size of 2.18 mm were, on average, 22.38, 30.86, and 22.3% more than the bed form roughened by sediment size of 0.51 mm, respectively.

Bridge piers, which are constructed across the rivers, are always prone to the erosion and scouring phenomena. Bed shear stress is one of the main factors of scouring around bridge piers in which there is a direct relationship between bed shear stress and scouring. In the present research work, firstly the laboratory results of cylindrical bridge piers were used for validation of the model, and then the effect of bridge pier’s shape on bed shear stress and other effective parameters were studied by numerical modeling and applying Flow-3D software. The cylinder pier experienced the highest amount of bed shear stress. In contrast, the case lenticular shape showed 26% reduction with cylinder shape occurs. The results show that different pier shapes have a perceptible effect on reducing the maximum amount of bed shear stress. This function affects streamlines around the bridge piers, and the formation, length, and power of wake and horseshoe vortexes. So that, streams having -0.1m/s behind the cylinder affected the length of 0.2m but in lenticular shape, 40% decrease appears. Additionally, the piers with a forehead causes a stronger down-flow. In those sections, because of the presence of horseshoe vortex, streamlines shift to the upstream, so that the negative velocity figures occur in this region.

Stilling basins are a short section of a floor of channel that is constructed as special structures at the end of the spillways and any other site that creates a supercritical flow to control the hydraulic jump, and the shorter their length, they will be the more economically viable. One of the most important topics in the study of reducing the destructive energy of the flow is hydraulic jump. Hydraulic jump control is carried out in order to reduce the damage of the downstream structures. A hydraulic jump is one of the most rapidly varied flows occurring when the flow is altered from a supercritical state to a subcritical one in a part of its path. Researchers have always tried to control the hydraulic jump to prevent severe damage to downstream hydraulic structures. The high cost of building recreational pools has encouraged researchers to reduce sequent depth, jump length and reduce energy losses. There are several methods to reduce the sequent depth and length of the hydraulic jump in addition to increasing the energy loss of the hydraulic jump, such as increasing the adverse slope of the bed or using roughness in the channel floor. In addition to reducing the dimensions of the hydraulic jump for the cost-effective construction of the stilling basin, the stabilization of the hydraulic jump at the pool site is also very important. A sudden drop in the bed (negative step) stabilizes a horizontal hydraulic jump of channel adjacent to the step for a wide range of sequent depths. This paper introduced a new solution for the hydraulic jump on adverse slope and negative step to the difference between the upstream and the downstream momentum fluxes.

The channel used in this research is a rectangular channel with a length of 8, a width of 0.4 and a height of 0.6 meters. The walls and floor of this channel were made of transparent Plexiglas sheet. This channel was fed by a pump with a maximum discharge of 50 liters per second. In order to increase the initial Froude number before the upstream sliding gate, the tank height is considered to be 1.25 m. To create the stairs at the beginning of the channel, Teflon sheets with thicknesses of 3 and 6 cm were used, taking into account the zero height, a total of 3 different heights were used for the stairs. In this study, three angles of zero, 3 and 6 degrees were used for adverse slope. To change the slope of a false plate with a length of 3 meters, its sloping part continued to a distance of 2 meters from the gate, the slope of which can be changed. This plate is located at the bottom of the channel, just after the initial gate, where the jump occurs completely in this part of the channel. To create the jump, a sliding gate was installed at the inlet of the flume and another valve was used at the end of the flume to stabilize the jump. In the experiments performed, the downstream gate was adjusted so that the start of the jump occurred at the point of change in the height of the channel. To perform this study, a total of 81 experiments were performed in the range of 4.5 to 9.5 Froude numbers.

In this research, the combined effect of using three inverse slopes and three elementary negative step elevations on hydraulic jump characteristics including sequent depths, jump length and energy loss in Froude numbers of 4.5 to 9.5 were studied. The results showed that by increasing the adverse slope of channel reduced the sequent depths, jump length and energy loss by 12.6%, 13.9% and 16.8%, respectively. When the negative step increased the ratio of sequent depths and jump length increased by 5.66% and 6.2%, respectively, and energy loss decreased by 2.6%. The combination of the adverse slope and negative step reduced the sequent depths, jump length and energy loss by 12.42%, 14.8% and 3%, respectively, and could also create a shear of 12.56 times equal to the conditions of a smooth bed. Therefore, the negative step only stabilizes the jump in the stilling basin, and the adverse slope makes this jump stabilization difficult.

As the water level rises in the main channel and crosses the floodplain boundary, because of different roughness coefficient and velocity between the main channel and the floodplain, the flow in the main channel move more acceleration than the flow in the floodplains and there will be strong momentum exchange between the flows in the intersection of the main channel and the floodplain. This exchange generates vortices in the intersection of the main channel and the floodplain. Momentum exchange reduces the high velocity flow energy of the main channel and adds to the low velocity flow energy of the floodplain. Momentum exchange also reduces the discharge of the main channel, increasing the discharge of the floodplain and decreasing river transmission capacity. At intersection of the main channel and the floodplain, due to the shear layer, the potential for instability and scour are high. Understanding parameters of the bed shear stress and momentum in the compound channel especially in the intersection of the main channel and floodplain make more accurate calculations related to the protection of the riverbed and determine the protection methods. In this study, the parameters of bed shear stress and flow momentum are quantified in conditions where the compound channel, bridge abutment length in the floodplain and the type of guide-wall simultaneously affect the flow pattern.

In the present study, three-dimensional numerical analysis is performed to investigate the bed shear stress and momentum exchanging between the floodplain and the main channel in a symmetric trapezoidal compound channel by using FLOW-3D. After validating of the mentioned model, the bed shear stress and momentum exchange between the floodplain and the main channel for different bridge abutment lengths in cases with and without elliptical guide-wall have been investigated.

The bed shear stress in the case with the elliptical guide wall has increased in the main channel and decreased in the floodplain compared to without guide-wall. Shear stress in the main channel has increased to 25.5% and decreased to 36.63% in the floodplain. The momentum exchange in the elliptical guide-wall case has increased up to 78.5% compared to without guide-wall case.

One of the remarkable results is that the bed shear stress in the floodplain decreases in the case of abutment with the elliptical guide-wall which improves the conditions at the bridge abutment. But increase the shear stress in the main channel causes the conditions of the abutment located in the main channel become more critical. Also, in the case of abutment with the elliptical guide-wall, the momentum exchange between the floodplain and the main channel increases compared to in the case of abutment without the guide wall, which increases the shear stress and creates vortices at intersection of the floodplain and the main channel.

The rivers, as the main watercourse and natural drainages, always play a significant role in the conveyance of flood flows. During floods, the water crosses the main section of the river and enters the floodplains. In this case the river crossing becomes a compound cross section. In present research work, by studying meandering compound channels, the effect of changing relative depth of flood currents on the hydraulic flow conditions and the rate of discharge are investigated. For this purpose, six channels with different sinusoidal rates at three relative depths with different flood rates were investigated by FLOW3D software. The results of numerical simulation show that by increasing relative depths from 0.26 to 0.45 (73% increase), the depth averaged velocity in all channels increased by 25% and the rate of discharge passing through the main channel decreased by 33%. Also, the results of this study show that the bed shear stress near the inner arch of the main channel is more than the outer arch and by reducing the relative depth in the compound channel, the amount of bed shear stress and flow velocity decreases. And the amount of shear stress of the inner arch wall of the main channel in all channels is higher than that of the outer arch wall, and by increasing the relative depth the amount of shear stress of the wall is increased.

Investigation of the flow pattern in meandering rivers is much more complicated than straight ones. Also, most rivers in the path of bends are not uniform due to channel width variations. Therefore, the use of a robust numerical model for simulating flow pattern in rivers is necessary. In this research, SSIIM three-dimensional numerical model was used to simulate the flow pattern in a meandering reach of ​​the Doab river in Chaharmahal and Bakhtiari province. For this purpose, after doing field measurements and model calibration the velocity values ​​calculated by the model were compared with the measured values in a relatively uniform section of bend by using RMSE and MAPE error criteria. Results obtained showed that the values of these two criteria are 0.081 and 0.075, respectively. This indicates that the model performance is good. Moreover, various flow characteristics including vertical and horizontal velocity profiles, the velocity lines in longitudinal and transverse directions and also the distribution of bed shear stress in location of two converged and diverged the bends are compared. The results of this study indicate that velocity values in longitudinal direction in converged bend is higher than diverged one. the high accuracy of the model in the study of flow pattern in the river bend. Furthermore, the density of the stream lines and the values of bed shear stress at the interval reach of two bends are maximum. As result, it can be said that flow pattern simulated well with the model

Interaction between water flow characteristics and bed erodibility plays an important role in sediment transport process. In order to reach stability, rivers with deposition or bottom erosion make different forms in the bed .The bed forms create extra resistance, which is called the bed resistance. The mutual interaction between the flow and the erodible bed through sediment transport phenomena in a sand-bed channel causes a variety of bed forms. Starting with ripples and gradually increasing in shear stress or water velocity, dunes, washed out dunes, flat bed, anti-dunes, and standing waves are formed. The most common boundary conditions in alluvial rivers are the mobile beds covered with Ripple and Dune. These forms, in many alluvial systems, play a critical role in contrast between the flow, discharge of sediment and morphology of bed. One way to identify the behavior of the rivers is to study the structure and the formation of bed forms within them. Ripples are among the smallest of the bed forms. The longitudinal cross-sections of ripples are usually asymmetrical. The upstream face of ripple is long and has a gentle slope, and the downstream face is short and steep. The height of ripples is usually between 0.5 cm and 2 cm, but not more than 5 cm. The wave lengths normally do not exceed to 30cm, and they are usually within the range of 1 cm to 15 cm. Some ripples that form in deep-water regions are symmetrical. Ripples are the smallest of the bed configurations. They are related to physical parameters near the river bed and have little correlation with the water depth. Their occurrence is the result of the unstable viscous layer near the boundary. They can form in both shallow and deep water. In plan, they either are parallel to each other or have a shape like fish scales. With increasing the flow velocity, the plan form of the ripples gradually develops form straight line to curves and then to a pattern like fish scales, symmetrical or unsymmetrical Resistance is a function of the geometrical dimensions of the bed forms and depth of water. Estimating of the flow resistance is one of the most important matters in planning, designing and operating of water resources projects, including water transfer and river system management. In this research, the effects of two different types of ripples (parallel and flake shape) on the hydraulic characteristics of flow were experimentally studied. The experiments flume located at the hydraulic laboratory of Shahrekord University, Iran. The flume used in this study was a Straight type that had the dimensions of 0.4 m wide and depth and 12 m long. This flume has vertical PVC sidewalls. Generally 48 tests in variety slopes of 0.0005 to 0.003 and variety discharges of 10 to 40 lit/s were conducted. Velocity and the shear stress were measured by using an Acoustic Doppler Velocimeter (ADV). Velocity measurements were performed with a frequency of 200 Hz, which provided accurate statistics on the mean flow and turbulence characteristics. Detailed velocity measurements were performed in 9 cross-sections in the Straight flume. The cross-sections were determined at: before crest, crest and after crest. Generally, in each section velocity was measured in 12 point (5, 20 and 35 cm from flume side and 4 points from bed). For All tests, flow depth was kept constant. In this study generally 48 test were tested under different hydraulic conditions. It was observed that the peak value of the bed shear stress appeared on the midpoint of upstream surface (before crest ripple) and the crest of the ripples had the lowest value of the bed shear stress. From the crest to trough (after crest) of the ripples, the general bed shear stress was in an increasing trend. It was generally found that with increasing Froude number and the bed shear stress increased. Also, in the case of parallel ripple bed form, the shear stress was about 26% more than that of plane bed and in the case of flake ripple bed form the increasing rate was 23%. It indicated that the shear stress was much affected by the parallel shape of ripple bed form compared with the flake shape, as it was 27% more than the flake shape.

'Transition' is a short hydraulic structure used to change the cross-sectional shape or flow shape. Transitions are commonly used both in the open-channels and natural waterways. The task of a transition is to connect a narrow channel at the upstream to a wider downstream channel or vice-versa. This, creates a disturbance region of turbulent flow causing energy losses. In general, the structure of a transition prevents the formation of wave and other turbulencies. In this case, the energy loss, due to the change in the amount of the momentum, will be minimized.

Transitions are commonly used structures in both natural and artificial open channels. With increasing the transition dimensions along the flow, the flow decelerates. Under subcritical steady state flow conditions, reducing the flow velocity increases the water pressure and reverses pressure gradient. This phenomenon creates separation zone and turbulent eddy flow, and causes the flow energy losses. Due to the complexity of flow pattern and scale effects, physical models can solely provide a clear understanding of the physical principles governing the flow field, so it is necessary to study flow pattern numerically along with the field and experimental studies. In this study, the flow pattern in a rectangular to rectangular expansive transition, has been simulated under subcritical flow with RSM turbulence model using Fluent software. Water surface and flow velocity profiles obtained by the two methods at different sections of transition were compared with experimental results. The results showed a good agreement between the simulated and experimental data. After validation of the numerical model, the effects of inflow Froude numbers on strength of secondary current, hydraulic efficiency of the transition, turbulent kinetic energy and bed shear stress at different cross sections were simulated. The results showed that with increasing the inflow Froude number, the strength of secondary current along the transition increased, while the hydraulic efficiency decreased. Maximum efficiency (60.83%) occurred in Fr1=0.40. Also, with increasing the Froude number, turbulent kinetic energy and bed shear stress increased, so that, from transition inlet to the outlet, turbulent kinetic energy for total Froude numbers decreased with a 30.71% reduction in turbulent power.

Hydraulic jump is a rapidly varied flow in open channels which performs an effective role in dissipation of the kinetic energy of flow at downstream of water structures. In the present study the characteristics of hydraulic jump, including sequent depth, energy dissipation, bed shear stress and velocity profiles at different sections, on rough bed with discontinuous roughness elements of lozenge shape have been investigated. The experiments were carried out for a range of Froude number values from 4.3 to 12.4 in three roughness densities and four combined arrangements of roughness elements. The results of this study showed that the velocity profiles on rough beds were similar but somehow different from those of wall jet on smooth bed. The dimensionless boundary layer thickness was 0.53 on rough bed that was more than corresponding value on smooth bed with the amount of 0.16. As the roughness density increased the bed shear stress produced by the interaction of upstream supercritical flow with the roughness elements enhanced so the energy loss increased and as a result the sequent depth decreased. In the arrangement of ternary combination of roughness elements with density of 10.67 % the maximum reduction value of the sequent depth was about 29.39%, the increase of the energy dissipation relative to hydraulic jump on smooth bed was10.94% and the bed shear stress coefficient on rough bed was about 13.54 times as much as its corresponding value on smooth bed.

In this study, the effect of the secondary flow strength and vorticity on variations of bed shear stress for different positions of spur dike are compared through a mild 90˚ bend along with a T-shaped spur dike in a rigid bed. To carry out these experiments, three dimensional velocimeters (ADV) have been used for measuring velocity. Moreover, a comparison has been made between velocity vectors and variations of streamlines along the bend; the secondary flow strength and vorticity values are estimated for various positions of spur dike, and their effects on bed shear stress variations have been analyzed. It is concluded that the maximum secondary flow strength is evident in a distance of 0.6 of spur dike’s length at upstream under all these different positions of spur dike. Also the maximum vorticity position corresponds to the position of the maximum secondary flow strength, in front of spur dike’s wing. According to these results, it is predicted that the maximum scour occurs near the position of maximum secondary flow strength and maximum vorticity. Besides, the path of sediments motion coincides with the maximum shear stress points locus.

Interaction between water flow and bed erodibility, which is associated with sediment transport and creat bed forms. Ripples are the smallest of the bed forms. Experiments were performed with variety slopes of 0.0005 to 0.003 and discharges of 10 to 40 L/S, on the artificial ripple in the hydraulic Laboratory of Shahrekord University.Velocity measurements were collected using an Acoustic Doppler Velocimeter. It was observed the peak value of bed shear stress was appear on the midpoint of upstream surface and the lowest value appear on the crest. From the crest to trough, the general bed shear stress was in a trend of increasing. It was also observed that in the case of ripples bed forms were consist of about 26% of total bed shear stress.

The goal of this paper is, study the effects of creating the surface waves in a uniform flow on bed load transport rate of deposited sediment in a Primal rigid rectangular channel. Experiments were carried out in a rectangular flume of 10m length, 0.2m width and 0.3m height, for two gradations of sediments with median diameters of 0.34 and 0.48 mm and specific weight of 2.67, and for five bed shear stresses (initiation of motion and multiplies of 1.16,1.25,1.39 and 1.43 of it for median diameter of 0.34 mm and initiation of motion and multiplies of 1.15,1.25,1.33 and 1.41 of it for median diameter of 0.48 mm) and two wave heights (1 and 1.6 cm) with four wave speeds. The results showed that creating surface waves increases the bed load transport rate. Furthermore, the bed load transport rate increases with Strouhal number, dimensionless wave height and bed shear stress. It can be concluded that creating artificial waves can increase the bed load transport rate in a rigid rectangular channel up to 280% compared to the no wave conditions.

This paper investigates the effects of roughness on flood plain on hydraulic overbank flow in a compound channels with non-prismatic floodplains. Experiments are done using three divergence angles and three-roughness size on the floodplain. The velocity was measured using a three dimensional acoustic Doppler velocimeter and directional current meter in a lattice along the divergence channel in three portions of the flume (entrance, middle and end of the divergence) with different depth ratio. Using the experimental data, the values of the shear stress, depth–averaged velocity, local and zonal roughness coefficients, turbulence parameters and divided discharge between main channel and floodplain were evaluated. The results show that: With increasing the floodplains roughness, in non-prismatic section, the ratio of the velocity at floodplain on main channel decreased about 31% and turbulence flow increased about 25%. Also under these conditions, the shear stress at interface zone was increased from 26% to 48. As well as with increasing depth ratio, the above mentioned values are markedly reduced.

Flow pattern in bends due to changing of hydraulic conditions causes scour and sedimentation. In this study، experiments were done in a 90 degree sharp rectangular bend with R/B=2 and constant flow depth 17 cm at three different flow discharges of 15، 25 and 35 lit/s having Froude numbers of 0. 17، 0. 28 and 0. 40، respectively، and in each Froude number، flume bed was covered by particles with uniform sizes، with three roughness height of 0. 5mm، 2mm and 5mm. Results indicate that with increasing roughness، shear stress increases and maximum shear stress in all of the conditions is located at 10-65 degree close to inner bank and from a location of 90 degrees is close to outer bank. Also، changes of strength of vortex along the channel has an absolute maximum and a relative maximum in the location of 50 and 80 degrees، respectively، which increasing the roughness cause the absolute maximum occur in head of bend (50 degree) and moreover hydraulic conditions of the bed (smooth or rough) are effective on the strength of the vortex. On the basis of changing of shear stress and the strength of the vortex، the location of 80-90 degree of bend in outer wall is diagnosed as erodible region.