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In the presented work, the lagrangian grid-free vortex element method is used for flow simulation in forced shear layer. Growth rate of instability and eddy formation is well simulated. Also, effect of eddy formation, pairing and the interaction among them are studied. In an unforced shear layer the growth rate is linear depending on the biggest instable frequency but in the forced shear layer other frequencies are taken part. The most important frequency is the forced frequency among other frequencies and the instability growth rate is no longer linear. The results show that the instability would accelerate due to increase of velocity ratio. The wavelength effect is well studied on the instability and is in good agreement with the experimental data.

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A passive scalar is a property that is affected by the flow field without affecting it. In this paper, first, the governing equations on the turbulent flows are solved and the property of a passive scalar in two dimensions is numerically studied. Having the values of the velocity components, the governing equation on transport of a passive scalar is solved. To compute the turbulent velocity field, the Large Eddy Simulation (LES) method using Smagorinsky subgrid scale is invoked. The flow in a cavity has been the basis to validate the accuracy of the generated computer code. To ensure the compatibility between the flow and the transport of passive scalar fields a similar LES approach is used. As a three-dimensional numerical solution for a turbulent flow fields needs a massive computational time and efforts, therefore a two-dimensional simulation used for a proper saving. Instead, to validate the numerical results, the range of the Reynolds number of the flow is kept within the range of the two-dimensional measurements. Comparison of the numerical results and the experimental measurements in two-dimension reveals the high accuracy of the results and compatibility between the flow and passive scalar fields. Ability of developed scheme to accurately handle transport of a passive scalar is promising to extend LES method into the transport of more species and hence to simulate reacting flows.

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In this article sub-grid modeling of Smagorinsky and Localized Smagorinsky Models are investigated. In modeling sub-grid scales, it is necessary to determine the Smagorinsky coefficient which is an experimental constant. Dynamic Models are developed to estimate this value more efficiently. In this research, the test filter is Gaussian, numerical method is based on the finite volume scheme, and a SIMPLE algorithm is used to evaluate the pressure. To perform computations on a personal computer, value of Reynolds number had chosen enough low to make a two dimensional modeling and comparison with respective experimental results possible. Comparison of numerical results shows high accuracy of the localized dynamic models. More numerical investigations reveal that although localized dynamic models need more computing time, but the higher resolution of the method makes it possible to use a coarser grid and hence compensate the extra CPU time.

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This paper discusses about the effects of square wave pulsation on the turbulent flow and heat transfer from slot jet impinging to a concave surface. The RNG k-ε turbulence model is applied for modeling the turbulent flow and heat transfer filed in the present 2-D slot jet flow. The effects of jet Reynolds number, nozzle to surface distance and pulsation frequency on time-averaged Nusselt number distribution are studied carefully. Results show that applying the pulsating jet in the range of 10 Hz to 50 Hz can increase heat transfer from the concave surface in comparison with the steady jet. Increasing jet Reynolds number ranged from 4740 to 9590 significantly increases the time-averaged local Nusselt number. Also, in steady jet, decreasing the nozzle to surface distance, consequences increasing the Nusselt number near the impingement zone. While in pulsating jet, it causes both increasing/ decreasing the Nusselt number all over the concave surface.

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In this research, interaction of a shock wave with incoming turbulent flow is investigated. To this end, different turbulent flows with different intensities and integral length scales are generated and impact of these turbulent flows on a shock wave are examined. In this study, two-dimensional Navier-Stokes equation is numerically solved using a high order spatial-temporal discretization. For spatial discretization, two different methods are implemented. In stream wise direction, i.e. perpendicular to the shock wave, a sixth-order accurate essentially non-oscillatory method (ENO) has been used which is able to capture the shock wave. In spanwise direction, i.e. parallel to the shock wave, a sixth-order Pade scheme has been used which is able to accurately capture small scale flow field structures. Time integration is performed using a third-order Runge-Kutta method. Overall, it has been observed that the turbulent kinetic energy increases across the shock wave. Fluctuations with larger integral length scale show higher turbulent kinetic energy increase across the shock wave. Further, it has been observed that although the integral length scale of the upstream fluctuations does not influence the location of the shock, the intensity of the upstream fluctuations have a profound effect on the shock wave location.

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Turbulent wind flow over buildings occurs due to the complexity like sharp corners, ground effect and different vortexes is one of the best choices to evaluate turbulence methods. DES and DDES are hybrid RANS-LES models for simulating turbulent flow which for their characteristic treat near wall as RANS and farther the wall act as LES model. Consequently computational time will decrease compared to traditional LES models. In this article to evaluate DES and DDES models, turbulent incompressible flow in Re = 22000 over 3D building is simulated using parallel processing facilities. For verification purpose other investigators experiment results are used. Also the mentioned models are compared with classic RANS and LES models, like k-ε and LES-Smagorinsky to depict their performance. Our results illustrate DES model with fine grid has good precision for simulating turbulent incompressible wind flow over building and decline of 26 percentage of computational time compared to LES-Smagorinsky model.

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The effect of bubbles on frictional drag reduction has been studied experimentally using a vertical Taylor-Couette system. Air bubbles are injected into water flow at the bottom of the system. The flow between cylinders is a fully turbulent flow and Taylor vortices are formed in annulus gap. In these experiments, the variations range of rotational Reynolds number is 5000<=Re_w<=70000 . The variations of drag reduction in the presence of bubbles have been investigated by measuring the exerted torque on the inner cylinder. The results show that increasing rotational Reynolds number up to a certain amount leads to enhancement of bubbles effects on drag reduction while the effects are inversed for higher rotational Reynolds number. In this work, the acquired maximum drag reduction is about 5%.

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Aims: This study aims to investigate the impact of shading devices and airflow velocity on the thermal (energy) performance and convective flow in a south-facing double-skin façade of a building located in Semnan, a hot and arid region in Iran. The research seeks to determine which type of shading device, including inclined and multi-layer shades, can most effectively reduce indoor temperature and enhance convective flow.

Methods: Numerical simulations were conducted using SolidWorks and COMSOL Multiphysics software for geometry modeling, fluid flow simulation, and heat transfer analysis, respectively. A two-dimensional double-skin façade with various shading configurations was considered, and turbulent natural ventilation flow within them was examined.

Findings: Simulation results demonstrated that the geometry and airflow velocity significantly influenced the velocity and turbulence of the airflow within the double-skin cavity. A geometry with a multi-layer (asymmetric) shading device exhibited an 18.5% temperature reduction at the same wind speed. The maximum temperature reduction occurred in a geometry with a multi-layer (asymmetric) shading device and an airflow velocity of 5 meters per second. In other words, the best thermal performance was observed in multi-layer shading devices.

Conclusion: This research indicated that the use of multi-layer (asymmetric) shading devices can effectively reduce indoor temperature and enhance convective flow. These findings suggest that the appropriate design of shading devices can be employed as a passive method to reduce energy consumption in buildings.
 

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Electrohydrodynamic actuator is one of the newest devices in flow control techniques which can delay separation point and reduce the drag coefficient by inducing external momentum to the boundary layer of the flow. In this paper, a 2-D numerical approach was implemented to analyze the presence of electrohydrodynamic actuator on the incompressible, turbulent, steady flow over a NACA 4412 asymmetric airfoil. In this regards, the flow field and aerodynamic characteristics such as the drag and pressure coefficient were evaluated through the variety of attack angles, applied voltages, the location of emitting electrode, and the distance from the upper surface of the airfoil. The numerical results indicate that the drag coefficient with the presence of an electric field decreases with the enhancement of the supplied voltage but increases when the attack angle is augmented. In addition, the location of separation point significantly depends on the position of emitting electrode and the distance between the emitting electrode and the collecting electrode. On the other hand, according to the results, the Electrohydrodynamic effects cause the diminution of the wake region over the airfoil.

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The nose and nasal cavity and sinuses are a parts of the upper respiratory system and study the air passage into the upper component of human airway is important to improve or cure deficiency in human respiration cycle. The nose performs many important physiological functions, including heating, humidifying and filtering inspired air, as well as sampling air to smell. Previously, numerical modeling of turbulent flow in nasal cavity, sinus, pharynx and larynx has rarely been employed Since the 1990s, with the development of computed tomography technology and computational fluid dynamics, a number of numerical studies on gas and particle flows in realistic nasal cavities have been conducted and provided precise data for deeper insight of the nature of nasal airflows. Also, most of pioneering studies in this field have been developed to the investigation of only nasal cavity without sinuses especially maxillary sinus So, this research is tried to study details of turbulent airflow through all spaces in human head that air can flow through. For this purpose, study has based on computed tomography scans image of a 26-years old female head, neck and chest without problems in her respiratory system from Shahid Chamran hospital, Shiraz, Iran. It is found that, nasal resistance was contribute up to half of the total airway resistance within the first 2-3 cm of the airway and the majority of the flow in this region remained close to the septum wall and only a small proportion reached the olfactory region.

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The present paper investigates turbulent flow of film cooling on model turbine blade leading edge using two scale-resolving attitude of turbulent flow modeling. In the first attitude the detached eddy simulation (DES) approach based on Spalart-Allmaras and in the second attitude the large eddy simulation (LES) approach will be used. Results show that the DES approach due to its hybrid nature and applying RANS models in near walls, predicts the Fluctuations of spanwise direction in coolant pipe lower. As a result, the coolant flow imports to the main flow with lower turbulence. Also DES approach predicts less turbulent kinetic energy lateral distribution and further turbulent heat flux in near walls. So, in DES approach the adiabatic effectiveness on turbine blade leading edge predicted lower than LES approach and experimental data. In addition, results show that mixture of coolant jet and mainstream hot gas in DES approach is estimated lower than LES approach. In total, it can be deducted that although DES approach provides acceptable results in far wall region, but in near wall region it has problems in correct prediction of turbulence Specifications. In addition, the main advantage of DES approach in comparison with LES approach is 40% reduction of computational cost that can explain using this approach.

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Effects of secondary sonic jet injection in divergent part of supersonic nozzle on flow field structure and thrust vector control performance has been numerically analyzed. Three dimensional multi-blocks extended numerical code has been used to model the complexity of turbulence flow by k-ω SST model. Structured computational domain has been applied and initial results of simulation validated with previous experimental results. The obtained numerical results are compared with the experimental ones, and the outcome shows acceptable agreement between the two. Different injection power generates by varying the injection surface and pressure ratio with respect to throat pressure. Injection power increment make changes in performance and also sometimes it lowers the performance. In the current research aside from complete complex flow features description, allowable power range to increase system performance has been presented. In this range, increasing the injection mass flow rate, decreases the amplification factor, but increases the deflection angle and axial thrust augmentation as most important performance parameters. Out of estimated range for allowable mass power injection, performance parameters different behavior differently that shows a drastic drop in performance.

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In this paper, the flow field between two straight groynes in shallow wide open channel has been measured using Particle Image Velocimetry method. Groynes with 25cm length, 5cm width and 7cm height with two aspect ratios of 1 and 2 have been located in the fully developed zone of a 18m length flume and velocity measurements carried out in order to study the circulating flow, structure of the mixing layer and downstream separation zone. Image processing is conducted using GPIV software and Westerweel and Brevis methods are used for filtering of the measured velocity fields. Results are presented in form of time averaged values, turbulence intensities and Reynolds stresses at various zones of the groyne field. Results showed that due to the flow shallowness, most of the turbulent structures are two dimensional. Development of a back flow from downstream zone to the groyne field enhances the complexity of the mixing layer and mas exchange phenomenon compared to the groyne series configuration. In this paper, the flow field between two straight groynes in shallow wide open channel has been measured using Particle Image Velocimetry method. Groynes with 25cm length, 5cm width and 7cm height with two aspect ratios of 1 and 2 have been located in the fully developed zone of a 18m length flume and velocity measurements carried out in order to study the circulating flow, structure of the mixing layer and downstream separation zone. Image processing is conducted using GPIV software and Westerweel and Brevis methods are used for filtering of the measured velocity fields. Results are presented in form of time averaged values, turbulence intensities and Reynolds stresses at various zones of the groyne field. Results showed that due to the flow shallowness, most of the turbulent structures are two dimensional. Development of a back flow from downstream zone to the groyne field enhances the complexity of the mixing layer and mas exchange phenomenon compared to the groyne series configuration. In this paper, the flow field between two straight groynes in shallow wide open channel has been measured using Particle Image Velocimetry method. Groynes with 25cm length, 5cm width and 7cm height with two aspect ratios of 1 and 2 have been located in the fully developed zone of a 18m length flume and velocity measurements carried out in order to study the circulating flow, structure of the mixing layer and downstream separation zone. Image processing is conducted using GPIV software and Westerweel and Brevis methods are used for filtering of the measured velocity fields. Results are presented in form of time averaged values, turbulence intensities and Reynolds stresses at various zones of the groyne field. Results showed that due to the flow shallowness, most of the turbulent structures are two dimensional. Development of a back flow from downstream zone to the groyne field enhances the complexity of the mixing layer and mas exchange phenomenon compared to the groyne series configuration. enhances the complexity of the mixing layer and mas exchange phenomenon compared to the groyne series configuration.

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abstract In this research the statistical parameter of the flow around T shaped spur dike located in a 90 bend was investigated. the experiments were conducted in a 90 bend at tarbiat modares university. the velocity was measured using ADV apparatuse. the frequency of the ADV set to 50 HZ this frequency was used by many previouse researchers. Two different submerged spur dike with submergence ratio equal to 5% and 50% were used. The submergence ratio is the ratio of the flow depth on crest of the spur dike to spure dike height. The spur dikes were located in 45 degree respect to the beginning of the bend and the experiments were done in freeze bed condition. The flow around the spur dike were investigated using parameters such as: Probability of the events (the ratio of the number of the events to the total of the events during the velocity measurment), variation of the events during the velocity measurments, shear Reynolds stresses in streamwise and lateral direction and angle of the events (the ratio of the vertical velocity fluctuation to the streamwise velocity fluctuation). The results showed that two secondary flow formed in lateral direction of the channel and the secondary flow affect on the upstream of the bend. These flow affect on sediment transport mechanism in lateral direction. In the upstream toe of the spur dike the Reynolds stresses in 5% submergence spur dike is greater compared to the 50% submergence spur dike since stronger horse shoe vortex due to the stronger downflow. The scour hole propagation along the shear layer in longitudinal direction may be due to the increasing the probability of the sweep and ejection events and variation of the interaction events, decreasing the angle of the events and increasing the Reynolds stresses. The Reynolds stresses along the shear layers located in 5% spur dike is greater that the 50% spure dike and larger scour process may be expected in this region. the stability of the events are more pronounced compared to the instability of the events. the angle of the sweep and ejection were minimum near the bed surface and maximum in the middle of the flow depth. Two different zone with maximum angle of sweep and ejection were observed that showed the fluid parcels deviate to the downstream recirculating zone. The angle of the events has different value near the over topping flow that passed from the crest of the spur dike. the flow measurments and analysis of the statistical parameters confirmes the previous researchers conclusions about larger scour hole around the spur dikes with lesser submergence. This may related to the difference of the flow structures around the submerged sour dikes especially near the upstram toe of the spur dike and sround the shear layer.

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Rivers have been always the main source of water for human kind and the basic element of population development. Study of the interaction between flow structure and bedforms is one way to understand the behavior of the rivers. Moreover,vegetation in natural rivers increases roughness of the main channel and flood plains which affects the geometry of channels, flow structure, bed resistance and consequently the pattern of sediment transport. Considering the role of bedforms on sediment transport, turbulence production and flow resistance, investigations on details of flow-bedforms interaction, vegetated banks and flow structure seem to be essential. In this study, the influence of straight crested gravel bedforms and vegetation of the banks of channels on flow turbulent characteristics are investigated based on model experimentation. For this purpose, seven fixed artificial 2-D straight crested bedforms were built inside a rectangular flume of 8 m long, 0.4 m wide and 0.6 m deep. The graded gravel particles used to create the bedforms had an average diameter of d50 = 10 mm. Johnson grasses with a diameter of 2.8 mm were used to simulate vegetation cover on the flume side-walls. Since, the fully developed flow was just observed after the fifth dune, experimental measurements were performed over the fifth and sixth dunes. Overall, three runs were performed over the dunes with a wave length, height, angle of repose and flow depth of 0.96 m, 0.04 m, 28 degrees and 0.28 m, respectively. In the first case 17 velocity profiles and in the second and the third cases 21 velocity profiles were measured. All the tests were performed with a constant discharge of 0.024 m3/s. The instantaneous three-dimensional velocity components were measured using a down-looking Acoustic Doppler Velocimeter ADV. Velocities were recorded for each point with a sampling rate of 200 Hz and the sampling volume of 5 mm. The sampling duration was at least 120 seconds. Overall, about 45400000 velocity data were collected, filtered by WinADV software. Results indicated no negative velocities for both cases of with and without vegetation cover. For no vegetation case, the least value of velocity was zero at a small region on the lee side of the dune. Whereas, for the case of vegetating the side-walls, the zero value of velocity was located at the dune's trough. Negative vertical velocity value in both cases of with and without vegetation along a dune confirmed that separation is not dominant for the case of straight crested dunes compared to the corresponding sharp-crested bedforms. The Reynolds stresses increase for the case of vegetating the side-walls compared to the case of without vegetation cover. This is in part due to the increase of flow resistance, while the side-walls are vegetated. Rivers have been always the main source of water for human kind and the basic element of population development. Study of the interaction between flow structure and bedforms is one way to understand the behavior of the rivers. Moreover,vegetation in natural rivers increases roughness of the main channel and flood plains which affects the geometry of channels, flow structure, bed resistance and consequently the pattern of sediment transport. Considering the role of bedforms on sediment transport, turbulence production and flow resistance, investigations on details of flow-bedforms interaction, vegetated banks and flow structure seem to be essential.

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In present study, heat transfer in double-tube heat exchanger filled with metal porous material has been investigated. In contrast to the most of previous studies, fluid flow is considered turbulent in heat exchanger which is in a good agreement with the practical performance of these exchangers in the industry. Fluid flow and heat transfer equations have been discretized on a collocated grid by the means of finite volume method with simple algorithm. Discretized equations are solved with a numerical program in FORTRAN language in order to study the effect of porous material parameters and Reynolds of fluid flow on the heat transfer in double-tube heat exchanger. According to the results and analysis of porosity in the range of 0.8 to 0.95 as well as pore diameter of 1 mm up to 6 mm and diverse types of porous material, it is observed that the decrease in porosity, the increase in pore diameter and use of copper porous material (with high heat conduction coefficient), increase heat transfer. In the best case, overall heat transfer coefficient enhances up to 7 times. Moreover, the results reveal that the heat transfer enhancement ratio have no distinct difference with changing Reynolds number of turbulent flow in the range of 10000 to 80000. Performance evaluation criteria, which investigate the effects of pump lost power and thermal power, depicts that with using porous material the value of the pump lost power is of major importance which can be decrease by increasing the porous pore diameter.

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In the present study, effect of nanofluid aluminum oxide-water on heat transfer in a pipe partially filled with porous material in a turbulent flow is investigated numerically. In this regard, the heat pipe is studied in four structures: without porous material, filled with porous material, boundary and central arrangement of porous material. The results of numerical solution show that use of nano particles with changing thermo physical base fluid's properties, enhances heat transfer in all of the above structures. However, with using of nanofluid, heat conduction enhancement ratio in porous medium is lower than clear medium (without porous material). As a result, heat transfer enhancement in boundary arrangement is less and in central arrangement is more.

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In this study, the effects of attack angle in opposing jet injection through supersonic blunt bodies on drag reduction and distribution of surface temperature is studied through developing a three dimensional multi-block code. Inviscid terms are calculated by AUSM scheme. The viscous terms is obtained by central difference method and using 4-stage Rung-Kutta algorithm, integral time is computed. Shear stress transport model is used to simulate the effects of turbulence. The effects of pressure ratio and properties of flow field have been verified and validated with experimental and numerical results of other researchers which is indicator of method accuracy. The results show that the sonic jet injection is able to significantly reduce drag nose by changing the shape of the bow shock and it also prevents a sharp increase in the surface temperature by covering the body. Increasing the total pressure ratio, improved performance of jet in both drag reduction and distribution of surface temperature. However due to the sharp increase in retro propulsion of jet there is a limitation in increasing the ratio of total pressure. In addition, the increase of pressure ratio will reduce the friction coefficient. Angle of attack of the free stream reduces the efficiency of the jet injection. Although in this situation the result can be improved to somehow by paralleling the jet and free stream.

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The growing and diverse applications of low Reynolds number operating vehicles impose the need for their accurate study. Optimization is an important part of computational science that can improve the performance and increase the efficiency of the initial geometry. most of the research studies on aerodynamic optimization were focused on high Reynolds number airfoils. But for aerodynamic devices that have small dimensions, like MAVs, usually the flow speed is low and thus the unsteady effects caused by boundary layer separation cannot be neglected. In this article, oscillating airfoil with pitching motion in turbulent and low Reynolds flow has been optimized with the continuous adjoint method. Drag to lift ratio was chosen to be the objective function and free form deformation parameters is adopted for the surface geometry perturbations. Since aerodynamic optimization generally consists of two parts, first solving the flow equation and then computing the gradient of the objective function, in this article in order to evaluate the accuracy of the optimization process both has been validated. The results show that the adjoint equation converges well and with specifying the suitable constraints, the designed shape approaches to the most optimized level without the loss of performance.

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In compound channels, in addition to shear flow originated from the bed (boundary layer flow), other forces are generated by momentum transform between the main channel and the floodplain (free shear layer). Due to such special type of momentum transport, a complicated three-dimensional flow structure forms in a compound channel. Previous studies showed that in a compound channel, secondary currents are enhanced for shallow overbank flow and consequently the complexity of flow structure increases. However, this complexity has not be described properly. To explore turbulent structure of a shallow overbank flow, flow field is measured in a compound channel with vertical walls using Particle Image Velocimetry. The results show that in the main channel, the maximum amount of streamwise velocity occurs below the floodplain level. Whereas in previous studies in compound channels with inclined transitional wall, turbulence intensities profiles in the main channel showed two different trends at lower and higher elevations of the floodplain invert, in the present study three different increasing or decreasing trends were observed for Reynolds shear stress and longitudinal turbulence intensity profiles and four different trends was observed for vertical turbulence intensity. Bed shear velocity was approximately constant in the floodplain but it increased near the interaction zone.