• 대한기계학회논문집A
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• 제26권9호
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• pp.1874-1881
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• 2002

Heat dissipation technology using the flow resonant phenomenon is a kind of a new concept in the heat transfer area. A vibration exciter is needed to enhance air flow mixing which has the natural shedding frequency of thermal system. A mechanical vibrating device for the air flow oscillation is introduced, which is driven by a moving coil actuator with a displacement estimator using strain gage. An analytical dynamic model for this mechanical vibration exciter is presented and its validity is checked 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. During the experimental verification phase, it turns out the high modal resonant characteristics of a vibrating plate are a major barrier against obtaining a high bandwidth vibration exciter.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2011년 춘계학술대회논문집
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• pp.311-317
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• 2011

The present paper deals with the continuous works of extending the multi-dimensional limiting process (MLP) for compressible flows, which has been quite successful in finite volume methods, into discontinuous Galerkin (DG) methods. From the series of the previous, it was observed that the MLP shows several superior characteristics, such as an efficient controlling of multi-dimensional oscillations and accurate capturing of both discontinuous and continuous flow features. Mathematically, fundamental mechanism of oscillation-control in multiple dimensions has been established by satisfaction of the maximum principle. The MLP limiting strategy is extended into DG framework, which takes advantage of higher-order reconstruction within compact stencil, to capture detailed flow structures very accurately. At the present, it is observed that the proposed approach yields outstanding performances in resolving non-compressive as well as compressive flaw features. In the presentation, further numerical analyses and results are going to be presented to validate that the newly developed DG-MLP methods provide quite desirable performances in controlling numerical oscillations as well as capturing key flow features.

• 대한기계학회논문집B
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• 제28권5호
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• pp.580-586
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• 2004

The ray effects of finite volume method (FVM) or discrete ordinate method (DOM) are known to show a non-physical oscillation in solution of radiative heat transfer on a boundary. This wiggling behavior is caused by the finite discretization of the continuous control angle. This article proposes a combined procedure of the Monte-Carlo and finite-volume method (CMCFVM) for solving radiative heat transfer in absorbing, emitting, and an-isotropically scattering medium with an isolated boundary heat source. To tackle the problem, which is especially pronounced in a medium with an isolated heat source, the CMCFVM is suggested here and successfully applied to a two-dimensional circular geometry.

• 한국항공우주학회지
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• 제34권7호
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• pp.1-11
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• 2006

본 논문에서는 불연속면이 포함된 다차원 유동에서 흔히 발생하는 수치적 진동현상을 막기 위해 기존의 TVD 제한자를 분석함으로써 새로운 형태의 다차원 제한 함수를 유도하였다. MLP 기법은 유도된 다차원 제한 함수를 기반으로 하며, 다차원 불연속면에서의 수치 진동을 효과적으로 제거하고 동시에 3차 이상의 공간 정확도 내삽기법과 함께 사용할 수 있다는 장점을 갖는다. 또한, 정상 유동의 경우 수치 진동이 제거됨으로써 수렴성이 향상됨을 확인할 수 있었고, 실제 코드에 적용하는 방법도 간단하다. MLP 기법을 적용함으로써 불연속 유동 뿐 만 아니라 연속 유동에서도 정확성, 효율성, 강건성 면에서 향상된 결과를 얻을 수 있음을 여러 가지 수치 실험을 통하여 확인하였다.

• Journal of electromagnetic engineering and science
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• 제15권2호
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• pp.82-88
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• 2015

The magnetron, a vacuum tube, is currently the usual high-power microwave power source used for microwave heating. However, the oscillating frequency and output power are unstable and noisy due to the low quality of the high-voltage power supply and low Q of the oscillation circuit. A heating system with enhanced reliability and the capability for control of chemical reactions is desired, because microwave absorption efficiency differs greatly depending on the object being heated. Recent studies on microwave high-efficiency power amplifiers have used harmonic processing techniques, such as class-F and inverse class-F. The present study describes a high-efficiency 100 W GaN-HEMT amplifier that uses a harmonic processing technique that shapes the current and voltage waveforms to improve efficiency. The fabricated GaN power amplifier obtained an output power of 50.4 dBm, a drain efficiency of 72.9%, and a power added efficiency (PAE) of 64.0% at 2.45 GHz for continuous wave operation. A prototype microwave heating system was also developed using this GaN power amplifier. Microwaves totaling 400 W are fed from patch antennas mounted on the top and bottom of the microwave chamber. Preliminary heating experiments with this system have just been initiated.

• 한국추진공학회지
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• 제7권3호
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• pp.22-29
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• 2003

KSR-III 추진기관 공급계는 각종 배관류, 밸브류 및 이를 제어하는 제어기로 구성되어 있으며, 엔진으로 유입되는 추진제의 제어를 담당하고 있다. 이러한 공급계 시스템의 특성을 검증하고, 구성부품의 성능을 확인하기 위하여 PTA-I 수류시험을 실시하였다. PTA-I 시험설비는 엔진과 헬륨가압탱크를 제외한 공급계 전체와 지원설비로 이루어져 있다. PTA-I에서는 밸브, 제어기와 같은 단품에 대한 시스템 시험, 배관특성시험, 추진제 유량조절 시험, 배관부 압력 변동폭 및 주파수 특성 시험, 레귤레이터 성능 시험등을 수행하였다. PTA-I 시험을 통하여 구성단품에 대한 문제점을 발견하고 이를 수정보완 하였으며, 공급계 시스템의 설계 데이터와 시험데이터 비교를 통한 설계 검증을 완료하였다. PTA-I 시험의 결과는 PTA-II 및 연소시험을 수행하기 위한 시스템 구성에 적용하였다.

• 한국인쇄학회지
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• 제23권2호
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• pp.117-128
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• 2005

Printing inks are basically dispersions of solid pigment particles in a vehicle. Pigment flocculation and/or colloidal aggregates created by thixotrope additives form a three- dimensional network in the inks. This structure complicates the flow behaviour of inks. However, if the internal structure is formed under control, the printing process will benefit from it because the ink must satify rheological requirements over a very wide range of shear conditions. The presence of internal structure results in the following prominent non-Newtonian rheological properties: viscoelasticity, yield stress, shear thinning and thixotropy. If the components of printing inks were changed, the rheological characteristics such as viscosity, yield stress, viscoelasticity and tack value were considerably varied. Thus, in this paper, the effects of changing the content of rosin modified phenolic resin on rheological properties of the vehicle will be studied. For that, the rheological properties were found by flow, yield stress, creep and oscillation measurements using Bohlin C-VOR Rotational Rheometer. And Emulsion rheology and its microstructure will be investigated.

• Smart Structures and Systems
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• 제7권1호
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• pp.41-57
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• 2011

A deeper understanding of the effectiveness of adopting devices mounting shape memory alloy (SMA) elements in applications targeted to the mitigation of vibrations is pursued via an experimental approach. During a seismic event, less than 1000 loading-unloading cycles of the alloy are required to mitigate the earthquake effects. However, the aging effects during the time of inactivity prior to the oscillations (several decades characterized by the yearly summer-winter temperature wave) should be considered in order to avoid and/or minimize them. In this paper, the results obtained by carrying out, in different laboratories, fatigue tests on SMA specimens are compared and discussed. Furthermore, the effects of seismic events on a steel structure, with and without SMA dampers, are numerically simulated using ANSYS. Under an earthquake excitation, the SMA devices halve the oscillation amplitudes and show re-centering properties. To confirm this result, an experimental campaign is conducted by actually installing the proposed devices on a physical model of the structure and by evaluating their performance under different excitations induced by an actuator.

• Journal of Electrical Engineering and Technology
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• 제13권5호
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• pp.1986-1995
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• 2018

In this paper, a novel neutral-point voltage balancing scheme for NPC three-level inverters using carrier-based sinusoidal pulse width modulation (SPWM) method is developed. The new modulation approach, based on the obtained expressions of zero sequence voltage in all six sectors, can significantly suppress the low-frequency voltage oscillation in the neutral point at high modulation index and achieve a fast voltage-balancing dynamic performance. The implementation of the proposed method is very simple. Another attractive feature is that the scheme can stably control any voltage difference between the two dc-link capacitors within a certain range without using any extra hardware. Furthermore, the presented scheme is also applicable to the single-phase NPC three-level inverter. It can maintain the neutral-point voltage balance at full modulation index and improve the voltage-balancing dynamic performance of the single-phase NPC three-level inverter. The performance of the proposed strategy and its benefits over other previous techniques are verified experimentally.

• Journal of Welding and Joining
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• 제34권4호
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• pp.48-54
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• 2016

The use of Zn coated steel has increased in the automotive industry due to its excellent corrosion resistance. Conventionally the BIW(body-in-white) structure and the hang-on parts have been made of Zn coated steel and more recently Zn coated steel began to be applied in the chassis parts. During gas metal arc (GMA) welding of the chassis part, lap fillet joint used to be adopted but spatter generation and porosities are most important concerns. In the industrial applications, an intentional joint gap was made to avoid the weld defects but it is not easy to control the size of joint gap. In this research, gas tungsten arc (GTA) is combined with GMA welding where GTA precedes GMA. As pulsed arc was adopted as GMA, GTA was oscillated along the longitudinal direction by pulsing GMA, but the arc oscillation did not disturb the molten droplet transfer of GMA welding. By increasing the distance between GTA and GMA, the length of weld pool increased and porosity could be reduced. Moreover porosity in the welds was fully removed when the distance between two arcs was 15 mm.