• International Journal of Naval Architecture and Ocean Engineering
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• 제3권1호
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• pp.53-64
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• 2011

The present study considers multi-level approach for the analysis of parametric roll phenomena. Three kinds of computation method, GM variation, impulse response function (IRF), and Rankine panel method, are applied for the multi-level approach. IRF and Rankine panel method are based on the weakly nonlinear formulation which includes nonlinear Froude-Krylov and restoring forces. In the computation result of parametric roll occurrence test in regular waves, IRF and Rankine panel method show similar tendency. Although the GM variation approach predicts the occurrence of parametric roll at twice roll natural frequency, its frequency criteria shows a little difference. Nonlinear roll motion in bichromatic wave is also considered in this study. To prove the unstable roll motion in bichromatic waves, theoretical and numerical approaches are applied. The occurrence of parametric roll is theoretically examined by introducing the quasi-periodic Mathieu equation. Instability criteria are well predicted from stability analysis in theoretical approach. From the Fourier analysis, it has been verified that difference-frequency effects create the unstable roll motion. The occurrence of unstable roll motion in bichromatic wave is also observed in the experiment.

• 과학교육연구지
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• 제34권2호
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• pp.432-439
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• 2010

이 연구의 목적은 전파가 전리층에서 반사되는 원리를 이해하고, 우주환경 변화가 전파 통신에 미치는 영향을 알아보는 것이다. 연구를 위하여 SID모니터를 설치하고, 안테나를 제작하였다. SID 모니터로 일본 Evino에서 송신하는 22.2kHz 전파를 수신하였다. 수신된 전파의 세기를 이용하여 전리층 D층의 상태를 분석하였다. 결과를 요약하면 다음과 같다. 저주파 통신은 규칙적인 일변화를 가지고 상태가 변하게 된다. D층의 생성, 소멸과 관련하여 저주파 통신은 일몰과 일출 직후 전파의 세기가 약해지는 현상이 나타난다. 태양의 남중고도에 따라 낮 시간 전파의 세기가 변하는 것을 관찰했다. 밤에는 D층이 사라져 매우 불규칙한 변화를 보인다. 전리층은 계절적인 변화를 비롯하여, 여러 요인에 의해 변하는 것으로 생각되며 보다 자세한 변화 과정을 이해하기 위해서는 긴 시간 동안의 전파 관측이 필요하다.

• 한국농공학회지
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• 제33권2호
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• pp.120-129
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• 1991

This study refers to temporal variation of physical properties of tilled soil under natural rainfalls. Field measurements of porosity, average hydraulic conductivity and average capillary pressure head on a tilled soil were conducted by soil sampler and air-entry permeameter respectively at regular intervals after tillage. Temporal variation of these physical properties were analysed by cumulative rainfall energy since tillage. Field experiment was conducted on a sandy loam soil at Suwon durging April~July in 1989. The followings are a summary of this study results ; 1. Average porosity just after tillage was 0.548cm$^3$/cm$^3$. As cumulative rainfall energy were increased in 0.1070, 0.1755, 0.3849 J/cm$^2$, average porosity were decreased in 0.506, 0.4]95, 0.468m$^3$/cm$^3$ respectively. 2. Average hydraulic conductivity just after tillage was 45.42cm/hr. As cumulative rainfall energy were increased in 0.1755, 0.2466, 0.2978, 0.3849J/cm$^2$ average hydraulic conductivity were decreased in 15.34, 13.47, 9.58, 8.65cm/hr respectively. 3. As average porosity were decreased in 0.548, 0.506, 0.495, 0.468cm$^3$/cm$^3$ average capillary pressure head were increased in 6.1, 6.7, 6.9, 7.4cm respectively. 4. It was found that temporal variation of porosity, average hydraulic conductivity on a tilled soil might be expressed as a function of cumulative rainfall energy and average capillary pressure head might be expressed as a function of porosity. 5. The results of this study may be helpful to predict infiltration into a tilled soil more accurately by considering Temporal variation of physical properties of soil.

• 한국전산구조공학회:학술대회논문집
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• 한국전산구조공학회 2005년도 춘계 학술발표회 논문집
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• pp.79-86
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• 2005

This paper deals with the static optimal shapes of simple beams which are subjected to a vertical point load. The area and second moment of inertia of the regular polygon cross-section of the tapered beams are determined, which have always same volume and same length for the parabolic taper. The differential equation governing the elastic curve is derived using the small deflection theory and solved numerically. By using the numerical results of deflections, rotations and bending stresses of such beams, the optimal shapes, namely, optimal section ratios, of the beams subjected to a single point load according to variation of load position parameters are determined and presented in the figures. Examples of the static optimal shapes for beams with a single load and multiple loads are reported. The design process of this study can be used directly for the minimum weight design of simple beams.

• 한국해양공학회지
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• 제6권1호
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• pp.43-51
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• 1992

The hydrodynamic performance of a floating-type OWC (Oscillating Water Column) chamber is studied numerically and experimentally in this study. The numerical approach based on two-dimensional linear theory of floating wave absorber was attempted to design an efficient wave energy absorber, while model test was performed in a wave basin to test a performance of designed model and validate the reliability of developed numerical code. The focus of study is placed mainly on the experimental study to evaluate the principal characteristics of the designed OWC chamber in regular waves. The effects of the variation of wave height on OWC device and of air pressure inside chamber are also presented. Finally, the measured results were compared with computed ones, and it was shown that the designed chamber works with high efficiency $(\eta_H>1$ over most of wave lengths covered by present study. It is therefore concluded that the developed code is capable of being successfully employed to design OWC chambers at various ocean environments, even though there exist some minor discrepancies between measured and computed results.

• 한국안전학회지
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• 제33권2호
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• pp.45-51
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• 2018

This study was conducted to assess the hazards of gasoline in relevance to the changes in octane numbers, and gasoline's spontaneous ignition temperature and instantaneous ignition temperature were measured. Spontaneous ignition temperature of regular gasoline was $301^{\circ}C$ for sample quantity of $100{\sim}125{\mu}{\ell}$. Spontaneous ignition temperature of middle gasoline was $380^{\circ}C$ for sample quantity of $125{\mu}{\ell}$ and that of premium gasoline was $400^{\circ}C$. As gasoline's octane numbers increased, their spontaneous ignition temperatures increased, and their instantaneous ignition temperature were almost identically $499^{\circ}C$ for sample quantity of $125{\mu}{\ell}$. In addition, activation energies of regular gasoline, middle gasoline, and premium gasoline were 10.48 Kcal/mol, 16.89 Kcal/mol, and 24.55 Kcal/mol respectively.

• International Journal of Naval Architecture and Ocean Engineering
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• 제8권1호
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• pp.13-21
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• 2016

Parametric pitch instability of a Deep Draft Semi-submersible platform (DDS) is investigated in irregular waves. Parametric pitch is a form of parametric instability, which occurs when parameters of a system vary with time and the variation satisfies a certain condition. In previous studies, analyzing of parametric instability is mainly limited to regular waves, whereas the realistic sea conditions are irregular waves. Besides, parametric instability also occurs in irregular waves in some experiments. This study predicts parametric pitch of a Deep Draft Semi-submersible platform in irregular waves. Heave motion of DDS is simulated by wave spectrum and response amplitude operator (RAO). Then Hill equation for DDS pitch motion in irregular waves is derived based on linear-wave theory. By using Bubnov-Galerkin approach to solve Hill equation, the corresponding stability chart is obtained. The differences between regular-waves stability chart and irregular-waves stability chart are compared. Then the sensitivity of wave parameters on DDS parametric pitch in irregular waves is discussed. Based on the discussion, some suggestions for the DDS design are proposed to avoid parametric pitch by choosing appropriate parameters. The results indicate that it's important and necessary to predict DDS parametric pitch in irregular waves during design process.

• Nutrition Research and Practice
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• 제4권5호
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• pp.433-437
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• 2010

In order to develop an obesity management program for teenagers, we compared obese and non-obese girls attending high schools in terms of their dietary practices related to snack consumption. Dietary records were collected for 7 days. No significant differences were found for the average daily energy intake between obese and non-obese girls. However, the highest energy intake was greater for obese girls while not much difference was found for the lowest amount of energy intake. Obese girls had significantly lower intakes in calcium (P < 0.01), vitamin A (P < 0.001) and folate (P < 0.01). Mean energy intake from snack ($594.1{\pm}312.1kcal$) was significantly higher for obese girls than for non-obese girls ($360.1{\pm}173.1kcal$) (P < 0.001). A significant, positive correlation was observed between energy intake from snack and total daily energy intake (r = 0.34 P < 0.01) only for obese girls. In case of dietary behaviors, obese adolescent girls consumed significantly greater number of items for snacks and fewer foods for regular meals compared to non-obese girls (P < 0.05). This study suggested that obesity management programs for adolescents should focus on providing strategies to reduce snack through enhancing balanced regular meals.

• 천문학회보
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• 제36권2호
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• pp.81.2-81.2
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• 2011

Motivated by recent attempts to derive geomagnetic activity from hourly mean data in long term studies, we test the recursive Kalman filter method to obtain the regular solar variation curve of the geomagnetic field. Using a simple algorithm, we are able to assign a quiet day curve to every day separately, without the need for additional input parameter(s) to define the geomagnetically quiet days. We derive a digital counterpart AhK of the analog range index Ak at the subauroral Sodankyl$\ddot{a}$ station and compare it to the earlier digital estimate Ah and the local Ak index. We find that the new method outperforms the former estimate in every aspect studied and provides a robust, straightforward manner of estimating and verifying the manually scaled Ak index, based on readily available hourly values. The model is independent of sampling; thus, for shorter term studies where high-sampling data are available, more accurate estimates can also be obtained when needed. Therefore, in contrast to other recent approaches, we do not provide a method to quantify irregular activity directly but derive the actual quiet day curves in the traditional manner. In future applications the same algorithm may be used to define a wide variety of geomagnetic indices (such as Ak, Dst, or AE).

• International Journal of Naval Architecture and Ocean Engineering
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• 제9권5호
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• pp.499-508
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• 2017

This paper uses optimization techniques to obtain bow hull form of a 66,000 DWT bulk carrier in calm water and in waves. Parametric modification functions of SAC and section shape of DLWL are used for hull form variation. Multi-objective functions are applied to minimize the wave-making resistance in calm water and added resistance in regular head wave of ${\lambda}/L=0.5$. WAVIS version 1.3 is used to obtain wave-making resistance. The modified Fujii and Takahashi's formula is applied to obtain the added resistance in short wave. The PSO algorithm is employed for the optimization technique. The resistance and motion characteristics in calm water and regular and irregular head waves of the three hull forms are compared. It has been shown that the optimal brings 13.2% reduction in the wave-making resistance and 13.8% reduction in the added resistance at ${\lambda}/L=0.5$; and the mean added resistance reduces by 9.5% at sea state 5.