• Wind and Structures
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• 제2권3호
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• pp.173-187
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• 1999

The flow around a structure has been an important subject in wind engineering research. There are various kinds of unstable aerodynamic phenomena with regard to a bluff body. In order to understand the physical mechanism of aerodynamic and aeroelastic instability of a bluff body, the relations between the flow around structures and the motion of body with various section shapes should be investigated. Based on a series of wind tunnel tests, this paper addresses the aerodynamic stability of square cylinder with various corner cuts and attack angles in the uniform flow. The test results show that the models with corner cut produced generally better behaviour for the galloping phenomenon than the original section. However, the corner cut method can not prevent the occurrence of the vortex-induced vibration(VIV). It is also shown that as the attack angle changes, the optimum size of corner cut changes also. This means that any one specific size of corner cut which shows the best aerodynamic behaviour throughout all the cases of attack angles does not exist. This paper presents an intensive study on obtaining the optimum size of corner cut for the stabilization of aerodynamic behaviour of cylinders.

• 한국기계가공학회지
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• 제18권7호
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• pp.63-70
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• 2019

An electro-aerodynamic lens for improving the performance of virtual impactor has been proposed in this study. ANSYS FLUENT Release 16.1 was used for numerical analysis of virtual impactor with and without the electro-aerodynamic lens, used to collimate the incoming aerosol particles into a particle beam before injecting the particles into the virtual impactor. Particles supplied to the electro-aerodynamic lens were assumed to be highly charged. By using an aerodynamic lens before the virtual impactor, without any electrostatic effect, it was found that the cut-off diameter of the virtual impactor was reduced from $4.2{\mu}m$ to $0.68{\mu}m$ and that the fine particle contamination problem became more serious. However, by employing the combined electrostatic and aerodynamic effects, that is, by applying electric voltage potential to the electro-aerodynamic lens, the cut-off diameter was found to be further reduced to $0.45{\mu}m$ and the fine particle contamination was eliminated.

• Wind and Structures
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• 제34권1호
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• pp.73-80
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• 2022

This paper presents a study on amplitude-dependent self-excited aerodynamic forces of a 5:1 rectangular cylinder through free vibration wind tunnel test. The sectional model was spring-supported in a single degree of freedom (SDOF) in torsion, and it is found that the amplitude of the free vibration cylinder model was not divergent in the post-flutter stage and was instead of various stable amplitudes varying with the wind speed. The amplitude-dependent aerodynamic damping is determined using Hilbert Transform of response time histories at different wind speeds in a smooth flow. An approach is proposed to extract aerodynamic derivatives as nonlinear functions of the amplitude of torsional motion at various reduced wind speeds. The results show that the magnitude of A2^*, which is related to the negative aerodynamic damping, increases with increasing wind speed but decreases with vibration amplitude, and the magnitude of A3^* also increases with increasing wind speed but keeps stable with the changing amplitude. The amplitude-dependent aerodynamic derivatives derived from the tests can also be used to estimate the post-flutter response of 5:1 rectangular cylinders with different dynamic parameters via traditional flutter analysis.

• Wind and Structures
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• 제36권4호
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• pp.249-261
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• 2023

Aerodynamic force coefficients are generally prescribed by an ensemble average of ten and/or twenty 10-minute samples. However, this makes it difficult to identify the exact probability distribution and exceedance probability of the prescribed values. In this study, 12,600 10-minute samples on three tall buildings were measured, and the probability distributions were first identified and the aerodynamic force coefficients corresponding to the specific non-exceedance probabilities (cumulative probabilities) of wind load were then evaluated. It was found that the probability distributions of the mean and fluctuating aerodynamic force coefficients followed a normal distribution. The ratios of aerodynamic force coefficients corresponding to the specific non-exceedance probabilities (C_f,Non) to the ensemble average of 12,600 samples (C_f,Ens), which was defined as an adjusting factor (C_f,Non/C_f,Ens), were less than 2%. The effect of coefficient of variation of wind speed on the adjusting factor is larger than that of the annual non-exceedance probability of wind load. The non-exceedance probabilities of the aerodynamic force coefficient is between P_C,nonex = 50% and 60% regardless of force components and aspect ratios. The adjusting factors from the Gumbel distribution were larger than those from the normal distribution.

• Wind and Structures
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• 제38권2호
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• pp.119-128
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• 2024

The aerodynamic damping is an essential factor that can considerably affect the dynamic response of the cable-stayed bridge induced by crosswind load. However, developing an accurate and efficient aerodynamic damping model is crucial for evaluating the crosswind load-induced response on cable-stayed bridges. Therefore, this study proposes a new method for identifying aerodynamic damping of the bridge structures under crosswind load using an extended Kalman filter (EKF) and the particle filter (PF) algorithm. The EKF algorithm is introduced to capture the aerodynamic damping ratio. PF technique is used to select the optimal spectral representation of the noise. The effectiveness and accuracy of the proposed solution were investigated through full-scale vibration measurement data of the crosswind-induced on the bridge's girder. The results show that the proposed solution can generate an efficient and robust estimation. The errors between the target and extracted values are around 0.01mm and 0.003^o, respectively, for the vertical and torsional motion. The relationship between the amplitude and the aerodynamic damping ratio is linear for small reduced wind velocity and nonlinear with the increasing value of the reduced wind velocity. Finally, the results show the influence of the level of noise.

• 한국자동차공학회논문집
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• 제24권2호
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• pp.198-204
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• 2016

In this study, a wind tunnel test was conducted to measure the aerodynamic characteristics of a streamline-designed high-speed bus with the change of wind direction and speed and the result is compared with the aerodynamic performance of a commercialized high-speed bus model (Model-0) manufactured by Zyle Daewoo Bus Corp. Aerodynamic performance of the existing rear-spoiler was tested to prove its aerodynamic effect on the test model bus. From the study, it was found that 24.6 % of the total drag of the original bus model (Model-0) was reduced on the streamline-designed model bus(model-1) without the rear-spoiler but only 14.3 % of the total drag was reduced with the spoiler on the streamlined model bus. It means that the rear spoiler does not work properly with the streamlined model bus (model-1) and should be noted that an optimum design of a rear-spoiler of a vehicle is important to reduce the induced pressure drag and increase the driving stability of a vehicle against yaw motion. The experimental outcome was also compared to the previous numerical research result to evaluate the reliability of the numerical algorithm of the aerodynamic performance analysis of a vehicle. The error rate (%) of the numerical result to the experimental output is about 5.4 % and it is due to the simplified body configuration of the numerical model bus. The drag increases at the higher yaw angle because the transparent frontal area of the model vehicle increases and the downward force increases with the yaw angle as well. It has a positive effect to the driving stability of the vehicle but the moderated downward force should be kept for the fuel economy of a vehicle.

• Wind and Structures
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• 제28권5호
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• pp.285-298
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• 2019

For super long-span bridges, the aerodynamic forces induced by the flow passing the box girder should be considered carefully. And the Reynolds number sensitively of aerodynamic characteristics is one of considerable issue. In the study, a numerical study on the Reynolds number sensitivity of aerodynamic characteristic (flow pattern, pressure distribution and aerodynamic forces) of a twin-box girder were carried out using large eddy simulation (LES) with the dynamic Smagorinsky-Lilly subgrid model. The results show that the aerodynamic characteristics have strong correlation with the Reynolds number. At the leading edge, the flow experiences attachment, departure, and reattachment stages accompanying by the laminar transition into turbulence, causing pressure plateaus to form on the surface, and the pressure plateaus gradually shrinks. Around the gap, attributing that the flow experiences stages of laminar cavity flow, the wake with alternate shedding vortices, and turbulent cavity flow in sequence with an increase in the Reynolds number, the pressures around the gap vary greatly with the Reynold number. At the trailing edge, the pressure gradually recovers as the flow transits to turbulence (the flow undergoes wake instability, shear layer transition-reattachment station), In addition, at relative high Reynolds numbers, the drag force almost does not change, however, the lift force coefficient gradually decreases with an increase in Reynolds number.

• Wind and Structures
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• 제6권2호
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• pp.141-150
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• 2003

This paper presents galloping analysis of multiple-degree-of-freedom (MDOF) structural roofs with multiple orientations. Instead of using drag and lift coefficients and/or their combined coefficient in traditional galloping analysis for slender structures, this study uses wind pressure coefficients for wind force representation on each and every different orientation roof, facilitating the galloping analysis of multiple-orientation roof structures. In the study, influences of nonlinear aerodynamic forces are considered. An energy-based equivalent technique, together with the modal analysis, is used to solve the nonlinear MDOF vibration equations. The critical wind speed for galloping of roof structures is derived, which is then applied to galloping analysis of roofs of a stadium and a high-rise building in China. With the aid of various experimental results obtained in pertinent research, this study also shows that consideration of nonlinear aerodynamic forces in galloping analysis generally increases the critical wind speed, thus enhancing aerodynamic stability of structures.

• 동력기계공학회지
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• 제9권3호
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• pp.44-49
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• 2005

The aerodynamic performance of an indoor room air-conditioner using a cross-flow fan is strongly influenced by the various design factors of a rear-guider and a stabilizer. The purpose of this study is to investigate the effects of a rear-guider and a stabilizer on the aerodynamic performance in the maximum flowrate range of a cross-flow fan. The design factors considered in this study are a rear-guider clearance, a stabilizer cutoff clearance, and a stabilizer setup angle, respectively. Aerodynamic performances including maximum flowrate and power show the biggest magnitude distribution in the case of $45^{\circ}$, the stabilizer setup angle as well as nearly similar magnitude distribution regardless of the stabilizer cutoff clearances. Moreover, the more a rear-guider clearance increases, the more the magnitude of maximum flowrate and power increases.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.435-438
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• 2002

In this study, a new passive aerodynamic control method is proposed. Control plate which is oscillated by TMD-like mechanism makes flutter stabilizing airflow. Effectiveness of proposed model is verified by experimental and analytical study. In addition, various parameters of the proposed system are investigated. Applicability to long span bridge is also examined. According to the research results, proposed model is very effective in suppressing flutter, and it also shows remarkable robustness.