مجله هیدرولیک
سال یازدهم شماره 4 (زمستان 1395)

In the present research, a 3D numerical model of the sediment grain movement has been developed based on the Eulerian-Lagrangian perspective. The forces which act on sediment grains are non-linear drag force, the shear lift force, the rotational lift (Magnus) force, the buoyancy force, the added mass force, the Basset history force and torque. Second-order nonlinear ordinary differential equations have been used to calculate linear and angular velocity vectors and also the position of sediment grains. Two parts including turbulent fluctuations in 3D space and a stochastic bed-particle collision model have been considered in the modeling procedure. The verification of the developed model has been examined for two important issues including the minimum number of jumps for statistical convergence and the appropriate value of the time step. The validation of the model has been performed using reliable experimental data in terms of the saltation height, length and streamwise particle velocity for the ranges of fine sand to coarse gravel. To evaluate the qualification of the conceptual model, the effects of the elimination of some forces and turbulent fluctuations, where do not exist consensus on the use of these factors in the previous researches, have been studied. The results indicated that the developed model can be considered as a numerical lab to study different aspects of the sediment transport in the prediction step, which will be performed in the part B of this series.

In the present research, the developed 3D Eulerian-Lagrangian model of the sediment transport in Part A of this series is used to predict the effects of the bed-roughness height, water temperature (viscosity) and density of sediment grains. The selected parameters for the analyses are important both for sediment transport hydraulics and also uncertainties of experimental data. The results indicated that increasing bed-roughness height causes decrease in the saltation characteristics and consequently bed-load transport rate. The effects of water temperature (viscosity) are significant in the range of sand size, and it is necessary to keep it in the control for reducing uncertainties of experimental data. The sediment density has significant effects in both sand and gravel ranges, and thus the use of grains with different density compared to natural sediment can impose significant effects on experimental results.

Few studies are made about the effect of environmental conditions on the leakage flow rate. Some of these researches show that environmental conditions play an important role in leakage flow rate. On the contrary, there are other researches opposing this idea. The purpose of this research is to investigate evidence of the effect of environmental conditions on leakage flow rate. Based on this idea, some tests were carried out in the college of engineering at the university of Tehran, using a circular experimental set up at high pressure discharging to the atmosphere. Various ranges of pressure up to 50 m of water were imposed on a 110 mm diameter aged steel pipe with some holes on the pipe surface. In the next step, by using the experimental results, an analytical model in ANSYS software was developed and outputs were compared with experimental data. Later a proper mathematical turbulent model was derived. Furthermore, the effect of submerged conditions was investigated by this appropriate numerical model. The results of this research showed that static pressure fluctuations in submerged jet had significant effect on discharge and the rate of leakage was reduced in comparison with atmospheric discharge. According to the results, pressure reduction in 5 m head pressure could affect the leakage discharge under submerged condition up to 55 percent when compared with the case of discharging to the atmosphere. With pressures above 20 m, smaller changes were observed. These changes were limited to less than 10 percent. The aforementioned reduction depended on the imposed head of water at leakage point and pressure value inside the pipe. Finally, a relationship between leakage outflow and internal pressure of pipe was proposed.

Coherent structures in wall turbulence boundary layer transport momentum and provide a means of producing turbulent kinetic energy. In fully developed flow and above viscous sub-layer, different coherent structures such as ejection and sweep and hairpin vortex can be formed which can be detected by various detection methods such as quadrant analysis and analysis of instantaneous velocity values. In the present study, we aim to explore the accuracy and efficiency of these methods in non-smooth bed free surface flows. To this end, laboratory measurements of flow field above a gravel bed were carried out using particle image velocimetry. Despite the fact that the results of both techniques above roughness elements are in agreement, it was not possible to see any coherency between the results of these techniques just between roughness elements. Such incoherency can be resulted by non-zero mean vertical velocity and three-dimensional nature of flow in non-smooth bed condition which makes application of quadrant analysis in this type of flow challenging. Therefore, using analysis of instantaneous velocity values or modified quadrant analysis can be suggested for correct estimation of sweep and ejection.

The effective length of spillway is an important factor in increasing its discharge capacity and consequently decreasing the head over the spillway crest and the dam height. On the other hand, increasing the straight crest length increases the chute width and volume of excavations. Increasing the effective length of the spillway in a limited width is a method to reduce the construction costs. Curved crest spillway is an example of such methods. In the present work, discharge coefficient of a curved crest ogee spillway is compared with a straight one, both with the same chute width in a physical model. Results showed that the discharge coefficient of the curved ogee crest is similar to the straight one. Therefore, discharge capacity of the curved ogee was more than the straight one due to its longer length. Based on the results of the present work, designers can increase the discharge capacity of ogee spillways by bending them in a constant chute width. It should be noticed that the transition between the curved ogee and the chute and also the upstream guide walls should be designed so that these transitions make no disturbance and shockwave.

Fariman dam is a historical dam located in Fariman City, Khorasan Razavi province, Iran. So far, experimental and field investigations has not been performed on this dam and its hydraulic structures such as stilling basin. Since the safety and preservation of this historical structure is necessary, its hydraulic performance must be evaluated under different design conditions. This dam has a special stilling basin for its bottom outlet structure. In this paper, the performance of the stilling basin is investigated by Flow-3D software. First, the accuracy of the software is demonstrated by an experimental model. Later, the numerical model of the stilling basin is constructed and two characteristics of flow: velocity and Froude number are verified in the stilling basin. It is shown that the the stilling basin did not have a proper performance under the design condition. Therefore, measures for improving its performance are recommended.