• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.49 no.8
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• pp.699-708
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• 2021

In this paper, the aerodynamic and aeroacoustic characteristics that vary depending on the rotation axial distance between the upper and lower rotor, which is one of the design parameters of the coaxial rotor, is analyzed in the hovering condition using the computational fluid dynamics. Aerodynamic analysis using the Reynolds Averaged Navier Stokes equation and the aeroacoustic analysis using the Ffowcs Williams ans Hawkings equation is performed and the results were compared. The upper and lower rotor of the coaxial rotor have different phase angle which changes periodically by rotation and have unsteady characteristics. As the distance between the upper and lower rotors increased, the aerodynamic efficiency of the thrust and the torque was increased as the flow interaction decreased. In the aeroacoustic viewpoint, the noise characteristics radiated in the direction of the rotational plane showed little effect by axis spacing. In the vertical downward direction of the axis increased, the SPL maintains its size as the frequency increases, which affects the increase in the OASPL. As the axial distance of the coaxial rotor increased, the noise characteristics of a coaxial rotor were similar with the single rotor and the SPL decreased significantly.

• Wind and Structures
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• v.36 no.3
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• pp.175-200
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• 2023

The large eddy simulation (LES) of the flow around a circular cylinder is not only affected by the sub-grid scale (SGS) model but also by the grid resolution of the computational domain. To study the influence of different grids on the LES results, the LES simulations of the flow around a circular cylinder with different grids at Reynolds number (Re) = 3900 was performed. A circular computational domain with different radial growth rates and circumferential and spanwise grid numbers was adopted for the simulations. Meanwhile, the aerodynamic forces, wind pressure coefficients, mean and instantaneous flow fields, and the effect of grid resolution on them were comprehensively analyzed. The results indicate that the lift coefficient, wind pressure coefficient, and recirculation length are significantly affected by the radial growth rate of the grid and the circumferential grid number. The spanwise grid number has a significant influence on the three-dimensionality of the flow and plays an important role in velocity fluctuations in the wake region. Nevertheless, the aerodynamic coefficients and recirculation length are not sufficiently sensitive to the grid number in the spanwise direction. By comparing the results, it can be concluded that suitable and reliable LES results can be obtained when the radial growth rate is 1.03 or 1.05, the circumferential grid number is 160, 200, or 240, and the spanwise grid number is 64. A radial growth rate 1.05, circumferential grid number 160, and spanwise grid number 64 are recommended to reduce the grid amount and further improve the efficiency.

• Journal of the Korean Society for Aviation and Aeronautics
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• v.29 no.1
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• pp.61-73
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• 2021

PAV is considered to improve quality of life and standards of living, improvement of which was caused by automobile hundred years ago. Comparative economic efficiency of PAVs is measured to compare each PAV. Specification and sales price of the PAVs are open to the public. BlackFly, Transition and Aeromobil 3.0 have competitive power in flying range, purchasing cost, and operational cost. Lift & cruise configuration and vectored thrust configuration PAVs are designed by many companies nowadays, and BlackFly which can be considered to be lift & cruise configuration is one of the most efficient PAVs. High battery price does not help multi-copter shaped PAVs to have economic efficiency. Aerodynamic wing, eVTOL, and low sale price are needed for PAVs to ride a wave of public interest as a new personal mobility. Under the conditions, the PAV can fly at downtown and can be purchased by people at large. Popularization of PAV could follow in the 100 years old footsteps of automobile.

• Journal of Bio-Environment Control
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• v.15 no.4
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• pp.289-295
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• 2006

A computational fluid dynamics (CFD) numerical model has been developed to effectively study the ventilation efficiency of multi-span greenhouses with internal crops. As the first step of the study, the internal plants of the CFD model had to be designed as a porous media because of the complexity of its physical shapes. In this paper, the results of the wind tunnel tests were introduced to find the aerodynamic resistance of the plant canopy. The Seogun tomato was used for this study which made significant effects on thermal and mass exchanges with the adjacent air as well as internal airflow resistance. With the main factors of wind speed, static pressure, and density of plant canopy, the aerodynamic resistance factor was statically found. It was finally found to be 0.26 which will be used later as an input data of the CFD model. Moreover, the experimental procedure of how to find the aerodynamic resistance of various plants using, wind tunnel was established through this study.

• Phonetics and Speech Sciences
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• v.15 no.4
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• pp.61-69
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• 2023

Sulcus vocalis is characterized by incomplete closure of the vocal folds, with a high mean airflow rate (MFR) as a distinctive feature. The MFR is measured using two aerodynamic analysis methods [the maximum sustained phonation protocol (MXPH) and voicing efficiency protocol (VOEF)] of the phonatory aerodynamic system (PAS), and the results may vary depending on the method. This study compared the differences in MFR before and after treatment (microsurgery and voice therapy) according to the MXPH and VOEF of the PAS in 30 patients with sulcus vocalis. Additionally, we examined whether there were differences in the subjective voice evaluation (voice handicap index, VHI), perceptual voice evaluation (GRBS), and fundamental frequency (F0) before and after treatment. The results showed significant differences between the two methods, both before and after treatment, in patients with sulcus vocalis. However, there were no significant differences by methods in the changes before and after treatment. The VHI and GRBS scores significantly decreased after treatment; however, F0 showed no significant differences before and after treatment. This study indicates that when evaluating MFR changes in patients with sulcus vocalis, it is acceptable to use either aerodynamic analysis (MXPH or VOEF).

• Journal of The Korean Society of Agricultural Engineers
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• v.50 no.1
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• pp.25-37
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• 2008

When livestock facilities in Korea have been changed larger and denser, rearing conditions have been getting worse and the productivity of animal production have been decreased. Especially in the cold season, the minimized ventilation has generally been operated to save energy cost in Korea resulting in very poor environmental condition and high mortality. While the stability, suitability, and uniformity of the rearing condition are the most important for high productivity, the ventilation configuration is the most important to improve the rearing condition seasonally. But, it is so difficult to analyze the internal air flow and the environmental factors by conducting only field experiment because the weather condition is very unpredictable and unstable as well as the structural specification can not be easily changed by the researchers considering cost and labor. Accordingly, an aerodynamic computer simulation was adopted to this study to overcome the weakness of conducting field experiment and study the aerodynamic itself. It has been supposed that the airflow is the main mechanism of heat, mass, and momentum transfers. To make the simulation model accurately and actually, simplified pig models were also developed. The accuracy of the CFD simulation model was enhanced by 4.4 % of errors compared with the data collected from field experiments. In this paper, using the verified CFD model, the CFD computed internal rearing condition of the mechanically ventilated pig house were analyzed quantitatively as well as qualitatively. Later, this developed model will be computed time-dependently to effectively analyze the seasonal ventilation efficiency more practically and extensively with tracer gas decay theory.

• The KSFM Journal of Fluid Machinery
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• v.10 no.6
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• pp.7-16
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• 2007

An axial-type fan which operates at the relative total pressure of 671Pa and static pressure of 560Pa with the flow rate of $416.6m^3/min$ is developed with an optimization technique based on the gradient method. Prior to the optimization of fan blade, a three-dimensional axial-type fan blade is designed based on the free-vortex method along the radial direction. Twelve design variables are applied to the optimization of the rotor blade, and one design variable is selected for optimizing a stator which is located behind of the rotor and is used to support a fan driving motor. The total and static pressure are applied to the restriction condition with the operating flowrate on the design point, and the efficiency is chosen as the response variable to be maximized. Through these procedures, an initial axial-fan blade designed by the free vortex method is modified to increase the efficiency with the satisfaction of the operating condition. The optimized fan is tested to compare the aerodynamic performance with an imported same class fan. The test result shows that the optimized fan operates with the satisfaction of restriction conditions, but the imported fan cannot. From the experimental and numerical test, they show that this optimization method improves the fan efficiency and operating pressures of a fan designed by the classical fan design method.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.40 no.6
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• pp.467-474
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• 2012

UAVs for reconnaissance and intelligence operations require long endurance capability, which demands high efficiency of the propulsion system. The distributed propulsion system(DPS) generates the thrust by replacing a large propulsion system with a number of small propulsion systems. A DPS distributed along the wing span can produce gains in propulsion efficiency by reducing ejection velocity. Also, the ingestion of boundary layers through the distributed DPS inlet and ejecting flow from the outlet can improve the lift to drag ratio of the vehicle. This study investigates the effects of locations and size of the inlet and outlet of the DPS on the blended-wing-body design based on Eppler 337 airfoil, with a CFD tool. The fans in the DPS are modeled as actuator disks for computational efficiency. The best location and aspect ratio of the inlet and outlet are found from lift-to-drag ratio and pitching moment considerations.

• Journal of Korean Society for Atmospheric Environment
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• v.15 no.6
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• pp.799-804
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• 1999

Particle collection characteristics of the MOUDI cascade impactor has been studied for coarse particles in the range of 2 to 20$mu extrm{m}$ in aerodynamic diameter. A vibrating orifice aerosol generator was empolyed to generate monodisperse test aerosols. The oleic acid and sodium chloride(NaCl) particles were used as test aerosols. Aluminum foil and Teflon filter were selected as impaction media. The sampling flow rate was changed from 25 to 35L/min. Particle collection efficiency for single stage was examined for liquid particles. The stage response was obtained experimentally for the cascade impactor composed of three stages and a backup filter. The results showed that most of particle collection efficiencies measured in this work are similar to the efficiency curves obtained by Marple et al.(1991). For particles less than cut-off size of the stage, the collection efficiencies of solid particles are similar to those of loquid particles. However, the collection efficiency of solid particles decreases with mereasing particle diameter for the particles greater than the actual cut-off size of the impactor. The particle collection efficiency increases with increasing sampling flow rate at the same particel size. However, the collection efficiency curves seem not to be greatly shifted with the flow rate. The stage responses obtained by direct measurements in this work are in good agreement with those derived from the collection efficiency curves for single stage.

• The KSFM Journal of Fluid Machinery
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• v.4 no.3 s.12
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• pp.46-52
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• 2001

This paper describes a turbine test program conducted at Seoul National University(SNU). To measure blades' aerodynamic performance, either linear(2-Dimensional) or annular(3-Dimensional) cascades are often used. However, neither cascade can consider effects such as those due to rotation or rotor-stator interaction. Therefore, a rotating test facility for axial turbines has been designed and built at SNU, and its description is given in this paper. The results from an axial turbine performance test are presented. At the design point, the measured efficiency agrees with the efficiency predicted by a meanline analysis. At off design points, however, the measured and predicted efficiencies diverge. The most likely cause is hypothesized to be the inaccuracy of correlations used in the meanline analysis at off design points.