• International Journal of Fluid Machinery and Systems
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• v.2 no.3
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• pp.197-205
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• 2009

Since large-scale commercial wind turbine generator systems such as MW-class wind turbines are becoming widely operated, the vibration and distortion of the blade are becoming larger and larger. Therefore the soft structure design instead of the solid-design is one of the important concepts to reduce the structural load and the cost of the wind turbine rotors. The objectives of the study are development of the fluid-structure coupled analysis code and evaluation of soft rotor-blade design to reduce the unsteady structural blade load. In this paper, fluid-structure coupled analysis for the HAWT rotor blade is performed by free wake panel method coupled with hinge-spring blade model for the flapwise blade motion. In the model, the continuous deflection of the rotor blade is represented by flapping angle of the hinge with one degree of freedom. The calculation results are evaluated by comparison with the database of the NREL unsteady aerodynamic experiment. In the analysis the unsteady flapwise moments in yawed inflow conditions are compared for the blades with different flapwise eigen frequencies.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.39 no.3
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• pp.210-217
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• 2011

Evolution of tip vortex generated by a model rotor blade which has a symmetric blade section has been investigated by use of the laser doppler anemometry. Swirl and axial velocity components of tip vortex were measured by the phase averaging technique within one revolution of a rotor blade. It was found that tip vortex becomes matured until 27 degrees and diffuses afterwards with diffusing rate becoming slower compared to the case of the asymmetric blade section, but the tip loss was expected to become more substantial. Swirl velocity components were well fit to n=2 model of Vatistas within measured wake ages, showing the self-similarity exists for the swirl velocity components. The axial components were followed with Gaussian profiles, but had much higher peak values than those of the symmetric blade section.

• 대한공업교육학회지
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• v.32 no.2
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• pp.87-103
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• 2007

The purpose of this study was to develop of the smoke flow visualization device of learning wind tunnel, teaching-learning materials in order to demonstrate air-flow around the fluid-flow field qualitatively and understand the resistance concepts of fluid-flow in secondary school. The contents of this study were consisted of the development and experiment of smoke flow visualization for learning wind tunnel. The main results of this study were as follows: First, this developed teaching-learning material here will help students understand the fundamental physical phenomena related with the resistance of fluid and the various patterns of air-flow in the field of transportation technology. Second, flow visualization has shown the same tendency in both of theoretical and experimental patterns. Third, the airfoil model has the smallest wake region meaning resistance against air-flow of circular cylinder and square rod model. Forth, flow separation point at leading edge and wide wake region began to show under the angle of attack of airfoil model ${\alpha}$ is $20^{\circ}$. Fifth, the wake width of the flow field behind a golf ball with dimple became slightly narrower than that without dimple. Sixth, the developed device was made to apply the teaching and learning materials for the experiment and practice in order to increase students' interest and attitude.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.2 s.245
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• pp.136-143
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• 2006

A particle image velocimetry (PlV) technique has been applied to measure the quantitative flow field characteristics behind a road vehicle with/without an air spoiler attached on its trunk and to estimate its effect on the wake. A vehicle model scaled in the ratio of 1/43 is set up in the mid-section of a closed-loop water tunnel. The Reynolds number based on the vehicle length is $10^5$. To investigate the three-dimensional structure of the recirculation zone and vortices, measurements are carried out on the planes both parallel and perpendicular to the free stream, respectively. The results show significant differences in the recirculation region and the vorticity distributions according to the existence of the air spoiler. The focus and the saddle point, appearing just behind the air spoiler, are disposed differently along the spanwise direction. Regarding the streamwise vortices, the air spoiler produces large wing tip vortices. They have opposite rotational directions to C-pillar vortices which are commonly observed in case that the air spoiler is absent. The wing tip vortices generate the down-force and as a result, they can make the vehicle more stable in driving.

• International Journal of Aeronautical and Space Sciences
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• v.10 no.2
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• pp.63-76
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• 2009

A study on the interference effects of overlapping tandem rotors in forward flight is conducted using the time-marching free-wake panel method which adopts field velocity boundary integral formulation. The conventional boundary integral formulation is numerically unstable for the cases when the blade and the wake are in close proximity to each other. In order to avoid this problem, this study applies the field velocity method and modifies the boundary integration formulation. The improved method is used for the parametric study on the advance ratio and the distance between the rotors. These are the parameters that most affect the interference of the tandem rotor in forward flight. Comparison of the aerodynamic performance shows that the horizontal distance between the rotors negligibly influences the overlap-induced power factor for high advance ratio. In addition, it shows that the overlap-induced power factor is inversely proportional to the squared vertical distance between the rotors, and that the overlap-induced power factor increases to a certain extent and decrease back as the advance ratio increases.

• Journal of Ship and Ocean Technology
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• v.1 no.2
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• pp.19-30
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• 1997

The accurate prediction of the flow and the pressure distribution near the tip of the blade is crucial in determining the tip vortex cavitation inception which usually occurs on the blade tip or inside the core of the tip vortex just downstream of the blade tip. An improved boundary element method is applied to the prediction of the flow around propeller blades, with emphasis at the tip region. In the method, the Blow adapted grid and a higher order panel method, which combines a hyperboloidal panel geometry with a hi-quadratic dipole distribution, are used in order to accurately model the trailing wake geometry and the highly rolled-up regions in the wake. The method is applied to several propeller geometries and the results have been found to agree well to the existing experimental data. Inviscid flow methods are able to predict the pressures at the tip as well as the shape of the trailing wake. On the other hand, they are unable to determine the flow inside the viscous core of the tip vortex, where cavitation inception often occurs. Thus, a method is presented that treats the flow inside the viscous core. The inner flow is treated with a 2-D Clavier-stokes solution without making any assumptions for axisymmetric flow and conicity of the flow along the tip trajectory. The method can thus allow the treatment of general propeller blade configurations. The velocity and pressure distributions inside the core are shown and compared to those from other numerical methods.

• Proceedings of the Korean Information Science Society Conference
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• 2010.06d
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• pp.360-365
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• 2010

In this paper, we present a distributed algorithm for detecting dynamic event regions in wireless sensor network with the consideration on energy saving. Our model is that the sensing field is monitored by a large number of randomly distributed sensors with low-power battery and limited functionality, and that the event region is dynamic with motion or changing the shape. At any time that the event happens, we need some sensors awake to detect it and to wake up its k-hop neighbors to detect further events. Scheduling for the network to save the total power-cost or to maximize the monitoring time has been studied extensively. Our scheme is that some predetermined sensors, called critical sensors are awake all the time and when the event is detected by a critical sensor the sensor broadcasts to the neighbors to check their sensing area. Then the neighbors check their area and decide whether they wake up or remain in sleeping mode with certain criteria. Our algorithm uses only 2 bit of information in communication between sensors, thus the total communication cost is low, and the speed of detecting all event region is high. We adapt two kinds of measure for the wake-up decision. With suitable threshold values, our algorithm can be applied for many applications and for the trade-off between energy saving and the efficiency of event detection.

• International Journal of Aeronautical and Space Sciences
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• v.14 no.3
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• pp.201-209
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• 2013

Much numerical and experimental research has been done for the flow around an oscillating airfoil. The main research topics are vortex shedding, dynamic stall phenomenon, MAV's lift and thrust generation. Until now, researches mainly have been concentrated on analyzing the wake flow for the variation of frequency and amplitude at a low angle of attack. In this study, wake structures and acoustic wave propagation characteristics were studied for a plunging airfoil at high angle of attack. The governing equations are the Navier-Stokes equation with LES turbulence model. OHOC (Optimized High-Order Compact) scheme and 4th order Runge-Kutta method were used. The Mach number is 0.3, the Reynolds number is, and the angle of attack is from $20^{\circ}$ to $50^{\circ}$. The plunging frequency and the amplitude are from 0.05 to 0.15, and from 0.1 to 0.2, respectively. Due to the high resolution numerical method, wake vortex shedding and pressure wave propagation process, as well as the propagation characteristics of acoustic waves can be simulated. The results of frequency analysis show that the flow has the mixed characteristics of the forced plunging frequency and the vortex shedding frequency at high angle of attack.

• Journal of the Society of Naval Architects of Korea
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• v.41 no.4
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• pp.30-37
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• 2004

Flow characteristics of the inflow ahead of a rotating propeller attached to a container ship model were investigated using a two-frame PIV (Particle Image Velocimetry) technique. Ensemble-averaged mean velocity fields were measured at four different blade phases. The mean velocity fields show the acceleration of inflow due to the rotating propeller and the velocity deficit in the near-wake region. The axial velocity distribution of inflow in the upper plane of propeller is quite different from that in the lower plane due to the thick hull boundary layer. The propeller inflow also shows asymmetric axial velocity distribution in the port and starboard side. As the inflow moves toward the propeller, the effect of phase angle variation of propeller blade on the inflow becomes dominant. In the upper plane above the propeller axis the inflow has very low axial velocity and large turbulent kinetic energy, compared with the lower plane. The boundary layer developed along the bottom surface of stern hull forms a strong shear layer affecting vortex structure of the propeller near-wake.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2004.11a
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• pp.938-941
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• 2004

Sound generation and radiation from the duct-rotor system are calculated numerically. The wake geometries of a two-bladed rotor are calculated by using a time-marching fiee-wake method without a non-physical model of the far wake. Acoustic free field due to a rotating rotor is obtained by Lowson's equation. Using Kirchhoff source, rotating sources are modeled as stationary ones and can be inserted in the thin body boundary element method. The Kirchhoff source is validated through calculation of acoustic pressure due to a rotating point force. The thin body boundary element method (thin body BEM) is validated through calculation of acoustic radiation of ducted dipole. Using Kirchhoff source and thin body BEM, acoustic radiation of a ducted rotating source is calculated. Acoustic shielding is observed by inserting a duct and shows different phenomena at each major frequency. Acoustic radiation of a real duct-rotor system is also calculated using this method and the ducted acoustic field is significantly different from rotor only.