• International Journal of Aeronautical and Space Sciences
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• v.12 no.4
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• pp.305-317
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• 2011

This paper presents an overview on flutter boundary prediction in tests which is principally based on a system stability measure, named Jury's stability criterion, defined in the discrete-time domain, accompanied with the use of autoregressive moving-average (AR-MA) representation of a sampled sequence of wing responses excited by continuous air turbulences. Stability parameters applicable to two-, three- and multi-mode systems, that is, the flutter margin for discrete-time systems derived from Jury's criterion are also described. Actual applications of these measures to flutter tests performed in subsonic, transonic and supersonic wind tunnels, not only stationary flutter tests but also a nonstationary one in which the dynamic pressure increased in a fixed rate, are presented. An extension of the concept of nonstationary process approach to an analysis of flutter prediction of a morphing wing for which the instability takes place during the process of structural morphing will also be mentioned. Another extension of analytical approach to a multi-mode aeroelastic system is presented, too. Comparisons between the prediction based on the digital techniques mentioned above and the traditional damping method are given. A future possible application of the system stability approach to flight test will be finally discussed.

• Journal of the Korean Society for Aviation and Aeronautics
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• v.21 no.4
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• pp.41-46
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• 2013

This paper deals with an experimental technique for the measurement of dynamic stability derivatives in the roll motion of aircraft. Experimental aquisition method for aircraft's dynamic stability derivatives is conducted on the oscillation condition of aircraft model in the subsonic wind tunnel. The oscillation of aircraft model was forced by the oscillation apparatus. The forced oscillation technique is the method getting data from the internal balance inserted into the aircraft model during oscillating it. Dynamic stability derivatives of rolling motion were calculated by data reduction from the measurements of rolling moment, frequency and amplitude of aircraft model due to forced oscillation under wind conditions. Results of experiment is obtained similar one with those of roll dynamic stability derivatives measured in other institutes.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.30 no.1
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• pp.9-19
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• 2002

Accuracy of AUSM-type schemes is closely related to a speed of in a cell-interface. Effect to accuracy by a speed of sound invastigated in the region of subsonic, transonic, and supersonic flows repectively. The advantage of the speed of sound in AUSMPW+ are summerized as the improvement of accuracy in capturing an oblique shock and the removal of an expansion shock to satisfy the entropy condition. They are proven by mathmatics and numerical result. Moreover AUSMPW+ is extended to a real gas flow and the speed of sound for equilibrium and nonequilibrium gas which could give exact soultions in an oblique shock is proposed.

• Journal of ILASS-Korea
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• v.7 no.3
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• pp.12-17
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• 2002

In this paper, spray and combustion characteristics of a liquid-fueled ramjet engine were experimentally investigated. The spray penetrations were measured to clarify the spray characteristics of a liguid jet injected transversely into the subsonic vitiated airstream, which is maintained a high velocity and temperature. The spray penetrations are increased with decreasing airstream velocity, increasing airstream temperature, and increasing air-fuel momentum ratio. To compensate our results of penetrations, the new experimental equation were modified from Inamura's equation. In the case of insufficient penetration, the combustion phenomenon in ram-combustor were unstable. Therefore, the temperature distribution was slanted to the low wall of the ram-combustor. These trends gradually disappeared as the length and air temperature of the combustor became longer. Combustion efficiency increased when the length of the combustor was long and the air temperature was high. Especially, stable flame region is enlarged when the length of the combustor was long and the air temperature was high. Type Abstract here. Type Abstract here.

• 유체기계공업학회:학술대회논문집
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• 2004.12a
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• pp.419-426
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• 2004

A three-dimensional computation was conducted to understand effects of the inlet boundary layer thickness on the loss mechanism in a low-speed axial compressor operating at the design condition(${\phi}=85\%$) and near stall condition(${\phi}=65\%$). At the design condition, the flow phenomena such as the tip leakage flow and hub comer stall are similar independent of the inlet boundary layer thickness. However, when the axial compressor is operating at the near stall condition, the large separation on the suction surface near the casing is induced by the tip leakage flow and the boundary layer on the blade for thin inlet boundary layer but the hub corner stall is enlarged for thick inlet boundary layer. These differences of internal flows induced by change of the boundary layer thickness on the casing and hub enable loss distributions of total pressure to be altered. When the axial compressor has thin inlet boundary layer, the total pressure loss is increased at regions near both casing and tip but decreased in the core flow region. In order to analyze effects of inlet boundary layer thickness on total loss in detail, using Denton's loss models, total loss is scrutinized through three major loss categories in a subsonic axial compressor such as profile loss, tip leakage loss and endwall loss.

• International Journal of Naval Architecture and Ocean Engineering
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• v.3 no.4
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• pp.242-253
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• 2011

Skin frictional drag reduction efficiency of "stiff" compliant coating was investigated in a wind tunnel experiment. Flat plate compliant coating inserts were installed in a wind tunnel and the measurements of skin frictional drag and velocity field were carried out. The compliant coatings with varying viscoelastic properties had been prepared using different composition. In order to optimize the coating thickness, the most important design parameter, the dynamic viscoelastic properties had been determined experimentally. The aging of the materials (variation of their properties) during half a year was documented as well. A design procedure proposed by Kulik et al. (2008) was applied to get an optimal value for the coating thickness. Along with the drag measurement using the strain balance, velocity and pressure were measured for different coatings. The compliant coatings with the thickness h = 7mm achieved 4~5% drag reduction within a velocity range 30~40 m/s. The drag reduction mechanism of the attenuation of turbulence velocity fluctuations due to the compliant coating was demonstrated. It is envisioned that larger drag reduction effect is obtainable at higher flow velocities for high speed trains and subsonic aircrafts.

• Proceedings of the KSR Conference
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• 2008.06a
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• pp.2224-2229
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• 2008

High-Speed train has been developed and it becomes faster and environmental friendly. As trains run faster, Noise of trains is generated mainly by aerodynamic disturbance. Pantograph, both ends of trains, and gaps of coaches which are thought to be aerodynamic noise's factors are primarily studied. Pantograph is a similarly shaped metal framework on the roof of an electric high speed train, transmitting current from an overhead electric catenary wire. Panhead which contacts electric wires directly looks like a bluff strut, goes through flows, is sensitive to external disturbances and is one of the most important factors which decide whole vehicles' driving ability. In this study, aerodynamically robust optimized pantograph panhead shape is designed and then evaluated through subsonic wind tunnel test. To compare these with existing panhead rectangular shapes or circular cylinder shapes, By visualizing strong vortex flow patterns which are main noise sources, characteristics are compared and analyzed

• Journal of Power System Engineering
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• v.22 no.1
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• pp.11-17
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• 2018

The steady-state, incompressible and three-dimensional numerical analysis was carried out to evaluate the internal flow fields characteristics of wind tunnel contractions made by Morel's curve equations. The turbulence model used in this study is a realizable ${\kappa}-{\varepsilon}$ well known to be excellent for predicting the performance of the flow separation and recirculation flow as well as the boundary layer with rotation and strong back pressure gradient. As a results, when the flow passes through the interior space of the analytical models, the flow resistance at the inlet of the plenum chamber is the largest at $Z_m=300$, 400 mm, but the smallest at $Z_m=700mm$. The maximum turbulence intensity in the test section is about 2.5% when calculated by the homogeneous flow, so it is improved by about 75% compared to the 10% turbulence intensity at the inlet of the plenum chamber due to the contraction.

• Journal of The Korean Society of Agricultural Engineers
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• v.55 no.6
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• pp.31-35
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• 2013

This study was carried out to present the pressure drop for various wind speeds through nine types of screens used in horticultural facilities. The screens have been widely used to prevent harmful insects from being entered into agricultural facilities, to reduce strong wind and to shade a light as well. Whatever the usage of the screens was, it was necessary to have good knowledge of how much the screen caused a pressure drop for wind speeds when analyzing both the inner thermal-flow distribution in the facility and the effect of reducing wind speed by using CFD. Furthermore, as for wind screens, the pressure drop for wind speeds was needed as a design load in evaluating the structural stability of the structures supporting the screens. Therefore, the pressure drop through the screens for wind speeds of 5~30 $m{\cdot}s^{-1}$ at about 5 $m{\cdot}s^{-1}$ interval and inflow angles of $0{\sim}45^{\circ}$ at an interval of $15^{\circ}$ was respectively measured in a subsonic wind tunnel. The relation of the pressure drop for various screens was well fitted as a secondorder polynomial expression.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.1
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• pp.46.2-46.2
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• 2013

The neutral component of the interstellar medium (ISM) is segregated into the cold neutral medium (CNM) and warm neutral medium (WNM) as a result of thermal instability. It was found that the CNM--WNM evaporation interface, across which the CNM undergoes thermal expansion, is linearly unstable to corrugational disturbances, in complete analogy with the Darrieus-Landau instability (DLI) in terrestrial flames. To explore dynamical consequences of the DLI in the ISM, we perform a linear stability analysis of the DLI including the effect of thermal conduction as well as nonlinear hydrodynamic simulations. We find that the DLI is suppressed at short length scales via heat transport. The linear growth time of the fastest growing mode is proportional to the square of the evaporation flow speed of the CNM relative to the interface and is typically >10 Myr. In the nonlinear stage, perturbations grow into cusp-like structure protruding toward the WNM, and soon reach a steady state where the evaporation rate is increased by a factor of 2 compared to the initial state. We demonstrate that the amplitude of the interface distortion and enhancement in evaporation rate are determined primarily by the density ratio between the CNM and WNM. Given quite a long growth time and highly subsonic velocities at saturation, the DLI is unlikely to play an important role in the ISM dynamics.