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Total 83 records

    RANS modelling of a NACA4412 wake using wind tunnel measurements

    , Article Fluids ; Volume 7, Issue 5 , 2022 ; 23115521 (ISSN) Tabatabaei, N ; Hajipour, M ; Mallor, F ; Örlü, R ; Vinuesa, R ; Schlatter, P ; Sharif University of Technology
    MDPI  2022
    Abstract
    Wake analysis plays a significant role in wind-farm planning through the evaluation of losses and energy yield. Wind-tunnel tests for wake studies have high costs and are time-consuming. Therefore, computational fluid dynamics (CFD) emerges as an efficient alternative. An especially attractive approach is based on the solution of the Reynolds-averaged Navier–Stokes (RANS) equations with two-equation turbulence closure models. The validity of this approach and its inherent limitations, however, remain to be fully understood. To this end, detailed wind-tunnel experiments in the wake of a NACA4412 wing section profile are compared with CFD results. Two-and three-dimensional RANS simulations are... 

    Wake interference of two identical oscillating cylinders in tandem: An experimental study

    , Article Ocean Engineering ; Volume 166 , 2018 , Pages 311-323 ; 00298018 (ISSN) Armin, M ; Khorasanchi, M ; Day, S ; Sharif University of Technology
    Elsevier Ltd  2018
    Abstract
    This study aims to assess the impact of the spacing between cylinders on their dynamic behaviour. Observations during this experiment have helped to identify the effect of spacing on excitation mechanism of each cylinder as well as establishing the relationship between cylinders response and the spacing between them. Arrays of cylindrical Structures in close proximity are common in variety of engineering structures, particularly in the offshore industry. If the cylinders are flexible, and are subject to excitation from fluid flow, then the dynamics is influenced by complex interactions between the cylinders, dependent on a variety of parameters. Two cylinders were flexibly mounted in tandem... 

    Flow visualization of internal waves and wakes of a streamlined body in a stratified fluid

    , Article Journal of Applied Fluid Mechanics ; Volume 9, Issue 2 , 2016 , Pages 635-641 ; 17353572 (ISSN) Bidokhti, A. A ; Bidokhti, A. A ; Sharif University of Technology
    Isfahan University of Technology  2016
    Abstract
    The wake and internal waves of a moving three dimensional (3D) airfoil body in a stratified fluid has been investigated in a large stratified tank with a finite depth using movies of shadowgraphs of the flow fields. Typical Reynolds and Froude numbers of the flow varied between 103 and 104, and 0.3 and 2 respectively. The flows are generated often by towing the body in a uniformly stratified flow, while limited cases are carried out with body stationary and the channel was in recirculating mode. For some experiments the density profile had a stepped like shape. The wake flow is often consisted of internal waves including random and coherent ones. Distortion of density fields was also... 

    Wind Farm Power Optimization Using Cooperative Game Control Methods

    , M.Sc. Thesis Sharif University of Technology Deljouyi, Nasim (Author) ; Nobakhti, Amin (Supervisor)
    Abstract
    This research studies the application of coopeartive control and game theoretic approaches in the field of wind farm optimization. The conventional (greedy) wind farm control strategy seeks to individually maximize the power of each turbine.However, due to the presence of aerodynamic interactions (wake effect) between turbines, this strategy will not lead to maximum overall farm power. Accordingly, this study formulates the wind farm power optimization problem as an identical interest game which is utilized to deal with cooperative control problems in general, and the wind farm control problem in particular. Two distributed model-free learning algorithms are then applied to obtain the... 

    Selective Wake-Up Receiver for Dense Environment

    , M.Sc. Thesis Sharif University of Technology Jafari Sharemi, Hamid (Author) ; Sharif Bakhtiar, Mehrdad (Supervisor)
    Abstract
    An ultra-low-power wake-up receiver is proposed, which reduces the power consumption significantly under the two-mode duty-cycling and with negligible latency. Due to the use of an accurate LO and high Q BB filters, the receiver shows a perfect selectivity in the frequency domain, and appropriately fits practical dense environment applications. In this receiver, four start frame bits (SFB) are received at the low data rate of 1 kbps in the MO mode, and after entering the ID mode, a long wake-up pattern is received at the high data rate of 100 kbps. Therefore, the latency is limited to the SFBs and is roughly 4 msec. The proposed wake-up receiver is designed and fabricated in TSMC 180 nm... 

    Numerical analysis of wake structure and performance of two oscillatory mechanisms of a foil: Pure pitching and undulating

    , Article Proceedings of the Institution of Mechanical Engineers Part M: Journal of Engineering for the Maritime Environment ; Volume 229, Issue 4 , April , 2015 , Pages 376-396 ; 14750902 (ISSN) Abbaspoor, M ; Ebrahimi, M ; Sharif University of Technology
    SAGE Publications Ltd  2015
    Abstract
    There are various propulsion, maneuvering, and stabilization mechanisms in nature, which can provide inspiration for similar mechanisms in man-made vehicles. This study aims to elucidate and compare the propulsive vortical signature and performance of a foil in two important natural mechanisms of pure pitching and undulatory oscillations. Governing equations are solved with a pressure-based finite volume method solver, in an arbitrary Lagrangian-Eulerian framework domain containing a NACA 0012 foil moving with prescribed kinematics. The results show that in a given Reynolds number (Re), the undulating mechanism produces thrust at a higher Strouhal number (St) and with smaller growth slope,... 

    A coupled prescribed wake-Euler solver for efficient computing helicopter rotor flows in hover

    , Article 25th Congress of the International Council of the Aeronautical Sciences 2006, Hamburg, 3 September 2006 through 8 September 2006 ; Volume 3 , 2006 , Pages 1670-1681 Hejranfar, K ; Mohammadi, M ; Sharif University of Technology
    Curran Associates Inc  2006
    Abstract
    A numerical solution technique is developed to simulate the flowfield of hovering helicopter rotor. The present method uses a coupled prescribed wake-Euler solver to efficiently allow the vortical wake effects. The threedimensional Euler equations written in a rotating coordinate frame are solved by using a cell-centered finite volume scheme that is based on the Roe's flux-difference splitting on unstructured meshes. High-order accuracy is achieved via the reconstruction of flow variables using the MUSCL interpolation technique. Calculations are carried out for an isolated rotor in hover for two operating conditions of subsonic and transonic tip Mach numbers, Mtip = 0.44 and Mtip = 0.877 ,... 

    A new reduced-order modeling approach based on fluid eigenmodes and boundary element method

    , Article 23rd AIAA Applied Aerodynamics Conference, Toronto, ON, 6 June 2005 through 9 June 2005 ; Volume 2 , 2005 , Pages 1245-1252 ; 10485953 (ISSN) Behbahani Nejad, M ; Nobari, A. S ; Shahverdi, H ; Haddadpour, H ; Sharif University of Technology
    American Institute of Aeronautics and Astronautics Inc  2005
    Abstract
    This paper presents a new reduced-order modeling approach based on boundary element method. In this approach the eigenvalue problem of the unsteady flows is defined based on the unknown wake singularities. By constructing this reduced-order model, the body quasi-static eigenmodes are removed from the eigensystem and it is possible to obtain satisfactory results without static correction technique when enough eigenmodes are used. In addition to the conventional method, Eigenanalysis and reduced-order modeling of unsteady flows over a NACA 0012 airfoil and a wing with NACA 0012 section are performed using this new ROM method. Numerical examples are presented that demonstrate the accuracy and... 

    Selection of a Proper Methodology and Field Activity for Micrositing of Wind Energy Production for a Selected Site

    , M.Sc. Thesis Sharif University of Technology Feizpour, Mahmood (Author) ; Abbaspour, Majid (Supervisor) ; Mozaffari, Ali Asghar (Supervisor)
    Abstract
    Because of advantages of using new sources of energy in the country, especially for generating green energy, the use of wind energy for power generating in wind Fields is of great interest of current communities. For developing a wind power plant the first step is to locate and confirm the areas which possess enough potential for installing the wind turbines. For this purpose in the first and second chapters of this thesis an introduction to the basic principles of wind energy is noted then in the third chaptermethods of grid connection, conversion systems and turnover problems in windfarm is evaluated.in the fourth chapter with the use of Windographer software and empirical data of standard... 

    Effect of Laser Pulse Shape on the Plasma Wakefield

    , M.Sc. Thesis Sharif University of Technology Zare, Somaye (Author) ; Sadighi Bonabi, Rasoul (Supervisor)
    Abstract
    Plasma-based accelerators are capable of producing compact and high energy electron sources in much shorter distances than conventional accelerators. Electron will be trapped and accelerated to higher energy by two pulses as compared to the single pulse case. The aim of the present work is to take up a detailed analytical study of a novel idea of wakefield amplification making use of two laser pulses copropagating one behind the other, with a fixed distance between them. However, we attempt for enhancing the wake field and hence the electron acceleration by employing different shapes of the laser pulses  

    Experimental Investigation of Reduced Frequency Effects on the Wake of an Airfoil in Plunging Motion

    , M.Sc. Thesis Sharif University of Technology Dallali, Maryam (Author) ; Soltani, Mohammad Reza (Supervisor)
    Abstract
    Wake aerodynamic of plunging airfoils have received less attention than the other unsteady phenomena. An experimental investigation was carried out to study the unsteady characteristics of flow field behind a model of a plunging airfoil in various conditions. The model has 0.25cm chord and is the 16m section of a 660kW wind turbine blade. All tests were conducted in the subsonic closed circuit wind tunnel in Iran. Invariable parameters were plunging amplitude, mean angle of attack and plunging frequency. The Wakes of plunging airfoil can be characterized as drag-producing, neutral, or thrust-producing .Our preliminary results show that the structure of vortex in the wake for the plunging... 

    Design and Fabrication of an Optimized Wake Equalizing Duct in Order to Improve Propulsion System Operation

    , M.Sc. Thesis Sharif University of Technology Ali Mirzazadeh, Siamak (Author) ; Seif, Mohammad S (Supervisor) ; Rad, Manouchehr ($item.subfieldsMap.e)
    Abstract
    Special geometrical sections, known as “duct”, which can be installed on merchant vessels stern in order to improve propulsion system and propeller performance, today have evolved considerably. The force which produces by the propeller depends on efficiency of propulsion system. In the way of improving the efficiency of propulsion systems, by using these special geometrical sections, to much research have been done from the mid-twentieth century. The Wake Equalizing Ducts (WEDs) in addition to conduct the more uniform fluid flow to the propeller disk (which causes to improve the propeller operation), affect on flow separation in ship stern zone and will decrease the separated flow zone.... 

    Utilising flags to reduce drag around a short finite circular cylinder

    , Article Journal of Turbulence ; Volume 19, Issue 3 , 2018 , Pages 229-251 ; 14685248 (ISSN) Javadi, K ; Kiani, F ; Tahaye Abadi, M ; Sharif University of Technology
    Taylor and Francis Ltd  2018
    Abstract
    This paper utilises flags to decrease the drag around a short finite circular cylinder. Wall-adapted large eddy simulation and two-way fluid–structure interaction methods were applied to resolve unsteady turbulent flow structure. The far-field Reynolds number of the current configuration based on the cylinder diameter was chosen to be 20,000. In addition, the length-to-diameter ratio of the cylinder was assumed to be L/D = 2 whereas the flexible flag had a width-to-diameter ratio of W/D = 1.5. The results were compared with the regular short finite circular cylinder and the rigid flagged cylinder in our previous work. The results indicate that utilising flags inside the near-wake region of... 

    An investigation into the effect of pressure source parameters and water depth on the wake wash wave generated by moving pressure source

    , Article Scientia Iranica ; Volume 25, Issue 4 , 2018 , Pages 2162-2174 ; 10263098 (ISSN) Javanmardi, M ; Binns, J ; Thomas, G ; Renilson, M ; Sharif University of Technology
    Sharif University of Technology  2018
    Abstract
    In this study, the effect of moving pressure source and channel parameters on the generated waves in a channel was numerically investigated; draught, angle of attack, and profile shape as parameters of pressure source, and water depth and blockage factor as channel parameters for wave height. Firstly, the chosen Computational Fluid Dynamics (CFD) approach was validated with the experimental data over a range of speeds. Then, the CFD study was conducted for further investigations. It was shown that that by enlarging draught, angle of attack, and beam of the pressure source, the wave height generated would be increased. Channel study showed that it was possible to increase the wave height... 

    Using the steepened plasma profile and wave breaking threshold in laser-plasma interaction

    , Article Contributions to Plasma Physics ; Volume 48, Issue 8 , 2008 , Pages 555-560 ; 08631042 (ISSN) Zobdeh, P ; Sadighi Bonabi, R ; Afarideh, H ; Yazdani, E ; Rezaei Nasirabad, R ; Sharif University of Technology
    2008
    Abstract
    In this work we evaluate the interaction of high intense laser beam with a steepened density profile. During laser interaction with underdense plasma by freely expanding plasma regime, modification of density profile is possible. In this paper we have investigated the ultra short laser pulse interaction with nonisothermal and collisionless plasma. We consider self-focusing as an effective nonlinear phenomenon that tends to increase when the laser power is more than critical rate. By leading the expanded plasma to a preferred location near to critical density, laser reflection is obtained, so the density profile will be locally steepened. The electromagnetic fields are evaluated in this new... 

    An Exp. Study of the Effect of Sweep on the Wake of a 2-D Wing

    , M.Sc. Thesis Sharif University of Technology Yousefi, Ramin (Author) ; Soltani, Mohammad Reza (Supervisor)
    Abstract
    In this experimental study, a series of wind tunnel tests were performed to examine the wake flow profile over three swept wings with sweep angles of 23, 33 and 40 degrees with similar aspect ratio at various conditions. The airfoil of wing was a NACA 6-series airfoil designed to provide a laminar flow on the front half of the airfoil. The tests, made at a mid aerodynamic chord Reynolds number of 8×〖10〗^5, covered an angle of attack range of -2 to 10 degree. The wing tip vortex effects are eliminated by placing an End-plate on the wing tip.The tests were carried out at two main section of wing and two various position from behind of wing. A total pressure rake and transducers was used to... 

    Measurements of velocity field in the wake of an oscillating wind turbine blade

    , Article Aeronautical Journal ; Volume 114, Issue 1158 , August , 2010 , Pages 493-504 ; 00019240 (ISSN) Soltani, M. R ; Mahmoudi, M ; Sharif University of Technology
    2010
    Abstract
    A series of tests were carried out to study the unsteady wake behaviour behind an aerofoil which is a section of a wind-turbine blade. The model is oscillated in pitch about its quarter chord axis at various reduced frequencies, amplitudes, and mean angles-of-attack. Instantaneous and mean velocity profiles were obtained using total and static pressure at 35 vertically aligned points behind the aerofoil via two similar rakes. The rakes were located at a distance of 1.5 chord length behind the model. An estimation of the real time and average variations of the linear momentum deficit during the oscillation cycle is obtained and has been compared with the corresponding static data. The results... 

    Experimental study on cavitation behavior of propellers in the uniform flow and in the wake field

    , Article Journal of the Brazilian Society of Mechanical Sciences and Engineering ; Volume 38, Issue 6 , 2016 , Pages 1585-1592 ; 16785878 (ISSN) Alimirzazadeh, S ; Roshan, S. Z ; Seif, M. S ; Sharif University of Technology
    Springer Verlag 
    Abstract
    Cavitation behavior of fully submerged propellers is an important issue in the marine engineering field. Cavitation tunnel is usually used for studying hydrodynamic performances and cavitation behavior of a propeller. In the present study, the cavitation behavior of the DTMB P4119 propeller was studied both in a uniform flow and in the wake field of a specific vessel. Considering the dimensions of the test section of K23 cavitation tunnel of Sharif University of Technology and related blockage effects, the propeller was scaled so that the model was 0.195 (m) in diameter. A wake screen was used to generate the desired wake field, which itself was primarily determined by CFD simulations around... 

    Numerical study of propulsion performance in swimming fish using boundary element method

    , Article Journal of the Brazilian Society of Mechanical Sciences and Engineering ; Volume 39, Issue 2 , 2017 , Pages 443-455 ; 16785878 (ISSN) Najafi, S ; Abbaspour, M ; Sharif University of Technology
    Springer Verlag  2017
    Abstract
    In this paper, hydrodynamic simulation of fish-like swimming for two types of aquatic animals including tuna fish and giant danio is presented. We employ an unsteady three-dimensional inviscid boundary element method including time stepping algorithm to capture the wake sheet and flow features around swimming fish in a straight course. At each time step, an unsteady Bernoulli equation was used to find the pressure distribution and thrust generated by the animal. To couple fluid solver with kinematic equations of flexible body, undulating motions of backbone were defined using a prescribed continuous function. Although the flexible motion mechanism controls the fish swimming but no structural... 

    Numerical investigation of flow pattern and hydrodynamic forces of submerged marine propellers using unsteady boundary element method

    , Article Proceedings of the Institution of Mechanical Engineers Part M: Journal of Engineering for the Maritime Environment ; Volume 233, Issue 1 , 2019 , Pages 67-79 ; 14750902 (ISSN) Najafi, S ; Abbaspoor, M ; Sharif University of Technology
    SAGE Publications Ltd  2019
    Abstract
    In this study, a numerical model of unsteady potential flow around submerged marine propellers has been developed. The boundary element approach in combination with time stepping method to model free wake dynamics has been implemented. An important feature of this method in the simulation of pressure-dominant problems is a proper balance between time and accuracy in the numerical process. Another advantage of time stepping method is that there is no need to define wake geometry before modeling. Due to inherent instability of boundary integral equations, a smoothing function to damp the effect of singularities is imposed to the solution. The main innovative idea of this work is that the...