• 한국산학기술학회논문지
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• 제20권9호
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• pp.150-155
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• 2019

공동현상은 유체의 속도 변화에 의한 압력변화로 인해 유체 내에 빈 곳이 생기는 현상을 말한다. 고속의 액체유동에서 액체의 압력이 포화증기압 이하로 낮아져서 액체 내에 증기 기포가 발생하는 현상이다. 본 연구에서는 선박 및 해양플랜트 분야에서 사용되어지는 유체기기에 대해 CFD 유동해석을 이용하여 선박용 프로펠러의 단독성능 해석기법의 타당성 확보 및 공동에 따른 유동양상을 파악하기 위해 FLUENT를 이용하여 전진비를 증가시키며 3차원 해석을 수행하고 MOERI의 실험 데이터와 비교분석하였다. 대형 컨테이너선용 KP505프로펠러의 사양을 기준으로 전진비에 따른 해석의 결과 전진비 0.7~0.8 구간에서 효율은 60% 수준으로 가장 높게 확인되었다. 압력면과 흡입면의 차이로 추력이 발생되는 것을 확인하였고 프로펠러 표면보다는 이면부근에서 Bubble이 많이 생성될 것으로 추정되며 공동현상이 더 많이 발생할 것이라 추정되었다. 또한 전진비가 증가함에 따라 공동현상은 급격히 감소함을 알 수 있었다. 추력계수와 토크계수는 MOERI 실험값과 비교했을 때 비교적 유사한 결과를 나타내었으며 전진비 1을 제외하고 모두 5%이내의 차이를 나타내었다. 따라서, CFD로 프로펠러 단독 성능에 대한 평가가 가능함을 확인하였다.

• 대한기계학회논문집B
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• 제30권3호
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• pp.278-286
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• 2006

Experiments were conducted to estimate the performance of drag force type vertical axis wind turbine with an opening-shutting rotor. It was operated by the difference in drag force generated on both sides of the blades. The rotational speed was measured by a tachometer in a wind tunnel and the tunnel wind speed was measured by using a pilot-static tube and a micro manometer. The performance test for a prototype was accomplished by calculating power, power coefficient, torque coefficient from the measurement of torque and rpm by a dynamometer controller. Various design parameters, such as the number of blades(B), blade aspect ratio(W/R), angle of blades$(\alpha)$ and drag coefficient acting on a blade, were considered for optimal conditions. At the experiment of miniature model, maximum efficiency was found at N=15, $\alpha=60^{\circ}$ and W/R=0.32. The measured test variables were power, torque, rotational speed, and wind speeds. The data presented are in the form of power and torque coefficients as a function of tip-speed ratio V/U. Maximum power was found in case of $\Omega=0.33$, when the power and torque coefficient were 0.14 and 0.37 respectively. Comparing model test with prototype test, similarity law by advance ratio for vertical axis wind turbine was confirmed.

• 대한기계학회논문집B
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• 제28권10호
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• pp.1178-1183
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• 2004

The purpose of this study was to investigate the influence of compression ratio on engine performance in a LPG(Liquefied Petroleum Gas) engine converted from a diesel engine. In ordor to determine the ideal compression ratio, a variable compression ratio 4-cylinder engine was developed. Retrofitting a diesel engine into a LPG engine is technically very complicated compared to a gasoline to LPG conversion. The cylinder head and the piston crown were modified to bum LPG in the engine. Compression ratios were increased from 8 to 10 in an increment of 0.5, the ignition timing was controlled to be at MBT(Minimum Spark Advance for Best Torque) for each case.

• Journal of Mechanical Science and Technology
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• 제17권8호
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• pp.1196-1202
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• 2003

The concept of hydrogen enriched LPG fuelled engine can be essentially characterized as low emissions and reduction of backfire for hydrogen engine. The purpose of study is obtaining low-emission and high-efficiency in LPG engine with hydrogen enrichment. In order to determine the ideal compression ratio, a variable compression ratio single cylinder engine was developed. The objective of this paper is to clarify the effects of hydrogen enriched LPG fuelled engine on exhaust emission, thermal efficiency and performance. The compression ratio of 8 was selected to minimize abnormal combustion. To maintain equal heating value, the amount of LPG was decreased, and hydrogen was gradually added. In a similar manner, the relative air-fuel ratio was increased from 0.8 to 1.3 in increment of 0.1, and the ignition timing was controlled to be at MBT each case.

• 한국기계연구소 소보
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• 통권11호
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• pp.95-101
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• 1984

The MAU-series propellers were designed and tested in japan. This report presents the polynomial coefficients of open water Characteristics for each standard MAU-series propellers, obtained by multiple polynomial regression analysis in terms of pitch-diameter ratio and advance coefficient. The limitation of applicability and the accuracy of the regression polynomial are also discussed.

• 로봇학회논문지
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• 제2권2호
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• pp.109-118
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• 2007

The thruster is the crucial factor of an underwater vehicle system, because it is the lowest layer in the control loop of the system. In this paper, we propose an accurate and practical thrust modeling for underwater vehicles which considers the effects of ambient flow velocity and angle. In this model, the axial flow velocity of the thruster, which is non-measurable, is represented by ambient flow velocity and propeller shaft velocity. Hence, contrary to previous models, the proposed model is practical since it uses only measurable states. Next, the whole thrust map is divided into three states according to the state of ambient flow and propeller shaft velocity, and one of the borders of the states is defined as Critical Advance Ratio (CAR). This classification explains the physical phenomenon of conventional experimental thrust maps. In addition, the effect of the incoming angle of ambient flow is analyzed, and Critical Incoming Angle (CIA) is also defined to describe the thrust force states. The proposed model is evaluated by comparing experimental data with numerical model simulation data, and it accurately covers overall flow conditions within 2N force error. The comparison results show that the new model's matching performance is significantly better than conventional models'.

• International Journal of Naval Architecture and Ocean Engineering
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• 제5권2호
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• pp.287-301
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• 2013

The present paper deals with the problems of yaw angle effects on podded propulsor performance. The study aims at providing insights on characteristics of podded propulsors in azimuthing condition. In this regard, a wide numerical simulation that concerned yaw angle effect measurement on podded propeller performance was performed. The Reynolds-Averaged Navier Stokes (RANS) based solver is used in order to study the variations of hydrodynamic characteristics of podded propulsor at various angles. At first, the propeller is analyzed in open water condition in absence of pod and strut. Next flow around pod and strut are simulated without effect of propellers. Finally, the whole unit is studied in zero yaw angle and azimuthing condition. Structured and unstructured mesh techniques are used for single propeller and podded propulsor. The performance curves of the propeller obtained by numerical method are compared and verified by the experimental results. The characteristic parameters including the torque and thrust of the propeller, the axial force and side force of unit are presented as function of velocity advance ratio and yaw angle. The results shows that the propeller thrust, torque and podded unit forces in azimuthing condition depend on velocity advance ratio and yaw angle.

• Tribology and Lubricants
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• 제34권6호
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• pp.284-291
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• 2018

Ball-milling for reactant powders in advance and using an induction heating system for Ni-Al intermetallic coating process are known to enhance the reactivity of combustion synthesis. In this work, the effects of the charging weight ratio of ball to powder in ball-milling for reactant Ni-Al powders and the synthesizing temperature in induction heating on sliding wear behavior of the coating layers are investigated. Sliding wear behavior of the coating layers is examined against a tool steel using a pin-on-disc type sliding wear machine. As results, wear of the coating layer ball-milled without ball was severely worn out at the sliding speed of 2m/s, regardless of the synthesizing temperature in induction heating. However, the wear rate of the coating layers at the sliding speed was remarkably decreased with increasing the charging weight ratio of ball in ball-milling for reactant powders. This can be explained by the fact that the void in the coating layer is disappeared and the coating layer is densified by the ball-milling. The evidence showed that pitting damages were disappeared on the worn surface of ball-milled coating layer. Consequentially, the Ni-Al intermetallic coating layer could have better wear resistance at all sliding speed ranges with the ball-milling for reactant powders in advance.

• 무역학회지
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• 제46권1호
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• pp.23-38
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• 2021

This paper investigated the preference change of payment terms in international trade along with counteroffer or first offer conditions of the other parties. Studies on trade payment terms have mainly focused on payment term determination factors such as firm size, product price level, country credit rate, etc. We tried to find other factors affecting payment terms choice, during the negotiation process. We applied behavioral economics theories such as 'Status Quo Bias' and 'Anchoring effect' to build our research model. To prove the existence of the above effects, we proceeded with field experiments to the exporting companies in Alibaba.com. Both 'Status Quo Bias' and 'Anchoring effect' were found in the field experiment. Most of the exporting companies preferred traditional payment methods to new payment methods. And an initial request for a low advance payment ratio led to a lower advance payment ratio. Also, the experience of using new payment methods could diminish status quo bias. This paper applied behavioral economics theories and field experiment methodology to the payment term studies in international trades. These attempts could contribute to expanding the diversity of methodology and scope of international trade studies.

• 대한조선학회논문집
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• 제48권2호
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• pp.141-146
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• 2011

This study numerically carried out the propeller open water test(POW) by solving Navier-Stokes equations governing the three-dimensional unsteady incompressible viscous flow with the turbulence closure model of the ${\kappa}-{\omega}$ SST model. Numerical simulations are performed at various range of advance ratios. Corresponding to Reynolds numbers of $5.89{\times}105{\sim}6.47{\times}105$ based on free stream velocity and the chord length at 0.7 propeller radius. The present results give a good agreement with those of the experiment. The propeller induced vortical structures have been analyzed by visualizing the resultant vorticity. As the advance ratio increases, the magnitude and length of the resultant vorticity decrease significantly. As the main focus of present study, the numerical model to present the ($r-{\theta}$) plane-averaged resultant vorticity along the streamwise direction for various advance ratios has been suggested.