Floodplain vegetation can alter the flow characteristics of a river through the application of redundant drag forces. In this study, turbulence characteristics and flow structure were investigated under the influence of partially double-layered vegetation in a compound channel. To investigate the phenomenon, a three-dimensional numerical model was used to solve the Navier-Stokes equations and track the evolution of the free surface. To ensure the performance of the model, the numerical results were validated using data from previous experimental studies. The validation results showed that this model captured the flow dynamics with high accuracy. In the next step, the model was used to predict the free surface fluctuations and velocity field of the steady flow in the layered vegetated floodplains. The modeling results showed that the formation of a velocity gradient at the interface between the main channel and the floodplain can lead to the development of secondary flows and the mass and momentum exchange at this interface. In addition, turbulent kinetic energy and turbulent dissipation of the flow through vegetation in floodplains was observed in the numerical results. It was concluded that the layered vegetation can increase the flow turbulence and the dissipation rate of the flow energy. The maximum values of turbulence kinetic energy and turbulence intensity were observed at the interface between the floodplain and the main channel. Therefore, the flow disturbance at the interface between the floodplain and the main channel may increase the mass and momentum exchange in this region.

Turbulence is a form of movement characterized by an irregular or agitated motion. Turbulent motions are very common in nature. Most flows in the lower atmosphere and in the upper ocean are turbulent. The Turbulence has long had a special attraction for physicists and mathematicians; it has been called “the last great unsolved problem of classical physics”.
In this study, hydrphysical measured data in the southern part of the Strait of Hormuz and with time step of half an hour during the period December 1996 to March 1998, by the University of Miami, and the meteorological station in island of Gheshm are used , then turbulence was simulated by General Ocean Turbulence Model (GOTM( . The results showed that, turbulent kinetic energy (TKE) in different seasons, with different penetration depths were appeared at during the year. In the cold season, the kinetic energy of the turbulent expands from surface to bottom and in the warm seasons because of existing the seasonal thermocline, depth penetration of TKE are limited, and only expands from surface to top of thermocline layer. In this study, investigation of the turbulent Prandtl number (Pr) shows that, effect turbulent viscosity Preference to the production buoyancy in the middle depth.

In this paper, the flow and turbulence structure in the boundary layer of an urbanizedregion with complex topography (Tehran) was studied using data from a meteorologicalstation, Sodar (PA1 Model) for heights above 50m, on days 13 through 24 of August,2002, and an ultrasonic anemometer, located in Tehran University Geophysics Institute,in August 2005. Days for observation were selected such that they were without anyactive synoptic system in the region, the skies were clear, wind speed at 10 m did notexceeded 5 meters per second, and the relative humidity was low. The data used for thevertical profiles are 12-day averages for 4 local hours, namely: 09:30, 15:30, 21:30 and03:30.The study of turbulence using the gradient Richardson number shows that, during theday, the boundary layer is generally turbulent while, at nights, in addition to the reductionof turbulence intensity, the depth of the turbulent region of the boundary layer alsodecreases. Tests done by Monti et al. (2002) have shown that the Rig is not sensitive to thetime of averaging in range30 s  Tav  900 s. The Sodar data are also 15 minutes averagedof the measurements. Additionally, Sodar data are spatial average of 25-meter, and sothis averaging may filter out some turbulent layers in the profiles. This averaging maycause large Richardson numbers.Graphs of u s, v s and w s show that nearly continuous turbulence occurs during the night,but the values are weaker with respect to daytime turbulence. The TKE diagram showsthat this quantity has a daytime maximum value and a nighttime minimum, indicating theeffect of stability on turbulence generation. Turbulent intensity componentsusu andusvare almost the same, and are also four times that ofusw during the night and nearly threetimes ofusw during the day. This shows that turbulent kinetic energy production is acombination of buoyancy and shear effects during the day and is mostly due to the effectsof shear during the night.A study of the behavior of turbulent quantity (sw) / z 3, in association with buoyancyand mechanical turbulent kinetic energy source production terms, explains the thermaland roughness effects on the characteristics of TKE. Values of diurnal z (sw) / 3 arereduced relative to height at different hours due to the increased production of mechanicaland buoyancy turbulence in the surface layer. Values of these quantities are low even inthe presence of wind shear that can be due to radiation-induced effects on these quantitiesduring the day, especially in relatively low wind conditions.Wavelet analysis of wind speed using ultrasonic anemometer data with one-minuteaveraging in the surface layer shows mostly wavy and continuous structures with periodsof 6 to 90 minutes that are related to turbulent fluctuations and both regular and irregularinternal waves.It seems that the topography-induced flows (down slope, upslope and drainage flows)and urban effects (flows from thermal islands and their interactions with artificialtopography such as high buildings, roads and vegetation) cause important changes in thecirculation of the wind flows of the region when synoptic systems are absent. Local flowsin the region with the effects of complex terrains are generated by pressure gradients andthermal forcing. Urban flows span a wide range of space and time scales. These factorschange turbulence and vertical wind profiles.The time series of various quantities show approximately diurnal variations. Verticalprofiles of turbulent quantities show that the flow is stratified in the lower section of theboundary layer (under 500m). The depth over which the katabatic flow occurs reachesabout 200 meters. This stratified lower section of the boundary layer possibly caused bythe effects of complex topography, the urbanization of the region and their circulationinteractions (especially during the night, when they reinforce each other). The height ofthis layer doubles during daytime. The layering of the wind profile may be due to airintrusion from various slopes originating from different sources according to Monti et al.(2002), or to the structure of generated internal waves.

Stepped spillways are a common structure for energy dissipation by creating frictional resistance to flow through the steps. Based on the studies and depending on flow conditions, the flow over a stepped spillway is usually categorized into three regimes: nappe, transition, and skimming. The stepped spillway is often designed for skimming flows. There were different studies investigating various aspects of stepped spillways, but what is important in this type of spillway is increasing the effectiveness of steps in the rate of energy dissipation. This can be done by a new type of step structure (i.e., inclination angles on steps or using a sill on the edge of a step and cases like that) or geometric alteration and change of steps called labyrinth stepped spillways. Therefore, it is scientifically beneficial to modify the shape of the step of the stepped spillway to increase its collision and roll to achieve energy dissipation. The present study deals with the design of step modification by creating cubic elements on the steps in different arrangements and different hydraulic conditions. This has been considered to improve the performance of stepped spillways by increasing the energy dissipation. For this purpose, using FLOW-3D software, the influence of geometric appendance elements on the steps on the velocity distribution, pressure, the turbulent kinetic energy (TKE), and finally the flow resistance and the energy dissipation on modified spillways was investigated and compared with the flat stepped spillway. The physical model for verifying the numerical results was carried out in a rectangular flume with a length of 12 m, a width of 1.2 m, and a height of 0.8 m. The experiments were conducted on a stepped spillway with a slope of 26.60° and consisted of 10 steps with step length (l) and height (h) of 0.06 and 0.12 m, respectively. Stepped spillway models in numerical study include flat models and models with cubic elements placed on the steps in four arrangements of two side, zigzag, center, and hybrid AE elements and two heights of elements h/2 and h/4 (h step height). The commercially available CFD program FLOW-3D was used for the numerical simulations. The RNG k-ε turbulence model was employed for the turbulence calculations. To obtain mesh-independent results, three different mesh sizes were used, and the grid convergence index (GCI) methodology was employed to select the appropriate mesh. As a result, the mesh consisting of a containing block with a cell size of 1.3 cm and a nested block of 0.95 cm was selected. In the fluid domain, the boundary conditions were set according to the experimental conditions. In the upstream of the domain, a discharge flow rate (Q) definition was set. The downstream section was treated as an outflow (O) boundary condition. The bottom and the sides behave as rigid walls (W). For the upper boundary, the atmospheric pressure boundary, and at the inner boundary conditions, symmetry (S) was used. The results showed that the appendance elements on the steps cause some fluctuations on the flow surface and increase the intensity of the current collision by deviating the flow from its parallel path. The result is reduced velocity by about 10%, an increase of 54% in TKE, and an increase of 6.42% in energy dissipation on modified models compared to the flat stepped model. There was no negative pressure on the horizontal plane of the steps, and the maximum pressure occurred in the middle of the steps and inclined to the end of the steps. The appendance elements reduce the negative pressure areas on the vertical surface of the steps and reduce the risk of cavitation. The hybrid element model performs best in other arrangements, and reducing the height of the elements improves their behavior. According to the obtained results, it can be concluded that the appendance elements on the steps improved the hydraulic performance of stepped spillways by increasing the roughness of the steps, increasing energy dissipation, reducing the flow velocity over the spillway and reducing the risk of cavitation by reducing the negative pressure in the vertical plane of the steps. The use of cube-shaped elements on the steps and in the hybrid arrangement is suggested.

Air pollution has harmful effect on human health and the environment. Accordingly, considerable effort has been put to analyze the air pollutants. One important issue is the spatial distribution of these pollutants. Dispersion of the pollutants released from sources on the ground is mostly driven by the planetary boundary layer where turbulent flow causes mixing of the content of the stationary ground layer of atmosphere with higher moving layers and thereby clears out the pollutants rapidly. To predict the dispersion of air pollution in the atmosphere, researchers investigate the behavior of plume rise under certain conditions. Two factors critically affect the result of the test: primary plume rise and its dispersion. In this work, we study the effective height of emission which determines the dispersion of the pollutants. The height of the plume has been subjected to semi-empirical and to more accurate numerical studies. These approaches suffer from some shortcomings. For instance, the gravitational effects and fluctuations of the wind speed are neglected. Numerical approaches are typically based on a Gaussian plume rise model so called ISC3 (published by EPA). In this paper plume rise of emission of an air pollutant is simulated using Fluent software which allows one to input natural wind velocity profile and to consider the gravity. Another advantage of this approach lies in the usage of turbulent kinetic energy (TKE) and temperature profiles. In this work, theoretical plume rise using the ISC3 model were being calculated at first. The calculated values of xf and ∆h are equal to 437.85 m and 50.87m, respectively. Then, the dispersion was being simulated accordingly using Fluent and the results were compared with those of the numerical studies.

Population growth، along with the rapid development in industrial and many urban sectors، and lack of sustainable development approach in urban planning are all caused great changes in environment in form of pollution and ravage. In urban environments، complexity of spaces and urban manmade phenomena as well as the lack of regular and continuous measurement of atmospheric elements and components such as surface fluxes، turbulence intensity، overnight stable layer depth and inversion layer، daily mixed and boundary layer and energy balance components which are generally the input of dispersion models have made uncertain mechanisms in dispersion of pollutants over Tehran. One of the important aspects in the study of air pollution is how the pollutants disperse from sources of emissions. Wind direction، turbulence conditions and fluxes in near surface atmosphere are the most important climatic factors that affect dispersion pattern and distribution of air pollutants leaving from emission sources. Study AreaTehran having a population of about 8،300،000 and a 15 million-plus metropolitan area is Iran''s largest city and urban area، and one of the largest cities in southwest Asia. Tehran (35° 42′ N، 51° 25′ E) covers an area of 750 km2 and is situated in a semi-enclosed basin south of the Alborz Mountains. Its location for a big city is unusual، since it is not near a river or even close to the sea. The average annual rainfall is approximately 230 mm، with most precipitation falling in autumn and winter months. Due to high elevation (approximately 1140 m)، aridity and latitude، the city experiences four seasons. Climate can be extremely hot in summer (with mid-day temperatures ranging between 30 oC to 40 oC)، and cold in winter when nighttime temperatures can fall below freezing point. Local precipitation is absent for six months of the year on the low-lying areas. Tehran suffers from severe air pollution and the city often covers by smog making breathing difficult and causing widespread pulmonary illnesses. It is estimated that about 27 people die each day from pollution-related diseases. According to local officials، 3،600 people died in a single month due to the hazardous air quality. 80% of the city''s air pollution is due to cars. The remaining 20% is due to industrial pollution. Other estimates suggest that motorcycles alone account for 30% of air and 50% of sound pollution in Tehran. Tehran is bound in the north by the massive Alborz mountain range that is stopping the flow of the humid Caspian wind. As a result، thermal inversion that traps Tehran''s polluted air is frequently observed. The methodology is based on undertaking literature review to develop theoretical foundations and explanation of the research method. In this research، three-hourly wind velocity and wind direction data and air temperature of 2006 were obtained from Geophysics، Mehrabad and the Shemal-e-Tehran weather stations (Table 1). Also atmospheric gridded data obtained from NCEP/NCAR reanalysis dataset for surface and 700 hPa levels for prepared synoptic maps using GrADS software (Table 2). Mehrabad upper level atmospheric data has been taken from the University of Wyoming are used to identify effective thermodynamic indices on intensified or mitigation air pollution in Tehran. Moreover، two site emission data available in SO2 of chimney exits south of Tehran including Tehran oil refinery source point and thermal power plant source point were used (Table 3). In this research، using EDMF thermodynamic index available in The Air Pollution Model (TAPM)، the dispersion pattern of pollutants in the air near the surface were studied under unstable weather and windy conditions over Tehran. Three-hourly meteorological data from three stations in Tehran were used to select two case studied. So two days with significant wind in all three weather stations were selected that they also include days of warm and cold periods. Then prevailing weather condition in synoptic-scale weather systems were evaluated by providing surface and upper level weather maps. Emission data from two point sources of Power plant and Tehran’s oil refinery in south of Tehran used as the input to model. Atmospheric model with three nested cells with 4، 3. 9، and 3 Km dimensions and pollution input with nets respectively 1000، 975، and 750 m were imported to model software. Pollution input is defined based on Eulerian and Lagrangian dispersion models، and their outputs were calculated for the innermost grid. In September 27 and March 21، a low pressure is seen in the surface in southeast of Caspian Sea and a high pressure is located over northwestern Iran. Barometric trough with contours of 1008 to 1011 hPa of it is drawn to Tehran area. Wind vectors represent weak surface western wind over the area. At 4 AM local time، the values of the vertical velocity (Omega) are positive which represent the stable conditions; however، at 4 PM local time omega becomes negative up to the level of 425 hPa and represents the unstable conditions (fig 3 and 4). Based on surface synoptic map in March 21، wind vectors represent high velocity and westerly direction. At 4 AM local time، vertical velocity is positive that indicates descending conditions، but in 4 PM it becomes negative and represents ascending condition (Fig 5 and 6). The result of model output for both dates show different atmospheric conditions in different time scales. Analysis of the Probability Density Function (PDF) for the summer wind condition (September 27) show that in 66% of winds in this day show 2 and 3 m/s velocity، and more than 53% of winds blow from northwest. Figures 7 to 11 show that the instability and turbulence conditions intensify in daytime and reduce at night. Figures 12 and 13 show the differences of spatial dispersion of pollutants in the daily average with (a) Eulerian and Lagrangian dispersion models for summer under windy conditions. Analysis of windy conditions in winter (March 21) by PDF is shown in figure (15). It shows westerly winds dominant and in this time more than 85% of winds velocity is more than 3 m/s. During winter condition، dispersion of pollutants in both Eulerian and Lagrangian models are the same in terms of density and wind direction of dispersions (fig 19). Simulation results indicate that in windy conditions of warm period، in south of Tehran with dry and hot air and soil، turbulent kinetic energy increases due to wind velocity and unstable atmospheric conditions and horizontal transfer of pollutants is associated with upward movement. Conversely، in the cold period، wind causes to increase the cooling air، so it result to decrease of turbulent kinetic energy in the atmosphere، hence pollutants are mostly transport horizontally.

Subgrid models are employed to predict the effect of small eddies in the LES method, and their performance is important in calculating the Reynolds stress and transferring the kinetic energy of turbulence to the grid. In this paper, the effect of sub-grid scale models in the LES method on the calculation of aerodynamic sound is investigated. For this purpose, five different sub-grid scale models including Smagorinsky-Lilly, WMLES, WMLES S-Omega, WALE, and KET have been used. First, the statistical convergence of computational methods has been investigated by employing aerodynamic coefficients, and the WMLES S-Omega model has been discarded due to aerodynamic instability. The near-wall flow study reveals models’ differences in the prediction of separation location and length-scale of vortices, which has changed the flow pattern. Aeroacoustic results obtained for the sound pressure level in the one-third octave band are validated using experimental data for the Naca-0012 3D airfoil. Based on the obtained results, the WALE model has presented the best approximation among different models in terms of the amount and trend of sound changes in different frequencies.

This research aims to optimize the mixing process in gas-lift anaerobic digesters of municipal sewage sludge since mixing and maintaining uniform contact between methanogenic bacteria and nutrients is essential. Wastewater municipal sludge sampling was performed at the Ahvaz West treatment plant (Chonibeh, Iran) during the summer of 2022. A Computational Fluid Dynamics (CFD) model was implemented to simulate, optimize, and confirm the simulation process using ANSYS Fluent software 19.0. The velocity of the inlet-gas into the digester was determined and a draft tube and a conical hanging baffle were added to the digester design. Different inlet-gas velocities were investigated to optimize the mixing in the digester. Furthermore, turbulence kinetic energy and other evaluation indexes related to the sludge particles such as their velocity, velocity gradient, and eddy viscosity were studied. The optimal inlet-gas velocity was determined to be 0.3 ms-1. The simulation results were validated using the Particle Image Velocimetry (PIV) method and the correlation between CFD and PIV contours was statistically sufficient (98.8% at the bottom corner of the digester’s wall). The results showed that the model used for simulating, optimizing, and verifying the simulation process is valid. It can be recommended for gas-lift anaerobic digesters with the following specifications: cylindrical tank with a height-to-diameter ratio of 1.5, draft tube-to-digester diameter ratio of 0.2, draft tube-to-fluid height ratio of 0.75, the conical hanging baffle distance from the fluid level equal to 0.125 of the fluid height, and its outer diameter-to-digester diameter of 2/3.

Most physical systems in nature have nonlinear dynamics and the system linearization of these physical systems is only a simplified assumption. The atmospheric motions, with the same philosophy, have a turbulent structure and ommiting the turbulent motions in the atmosphere is only a problem simplification assumption. When a buoyant jet of a chimney enters the atmosphere, it behaves like a turbulent flow, in which the atmospheric turbulence and self-generated turbulence of the plume play major parts. The present survey aimed at demonstrating the effects of turbulence on plume dynamics through computer simulation. The well-known turbulent flow simulation methods commonly used to simulate plume dynamics and atmospheric processes are: Direct numerical simulation (DNS), Reynolds averaged Navier-Stokes (RANS) and large eddy simulation (LES), which most distinguishing feature is their way of parameterizing the turbulence. As far as computational requirements, accuracy and turbulence simulation, the former two models are the two extremes while LES occupies an intermediate position between them, directly simulating the large-scale eddies and parameterizing the less important sub-grid scale (SGS) dissipative processes using sub-grid models (SGM). Most often, LES can predict the unsteadiness and intermittency of the turbulence structure, which is the most important feature of a buoyancy-driven jet. It should be noted that it is not efficient to employ full LES method when tackling an issue with certain unimportant zones. Furthermore, in the case of strong turbulent motions, the scale of flow structures near the rigid bodies are small and LES method method requires very fine grids that can increase its computational cost as large as DNS. To surmount this drawback, a hybrid RANS-LES method with a new mixed scale sub-grid parameterization model was applied to simulate the turbulent plume dynamics in ANSYS Fluent 14.5 software. The effectiveness and the accuracy of the mentioned turbulence simulation method was demonstrated through simulation study and experimental data in the neutral atmospheric conditions. Comparing the simulation results of the RANS method, the default hybrid RANS-LES method with static sub-grid scale parameterization and the new RANS-LES method with dynamic mixed scale parameterization indicated that the mean temperature profile at stack downstream was more accurately predicted by the new hybrid method. The root mean square error of plume rise estimation of the RANS method, the default RANS-LES method and the new hybrid RANS-LES method were 0.0437, 0.054 and 0.0323, respectively. It was further demonstrated that the Briggs integral plume rise model could not properly predict the plume rise in the presence of turbulence, because it does not consider the updraft and downdraft turbulence-induced motions in the atmosphere. Ultimately, we checked the capability of the new hybrid method to resolve the substantial parts of the turbulent motions. The turbulent energy transfer from the energy containing scales to inertial sub-range followed the well-known law. The capability of the new hybrid method in predicting the mean profile and the turbulent structure can be employed in the study of the effects of turbulent parameters on plume rise in different atmospheric stability classes.

In this article, a set of 2D numerical simulations were performed for different stages of a real Czochralski (Cz) growth of germanium crystal using Finite Element Method. The results of temperature distribution, melt and gas flow field, and crystal-melt interface were obtained numerically using the Reynolds-averaged turbulence modelling approach. The obtained results show that, as the crystal length is increased (i.e., the melt depth is decreased), the constant rotation rates of crystal and crucible increase both the melt Reynolds number and its turbulent kinetic energy, because of transferring constant momentum and energy to the remaining Ge melt. In addition, the computed crystal-melt interface shape for germanium crystal having 9 cm height was in a good agreement with the experimental data and observations in the crystal growth lab.