In the present research, the flow on a free dam is modeled in 3D environment of Flow-3D software, then the software results were validated in comparison with results obtained from the physical model. Finally, the effect of various types of ramps on effective factors of vacuum creation was scrutinized. The results for vacuum creation index and air concentration existing within the flow revealed that a spillwaywith ramp at the bed and a wall equipped with duct, and a spillway with ramp at bed and wall, leads to %22, %22 and %19 increase in vacuum creation index, respectively...

Oxidation ditch process is used to treat the municipal and domestic sewage. The untreated sewage generates fouling smell, harmful bacteria, etc. which is hazardous for public health and degrades the environment by librating noxious gases. This paper examines the use of curved blade aerator for oxidation ditch process. The rotor, which controls the aeration, is the main component of the aeration process. Therefore, the objective of this study is to find out the variations in overall oxygen transfer coefficient and aeration efficiency for different configurations of aerators by varying the parameters like speed of aerator, depth of immersion and blade tip angles so as to yield higher values of overall oxygen transfer coefficient and aeration efficiency. Four aerators of different configuration were developed and fabricated in the laboratory and were tested for above mentioned parameters. A mathematical model is developed for predicting the values of kLa and aeration efficiency which has R2 values of 0.97 and 0.99 for experimentally determined and calculated values. In laboratory studies, the optimum value of overall oxygen transfer coefficient and aeration efficiency were observed to be 10.33/h and 2.269 kg O2/kWh for aerator speed of 48 rpm, 5.5 cm depth of immersion and 47o blade tip angle for curved blade aerator.

Soil aeration by imported machines were reportedly unsatisfactory due to creation of shallow holes, taking off some patches of the turf and time consuming work needed for collecting the excavated cores. Considering these problems, a machine was developed to suit Iran’ s lawn spaces. To develop an appropriate aerator, the shape of the plugs was studied at the first step and for this purpose six types of plugs with conical and cylindrical cross section (with and without slot) with tips cut diagonally and perpendicularly were built. The plugs were compared in an experiment based on complete randomized blocks design with 3 replication in an urban lawn space with clay soil in terms of the required static force for cutting the thatch and penetrating to the soil as well as the length of projected cores. The results showed that the conical plugs with beveled tips had a 28 millimeter longer cores than the cylindrical plugs with similar tip using approximately equal forces for insertion. Having chosen the appropriate shape of plug, a split roller with a relatively large diameter were constructed for installing plugs around it and the whole device attached to a walking tractor. Comparing the developed machine with a commercial self-propelled aerator revealed that the developed aerator improved the length of the cores and depth of the holes. The split roller in the machine which windrowed the cores also caused a 65% decrease in labor requirement for collecting the cores and did not have dug patches of turf.

Introduction: To protect hydraulic structures like spillways, chutes, and bottom outlets against damage caused by cavitation, the air is usually pumped into the regions with a cavitation index below the critical value. Using the aerators, the erosion of the spillway surfaces caused by cavitation can be eliminated. The aerators are usually mounted on the bottom or the lateral walls of the spillway and cause the separation of high speed flows from the spillway surface and prevent erosion on the rigid surfaces. Most of the experiments have focused on the average air concentration of flow, while the amount of the air and the way it comes out the flow must be determined. Therefore, in the present study, the experimental data presented by Pfister (2007) for numerical simulation of flow over the aerator was used to investigate variations of the air concentration along the chute bottom. FLUENT software was used for the simulation of the two-phase air-water flow. Jump length has been considered as an important and effective factor in entering air into the flow and a criterion for verification. Methodology: According to the importance of the determination of the minimum volume of required air to prevent cavitation damages, the numerical effects of the air concentration during chute has been studied in this research. Eulerian and K-ɛ (RNG) models have been selected for two-phase simulation and for studying the turbulence effect respectively. The structured and unstructured mesh has been reviewed for the meshing model, finally, the use of structured mesh has been considered. As mentioned, the result of Pfister (2007) model has been utilized for validation and the length of the jet in crossflow was the criteria of validation. Therefore, parameters affecting the length of jet flow include the ramps with angels of 5. 7, 8. 1, and 11. 3 degrees, the steps with the heights of 23, 25, 44, 45, and 100 millimeters, the combination of the ramp and the steps, various Froude numbers in the range of 5. 8 to 10. 4, different ramp heights include 0, 6. 7, 13. 3, and 26. 7 millimeters and the chute slopes of 12, 30, and 50 degrees have been studied. The proper estimate of the jump length from the aerators has been simulated for 93 models. Additionally, the bed air concentration during chute and the air concentration at the depth, in the downstream of the impact point, has been modeled using Computational Fluid Dynamics and FLUENT software that can be employed in determining the distance of aerators. Results and Discussion: In this research, the effects of Froude number, the slope of the ramp, the initial height of the flow, the height of the step, and the ramp height on coefficient m has been studied considering the equation of changes in the bed air concentration ( (( / ) 1) m x L jet C C e ). In this equation, Cx/Ljet-1 represents the bed air concentration in the ( / 1) b x L jet point of impact to the chute bottom and m is a coefficient for which sensitivity has been tested. According to the result, air outflow decreases by increasing the Froude number. This shows a reverse trend for aerators with the ramp. Increasing the height of the step speeds up the increasing amount of m as well. The air outflow gradient goes up with the increasing the slope of the ramp so that the air outflow gradient has a significant rise by increasing the step height. The result does not show the specific trend for air outflow gradient as a function of the initial height of the flow. An increase in the step height results in increasing the length of the jump and increases the amount of the air entering the flow and the growth of air outlet from flow also increases. The air outlet gradient rises by increasing the ramp height. After determining the jump length of jet, the rate of the air entering the flow and the variability of the bottom airflow can be investigated by the equations suggested by the authors, accordingly, the appropriate distance between two aerators can be defined. The location of the first aerator is the initial point of cavitation and the distance of the second aerator can be affected by factors as follows: 1. The rate of the air coming out of the downstream of the first aerator 2. Natural aeration of the flow from the free surface. Conclusion: A comparison of the results indicates the numerical and experimental models are compatible. According to the importance of the point of the impact, where the flow collides at the chute bottom (sudden outlet of air due to collision), the point was considered as the reference point for the calculations in the equation. Generally, the results showed that the air concentration downstream of the aerators increases with an increase in Froude Number, ramp height, steps height, and ramp angle. This decreases as the height of water upstream of the aerator increases.

Due to the high speed of the air inside the combustion chamber of Scramjet engines, the proper mixing of fuel and air in the combustion chamber is one of the challenges and key issues in the design and analysis of such engines. To improve the mixing, various methods are used, including the implementation of a ramp ahead of the injection port. In the present work, the effect of ramp height on the area of double hydrogen jet cross-jet flow in supersonic airflow has been investigated numerically and the effect of ramp height on parameters such as mixing efficiency, effective mixing area ratio and stagnation pressure losses has been investigated. Numerical simulations have been performed using the solution of the three-dimensional Reynolds-Averaged Navier-Stokes equations with the two-equation turbulence model k-ω,sst. Initially, the results of numerical solution were validated with the experimental data, which comparison of numerical solution results with experimental data shows their good agreement with each other. Then, the effect of the presence of a ramp upstream of the injection port is numerically investigated for several ramps with different heights.

This paper studies the effects of queue formation in the bottlenecks at off-ramps on the capacity of the freeways. Six expressway exit-ramps throughout the city of Tehran, Iran were selected and their traffic flows were observed in thirty-minute intervals during which the queue formation and queue elimination occurred. Assuming that in the absence of the queue, the traffic flow is in its normal state, the changes in the volume of through vehicles has been modeled as an average estimator of the change in the expressway capacity.The developed models prove that the changes in freeway capacity are due to queue formation at the off-ramp sections. However, the estimated figures are different from those obtained from the theory of freeway capacity. The conclusion is that lane blockage is only one of many factors that affect the freeway capacity while the queue forms. Since it is not possible to quantify all those factors individually, the resulting models are macroscopic estimates of the phenomenon.

Probabilistic designation is a powerful tool in hydraulic engineering. The uncertainty caused by random phenomenon in hydraulic design may be important. Uncertainty can be expressed in terms of probability density function, confidence interval, or statistical torques such as standard deviation or coefficient of variation of random parameters. Controlling cavitation occurrence is one of the most important factors in chute spillways designing due to the flow’ s high velocity and the negative pressure (Azhdary Moghaddam & Hasanalipour Shahrabadi, 2020). By increasing dam’ s height, overflow velocity increases on the weir and threats the structure and it may cause structural failure due to cavitation (Chanson, 2013). Cavitation occurs when the fluid pressure reaches its vapor pressure. Since high velocity and low pressure can cause cavitation, aeration has been recognized as one of the best ways to deal with cavitation (Pettersson, 2012). This study, considering the extracted results from the Flow-3D numerical model of the chute spillway of Darian dam, investigates the probability of cavitation occurrence and examines its reliability. Hydraulic uncertainty in the design of this hydraulic structure can be attributed to the uncertainty of the hydraulic performance analysis. Therefore, knowing about the uncertainty characteristics of hydraulic engineering systems for assessing their reliability seems necessary (Yen et al., 1993). Hence, designation and operation of hydraulic engineering systems are always subject to uncertainties and probable failures. The reliability, ps, of a hydraulic engineering system is defined as the probability of safety in which the resistance, R, of the system exceeds the load, L, as follows (Chen, 2015): p_s=P(L≤ R) (1) Where P(0) is probability. The failure probability, p_f, is a reliability complement and is expressed as follows: p_f=P[(L>R)]=1-p_s (2) Reliability development based on analytical methods of engineering applications has come in many references (Tung & Mays, 1980 and Yen & Tung, 1993). Therefore, based on reliability, in a control method, the probability of cavitation occurrence in the chute spillway can be investigated. In reliability analysis, the probabilistic calculations must be expressed in terms of a limited conditional function, W(X)=W(X_L, X_R)as follows: p_s=P[W(X_L, X_R)≥ 0]= P[W(X)≥ 0] (3) Where X is the vector of basic random variables in load and resistance functions. In the reliability analysis, if W(X)> 0, the system will be secure and in the W(X) <0 system will fail. Accordingly, the eliability index, β , is used, which is defined as the ratio of the mean value, μ _W, to standard deviation, σ _W, the limited conditional function W(X) is defined as follows (Cornell, 1969): β =μ _W/σ _W (4) The present study was carried out using the obtained results from the model developed by 1: 50 scale plexiglass at the Water Research Institute of Iran. In this laboratory model, which consists of an inlet channel and a convergent thrower chute spillway, two aerators in the form of deflector were used at the intervals of 211 and 270 at the beginning of chute, in order to cope with cavitation phenomenon during the chute. An air duct was also used for air inlet on the left and right walls of the spillway. To measure the effective parameters in cavitation, seven discharges have been passed through spillway. As the pressure and average velocity are determined, the values of the cavitation index are calculated and compared with the values of the critical cavitation index, σ _cr. At any point when σ ≤ σ _cr, there is a danger of corrosion in that range (Chanson, 1993). In order to obtain uncertainty and calculate the reliability index of cavitation occurrence during a chute, it is needed to extract the limited conditional function. Therefore, for a constant flow between two points of flow, there would be the Bernoulli (energy) relation as follows (Falvey, 1990): σ = ( P_atm/γ-P_V/γ +h cos⁡ θ )/(〖 V_0〗 ^2/2g) (5) Where P_atm is the atmospheric pressure, γ is the unit weight of the water volume, θ is the angle of the ramp to the horizon, r is the curvature radius of the vertical arc, and h cos⁡ θ is the flow depth perpendicular to the floor. Therefore, the limited conditional function can be written as follows: W(X)=(P_atm/γ-P_V/γ +h cos⁡ θ )/(〖 V_0〗 ^2/2g)-σ _cr (6) Flow-3D is a powerful software in fluid dynamics. One of the major capabilities of this software is to model free-surface flows using finite volume method for hydraulic analysis. The spillway was modeled in three modes, without using aerator, ramp aerator, and ramp combination with aeration duct as detailed in Flow-3D software. For each of the mentioned modes, seven discharges were tested. According to Equation (6), velocity and pressure play a decisive and important role in the cavitation occurrence phenomenon. Therefore, the reliability should be evaluated with FORM (First Order Reliable Method) based on the probability distribution functions For this purpose, the most suitable probability distribution function of random variables of velocity and pressure on a laboratory model was extracted in different sections using Easy fit software. Probability distribution function is also considered normal for the other variables in the limited conditional function. These values are estimated for the constant gravity at altitudes of 500 to 7000 m above the sea level for the unit weight, and vapor pressure at 5 to 35° C. For the critical cavitation index variable, the standard deviation is considered as 0. 01. According to the conducted tests, for the velocity random variable, GEV (Generalized Extreme Value) distribution function, and for the pressure random variable, Burr (4P) distribution function were presented as the best distribution function. The important point is to not follow the normal distribution above the random variables. Therefore, in order to evaluate the reliability with the FORM method, according to the above distributions, they should be converted into normal variables based on the existing methods. To this end, the non-normal distributions are transformed into the normal distribution by the method of Rackwitz and Fiiessler so that the value of the cumulative distribution function is equivalent to the original abnormal distribution at the design point of x_(i*). This point has the least distance from the origin in the standardized space of the boundary plane or the same limited conditional function. The reliability index will be equal to 0. 4204 before installing the aerator. As a result, reliability, p_s, and failure probability, p_f, are 0. 6629 and 0. 3371, respectively. This number indicates a high percentage for cavitation occurrence. Therefore, the use of aerator is inevitable to prevent imminent damage from cavitation. To deal with cavitation as planned in the laboratory, two aerators with listed specifications are embedded in a location where the cavitation index is critical. In order to analyze the reliability of cavitation occurrence after the aerator installation, the steps of the Hasofer-Lind algorithm are repeated. The modeling of ramps was performed separately in Flow-3D software in order to compare the performance of aeration ducts as well as the probability of failure between aeration by ramp and the combination of ramps and aeration ducts. Installing an aerator in combination with a ramp and aerator duct greatly reduces the probability of cavitation occurrence. By installing aerator, the probability of cavitation occurrence will decrease in to about 4 %. However, in the case of aeration only through the ramp, the risk of failure is equal to 10%.