• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.26 no.1
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• pp.13-19
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• 2016

The flow-excited Helmholtz resonance phenomenon was investigated numerically using Reynolds averaged Navier-Stokes approach. The fundamental cause of the Helmholtz resonance phenomenon is known as shedding of a single discrete vortex from orifice edge that travels during one period of the oscillation. In this study, serrated deflector, which is biomimetic design of the owl's feather, is used to split a single vortex into small vortices. Rectangular deflector and serrated deflector are compared with numerical results of pressure and streamline inside the cavity. Consequently, the serration breaks the shedding period of vortex core and eliminates the resonance. Also, it changes the flow pattern in according to the location of different serration height. By making inflows and outflows occur simultaneously in spanwise direction in the cavity, the period of Helmholtz resonance disappears. Comparing between rectangular deflector and serrated deflector, the serrated deflector can deal with the Helmholtz resonance more effectively.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2003.11a
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• pp.446-451
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• 2003

This paper presents some examples of active control of flow-induced vibration using piezoelectric actuators. The flutter phenomenon, which is the dynamic instability of structure due to mutual interaction among inertia, stiffness, and aerodynamic forces, may cause catastrophic structural failure, and therefore the active flutter suppression is one of the main objectives of the aeroelastic control. Active flutter control has been numerically and experimentally studied for swept-back lifting surfaces using piezoelectric actuation. A finite element method, a panel aerodynamic method, and the minimum state space realization are involved in the development of the governing equation, which is efficiently used for the analysis of the system and design of control laws with modern control framework. The active control suppressed flow-induced vibrations and extended the flutter speed around by 10%. Another representative flow-induced vibration phenomenon is the oscillation of blunt bodies due to the vortex shedding. In general, it is quite difficult to set up the numerical model because of the strong non-linearity of the vortex shedding structure. Therefore, we applied adaptive positive position feedback controller, which requires no pre-determined model of the plant, and successfully suppressed the flow-induced vibration.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2010.05a
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• pp.411-414
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• 2010

In supersonic propulsion system, the inlet buzz phenomenon in the subcritical operation arises large pressure oscillation, combustion instability, and thrust loss, etc. Inlet Buzz phenomenon and the spray injection/combustion are figured out by the unified unsteady numerical analysis. TAB(Taylor Analogy Breakup) model was applied. Acoustic mode of the entire engine was investigated by detail analysis of pressure fluctuation at each location of the engine.

• Journal of ILASS-Korea
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• v.27 no.3
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• pp.134-143
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• 2022

When a recess is applied to a swirl coaxial injector that uses liquid and gas propellants, a self-pulsation phenomenon in which the spray oscillates at regular intervals may occur. The phenomenon is caused by the interaction between the liquid and gas propellants inside the injector recess region. The propellants' kinetic energies are expected to affect significantly the spray oscillation. Therefore, cold-flow tests using helium as a gas-simulating propellant were conducted and compared with the results of the previous study using air. Dynamic pressure was measured in the injector manifold and frequency characteristics were investigated through the fast Fourier transform analysis. In the experimental environment, the helium density was about seven times lower than the air density. Accordingly, the intensity of pressure fluctuations was confirmed to be greater when air was used. At the same kinetic energy condition, the perturbation frequency was almost identical in the low flow rate conditions. However, as the flow rate increased, the self-pulsation frequency was higher when helium was used.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.64 no.11
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• pp.1543-1550
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• 2015

Ferroresonance is a non-linear vibrational phenomenon that is generated by the electrical interaction of the inductance component with the capacitor component of a certain capacitance as the device of the inductance component such as a transformer is saturated due to the degradation, the waveform distortion of current and voltage, and the oscillation of overcurrent and overvoltage in a system. Recently, ferroresonance was generated from the waveform distortion of current and voltage, or the overvoltage or undervoltage phenomenon caused by the nature of an electrical power system and design technology of the transformer in the three phase transformer system. Hence, in general, ferroresonance analyzed by converting to the LC equivalent circuit. However, in general, the aforementioned analytical method only applies to the resonance phenomenon that is generated by the interaction of the capacitance of bussbar and grounding, and switching as the capacitor component with PT and the transformer as the inductance component in a system. Subsequently, the condition where ferroresonance was generated since overvoltage was supplied as line voltage to the phase voltage and thus the iron core is saturated due to the interconnection between grounded and ungrounded systems could not be analyzed when single phase PT was connected in a ${\Delta}$/Y connection system. In this study, voltage swell in the configuration of grounded circuit of a step-up transformer with the ${\Delta}-{\Delta}$ connection linked to PT for control power and the ferroresonance generated by overvoltage when the line voltage of the ${\Delta}-{\Delta}$ connection was connected to the phase voltage of the grounded Y-Y connection were analyzed using PSCAD / EMTDC through the failure case of the transformer caused by ferroresonance in the system with the ${\Delta}-{\Delta}$/Y-Y connection, and subsequently, the preventive measure of ferroresonance was proposed.

• 한국연소학회:학술대회논문집
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• 2006.10a
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• pp.73-78
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• 2006

The lifted oscillating flame has been studied using experiments of inverse diffusion flames that the air jet injected into a methane background. To find out the characteristics of inverse diffusion flames, fundamentally flame stabilized diagram is investigated with various air and fuel jet velocities. It has five regions - flame extinction, stable attached flame, anchored flame, liftoff flame and blow off region. In inverse diffusion flame, lifted flames were observed near the blow off region. As long as flames lift off, flames oscillate by periods. In this oscillating lifted flame region, the frequency of 1 and under were observed in various air and methane jet velocities. Characteristics of lifted flames are also examined by using the ICCD direct image. And intensity of flame chemiluminescence is very different in rising and falling period from photographs. For the present, it is predicted that the changes of flame structure are related with flame oscillation, but more experiments will be needed to make clear the phenomenon.

• Journal of Industrial Technology
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• v.21 no.B
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• pp.125-132
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• 2001

Heat dissipation technology using flow resonant phenomenon is a kind of new concept in heat transfer area. A vibration exciter is needed to generate air turbulence which has the natural shedding frequency of heat system. A mechanical vibrating device for the air flow oscillation is introduced, which is driven by a moving coil actuator. An analytical dynamic model for this mechanical vibration exciter is presented and its' validity is verified by the comparison with experimental data. Values of some unknown system parameters in the analytic model are estimated through the system identification approach. Based on this mathematical model, the vibration exciter using strain displacement estimator is developed. And in the experiment, the feedback control is used. During the experimental verification phase, it turns out the high modal resonant characteristics of vibrating plate are the major barrier against obtaining a high bandwidth vibration exciter.

• Steel and Composite Structures
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• v.18 no.4
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• pp.909-924
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• 2015

This paper investigates the vibration phenomenon of a nanobeam subjected to a time-dependent heat flux. Material properties of the nanobeam are assumed to be graded in the thickness direction according to a novel exponential distribution law in terms of the volume fractions of the metal and ceramic constituents. The upper surface of the functionally graded (FG) nanobeam is pure ceramic whereas the lower surface is pure metal. A nonlocal generalized thermoelasticity theory with dual-phase-lag (DPL) model is used to solve this problem. The theories of coupled thermoelasticity, generalized thermoelasticity with one relaxation time, and without energy dissipation can extracted as limited and special cases of the present model. An analytical technique based on Laplace transform is used to calculate the variation of deflection and temperature. The inverse of Laplace transforms are computed numerically using Fourier expansion techniques. The effects of the phase-lags (PLs), nonlocal parameter and the angular frequency of oscillation of the heat flux on the lateral vibration, the temperature, and the axial displacement of the nanobeam are studied.

• Journal of Mechanical Science and Technology
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• v.18 no.8
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• pp.1461-1469
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• 2004

Acoustic characteristics in an industrial gas-turbine combustor are numerically investigated by a linear acoustic analysis. Spatially non-homogeneous temperature field in the combustor is considered in the numerical calculation and the characteristics are analyzed in view of acoustic instability. Acoustic analyses are conducted in the combustors without and with acoustic resonator, which is one of the acoustic-damping devices or combustion stabilization devices. It has been reported that severe pressure fluctuation frequently occurs in the adopted combustor, and the measured signal of pressure oscillation is compared with the acoustic-pressure response from the numerical calculation. The numerical results are in good agreement with the measurement data. In this regard. the phenomenon of pressure fluctuation in the combustor could be caused by acoustic instability. From the numerical results for the combustor with present acoustic resonators installed, the acoustic effects of the resonators are analyzed in the viewpoints of both the frequency tuning and the damping capacity. It is found that the resonators with present specifications are not optimized and thus, the improved specification or design is required.

• Journal of the Korean Society for Precision Engineering
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• v.18 no.6
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• pp.141-147
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• 2001

A heat dissipation technology using flow resonant phenomenon is a kind of new concept in heat transfer area. A vibration exciter is needed to generate an air turbulence which has the natural shedding frequency of a heat system. A mechanical vibrating device for the air flow oscillation is introduced, which is driven by a moving coil actuator. An analytical dynamic model for this mechanical vibration exciter is presented and its validity is verified by the comparison with experimental data. Values of some unko주 system parameters in the analytic model are estimated through the system identification approach. based on this mathematical model, a high bandwidth vibration exciter is designed using feedback control. During the experimental verification phased, it turns out the high frequency modal resonant characteristics of vibrating plate are the major barrier against obtaining a high bandwidth vibration exciter.