Showing 16 results for Large Eddy Simulation
Ghasem Heidarinejad, Tohid Sedaghat,
Volume 10, Issue 2 (9-2010)
Abstract
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.
Ghassem Heidarinejad, Hadi Pasdarshahri, Kiumars Mazaheri,
Volume 13, Issue 4 (7-2013)
Abstract
A numerical study for the simulation of induced-flow by a two-room compartment fire has been accomplished using a fully-coupled Large Eddy Simulation (LES) model which incorporates Smagorinsky and One-Equation Sub-Grid Scale (SGS) turbulence models. Also, modified Eddy Dissipation Concept (EDC) and Discrete Ordinate Methods (DOM) are used for incorporating combustion and radiation, respectively. The models are applied for a range of total heat release rate (HRR) for fire source in the center and corner of the fire room. Numerical results of prediction by each SGS model are validated and compared against well-known available experimental data. The predicted time-averaged temperature profiles at different location of the compartment for each case have been calculated and found to be in good agreement with the experimental data. The results also show that the accuracy of One-Equation SGS model for the prediction of the characteristics of fire is higher than those obtain by Smagorinsky SGS model. The air mixture at the fire room opening is higher for the centrally located fire source than the corner one.
Maryam Mapar, Ghassem Heidarinejad, Hadi Pasdarshahri,
Volume 13, Issue 15 (3-2014)
Abstract
In this paper a simulation of two fires in a tunnel in which varied arrangements of different sized vehicles at the upstream of fires has been accomplished using FDS. The results reveal that the behavior of two fires is directly influenced by distance between them. The calculated critical velocity is not affected by the variations on the small vehicles arrangement and distance between the vehicles and fires. Interestingly, the presence of medium vehicles leads ventilation flow to strengthen inertial force rather than buoyant force of fire plume in tunnel. Accordingly, when there is a short distance between fires and obstructions, less air ventilation is needed to prevent smoke backlayering. Eventually, far distance between the vehicles and the fires results in vanishing obstruction effects. Consequently, the critical velocity is the same as the case in which there is no vehicle in the tunnel.
Alireza Razavi, Ali Ashrafizadeh,
Volume 14, Issue 6 (9-2014)
Abstract
Tornado is a destructive phenomenon which causes severe damage every year. To improve resistance of structures which face tornado, the flow field and factors which affect damage patterns of tornado need to be investigated. In this paper, numerical simulations of stationary and translating tornadoes are carried out using Ward-type simulator results and large eddy turbulence model. Validation for stationary case has been done with experimental work of Baker. The effects of peak winds, duration of intense winds and acceleration of translating tornado on damage patterns have been investigated. Results show that destruction is more intense at the side of the tornado that translational velocity and tangential wind velocity are added up. Moreover, peak wind velocity and duration of intense winds are important factors that have important effects on the destruction pattern of tall structures. However, the value of the translational acceleration of tornado is important for the design of all structures regardless of their heights.
Farzad Bazdidi-Tehrani, Seyed Majid Mousavi, Mohammad Jadidi,
Volume 15, Issue 8 (10-2015)
Abstract
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.
Mohsen Boojari, Esmail Mahmoodi, Ali Abbas Nejad, Sasan Sarmast,
Volume 16, Issue 9 (11-2016)
Abstract
Wind turbines are highly complex structures for numerical flow simulation. Today, developments and increasing the use of wind energy in the world has created a demand for increasingly accurate and efficient models for wind applications. Wind turbine wakes have significant effects on decreasing the produced power and blades fatigue loads. thus, the wakes study has great importance in wind turbine simulations. Actuator line model (ALM) is one of the most accurate models for characterization of the flow field and the turbulent wakes created by the turbines. AL model does not require boundary layer resolution and is thus significantly more efficient than the fully-resolved computations. this model can accurately simulate the wakes of wind turbines operating in a flow field without any need to create or import the CAD models of turbine and just by using turbine parameters. In this paper, AL method implemented in openFOAM solver and a new method used to spread forces on actuator lines. in order to validate the results, MEXICO rotor was modeled and large eddy simulation’s turbulence model is used to investigate the flow field around wind turbine. Simulation has been done for two different conditions include design conditions and stalled conditions. Results obtained for predicted wakes and performance parameters, were compared to experimental data and it was observed that the ALM results agree well with measured data. Stall condition’s results were in better agreement with experimental dada so that the thrust had 8.5% difference and the toque and power had 2.8 and 2.4% respectively.
Amin Allah Veisi, Mohammad Hossein Shafiei Mayam,
Volume 16, Issue 12 (2-2017)
Abstract
In this study Large Eddy Simulation method has been employed in order to investigate the effects of blade rotation direction of downstream turbine in two co-rotating and counter-rotating configurations. The acquired results are in good agreement with presented experimental data in literatures. Counter-rotating configuration is used in order to investigate the effect of blade rotation on the efficiency of downstream wind turbine. The results show that the efficiency of downstream wind turbine is increased about 4 percent without any change in wind farm layout and type of wind turbines. The upstream wind turbine absorbed a portion of wind energy. Hence stream wise velocity is decreased and lateral velocities are increased in downstream direction. The flow behind the upstream turbine is rotated in same direction with downstream turbine in a counter-rotating configuration. This is why the efficiency of downstream turbine is increased in a counter-rotating configuration. The results of the present study show that streamwise velocity profile is almost identical in both configurations, while, lateral velocities are changed considerably. In other words, a better efficiency of wind farm could be due to the lateral velocities. Hence, the efficiency of wind farm could be increased by decreasing the distance between two consecutive wind turbines in a counter-rotating configuration.
Hossein Mohammadi, Mahdi Ramezanizadeh,
Volume 17, Issue 5 (7-2017)
Abstract
The objective of this paper is to obtain acoustical behavior of air flow around 65° sweep back cropped, 66°/42° cranked arrow and 53° diamond delta wing configuration by using large eddy simulation approach with 0.17 of smagorinsky-lilli coefficient. It is worthy to mention that the root chord of the wings is 360 mm. Validation of the aerodynamics coefficient is performed using available experimental results which showed good agreements. CFD simulations were performed for 15 degree angles of attack at a Mach number of 0.147 and a Reynolds number of 1.2 million based on the root chord. Acoustic measurements such as power spectral density, acoustic pressure, sound pressure level and sound amplitude, were taken using 3 microphones in the wake region of the mentioned wings. The amount of sound pressure level of microphone which is placed at 1.835 meter from apex of above wings in the range of Strouhal 0 to 1 is, 22 to 66, 10 to 73 and 9 to 44 which indicates aeroacoustic behavior of the diamond delta wing, is more better than cropped and cranked arrow delta wing.
Ghasem Heidarinejad, Mohammad Hosein Roozbahani,
Volume 17, Issue 6 (8-2017)
Abstract
One of the focused problem in airway flow simulation is pulmonary airways modeling. There are two kind of Lung models, one is created anatomically based on bronchial data and second is realistic model which is created based on CT scan images. Unfortunately cause of modeling process or simplification cause of restriction of CPU and time, the result model is different from a really pulmonary airways. Anatomically model are many simplification and realistic model from CT scan have major limitation in CT image resolution and smoothing stage of make out the 3D model. Anyway the lung has many rough and the first thing that is vital on this way is cartilage rings as macro scale roughness. So the presented work, compared the airflow in both simple and modified Horsfield model by cartilage rings in term of time averaged wall shear stress which are important in engineering of Cell-Fluid Interactions (CFI). This is shown that cartilage rings affected the trachea and second generation of brunches so this is not reasonable to neglect the cartilage rings.
Hamid Yousefi, Ehsan Khavasi, Saba Teymouri, Parsa Nazmi, Zahra Mashhadi,
Volume 18, Issue 1 (3-2018)
Abstract
Density currents flow due to the density difference between the current and surrounding environment. An important category of density currents is called turbidity currents, which density difference created as a result of suspended solid particle presence in fluid. In the present study, it is tried to use both Eulerian-Eulerian and Eulerian-Lagrangian methods, to take advantage of each one. In this way, the larger particle that have a more effective role in sedimentation mechanism due to the more falling velocity are calculated as Lagrangian and smaller particles by the Eulerian method. In order to obtain a criterion for particle assortment, seven currents with different particle sizes in the Eulerian-Eulerian model have been numerically simulated in a simple channel and it is compared with no particle case, and also the Eulerian-Eulerian method has been verified with experimental results and identified when the particle sizes is less than 12 micron, the sedimentation process is not appreciable, and the presence effect of these kind of particle can be ignored. Therefore, the Eulerian-Eulerian method is a suitable method for this case. The Eulerian-Lagrangian method validation has been performed with experimental results. Finally, the current inside the channel with a spectrum of particle dimensions is simulated and described the results by the proposed method (the combination of two methods). To perform numerical simulations, the development of open-source OpenFOAM codes has been used to take into account the effect of particle. Due to the current’s turbulence, a Large Eddy Simulation method has been used for turbulent modeling.
Arezoo Najafian, Hamid Parhizkar, Sajjad Ghasemlooy, Abbas Tarabi,
Volume 18, Issue 3 (5-2018)
Abstract
In the present study, the numerical solution of the Ansys Fluent software has been used to calculate the sound produced by the high-speed flow on a cylinder using the Lighthill acoustic analogy. The calculations were carried out on a cylinder (part of the landing gear) at a speed of 70 m/s (take-off and landing speeds of airliners). The problem is initially caried out as a regular unsteady numerical solution. During the solution, aerodynamic noise data sources are stored as inputs of acoustic analyzes in files. Then, by solving the acoustic equations, the volume of produced sound (in decibel) is calculated at points that are pre-defined as the microphone in the desired coordinates. The purpose of this study is to study the ability of Fluent solution to calculate the sound generated by the flow, in addition of using a method for estimating the amount of sound increase by increasing the length of the cylinder. In the other words, due to the timing of the numerical solution, one can calculate sound generated by small length cylinder, and then, using engineering approximation, it estimates the sound of the flow around the larger-length cylinder. After the necessary calculations, results are provided as sound pressure level curves using the acoustic analogy and fourier spectral analysis. The results show that large eddy simulation turbulence model is most appropriate model for acoustic simulations. Also, the approximate method for evaluating the effect of increasing the length of the cylinder is in good agreement with the experimental results.
Mahmood Asgari Savadjani, Behzad Ghadiri,
Volume 18, Issue 3 (5-2018)
Abstract
The numerical simulation of near-stall condition in a passage of an isolated subsonic rotor is studied in detail. The requirements of numerical simulation in order to resolve turbulent spectra around the blade are studied. According to the fact that most of unsteady aerodynamic phenomena incept from blades leading edge, and the role of this part in types and intensity of instabilities, the goal of this paper is to investigate the effects of changes in radius of leading edge of airfoil on flow phenomena in different scales of wave numbers. The governing equations of flow-field are solved using different numerical approaches. Resolution characteristics of different modeling and simulation techniques are investigated. The primary geometry of blade uses a standard NACA-65 series airfoil, which has been tolerated by 50% variation in circular leading edge radius. Mesh requirements of flow simulation for intended purposes are studied in detail and some recommendations are proposed to be implemented in numerical aeroelastic simulations. Accuracy and fidelity of LES results are studied with extraction of power spectra around the blade and the portion of resolved energy is also estimated. Results suggest that the order of accuracy and grid density highly affect the small-scale flow phenomena. The variations in leading edge radius also have great effect on energy distribution among resolved scales.
M. Shirzadeh Germi, H. Eimani Kalehsar,
Volume 19, Issue 3 (3-2019)
Abstract
The application of computational fluid dynamics is being developed in recent years in order to evaluate the numerical impact of wind damage on high-rise buildings due to the increasing computing power of computers. With regard to the turbulent downturns around flexible, slender and long-winded buildings with relatively high Reynolds numbers, the study of aeroelastic behavior of tall buildings is essential. In this paper, the turbulent wind flow is simulated numerically with four different velocities around the high standard CAARC building. Large Eddy Simulation has been used to solve the turbulence effect in solving fluid flow equations and the response of tall buildings to wind forces is determined by solving the differential equation of motion. A two-way coupling method is used to transfer data between two areas of fluid and structural solution in each step of time. According to the results of the numerical simulation, the pressure coefficients, streamlines and instantaneous pressure field around the tall building are in good agreement with the common characteristics of the flow around the airborne objects. The critical speed corresponding to the lock in phenomenon in this problem is calculated using a Strouhal number equal to 100m/s. Also, the history of displacement of the roof of the building in the direction of the wind and perpendicular to its length have been extracted for different wind velocities and the mean and their standard deviations respectively have been calculated. The continuous increase in the range of the fluctuations of the building under the wind blowing at 100m/s is observed. This point indicates the efficiency and capability of the numerical process in detecting aeroelastic instability with a predicted speed.
H.r. Talesh Bahrami, H. Parhizkar, S. Ghasemlooy,
Volume 19, Issue 5 (5-2019)
Abstract
one of the key issues in the design of high-speed modern devices such as giant aircraft and high-speed trains. In this regard, it is to design these devices in such a way to have at least aerodynamic noise. The cylinder, as a bluff body, is widely used in the design of various devices, such as a landing gear. Therefore, the reduction of cylinder noise can be widely used. In the present study, numerical solution is used to present a method for reducing the noise generated by flow on the cylinder. This is done by flow suction from the grooves the cylinder. Acoustic numerical calculations were performed, using LightHill's acoustic analog approach in the form of wave equations of Ffowcs-Williams & Hawkings model. The numerical solution is performed in the three-dimensional unsteady form, using the large eddy simulation turbulence model. The characteristics of the grooves, such as their dimensions and distance the generated acoustic noise have been studied. The results show that the active control method presented in this paper is an effective and yet simple way to control noise. The cylinder used in the present study produces a noise of about 110 dB at a speed of 250 km/h. According to the results, it can be said that by optimally arranging the number of slots and creating a proper flow suction, its sound level can be reduced to about 60 dB.
D. Zhaleh, F. Ommi, Z. Saboohi,
Volume 20, Issue 1 (1-2020)
Abstract
The idea of designing new geometries for catalytic bed in the decomposition chamber of monopropellant thrusters is introduced with numerical simulations of pore-scale turbulent flows. The LES numerical technique is used for simulation of turbulent structures in the flow-field. The efficiency and reliability of the results obtained from numerical simulation have been determined by solving a benchmark problem of turbulent flow over the pack of cubes. The results show very good agreement with the experimental data, indicating the accuracy of the used model and numerical solution process. The characteristics of turbulent flow over two different geometries have been investigated using the numerical method. The results have been analyzed to evaluate the effectiveness of geometrical changes on the parameters associated with the catalytic reaction. All simulations have been conducted for cold flow, and the exact effects of the geometrical design of porous bed on reactive flow have not been quantified. The eddy dissipation and length scales of turbulence have been considered as the main parameters, because of their effect on rates of turbulent mixing and rate of reaction. The difference between the turbulent dissipation and length scales in the investigated flows in two different geometries indicates the effectiveness of the geometrical changes of the porous bed on the flow characteristics. Coherent structures are seen in the new geometry and the wall shear stress is reduced significantly, which increases the life of the catalytic coating.
M. Sangbori, A. Nejat, K. Gharali,
Volume 20, Issue 3 (2-2020)
Abstract
In this article, noise generation mechanisms are studied at different Reynolds numbers and angles of attack. Tonal noise is the major part of airfoil noise at low Reynolds numbers. Studying the tonal noise and the effects of Reynolds number and angle of attack is challenging in aeroacoustics. 3D numerical simulation is conducted using the large eddy simulation method on SD7037 airfoil. Sound propagation is computed using the Ffowcs Williams-Hawkings (FW-H) analogy. The numerical results are validated using available experimental results. Some discrete peaks and a dominant peak exist in frequency spectra at low angles of attack. Increase of Reynolds number and the angle of attack decreases the number of discrete peaks and at high angles of attack and the dominant peak is diminished too. Studying the flow features shows that when a laminar boundary layer covers a vast area of the suction side, it can amplify acoustic waves that are generated in wake of the airfoil and this mechanism causes a dominant peak in the acoustic spectrum. Amplifying Tollmien-Schlichting waves by shear layer in laminar separation at suction side cause the discrete peaks and when a transition occurs in the airfoil suction side, discrete peaks are diminished. In the original semi-empirical Brooks, Pope and Marcolini (BPM) formulation, the boundary layer thickness of the pressure side is usually used as the length scale and it is replaced by the suction side boundary layer thickness. The results predict the frequency and amplitude of tonal noise successfully.
