• Wind and Structures
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• 제27권5호
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• pp.293-309
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• 2018

The prediction of multimode flutter relies, to a larger extent than bimodal flutter, on accurate modeling of the self-excited forces since it is challenging to perform experimental validation by using aeroelastic tests for a multimode case. This paper sheds some light on the accuracy of predicted self-excited forces by comparing numerical predictions of self-excited forces with measured forces from wind tunnel tests considering the flutter vibration mode. The critical velocity and the corresponding flutter vibration mode of the Hardanger Bridge are first determined using the classical multimode approach. Then, a section model of the bridge is forced to undergo a motion corresponding to the flutter vibration mode at selected points along the bridge, during which the forces that act upon it are measured. The measured self-excited forces are compared with numerical predictions to assess the uncertainty involved in the modeling. The self-excited lift and pitching moment are captured in an excellent manner by the aerodynamic derivatives. The self-excited drag force is, on the other hand, not well represented since second-order effects dominate. However, the self-excited drag force is very small for the cross-section considered, making its influence on the critical velocity marginal. The self-excited drag force can, however, be of higher importance for other cross-sections.

• Journal of Advanced Marine Engineering and Technology
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• 제28권7호
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• pp.1111-1122
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• 2004

An idea to suppress the self-excited combustion oscillation was applied to the flames. The characteristics of unsteady combustion were examined and the unsteady combustion was driven by forced pulsating mixture supply that can modulate its amplitude and frequency. The self-excited combustion oscillation having weaker flow velocity fluctuation intensity than that of the forced pulsating supply can be suppressed by this method. The effects of the forced pulsation amplitude and frequency on controlling self-excited combustion oscillations were also investigated comparing with the steady mixture supply. The unsteady combustion used in this experiment plays an important role in controlling self-excited combustion oscillation. Symptoms of self-excited combustion oscillation were also studied in order to predict the onset of combustion oscillation before it proceeded to a catastrophic failure For the purpose, the unique measures to observe the onset of self-excited combustion oscillations based on the careful statistics of fluctuating properties in flames, such as pressure or emission of OH radicals, have been proposed.

• Journal of Mechanical Science and Technology
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• 제17권11호
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• pp.1820-1831
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• 2003

Characteristics of self-excited combustion oscillation are experimentally studied using confined premixed flames stabilized by a rearward-facing step. A new idea to suppress combustion oscillation was applied to the flames. The characteristics of unsteady combustion were examined, which is driven by forced pulsating mixture supply that can modulate its amplitude and frequency. The self-excited combustion oscillation having weaker flow velocity fluctuation intensity than that of the forced pulsating supply can be suppressed by the method. The effects of the forced pulsation amplitude and frequency on controlling self-excited combustion oscillations were also investigated comparing with the steady mixture supply. The unsteady combustion used in this experiment plays an important role in controlling self-excited combustion oscillations, and it also exhibits desirable performances, from a practical point of view, such as high combustion load and reduced pollutant emissions of nitric oxide.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2007년도 춘계학술대회논문집
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• pp.103-108
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• 2007

This paper presents a friction model to realize self-excited vibration of multi-body systems. The friction coefficient is modeled with a spline function in most commercial codes. Even if such a function resolves the problem of discontinuity in friction force, it cannot realize self-excited vibration phenomena. Furthermore, as the relative velocity approaches zero, the friction coefficient approaches zero with the conventional model. So, slip occurs when small force is applied to the system. To avoid these problems a new friction model is proposed in this study. With the new friction model, the self-excited vibration can be realized since the friction coefficient changes with the relative velocity. Furthermore, the slip phenomena could be reduced significantly with the proposed model.

• 한국소음진동공학회논문집
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• 제17권6호
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• pp.524-530
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• 2007

This paper presents a friction model to realize self-excited vibration of multi-body systems. The friction coefficient is modeled with a spline function in most commercial codes. Even if such a function resolves the problem of discontinuity in friction force, it cannot realize self-excited vibration phenomena. Furthermore, as the relative velocity approaches zero, the friction coefficient approaches zero with the conventional model. So, slip occurs when small force is applied to the system. To avoid these problems a new friction model is proposed in this study. With the new friction model, the self-excited vibration can be realized since the friction coefficient changes with the relative velocity. Furthermore, the slip phenomena could be reduced significantly with the proposed model.

• 한국정밀공학회지
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• 제18권2호
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• pp.119-125
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• 2001

This paper presents an experimental study on active control of self-excited vibration for a high speed turbomachinery. In order to suppress the self-excited vibration, it is necessary to actively control the air film pressure or the air film thickness. In this study, active pads are used to control the air film thickness. Active pads are supported by pivots containing piezoelectric actuators and their radial position can be actively controlled by applying voltage to the actuators. The transfer characteristics from actuator inputs to shaft vibration outputs are experimentally investigated. In a tilting-pad gas bearing (TPGB), a shaft is supported by the pressurized air film. Four gap sensors were used to measure the vibration of the shaft and PID was used in the feedback control of the shaft vibration. The experimental results show that the self-excited vibration of the rotor can be effectively suppressed if the PID controller gains are properly chosen. As a result we find that the feedback control is effective for suppressing the self-excited vibration of a rotor system using stack-type PZT actuators.

• International Journal of Fluid Machinery and Systems
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• 제6권4호
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• pp.206-212
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• 2013

Self-excited pulse jet is a specific nozzle with a closed chamber which can change a continuous jet into a pulse one. Energy of the pulse jet can be output not only unevenly but also with multifrequency. With the peak pressure of pulse jet, the hitting power would be 2~2.5 times higher than that of continuous jet. In order to reveal the correlation between the self-excited pulse frequency and nozzle diameter ratio, nozzle spacing and operating pressure, the model of 3D unsteady cavitation model has been used. We found that with the same nozzle structure parameters and the different operating pressure, the self-excited frequency and the width of peak crest are different, but the wave profiles are similar. With FFT, we also found that the less bandwidth of amplitude in low frequency range will lead to the wider wave crest of outlet velocity in its time domain, and the larger force of the strike will be gained. By studying the St of self-excite nozzle, not only the frequency of a certain nozzle can be predicted, but also a nozzle structure with a certain frequency can be designed.

• Journal of Mechanical Science and Technology
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• 제18권7호
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• pp.1246-1257
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• 2004

The objective of this paper is to characterize dynamic pressure traces measured at self-excited combustion instabilities occurring in two combustion systems of different hardware. One system is a model lean premixed gas turbine combustor and the other a fullscale bipropellant liquid rocket thrust chamber. It is commonly observed in both systems that low frequency waves at around 300㎐ are first excited at the onset of combustion instabilities and after a short duration, the instability mode becomes coupled to the resonant acoustic modes of the combustion chamber, the first longitudinal mode for the lean premixed combustor and the first tangential mode for the rocket thrust chamber. Low frequency waves seem to get excited at first since flame shows the higher heat release response on the lower frequency perturbations with the smaller phase differences between heat release and pressure fluctuations. Nonlinear time series analysis of pressure traces reveals that even stable combustion might have chaotic behavior with the positive maximum Lyapunov exponent. Also, pressure fluctuations under combustion instabilities reach a limit cycle or quasi-periodic oscillations at the very similar run conditions, which manifest that a self-excited high frequency instability has strong nonlinear characteristics.

• International Journal of Automotive Technology
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• 제7권5호
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• pp.535-545
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• 2006

A nonlinear analysis of torsional self-excited vibration in the driveline system for wheeled towing tractors was presented, with a 2-DOF mathematical model. The vibration system was described as a second-order ordinary differential equation. An analytical approach was proposed to the solution of the second-order ODE. The mathematical neighborhood concept was used to construct the interior boundary and the exterior boundary. The ODE was proved to have a limit cycle by using $Poincar\'{e}-Bendixson$ Annulus Theorem when two inequalities were satisfied. Because the two inequalities are easily satisfied, the self-excited vibration is inevitable and even the initial slip rate is little. However, the amplitude will be almost zero when the third inequality is satisfied. Only in a few working modes of the towing tractor the third inequality is not satisfied. It is shown by experiments that the torsional self-excited vibration in the driveline of the vehicle is obvious.

• 한국초전도ㆍ저온공학회논문지
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• 제20권4호
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• pp.55-59
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• 2018

Currently, the development of industry makes needs larger electric supply. Providers must consider the efficiency about losses and reliability of the system. In this case, DC power system can save electrical energy; long-distance transmission line losses. Relevance to switch technology with a voltage-source converter (VSC) in AC-DC conversion system have been researched. But, protection device of DC-link against fault current is still needed to study much. VSC DC power system is vulnerable to DC-cable short-circuit and ground faults, because DC-link has a huge size of capacitor filter which releases extremely large current during DC faults. Furthermore, DC has a fatal flaw that current zero crossing is nonexistence. To interrupt the DC, several methods which make a zero crossing is used; parallel connecting self-excited series LC circuit with main switch, LC circuit with power electronic device called hybrid DC circuit breaker. Meanwhile, self-excited oscillator needs a huge size capacitor that produces big oscillation current which makes zero crossing. This capacitor has a quite effective on the price of DCCB. In this paper, hybrid self-excited type superconducting DCCB which are using AC circuit breaker system is studied by simulation tool PSCAD/EMTDC.