• 대한수학회보
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• 제46권5호
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• pp.931-940
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• 2009

Given a system of smooth 1-forms $\theta$ = ($\theta^1$,...,$\theta^s$) on a smooth manifold $M^m$, we give a necessary and sufficient condition for M to be foliated by integral manifolds of dimension n, n $\leq$ p := m - s, and construct an integrable supersystem ($\theta,\eta$) by finding additional 1-forms $\eta$ = ($\eta^1$,...,$\eta^{p-n}$). We also give a necessary and sufficient condition for M to be foliated by reduced submanifolds of dimension n, n $\geq$ p, and construct an integrable subsystem ($d\rho^1$,...,$d\rho^{m-n}$) by finding a system of first integrals $\rho=(\rho^1$,...,$\rho^{m-n})$. The special case n = p is the Frobenius theorem on involutivity.

• 대한수학회지
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• 제54권5호
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• pp.1411-1440
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• 2017

We study free actions of finite groups on 3-dimensional nil-manifolds with the first homology ${\mathbb{Z}}^2{\oplus}{\mathbb{Z}}_p$. By the works of Bieberbach and Waldhausen, this classification problem is reduced to classifying all normal nilpotent subgroups of almost Bieberbach groups of finite index, up to affine conjugacy.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2007년도 춘계학술대회논문집
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• pp.283-290
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• 2007

Recently, plastic products in air-intake parts of automotive engines have become very popular due to advantages that include reduced weight, constricted cost, and lower intake air temperature. However, flow-induced noise in air-intake parts becomes a more serious problem for plastic intake-manifolds than for conventional aluminum-made manifolds. This is due to the fact that plastic manifolds transmit more noise owing to their lower material density. Internal aerodynamic noise from an Idle Speed control Actuator (ISA) is qualitatively analyzed by using a scaling law, which is expressed with some flow parameters such as pressure drop, maximum flow velocity, and turbulence kinetic energy. First, basic flow characteristics through ISA passage are identified with the flow predictions obtained by applying Computational Fluid Dynamics techniques. Then, the effects on ISA passage noise of each design factors including the duct turning shape and vane geometries are assessed. Based on these results, the preliminary low noise design for the ISA passage are proposed. The current method for the prediction of internal aerodynamic noise consists of the steady CFD and the scaling laws for the noise prediction. This combination is most cost-effective, compared with other methods, and therefore is believed to be suited for the preliminary design tool in the industrial field.

• 대한기계학회논문집A
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• 제34권7호
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• pp.905-910
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• 2010

일체형 배기다기관은 하이드로 포밍 대량 생산이 가능한 부품으로 국내, 국제적인 환경문제에 따른 배출 규제와 매우 밀접한 관계에 있기 때문에, 국내 완성차 업체에서 큰 관심을 가지고 있다. 일반적인 주조부품과 비교해서 관형 배기다기관은 용접공정 감소에 따른 단순화된 제작 공정 뿐만아니라, 엔진 시동시 가스 분해능이 우수한 특징을 가지고 있다. 본 연구는 부품, 공정, 금형 설계를 포함하는 배기다기관을 개발하는데 목적이 있다. 배기 시스템의 성능은 유동해석, 열전달, 열응력, 내구해석 시뮬레이션을 통해 평가하였다.

• 한국소음진동공학회논문집
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• 제17권8호
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• pp.683-692
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• 2007

Recently, plastic products in air-intake parts of automotive engines have become very popular due to advantages that include reduced weight, constricted cost, and lower intake air temperature. However, flow-induced noise in air-intake parts becomes a more serious problem for plastic intake-manifolds than for conventional aluminum-made manifolds. This is due to the fact that plastic manifolds transmit more noise owing to their lower material density. Internal aerodynamic noise from an idle speed control actuator(ISA) is qualitatively analyzed by using a scaling law, which is expressed with some flow parameters such as pressure drop, maximum flow velocity, and turbulence kinetic energy. First, basic flow characteristics through ISA passage are identified with the flow predictions obtained by applying computational fluid dynamics techniques. Then, the effects on ISA passage noise of each design factors including the duct turning shape and vane geometries are assessed. Based on these results, the preliminary low noise design for the ISA passage are proposed. The current method for the prediction of internal aerodynamic noise consists of the steady CFD and the scaling laws for the noise prediction. This combination is most cost-effective, compared with other methods, and therefore is believed to be suited for the preliminary design tool in the industrial field.

• 한국음향학회:학술대회논문집
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• 한국음향학회 2002년도 하계학술발표대회 논문집 제21권 1호
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• pp.469-474
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• 2002

In recent years, modularization of engine parts has increased the application of plastic products in air intake systems. Plastic intake manifolds provide many advantages including reduced weight, contracted cost, and lower intake air temperatures. These manifolds, however, have some weakness when compared with customary aluminium intake manifolds, in that they have low sound transmission loss because of their lower material density. This low transmission loss of plastic intake manifolds causes several problems related to flow noise, especially when the throttle is opened quickly. The physical processes, responsible for this flow noise, include turbulent fluid motion and relative motion of the throttle to the airflow. The former is generated by high-speed airflow in the splits between the throttle valve and the inner-surface of the throttle body and surge-tank, which can be categorized into the quadrupole source. The latter induces the unsteady force on the flow, which can be classified into the dipole source. In this paper, the mechanism of noise generation from the turbulence is only investigated as a preliminary study. Stochastic noise source synthesis method is adopted for the analysis of turbulence-induced, i.e. quadrupole noise by throttle at quick opening state. The method consists of three procedures. The first step corresponds to the preliminary time-averaged Navier-Stokes computation with a $k-\varepsilon$ turbulence model providing mean flow field characteristics. The second step is the synthesis of time-dependent turbulent velocity field associated with quadrupole noise sources. The final step is devoted to the determination of acoustic source terms associated with turbulent velocity. For the first step, we used market available analysis tools such as STAR-CD, the trade names of fluid analysis tools available on the market. The steady state flows at three open angle of throttle valve, i.e. 20, 35 and 60 degree, are numerically analyzed. Then, time-dependent turbulent velocity fields are produced by using the stochastic model and the flow analysis results. Using this turbulent velocity field, the turbulence-originated noise sources, i.e. the self-noise and shear-noise sources are synthesized. Based on these numerical results, it is found that the origin of the turbulent flow and noise might be attributed to the process of formulation and the interaction of two vortex lines formed in the downstream of the throttle valve. These vortex lines are produced by the non-uniform splits between the throttle valve and inner cylinder surface. Based on the analysis, we present the low-noise design of the inner geometry of throttle body.

• 대한수학회지
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• 제44권5호
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• pp.1079-1092
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• 2007

This paper studies the deformation theory of a holomorphic surjective map from a normal compact complex space X to a compact $K\"{a}hler$ manifold Y. We will show that when the target has non-negative Kodaira dimension, all deformations of surjective holomorphic maps $X{\rightarrow}Y$ come from automorphisms of an unramified covering of Y and the underlying reduced varieties of associated components of Hol(X, Y) are complex tori. Under the additional assumption that Y is projective algebraic, this was proved in [7]. The proof in [7] uses the algebraicity in an essential way and cannot be generalized directly to the $K\"{a}hler$ setting. A new ingredient here is a careful study of the infinitesimal deformation of orbits of an action of a complex torus. This study, combined with the result for the algebraic case, gives the proof for the $K\"{a}hler$ setting.

• Journal of Welding and Joining
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• 제32권2호
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• pp.14-17
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• 2014

Ferritic stainless steels are widely used in the construction industry and in exhaust manifolds due to their low cost and relatively superior stress corrosion cracking resistance and pitting corrosion resistance compared to austenite stainless steels. Ferritic stainless steels are currently welded by various welding process including gas tungsten arc welding (GTAW), electron resistance welding (ERW) and laser beam welding. However, when these stainless steels are welded by fusion welding, some problems occur in the fusion zone (FZ) and heat affected zone (HAZ). First, the ductility of the weld is reduced due to the grain growth in the FZ and HAZ. Second, as its HAZ is frequently sensitized during welding, corrosion resistance deteriorates in this region due to the Cr depletion zone. To prevent these problems, it is recommended that ferritic stainless steels be welded with a low heat input. In this study, recent researches in the view of friction stir welded ferritic stainless steels are briefly reviewed.

• 한국자동차공학회논문집
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• 제21권1호
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• pp.43-50
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• 2013

The stringent emission regulation and future shortage of fossil fuel motivate the research of alternative powertrain. In this study, a system of proton exchange membrane fuel cell has been modeled to analyze the performance of the fuel cell system for automotive application. The model is composed of the fuel cell stack, air compressor, humidifier, and intercooler, and hydrogen supply which are implemented by using the Matlab/Simulink(R). Fuel cell stack model is empirical model but the water transport model is included so that the system performance can be predicted over various humidity conditions. On the other hand, the model of air compressor is composed of motor, static air compressor, and some manifolds so that the motor dynamics and manifold dynamics can be investigated. Since the model is concentrated on the strategic operation of compressor to reduce the power consumption, other balance of components (BOP) are modeled to be static components. Since the air compressor model is empirical model which is based on curve fitting of experiments, the stack model is validated with the commercial software and the experiments. The dynamics of air compressor is investigated over unit change of system load. The results shows that the power consumption of air compressor is about 12% to 25% of stack gross power and dynamic response should be reduced to optimize the system operation.