Sharif Digital Repository

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... 

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... 

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... 

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,... 

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... 

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... 

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... 

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  

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... 

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... 

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... 

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...