• 상하수도학회지
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• 제24권3호
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• pp.277-285
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• 2010

We examined the optimal shape of blades and efficiency of a self-power generator when the self-power generator using flow of the water in pipe as the power source was installed. Selected factors were the shape of blades, the number of blades, pitch angle, and the existence of separator. GAMBIT2.4 was used as a modeling program, FLUENT6.3, which is computational fluid dynamics simulation program, was used as an analytical model. In the case of a viscous model, k-epsilon standard model was chosen. As a result, when the number of blades was increased, the efficiency and maximum moment were enhanced slightly. The pitch of blades went up, and maximum moment was also increased. The optimal pitch of blade was 62.5 degree and the efficiency was increased by 30%. The efficiency was also increased when a separator was installed.

• 설비공학논문집
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• 제8권1호
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• pp.120-139
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• 1996

Experiment was performed to study the heat transfer characteristics in 27 kinds of 15 : 1 scale models of multi-louverred fin heat exchangers with a wide range of variables(R $e_{Lp}$ =100~1, 800, $L_p$/F$p$=0.3~0.9, $\theta$=20$^{\circ}$~40$^{\circ}$). Thermofoil heaters were used to heat the louver fins and the local average Nusselt number for each louver in the louver array was obtained at constant wall temperature conditions. Correlations are developed to predict the heat transfer characteristics and drag coefficients. Generally, the heat transfer characteristics in the multi-louvered fins is shown to be similar to those of the laminar heat transfer on a flat plate. As the Reynolds number, the louver pitch to fin pitch ratio$L_p$/F$p$and the louver angle($\theta$) increase respectively, the average Nusselt number increases, but the variation of average Nusselt number as a function of the louver angle is smaller than that as a function of the louver pitch to fin pitch ratio. In case of$L_p$/F$p$ <0.5, the average Nusselt number of the 3rd louver is especially lower than the others, it is expected that it is due to the flow structure such as a recirculation flow and a flow separation.

• 천문학회보
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• 제44권1호
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• pp.34.2-34.2
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• 2019

Several observational results show a tighter pitch angle at wavelengths of optical and near-infrared than those that are associated with star formation, which is in agreement with the prediction of the density wave theory. In my recent numerical studies, the dependence of the shock positions relative to the potential minima is due to the tendency that stronger shocks form farther downstream. This causes a systematic variation of the perpendicular Mach number, with radius and makes the pitch angle of the gaseous arms smaller than that of the stellar arms, which supports the prediction of the density-wave theory, independently. However, some observations still give controversial results which show similar pitch angles at wavelengths, and there is no statistical study comparing observations and numerical models directly. By analyzing optical image of disk galaxies in the Carnegie-Irvine Galaxy Survey (CGS), I measured the physical values of stellar and gaseous arms such as their strength, length, and pitch angles. For direct comparison with numerical results, I analyzed more than 30 additional numerical models with varying the initial parameters in model galaxies. In this talk, I will present results both of observational and numerical samples and discuss the physical properties of spiral structures based on the density-wave theory.

• International Journal of Precision Engineering and Manufacturing
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• 제9권3호
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• pp.13-17
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• 2008

This paper deals with the directional mobility of droplets on structured surfaces. Structured surfaces were micro-patterned with rectangular lines and spaces of varying pitch and height in the sub-millimeter range. The material used was polydimethylsiloxane, which is hydrophobic and wettable by oil. First, we studied the effect of the structural design on the sliding angle of pure water or oil through experiments. For pure water droplets, we found that a wider pitch enhanced the directionality. On the other hand, oil droplets spread along the groove because of their low surface tension and strong capillary force. The directionality of the sliding angle of oil droplets was larger than that of pure water, especially when the groove was narrower and deeper. Second, we poured a large amount of liquid on the structure and evaluated the removal rate on the tilted surface. We found that a parallel structure enhanced the liquid mobility for both pure water and oil.

• 한국기계가공학회지
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• 제17권5호
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• pp.91-96
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• 2018

The oleophobic property of surfaces modified with micro-post structures are investigated for a range of micro-post diameter ($11-23{\mu}m$) and pitch ($20-40{\mu}m$). The contact angle of an oil droplet on surfaces with various micro-post dimensions was calculated using the Cassie-Baxter model and did not show a good agreement with the measured contact angle. From measurement, the micro-post with diameter of $23{\mu}m$ and pitch of $32{\mu}m$ was found to have the highest contact angle ($134.3^{\circ}$).

• 한국항공우주학회지
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• 제46권1호
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• pp.1-9
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• 2018

람다 날개 형태의 무인전투기는 동체-날개 융합익의 형태를 띄고 있어 일반적인 항공기에 비해 상대적으로 항력이 작고 레이더 반사 면적이 작아 우수한 스텔스 성능을 갖는다. 그러나 앞전 후퇴각에 의해 생성되는 앞전 와류의 영향으로 특정 받음각에서 피칭모멘트가 급격히 증가하는 현상이 나타난다. 본 연구에서는 무미익 람다 날개 형상을 기반으로 한 UCAV 1303 모델을 사용하여 풍동시험과 전산해석을 수행하였다. 실험 풍속은 50 m/s, 받음각 범위는 $-4^{\circ}{\sim}28^{\circ}$ 으로 하였으며 전산해석 또한 실험 조건과 동일하게 연구를 수행하였다. 본 연구를 통해 UCAV의 날개 비틀림이 피칭모멘트의 안정성에 미치는 영향을 확인하였다. 그 결과 날개에 음의 비틀림 각을 적용하였을 때 날개 바깥쪽에서의 유동 박리가 지연되면서 Pitch-break가 발생하는 받음각이 증가하였고, 양의 비틀림 각을 적용하였을 때 Pitch-break가 발생하는 받음각이 감소하였지만 양항비가 증가하는 것을 확인하였다.

• Structural Engineering and Mechanics
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• 제49권3호
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• pp.311-327
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• 2014

The safety relating to leakage of water and pitch instability of ship chambers of the full balanced hoist vertical shiplifts has been the focus of adoption of the type of vertical shiplifts. This paper aims to remove the doubts through theoretical and engineering researches. The leakage and pitch stability of ship chambers of full balanced hoist vertical ship lifts are investigated on the basis of theoretical analysis and exploration of engineering measures. Regarding the issue of leakage of ship chambers, a mathematical model on leaking process is built and corresponding formula and coping measures are obtained which can be applied in control program of ship lifts by linking with monitoring. The concept of safety grade is put forward to seek the best technical and economic index and the corresponding technical measures are for different grades of ship lift is suggested. For the issue of pitch instability, a methodology of combining theoretical deduction and summary of achievements of design and operation of the type of the full balanced hoist shiplifts is adopted, and the formula for design about pitch stability of ship chambers is derived.

• 제어로봇시스템학회논문지
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• 제15권2호
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• pp.240-248
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• 2009

Modem version of supersonic jet fighter aircraft must have been guaranteed appropriate controllability and stability in HAoA(High Angle of Attack). The HAoA flight control law have two parts, one is control law of departure prevention and the other is control law of departure recovery support. The control laws of departure prevention for advanced jet trainer consist of HAoA limiter, roll command limiter and rudder fader. The control laws of departure recovery support are consist of yaw-rate limiter and MPO(Manual Pitch Override) mode. The guideline of pitch rocking using MPO mode is simple, but operating skill of pitch rocking is very difficult by the pilot with inexperience of departure situation. Therefore, automatic deep stall recovery system is necessary. The system called the "Automatic Pitch Rocker System" or APRS, provided a pilot initiated automatic maneuver capable of an aircraft recoveries in situations of deep stall, speed and altitude. This paper addresses the design and validation for APRS to recovery of an deep stall without manual pitch rocking by the pilot. Also, this system is designed to recovery of speed, attitude and altitude after deep stall recovery using ATCS (Automatic Thrust Control System) and autopilot. Finally, this system is verified by real-time pilot evaluation using HQS (Handling Quality Simulator).

• 한국항공우주학회지
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• 제35권10호
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• pp.868-874
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• 2007

정지비행시의 헬리콥터 로터 모형의 블레이드의 피치각 변화가 소음방사에 미치는 영향을 수치해석을 통해 파악하였다. 공력 자료는 비정상 패널법과 경험후류 방법을 이용하여 구하였으며, $0^{\circ}$에서 $9^{\circ}$까지 등간격으로 $1.5^{\circ}$ 씩 피치각을 증가시키면서 블레이드 표면상의 공력 하중 분포를 얻어내었다. 수치해석을 통해 얻어낸 두께 소음은 피치각에 무관한 결과를 보였으나, 하중소음은 피치각이 $1.5^{\circ}$ 씩 증가할 때 마다 대략 3~4dBA정도로 소음의 세기가 증가하는 경향을 보였으며, 이정도의 증가분은 소음이 더 커졌음을 감지할 수 있는 충분한 크기라 할 수 있다. 또한 하중소음의 방향성 결과로부터 블레이드의 윗면 보다는 아랫면에서의 소음의 세기가 더 크게 나옴을 알 수 있었다.

• 한국정밀공학회지
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• 제31권9호
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• pp.839-846
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• 2014

Recent trends in the miniaturization and weight reduction of portable electronic parts have driven the use of subminiature screws with a micrometer-scale pitch. As both screw length and pitch decrease in subminiature screws, the resulting clamping force becomes diminishes. In this work, Finite element (FE) analysis is performed to evaluate clamping force of a screw assembly, with a comparison with experimental result. To improve clamping force of subminiature screws, a new screw design is considered by modifying screw thread angle: the thread angle is varied as an asymmetric way unlike the conventional symmetric thread angle. FE analyses are then performed to compare the clamping characteristics of each subminiature screw with different thread angle. The effect of thread angles on the clamping force is then discussed in terms of structural safety for both positive and negative screws.