• Proceedings of the KSR Conference
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• 2011.10a
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• pp.1320-1326
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• 2011

The maximum speed of high-speed rail is restricted to various factors such as track condition including slope and radius, tunnel and dynamic stability of vehicle. Among the various factors, traction effort and resistance to motion is principal and basic factor. In addition, at high speed over 300km/h, aerodynamic drag amounts up to 80% of resistance to motion, that it can be said that aerodynamic drag is the most important factor to decide the maximum speed of high-speed rail system. This paper deals with a measure to increase the maximum speed of high-speed train by reducing aerodynamic drag. The traction effort curve and resistance to motion curve of existing high-speed train under development has been employed to set up the target of aerodynamic drag reduction to reach up to 500km/h without modification traction system. In addition, the contribution of various sources of aerodynamic drag to total value has been analyzed and the strategy for implementation of aerodynamic drag reduction has been discussed based on the aerodynamic simulation results around the train using computational fluid dynamics.

• Journal of the Korea Institute of Military Science and Technology
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• v.14 no.3
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• pp.397-404
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• 2011

The aerodynamic characteristics of a separated captive body in flow field around aircraft are studied to observe aerodynamic stability for safe separation from aircraft. Since the captive body separated from aircraft is initially exposed to unsteady flow pattern, the change of aerodynamic forces and moments should be measured to analyze how the flow pattern affects on the captive body at the vicinity of aircraft. Aerodynamic forces and moments of the separated captive body are measured at selected positions along predictable dropping trajectories. The measuring trajectories, generated by the free drop test of the dropping model in the wind tunnel, are consisted of 9 possible lines by free dropped trajectories. Experimental results show that the aerodynamic forces and moments are significantly varied with the distance between the captive body and aircraft. In conclusion, the change of aerodynamic characteristics within flow field around aircraft should be considered to simulate trajectories of the separated captive body from aircraft.

• Wind and Structures
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• v.25 no.5
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• pp.415-432
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• 2017

The nonlinear aerodynamic instability of a tensioned plane orthotropic membrane structure is theoretically investigated in this paper. The interaction governing equation of wind-structure coupling is established by the Von $K\acute{a}rm\acute{a}n's$ large amplitude theory and the D'Alembert's principle. The aerodynamic force is determined by the potential flow theory of fluid mechanics and the thin airfoil theory of aerodynamics. Then the interaction governing equation is transformed into a second order nonlinear differential equation with constant coefficients by the Bubnov-Galerkin method. The critical wind velocity is obtained by judging the stability of the second order nonlinear differential equation. From the analysis of examples, we can conclude that it's of great significance to consider the orthotropy and geometrical nonlinearity to prevent the aerodynamic instability of plane membrane structures; we should comprehensively consider the effects of various factors on the design of plane membrane structures; and the formula of critical wind velocity obtained in this paper provides a more accurate theoretical solution for the aerodynamic stability of the plane membrane structures than the previous studies.

• Journal of Bio-Environment Control
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• v.5 no.2
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• pp.152-159
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• 1996

Experiment was performed in a newly developed wind tunnel with light system to determine the aerodynamic resistance and eddy diffusivity above the plug stand under artificial light. Maximum air temperature appeared near the top of the plug stand under artificial light. Since Richardson number was ranged from -0.07 to +0.01, the atmosphere above the plug stand in wind tunnel was in an unstable or near- neutral stability state. The average aerodynamic resistance at rear region of plug stand was 25 % higher than that at middle region. Eddy diffusivity($K_{M}$) linearly increased with the increasing air current speed. $K_{M}$ at air current speed of 0.9 m.$s^{-1}$ was about two times as many as that at air current speed of 0.3 m.$s^{-1}$. And average $K_{M}$ at the rear region was 15% lower than that at the middle region. These results indicated that the diffusion of heat and mass along the direction of air current inside the plug stand was different. It might cause the lack of uniformity in the growth and quality of plug seedlings. The wind tunnel developed in this study would be useful to investigate the effects of air current speed on microclimates and the growth of plug seedlings under artificial light in a semi- closed ecosystem.

• Wind and Structures
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• v.30 no.5
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• pp.499-509
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• 2020

The aeroelastic stability of a long-span suspension footbridge with a bluff deck (prototype section) was examined through static and dynamic wind tunnel tests using a 1:10 scale sectional model of the main girder, and the corresponding aerodynamic countermeasures were proposed in order to improve the stability. First, dynamic tests of the prototype sectional model in vertical and torsional motions were carried out at three attack angles (α = 3°, 0°, -3°). The results show that the galloping instability of the sectional model occurs at α = 3° and 0°, an observation that has never been made before. Then, the various aerodynamic countermeasures were examined through the dynamic model tests. It was found that the openings set on the vertical web of the prototype section (web-opening section) mitigate the galloping completely for all three attack angles. Finally, static tests of both the prototype and web-opening sectional models were performed to obtain the aerodynamic coefficients, which were further used to investigate the galloping mechanism by applying the Den Hartog criterion. The total damping of the prototype and web-opening models were obtained with consideration of the structural and aerodynamic damping. The total damping of the prototype model was negative for α = 0° to 7°, with the minimum value being -1.07%, suggesting the occurrence of galloping, while that of the web-opening model was positive for all investigated attack angles of α = -12° to 12°.

• Journal of the Korean Society of Hazard Mitigation
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• v.8 no.6
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• pp.21-30
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• 2008

In this study, to evaluate the aerodynamic stability of suspension bridge hanger ropes, the wind tunnel tests are carried out. It is found that the vortex induced vibration is detected only in single PE-coated PWS cable case. And the wake galloping is occurred in twin cables spaced $3\sim6$ cable diameters of cable center to center when the incidence angle of wind is only zero degree. In case of other incidence angles of wind except zero degree, the wake galloping or the wake flutter are showed in twin cables even outside range of the bounds of $3\sim6$ cable diameters. CFRC cable shows very stable for the twin cables regardless of the distance between two cables, and also for various incidence angles of wind. Thus the characteristic of CFRC rope overwhelms one of PWS cable in aerodynamic stability.

• 한국전산유체공학회:학술대회논문집
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• 2008.03b
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• pp.51-53
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• 2008

The vehicle aerodynamic crosswind characteristics are mainly governed by the coefficient of side force and yawing moment. These performances affect not only the driving comfort which can be felt by driver but also the safety due to the instability of vehicle. The aims of this investigation are to improve the aerodynamic crosswind performance of sedan vehicle under the crosswind conditions. In order to improve the crosswind stability, numerical analysis has been performed by modifying the rear body shape of vehicle. As the results, we observed about 20% reduction of yawing moment coefficient relative to the base vehicle.

• International Journal of Aeronautical and Space Sciences
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• v.12 no.1
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• pp.78-83
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• 2011

Traditional autopilot design requires an accurate aerodynamic model and relies on a gain schedule to account for system nonlinearities. This paper presents the control architecture applied to a dynamic model inversion at a single flight condition with an on-line neural network (NN) in order to regulate errors caused by approximate inversion. This eliminates the need for an extensive design process and accurate aerodynamic data. The simulation results using a developed full nonlinear 6 degree of freedom model are presented. This paper also presents the stability evaluation for control systems to which NNs were applied. Although feedback can accommodate uncertainty to meet system performance specifications, uncertainty can also affect the stability of the control system. The importance of robustness has long been recognized and stability margins were developed to quantify it. However, the traditional stability margin techniques based on linear control theory can not be applied to control systems upon which a representative non-linear control method, such as NNs, has been applied. This paper presents an alternative stability margin technique for NNs applied to control systems based on the system responses to an inserted gain multiplier or time delay element.

• Wind and Structures
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• v.12 no.4
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• pp.285-298
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• 2009

For long-span suspension bridges with their intrinsic limit in flutter, some counter measures, for example, central stabilizers, should be adopted to improve aerodynamic stability to meet with the appropriate wind resistance requirements. The present paper introduces aerodynamic stabilization for long-span suspension bridges with box girders by using central stabilizers based on Xihoumen Bridge with the main span of 1650 m. The aerodynamic stabilization study covers experimental investigation of sectional model testing, comprehensive evaluation of three central stabilizers and theoretical analysis of stabilizing mechanism related to flutter derivatives, aerodynamic damping and degree participation.

• 제어로봇시스템학회:학술대회논문집
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• 1986.10a
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• pp.256-261
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• 1986

The sensitiveness of the stability characteristics of the aircraft with respect to changes in the stability parameters as predicted by the linear analysis is applied for the two aerodynamic models proposed. The results give the detailed information for an aircraft dynamic behavior especially at complicated flight envelope.