Search for: reduced-frequency
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    Experimental investigation of aerodynamic performance of a wind turbine blade undergoing pure pitch and plunge motions

    , Article Journal of Advanced Research in Fluid Mechanics and Thermal Sciences ; Volume 67, Issue 1 , 2020 , Pages 105-118 Razavi Dehkordi, M. H ; Soltani, M. R ; Davari, A. R ; Sharif University of Technology
    Penerbit Akademia Baru  2020
    In this paper, extensive experimental tests were carried out for an aerodynamic characterization of the airfoil cross-section of a wind turbine blade. The wind turbine blade cross-sections are usually exposed to unsteady pitch and plunge oscillations. One way to increase the power of blade, is to improve it's aerodynamic performance, so it is necessary to study and compare the unsteady aerodynamic behavior of the airfoil. The aerodynamic behavior of turbine blades are dependent on several parameters including initial angle of attack, reduced frequency and oscillation amplitude. In this study, reduced frequency and oscillation amplitude assumed constant and the initial angle of attack of the... 

    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)
    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... 

    Unstaedy Boundary Layer Measurement over an Airfoil in Plunging Motion

    , M.Sc. Thesis Sharif University of Technology Abdollahi, Rezvan (Author) ; Soltani, Mohammad Reza (Supervisor)
    An experimental investigation of unsteady boundary layer over a wind turbine blade conducted in a subsonic wind tunnel. The experiment performed at different positions along the airfoil chord. Model was oscillated with amplitude of ±5 cm. Reynolds number, based on airfoil chord, ranged 3˟105 upto 6.5˟105 and the reduced frequency ranged from 0.0055 to 0.0981. Velocity profile measured with a total pressure rake. The data were reduced via a low-pass filter. Increasing the reduced frequency, the velocity profile increases the level on turbulence fluctuation and becomes more attached to the surface.


    Study of Reduced Frequency Effects on Wind Tunnel Wall Pressure for a Plunging Airfoil

    , M.Sc. Thesis Sharif University of Technology Mohammadi, Zeynab (Author) ; Soltani, Mohammad Reza (Supervisor)
    The unsteady phenomena occur in rotors of helicopters, wind turbines and compressors. Investigation of the unsteady flow is a very important test, since it can affect and restrict the performance of rotors of helicopters and wind turbine's blades when operating at high angles of attack. Dynamic tests, such as pitching and plunging oscillations are one of the best methods for studying these phenomena due to the complexity of the unsteady flows. In analyzing the results of dynamic tests one must consider the interference effects of wind tunnel walls. Different investigations show that in dynamic tests, the interference effects of flow field around the model, wind tunnel walls and support of... 

    Experimental Investigation of Reduced Frequency Effects on the Boundary-Layer Profile of an Oscillating Airfoil in Pitching Motion

    , M.Sc. Thesis Sharif University of Technology Ghorbani Shohrat, Faegheh (Author) ; Soltani, Mohammad Reza (Supervisor)
    Application of wind energy and specially wind turbines as a green source of energy is getting more popular and as a result, the knowledge of wind energy has been improved dramatically. The purpose of this research is to determine the flow characteristics in the boundary layer profile over an airfoil which is used in a wind turbine blade. The model is sinusoidally oscillated in pitch. For the present study, the critical cross section of the wind turbine blade has been selected and the boundary layer velocity profiles over its airfoil have been investigated experimentally. Dynamic motion is produced with sinusoidal oscillator and the reduced frequency is varied in the range of 0.054 to 0.1.... 

    Experimental Study of Transition on an airfoil in Plunging Motion

    , Ph.D. Dissertation Sharif University of Technology Rasi Marzabadi, Faezeh (Author) ; Soltani, Mohammad Reza (Supervisor)
    Prediction of the location of transition point is important for wind turbine applications. The laminar/turbulent properties of the flow field have an important influence on the skin friction and separation location that affect both lift and drag characteristics of the blade significantly. In this study, a series of experiments were conducted on a section of a 660 kW wind turbine blade in subsonic flow. Boundary layer transition point was measured using multiple hot-film sensors. Hot-film data were analyzed in both time and frequency domains. The experiments involved static and dynamic tests where the airfoil motion was of plunging type oscillation. The effects of several parameters such as... 

    Experimental Investigation of Reduced Frequency Effects on the Aerodynamic Efficiency of a New Wind Turbine Blade Section in Pitching Motion

    , M.Sc. Thesis Sharif University of Technology Khorshidian, Nima (Author) ; Soltani, Mohammad Reza (Supervisor)
    In recent years, renewable energy equipments such as wind turbines has become more common for electric power generation. Due to the complexity of the aerodynamics of wind turbines,experimental studies in this field are very important. The purpose of this study is to investigate the aerodynamic performance of a new wind turbine airfoil in static mode and pitching motion.The airfoil is a section of 2 MW wind turbine blade, pitching around its quarter-chord axis .All experiments are carried out in a subsonic wind tunnel. In these experiments, the reynolds number is 140,000 and the reduced frequency varies from 0.034 to 0.126. Pressure transducers have been used to determine the pressure... 

    Investigation of the Effects of Reduced Frequency and Mean Angle of Attack on the Surface Pressure of a Supercritical Airfoil in Pitching Oscillation

    , M.Sc. Thesis Sharif University of Technology Shokri, Hossein (Author) ; Soltani, Mohammad Reza (Supervisor)
    Supercritical airfoils have been designed for transonic flow, however due to different flight conditions, the airfoil behavior during take-off and landing of the aircraft in subsonic flow regime must be known. Experimental and numerical information derived from supercritical airfoils are less than those of conventional airfoils. Because of the special geometry of supercritical airfoils and different phenomena that occurs through flow, using such airfoils even in steady states requires precise study/analysis of the flow. Therefore, one could benefit from experimental tests to better understand flow phenomena. The objective of the present research is the study effect of mean angle of attack... 

    Viscous damping effect on the aeroelastic stability of subsonic wings: Introduction of the U–K method

    , Article Journal of Fluids and Structures ; Volume 73 , 2017 , Pages 1-15 ; 08899746 (ISSN) Beheshtinia, F ; Dehghani Firouz Abadi, R ; Rahmanian, M ; Sharif University of Technology
    This study aims at introduction of a novel method for evaluating the effect of viscous damping on the aeroelastic stability boundaries. The K-method is well-known for being one of the fastest methods in determining the instability conditions (i.e. critical speed and its corresponding frequency). However, formulation of the K-method is developed for aeroelastic systems without viscous damping and solution is valid where the introduced artificial damping is zero. Taking into account the framework of the K-method in general, this study has tried to remove the major shortcoming of the K-method, i.e. investigation of the effect of viscous damping on the aeroelastic stability boundaries. The... 

    Impact of reduced frequency on the time lag in pressure distribution over a supercritical airfoil in a pitch-pause-return motion

    , Article Chinese Journal of Aeronautics ; Volume 32, Issue 2 , 2019 , Pages 243-252 ; 10009361 (ISSN) Eslami, H. Z ; Davari, A. R ; Soltani, M. R ; Sharif University of Technology
    Chinese Journal of Aeronautics  2019
    Effects of reduced frequency, stop angle, and pause duration have been studied on a thin supercritical airfoil undergoing a pitch-pause-return motion, which is one of the classic maneuvers introduced by the AIAA Fluid Dynamics Technical Committee. Experiments were conducted in a low-speed wind tunnel at both a constant mean angle of attack and an oscillation amplitude with a reduced frequency ranging from 0.01 to 0.12. The desired stop angles of the airfoil were set to occur during the upstroke motion. The unsteady pressure distribution on the airfoil was measured for below, near, and beyond static stall conditions. Results showed that the reduced frequency and stop angle were the dominant... 

    Effect of surface roughness on an airfoil in pitching motion

    , Article 36th AIAA Fluid Dynamics Confernce, San Francisco, CA, 5 June 2006 through 8 June 2006 ; Volume 4 , 2006 , Pages 2785-2792 ; 1563478102 (ISBN); 9781563478109 (ISBN) Seddighi, M ; Soltani, M. R ; Sharif University of Technology
    Extensive experimental tests have been conducted to examine the roughness effects on the aerodynamic performance of a model in an oscillatory motion. All tests were carried out in a 0.8×0.8 m 2 low-speed wind tunnel. The model had 0.25m chord and was pitched about its quarter-chord. Data were acquired at various Reynolds number and reduced frequencies. Surface roughness was applied at two different locations of the model upper surface, leading edge and the point of maximum thickness. Furthermore, tunnel turbulent was varied and all tests were repeated. The results indicate that the width of the hysteresis loop, position of the figure eight shape, slope of the normal force coefficient curve,... 

    Surface pressure variation on an airfoil in plunging and pitching motions

    , Article 25th Congress of the International Council of the Aeronautical Sciences 2006, Hamburg, 3 September 2006 through 8 September 2006 ; Volume 2 , 2006 , Pages 962-971 ; 9781604232271 (ISBN) Soltani, M. R ; Rasi Marzabadi, F ; Seddighi, M ; Sharif University of Technology
    Unsteady aerodynamic experiments were conducted on an oscillating airfoil in a subsonic wind tunnel. The model was oscillated in two types of motion, pitch and plunge, at a range of reduced frequencies, k=0.029-0.1. In addition, steady data were acquired and examined to furnish a baseline for analysis and comparison. The unsteady surface pressure is measured along the chord for both upper and lower surfaces of the model. Particular emphases were placed on the effects of different types of motion on the unsteady pressure distribution of the airfoil at pre-stall, near stall, and post stall conditions. It was found that the variations of the pressure distribution with angle of attack have... 

    Statistical analysis on the effect of reduced frequency on the aerodynamic behavior of an airfoil in dynamic physical motions

    , Article Physica A: Statistical Mechanics and its Applications ; Volume 535 , 2019 ; 03784371 (ISSN) Razavi Dehkordi, M. H ; Soltani, M. R ; Davari, A. R ; Sharif University of Technology
    Elsevier B.V  2019
    In this paper, an extensive experimental tests were performed to determine the aerodynamic characteristics of an airfoil undergoing static, dynamic pitch, dynamic plunge and dynamic combined pitch and plunge motions for various cases. This paper, however, focuses on the effects of reduced frequencies and mean angles of attack on the surface pressure distribution and on the corresponding lift of the airfoil oscillating in either pure pitch or in combined pitch–plunge motions. The angles of attack variations were set such that the model motion would be ceased lower than the static stall, near the static stall and beyond the static stall angles of attack. All tests were conducted at a constant... 

    Analysis of Unsteady Flow Field around a Pitching Finite Wing

    , M.Sc. Thesis Sharif University of Technology Kavianpour, Sina (Author) ; Farahani, Mohammad (Supervisor)
    The characteristics of unsteady flow field around a low-aspect-ratio pitching wing with an airfoil section used in horizontal axis wind turbine are investigated at Reynolds number 2.8×105 and Mach number 0.06. The major objective of this study was to investigate the effects of reduced frequency, oscillating amplitude, mean angle of attack and 3D effects on aerodynamic coefficients. Using aerodynamic coefficients, the unsteady aerodynamics in pre-, near-, and post-static stall regions are analyzed and the results are validated with the corresponding experimental data. The two-dimensional simulation results demonstrate the significant effects of reduced frequency, mean angle of attack and... 

    The unsteady behavior of subsonic wind tunnel wall pressure during pitching motion of the model

    , Article Scientia Iranica ; Vol.21, issue.1 , 2014 , p. 192-202 ; 10263098 Davari, A. R ; Soltani, M. R ; Ghaeminasab, M ; Sharif University of Technology
    Extensive low speed wind tunnel experiments have been undertaken to measure the test section, oor wall pressure distribution, in the presence of a 2D wing inside the test section. The experiments were performed for both the static and dynamic pitching motion of the model under difierent conditions. In these measurements, the efiects of the existence and oscillations of a 2D wing on the oor wall pressure at various locations were studied. According to the results, as the oscillation parameters, such as mean angle of attack and frequency, change, wall pressures at the points located in the front part of the test section, in the upstream region, exhibit difierent behavior from those in the... 

    Effect of reduced frequency on the boundary layer of a plunging airfoil

    , Article ASME 2010 10th Biennial Conference on Engineering Systems Design and Analysis, ESDA2010, 12 July 2010 through 14 July 2010, Istanbul ; Volume 1 , 2010 , Pages 185-190 ; 9780791849156 (ISBN) Rasi Marzabadi, F ; Soltani, M. R ; Masdari, M ; Sharif University of Technology
    This investigation addresses the boundary layer study of a plunging airfoil. It specifically concerns the effect of reduced frequency on transition and separation/reattachment of the unsteady boundary layer. The wind tunnel measurements were conducted using multiple hot-film sensors, pressure transducers and a boundary-layer rake, at Reynolds numbers of 0.42 to 0.84 million, and over reduced frequencies from 0.05 to 0.11. It was observed the boundary layer transition occurs by a laminar separation bubble. The unsteady laminar separation is promoted (delayed) by the increase of the reduced frequency in upstroke (downstroke) portion of the equivalent angle of attack  

    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
    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... 

    On wind tunnel wall pressure fluctuations during an unsteady test of a fighter model

    , Article 35th AIAA Fluid Dynamics Conference and Exhibit, Toronto, ON, 6 June 2005 through 9 June 2005 ; 2005 ; 9781624100598 (ISBN) Davari, A. R ; Soltani, M. R ; Sharif University of Technology
    An intensive experimental work was conducted to investigate variations of the wall pressure time history at different locations during the test of a fighter aircraft model in a sinusoidal pitching motion. The model used in this research was a standard dynamics model known as SDM. Dynamic data were taken at various combinations of reduced frequencies, mean angles of attack and oscillation amplitudes. Time variations of the static pressure at several locations on the wind tunnel wall have been recorded using sensitive pressure transducers. The results show that the wall pressure oscillation is highly affected by the model motion and varies with the free stream Mach number, oscillation... 

    Effect of leading-edge roughness on boundary layer transition of an oscillating airfoil

    , Article Scientia Iranica ; Volume 20, Issue 3 , 2013 , Pages 508-515 ; 10263098 (ISSN) Marzabadi, F. R ; Soltani, M. R ; Sharif University of Technology
    A series of experiments were conducted to study the effect of leading-edge roughness on the state of the boundary layer of a wind turbine blade section using multiple hot-film sensors. The experiments involved static and dynamic tests, where airfoil motion was of plunging type oscillation. The application of surface grit roughness simulates surface irregularities that occur on the wind turbine blades. The measurements showed that increasing the angle of attack results in movement of transition locations toward the leading edge. Surface roughness moved the transition point toward the leading edge and caused early trailing edge turbulent separation, which resulted in reducing the effectiveness... 

    Experimental study of the boundary layer over an airfoil in plunging motion

    , Article Acta Mechanica Sinica/Lixue Xuebao ; Volume 28, Issue 2 , 2012 , Pages 372-384 ; 05677718 (ISSN) Rasi Marzabadi, F ; Soltani, M. R ; Sharif University of Technology
    This is an experimental study on the boundary layer over an airfoil under steady and unsteady conditions. It specifically deals with the effect of plunging oscillation on the laminar/turbulent characteristics of the boundary layer. The wind tunnel measurements involved surface-mounted hot-film sensors and boundary-layer rake. The experiments were conducted at Reynolds numbers of 0.42×10 6 to 0.84 × 10 6 and the reduced frequency was varied from 0.01 to 0.11. The results of the quasi-wall-shear stress as well as the boundary layer velocity profiles provided important information about the state of the boundary layer over the suction surface of the airfoil in both static and dynamic cases. For...