• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.41 no.11
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• pp.855-864
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• 2013

Active tab is one of the promising technology for the BVI (blade-vortex interaction) noise reduction, and analysis of noise reduction performance is very important phase of the technology development. For the purpose of analysing the performance of noise reduction using active tab, CAMRAD II model for a model-scale rotor system was constructed utilizing structural design result and airfoil aerodynamic data generated by CFD (computational fluid dynamics) calculation. HHC strategy was applied to descent flight condition and air-load was calculated by CAMRAD II then variations of BVI noise was calculated by in-house program. Calculation result with respect to tab length and control phase changes showed BVI noise could be reduced by -3.3dB.

• Wind and Structures
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• v.26 no.5
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• pp.331-341
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• 2018

A numerical study on the flow over a square cylinder in the vicinity of a wall is conducted for different Couette-Poiseuille-based non-uniform flow with the non-dimensional pressure gradient P varying from 0 to 5. The non-dimensional gap ratio L (=$H^{\ast}/a^{\ast}$) is changed from 0.1 to 2, where $H^{\ast}$ is gap height between the cylinder and wall, and $a^{\ast}$ is the cylinder width. The governing equations are solved numerically through finite volume method based on SIMPLE algorithm on a staggered grid system. Both P and L have a substantial influence on the flow structure, time-mean drag coefficient ${\bar{C}}_D$, fluctuating (rms) lift coefficient ($C_L{^{\prime}}$), and Strouhal number St. The changes in P and L leads to four distinct flow regimes (I, II, III and IV). Following the flow structure change, the ${\bar{C}}_D$, $C_L{^{\prime}}$, and St all vary greatly with the change in L and/or P. The ${\bar{C}}_D$ and $C_L{^{\prime}}$ both grow with increasing P and/or L. The St increases with P for a given L, being less sensitive to L for a smaller P (< 2) and more sensitive to L for a larger P (> 2). A strong relationship is observed between the flow regimes and the values of ${\bar{C}}_D$, $C_L{^{\prime}}$ and St. An increase in P affects the pressure distribution more on the top surface than on bottom surface while an increase in L does the opposite.

• Journal of the Korea Institute of Military Science and Technology
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• v.4 no.2
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• pp.256-262
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• 2001

Viscous solutions of supersonic side jet nozzle and supersonic jet impinging on a flat plate are simulated using three-dimensional Navier-Stokes solver. For rapid and abrupt control of a missile in supersonic flight, side jet on a missile body is found to be a useful devise as evidenced by recent missile development at several nations. The magnitude of the side jet and the duration of it decide the level of control of such a missile system. The aerodynamic characteristics of the side jet devise itself are examined in terms of key parameters such as the side jet nozzle geometry, the chamber pressure and temperature. On the other hand, the jet impinging flow structure exhibits such complex nature as shock shell, plate shock and Mach disk depending on the flow parameters. Among others, the dominant parameters are the ratio of the nozzle exit pressure to the ambient pressure and the distance between the nozzle exit plane and the impinging plane. As the plate is placed close to the nozzle, the computed wall pressure at or near the jet center oscillates with large amplitude with respect to the mean value. The amplitude of wall pressure fluctuations subsides as the plate/nozzle distance increases, and the frequency of the wall pressure is estimated on the order of 10.0 KHz. Objectives of this paper are to show accurate simulation of nozzle flow itself and to demonstrate the jet flow structure when the jet interacts with a wall at a close range.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.4 s.247
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• pp.298-305
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• 2006

The flow around a circular cylinder which oscillates rotationally with a relatively high forcing frequency has been investigated experimentally using flow visualization and hot-wire measurements. Dominant parameters are Reynolds number (Re), oscillation amplitude $({\theta}_A)$, and frequency ratio $F_R=f_f/f_n$, where $f_f$ is the forcing frequency and $f_n$ is the natural frequency of vortex shedding. Experiments were carried out under the conditions of $Re=4.14{\times}10^3,\;{\theta}_A={\pi}/6$, and $0{\leq}F_R{\leq}2$. The effect of frequency ratio $F_R$ on the flow structure of wake was evaluated by measuring wake velocity profile and spectral analysis of hot-wire signal. Depending on the frequency ratio $F_R$, the cylinder wake has 5 different flow regimes. The vortex formation length and vortex shedding frequency are changed significantly before and after the lock-on regime. The drag coefficient was reduced under the condition of $F_R<1.0$ and the maximum drag reduction is about 33% at $F_R=0.8$. However, the drag is increased as $F_R$ increases beyond $F_R=1.0$. This active flow control method can be effective in aerodynamic applications, if the forcing parameters are selected optimally.

• Wind and Structures
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• v.23 no.2
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• pp.127-142
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• 2016

Aerodynamic effects, such as drag force and flow-induced vibration (FIV), on civil engineering structures can be minimized by optimally modifying the structure shape. This work investigates the turbulent wake of a square prism with its faces modified into a sinusoidal wave along the spanwise direction using three-dimensional large eddy simulation (LES) and particle image velocimetry (PIV) techniques at Reynolds number $Re_{Dm}$ = 16,500-22,000, based on the nominal width ($D_m$) of the prism and free-stream velocity ($U_{\infty}$). Two arrangements are considered: (i) the top and bottom faces of the prism are shaped into the sinusoidal waves (termed as WSP-A), and (ii) the front and rear faces are modified into the sinusoidal waves (WSP-B). The sinusoidal waves have a wavelength of $6D_m$ and an amplitude of $0.15D_m$. It has been found that the wavy faces lead to more three-dimensional free shear layers in the near wake than the flat faces (smooth square prism). As a result, the roll-up of shear layers is postponed. Furthermore, the near-wake vortical structures exhibit dominant periodic variations along the spanwise direction; the minimum (i.e., saddle) and maximum (i.e., node) cross-sections of the modified prisms have narrow and wide wakes, respectively. The wake recirculation bubble of the modified prism is wider and longer, compared with its smooth counterpart, thus resulting in a significant drag reduction and fluctuating lift suppression (up to 8.7% and 78.2%, respectively, for the case of WSP-A). Multiple dominant frequencies of vortex shedding, which are distinct from that of the smooth prism, are detected in the near wake of the wavy prisms. The present study may shed light on the understanding of the underlying physical mechanisms of FIV control, in terms of passive modification of the bluff-body shape.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.35 no.1
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• pp.66-72
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• 2007

an approach to air-vehicle design, an application of the radio-controlled model flight test techniques has been presented. The approach presented in this study is to validate the air-vehicle design configuration by analyzing the flight test results of scale model with dynamic similarities, and then to apply the analyzed results to the aerodynamic design process in early stage of the air-vehicle development. To develop practically applicable similarity laws for the subscale flying model design, the air-vehicle motions are decoupled into rotational motions for stability & control similarities and translational motions for flight performance similarities. Also, detail techniques for radio-controlled model flight test have been developed. Based on the results obtained from the radio-controlled flight test, the present approach for air-vehicle design has shown to be useful to validate the air-vehicle design configuration.

• International Journal of High-Rise Buildings
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• v.7 no.1
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• pp.17-32
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• 2018

This paper explores the function of a structural skin with an embossed surface applicable to use for tall building structures. The major diagrid system with a secondary embossed surface structure provides an enhanced perimeter structural system by increasing tube section areas and reduces aerodynamic loads by disorienting major organized structure of winds. A parametric study used to investigate an optimized configuration of the embossed structure revealed that the embossed structure has a structural advantage in stiffening the structure, reducing lateral drift to 90% compared to a non-embossed diagrid baseline model, and results of wind load analysis using computational fluid dynamics, demonstrated the proposed embossed system can reduce. The resulting undulating embossed skin geometry presents both opportunities for incorporating versatile interior environments as well as unique challenges for daylighting and thermal control of the envelope. Solar and thermal control requires multiple daylighting solutions to address each local façade surface condition in order to reduce energy loads and meet occupant comfort standards. These findings illustrate that although more complex in geometry, architects and engineers can produce tall buildings that have less impact on our environment by utilizing structural forms that reduce structural steel needed for stiffening, thus reducing embodied $CO^2$, while positively affecting indoor quality and energy performance, all possible while creating a unique urban iconography derived from the performance of building skin.

• Journal of Power Electronics
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• v.15 no.4
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• pp.1047-1053
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• 2015

A time-domain simulation tool to predict the dynamic power output of wind turbines in an offshore wind farm was developed in this study. A wind turbine model consisting of first or second order transfer functions of various wind turbine elements was combined with the Ainslie's eddy viscosity wake model to construct the simulation tool. The wind turbine model also includes an aerodynamic model that is a look up table of power and thrust coefficients with respect to the tip speed ratio and pitch angle of the wind turbine obtained by a commercial multi-body dynamics simulation tool. The wake model includes algorithms of superposition of multiple wakes and propagation based on Taylor's frozen turbulence assumption. Torque and pitch control algorithms were implemented in the simulation tool to perform max-Cp and power regulation control of the wind turbines. The simulation tool calculates wind speeds in the two-dimensional domain of the wind farm at the hub height of the wind turbines and yields power outputs from individual wind turbines. The NREL 5MW reference wind turbine was targeted as a wind turbine to obtain parameters for the simulation. To validate the simulation tool, a Danish offshore wind farm with 80 wind turbines was modelled and used to predict the power from the wind farm. A comparison of the prediction with the measured values available in literature showed that the results from the simulation program were fairly close to the measured results in literature except when the wind turbines are congruent with the wind direction.

• Journal of IKEEE
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• v.26 no.4
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• pp.596-601
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• 2022

The application of the EGR system is increasing according to the recent trend of conversion to green-ships. EGR blower, one of the core parts of the EGR, consists of aerodynamic system and e-motor and inverter and etc. For the e-motor, a permanent magnet type synchronous motor with high energy density and excellent efficiency is applied recently. Small and medium-sized enterprises trying to develop the e-motors, however, for marine inverters mostly developed by global advanced companies due to the rigid classification certification and technical difficulties. One of disadvantage of universal inverters is that when optimal control fails, it is difficult to find the cause from user's point of view. Therefore, in this study, optimal controllers(Current vector contol and Tracking observer) for SPMSM for EGR blower was designed and verified to analyze the causes of failure of optimal control of universal inverter.

• Journal of the Korean Society of Laryngology, Phoniatrics and Logopedics
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• v.18 no.1
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• pp.39-43
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• 2007

Background and Objectives: There are few studies reported that specifically examine the phonetic characteristics and voice handicap index (VHI) in patients with Allergic Rhinitis. This study was designed to examine phonetic characteristics and VHI in adult patients with allergic rhinitis. Materials and Methods: Forty-two male patients diagnosed as allergic rhinitis were given skin-prick test and others, aged from 20 to 56 years, and were compared with a 16 male control group with no pathology and in the same age group. The VHI was used to measure the changes of patient's perception. Acoustic and aerodynamic analysis test were done, and a nasalance test performed to measure rabbit, baby, and mother passage. Acoustic rhionometry (AR) was performed to evaluate nasal volume and nasal crosssectional area. Statistical analysis was done using independent sample t-test. Results: VHI showed significantly different score in the studied group, higher than that of control group. AR graph showed that there was no significant differences of nasal volume and nasal cross-sectional area. The Shimmer and SFF value in the group of allergic patients were higher than in the control group. MPT value in the group of allergic patients was lower than in the control group. Nasalance in allergic patients showed hypernasality all passage. Conclusion: We suggest that patients with allergic rhinitis have considerable voice problems. Most of them have hypernasality, which may be a compensatory mechanism by nasal obstruction.