چكيده به لاتين
Abstract:
The creation and expansion of local scouring in bridge abutments is one of the most important hydraulic causes of bridge destruction. In order to control the amount of local scouring, different methods have been presented, and one of the most important methods is to increase the resistance of the materials that make up the bed and reduce the strength of erosive agents. One of the most important natural controls that is of interest today is the use of vegetation in order to reduce erosion factors and control erosion by increasing the strength of the substrate materials around submerged structures, which can affect the improvement of structural engineering conditions and structure-substrate-water interaction. . For this purpose, in the present study, this sensitive issue has been taken into consideration and an experimental study has been done on the conditions without cover and with vegetation cover around the semi-circular supports. Vegetation conditions are irregular so that the effect of randomness in controlling turbulent flow and local erosion can be investigated. Based on the results, it has been determined that the use of vegetation has been very effective in reducing erosion in the area of the supports. This issue has been considered in this research and an attempt has been made to measure the effect of vegetation on reducing erosion.
The purpose of this research is to investigate the flow pattern around the abutment with the shape of a semicircle with a radius of 6 cm, as well as the effect of irregular and submerged vegetation on the structure of the flow around the abutment. Laboratory studies were carried out in a channel with a length of 14 meters, width of 0.9 meters, height of 0.6 meters and a flow rate of 30 liters per second with a rectangular cross section. The first stage of the experiments was carried out in clear water scouring conditions and on sand particles with a diameter of 0.78 mm and at two flow depths of 19 cm and 14 cm in order to investigate the effect of water flow speed on scouring. In the second stage of the research, the effect of floor vegetation on the characteristics of the flow with a depth of 14 cm was investigated. A total of 65 speed profiles were taken, and in each profile between 15 and 25 points were collected depending on the depth of the scour. The measurement sections were chosen in such a way that the flow was fully developed. Average velocity components, turbulence values, Reynolds stresses, kinetic energy were measured and calculated using instantaneous velocities at different points and in all three dimensions by ADV speedometer. The results show the simultaneous presence of horseshoe vortices and downward currents upstream of the support, and the presence of rising vortices downstream has caused many irregularities in the flow structure. The presence of bottom vegetation reduces the depth and equilibrium time of scour, so that the time to reach equilibrium according to the criteria of Melville and Chiu, the experiment (first series) without vegetation and with a flow depth of 19 cm in 60 hours, the experiment (second series) without Vegetation and with a flow depth of 14 cm in 50 hours and the experiment (third series) with vegetation and a flow depth of 14 cm in 40 hours reached the washing equilibrium depth. Also, the depth of water washing in the state without vegetation and with a flow depth of 19 cm compared to the same state but with a flow height of 14 cm is reduced by about 20%, as well as the depth of water washing in the state without vegetation and with a flow height of 14 cm compared to the state with Vegetation and the height of the flow decreases by about 40%. The cause of which was estimated to be the presence of horseshoe eddies and stronger downward currents in the upstream of the stream, and also the presence of rising eddies in the downstream, creates many irregularities in the flow structure. By examining the flow in the state with vegetation, it was found that the presence of vegetation has weakened the primary eddies upstream of the support, which is caused by two important effects of the subsurface part of the vegetation, i.e. the roots, which stabilize the bed more and more. The upper part of the vegetation surface means the body, which lowers the flow speed around the support. The drawn diagrams show that for the flow approaching the fulcrum, the velocity profiles u have become skewed profiles. These deviations are observed near the bottom and the velocity distribution above the bed is often similar to the velocity distribution in turbulent flow in smooth beds. The reason for the significant deviations in the velocity profiles is the presence of pressure gradients as a result of strong vortices that have created a vortex and spiral flow around the support. Also, the presence of strong rotations in the vicinity of the support and in the bottom of the pit greatly reduced the velocities in the direction of the u flow near the bottom, and this effect was reduced by moving away from the axis of the support.The results show that at the top of the scour pit, the changes in the horizontal speed of the current (u) are almost logarithmic and reach their maximum value at the water level. In this study, the flow velocity was measured using an acoustic speedometer device, and by using quadrant analysis, the contribution of each of the four occurrences of the spalling phenomenon in the scour hole around a semi-circular foundation in a laboratory channel was investigated and determined. The results showed that the dominant phenomenon is reciprocating flow (Q4), then retraction flow (Q2), followed by external flow (Q1) and followed by internal flow (Q3) in the entire flow depth, except for a few nearby points. The surface of the bed and inside the scour hole, which with the increase of the depth of the flow, the contribution of the retraction flow becomes more and more important than the contribution of the reciprocating flow. Another point is the reduction of the contribution of the reciprocating flow and the retraction flow by moving away from the surface of the bed and the increase of the contribution of the events of the first and third quadrant in the production of Reynolds stress, so that the contribution of all four events is almost the same near the water surface. It was also found that moving away from the base does not change the share of spalling events. Therefore, lowering the water speed near the support and also using floor vegetation can be proposed as a completely new and low-cost protection method for the supports, especially during floods.
