• International Journal of Aeronautical and Space Sciences
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• 제16권4호
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• pp.571-580
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• 2015

In this paper, a computational study was conducted in order to investigate the rotor blade sweep effect on the aerodynamics of a small axial supersonic impulse turbine stage. For this purpose, three-dimensional unsteady RANS simulations have been performed with three different rotor blade sweep angles ($-15^{\circ}$, $0^{\circ}$, $+15^{\circ}$) and the results were compared with each other. Both NTG (No tip gap) and WTG (With tip gap) models were applied to examine the effect on tip leakage flow. As a result of the simulation, the positive sweep model ($+15^{\circ}$) showed better performance in relative flow angle, Mach number distribution, entropy rise, and tip leakage mass flow rate compared with no sweep model. With the blade static pressure distribution result, the positive sweep model showed that hub and tip loading was increased and midspan loading was reduced compared with no sweep model while the negative sweep model ($-15^{\circ}$) showed the opposite result. The positive sweep model also showed a good aerodynamic performance around the hub region compared with other models. Overall, the positive sweep angle enhanced the turbine efficiency.

• 유체기계공업학회:학술대회논문집
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• 유체기계공업학회 2004년도 유체기계 연구개발 발표회 논문집
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• pp.578-582
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• 2004

This paper presents the response surface optimization method using three-dimensional Navier-Stokes Analysis to optimize the shape of a axial flow fan. Reynolds-averaged Navier-Stokes equations with k-$\epsilon$ turbulence model are discretized with finite volume approximations. Regression analysis is used for generating response surface, and it is validated by ANOVA. Five geometric variables, i.e., distribution of sweep angle at mean and tip, lean angle at mean and tip, and spanwise location of mean were employed to optimize the efficiency. The computational results are compared with experiment data. As a main result of the optimization, the efficiency was successfully improved.

• 한국유체기계학회 논문집
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• 제11권6호
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• pp.7-17
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• 2008

Fan performances are obtained with various tip clearance gaps and stagger angles of the rotor. A tested fan is an axial-type fan of which the casing diameter is 806 mm. Two different rotors are applied to this test. One is designed on the basis of the free vortex method along the radial direction and the other is designed using the forced vortex method. The operating conditions are varied to the ultimate off-design point as well as the deign point. Overall efficiency, total pressure and input power are compared with the tip clearance gaps and different stagger angle. The experimental results show that changing of the stagger angle has minor influence to the performance when the same rotor is applied. When the tip clearance gap is less than 5% of the rotor span, the overall efficiency, total pressure loss and input power reduction are varied linearly with the variation of the tip clearance gaps. On the design point, the overall efficiency is decreased to the rate of 2.8-2.9 to the increasing of the tip clearance, but the changing rate of the overall efficiency is alleviated when the fan operates at off-design points. In particular, this rate is more quickly declined on a fan with the rotor designed using the forced vortex method. The result of the total pressure shows that the pressure reduction rate is a 0.08-0.1 according to the tip clearance, and additionally the input power reduction rate is a 0.045-0.065 at design point.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년 영문 학술대회
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• pp.743-750
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• 2008

Asymmetric tip clearance in an axial compressor induces pressure and velocity redistributions along the circumferential direction in an axial compressor. This paper presents the mechanism of the flow redistribution due to the asymmetric tip clearance with a simple numerical modeling. The flow field of a rotor of an axial compressor is predicted when an asymmetric tip clearance occurs along the circumferential direction. The modeling results are supported by CFD results not only to validate the present modeling but also to investigate more detailed flow fields. Asymmetric tip clearance makes local flow area and resultant axial velocity vary along the circumferential direction. This flow redistribution 'seed' results in a different flow patterns according to the flow coefficient. Flow field redistribution patterns are largely dependent on the local tip clearance performance at low flow coefficients. However, the contribution of the main flow region becomes dominant while the tip clearance effect becomes weak as the flow coefficient increases. The flow field redistribution pattern becomes noticeably strong if a blockage effect is involved when the flow coefficient increases. The relative flow angle at the small clearance region decreases which result in a negative incidence angle at the high flow coefficient. It causes a recirculation region at the blade pressure surface which results in the flow blockage. It promotes the strength of the flow field redistribution at the rotor outlet. These flow pattern changes have an effect on the blade loading perturbations. The integration of blade loading perturbation from control volume analysis of the circumferential momentum leads to well-known Alford's force. Alford's force is always negative when the flow blockage effects are excluded. However when the flow blockage effect is incorporated into the modeling, main flow effects on the flow redistribution is also reflected on the Alford's force at the high flow coefficient. Alford's force steeply increases as the flow coefficient increases, because of the tip leakage suppression and strong flow redistribution. The predicted results are well agreed to CFD results by Kang and Kang(2006).

• 한국수자원학회논문집
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• 제38권2호
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• pp.143-153
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• 2005

최근 들어 자연형 하천과 하천생태계 복원에 대한 관심이 점증하면서 수리학적 기능 외에, 다양한 생태환경을 제공하고 하안 부근의 경관을 개선하는 기능을 가는 수제에 대한 관심이 커지고 있다. 그러나 현재 국내에는 수제설치에 대한 설계지침이 충분치 않을 뿐 만 아니라 국내 수행된 연구 자료가 매우 부족한 실정이다. 이에 본 연구에서는 수제 설계인자 해석을 위한 수리모형실험을 수행하였다. 실험조건은 수제선단 흐름변화와 수제하류부 재순환영역에 대한 고정상실험으로 수제길이와 투과율을 조절하여 수행하였다. 주요 분석은 수제 선단에서는 중앙부 유속증가와 세굴의 영향인자인 유속과 편향각을 측정하여 수제 선단유속비와 면적비에 대한 실험식을 제안하였다. 또한 수제하류부 재순환 영역에서는 수제 설치의 주요요인인 수제간격의 기초 자료를 해석하기 위해 흐름분리 길이와 입사각의 변화를 분석하였으며 투과율과 수제 설치각에 대한 실험식을 제안하였다.

• 한국농공학회논문집
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• 제54권4호
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• pp.137-145
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• 2012

Sand is one of the essential materials used for social infrastructure construction such as embankment, landfill and backfill. It was known that mechanical properties and shear strength of sand are closely related to relative density. Therefore it is very important to determine accurate relative density. In this study, Portable Cone Penetration Tester (PCPT) was used to estimate the relative density and the internal friction angle of sand. PCPT cone-tip resistance ($q_c$) was measured changing the relative density of the two soil samples.Standard sand (JMJ) and Busan sand (BS). Also, a direct shear test was performed to investigate relationship between relative density and internal friction angle. The size and shape of soil particles were confirmed by using Scanning Electron Microscope (SEM). As a result, the log value of $q_c$ was linearly correlated with relative density and internal friction angle. In particular, the internal friction angle of BS sample was greater than that of JMJ, which was due to difference of the shape and mean size of particles. This result shows that it is important to determine the shape and size of particles as well as relative density to define mechanical property of sand. Through this study, it can be more effectively and conveniently to investigate relative density and shear strength of sand by using PCPT in situ.

• 한국안전학회지
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• 제15권4호
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• pp.15-19
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• 2000

The exact estimation of the ductile crack growth in a thin sheet would be needed in part of the commercial transport aircraft industry fields. A 2-dimensional elastic plastic finite element analysis was carried out to simulate a stable crack extension in a thin sheet 2024 aluminium alloy. Two kinds of crack modeling were used to evaluate curves of the stable crack extension. And then CTOA(crack tip opening angle) and CTED(crack tip energy density) were calculated in order to determine whether they can be used as useful crack extension criterions in a thin sheet. Results indicate that stable crack extension behaviors were simulated well and CTED is more admirable even though CTOA also is reasonable as a criterion for a stable crack extension in a thin 2024 aluminium alloy sheet.

• 설비공학논문집
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• 제13권2호
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• pp.134-143
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• 2001

This paper presents the characteristics of flank and tip seal leakage in a scroll compressor for air-conditioners with R-22 under actual operating conditions. It is well known that the leakage has significant effect on the performance of the scroll compressor. Experiments were performed by using indirect method for measuring mass flow rate passing through flank and tip seal under actual operating conditions, In addition, an analytical model for tip seal leakage was developed to investigate tangential and radial leakage observed at grooves and contact points of tip seals. For low oil concentration, theoretical results were compared with experimental data to verify the analytical model. As a result, leakages through flank and tip seal parts were evaluated as afunction of pressure ratio, orbiting angle, discharge pressure, tip clearance, and leakage point. It was also found that the tip seal leakage was considerable even though the tip seal provided adequate sealing effect.

• Tribology and Lubricants
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• 제33권6호
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• pp.251-255
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• 2017

In this study, we aimed to clarify the wettability and anisotropic flow characteristics of rice leaves as a basic study for engineering applications of anisotropic flow characteristics of rice leaf surface. To investigate the surface structure of rice leaf, the micro grooves and asperities of rice leaves were analyzed and quantified by scanning electron microscope, Confocal laser scanning microscopy, and stylus profilometer. The analysis of the structure of rice leaf surface confirmed that asymmetrical cone - like protrusions in leaf veins were inclined toward the leaf tip. The static contact angle test showed that the contact angle at the midline vein or leaf vein location where the micropapilla is concentrated is about $20^{\circ}$ higher than the leaf blade position. The contact angles of fresh and dried rice leave were also compared. The dried rice leaves showed a contact angle of about $5^{\circ}$ to $15^{\circ}$ higher than that of fresh leaves, suggesting that the volume of the protrusions decreased as the water was removed, thus reducing the contact area with the droplet. In the contact angle history test the hysteresis in the leaf tip direction was found to be much lower than that in the leaf petiole direction. This results can be explained that asymmetrical cone - like protrusions had a significant effect on the droplet flow characteristics through contact angle hysteresis experiment.

• 한국분무공학회지
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• 제6권3호
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• pp.38-44
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• 2001

The effect of four diesel fuels with oxygenated agents fuels on spray properties from plain-orifice atomizer was investigated. The oxygenates evaluated were diglyme, MTBE, DEE and DMM and were blended in weights of 5, 10, 15, 20 and 30% in a baseline diesel fuel. The physical properties such as surface tension, density and viscosity are also measured for each blended oxygenated fuels. It was found that changes in physical properties of fuels considered are enough to influence spray properties, i.e. spray angle, spray tip penetration and mean drop size. Spray properties were measured by PMAS(particle motion analysis system) which is employing a point measurement technology. Spray angle increased with increase in oxygenate content. The effect, however, was not great in the higher blend level. The oxygenated fuels produced more shorter spray tip penetration than diesel fuels. SMD was decreased with the increase in blending percent. SMD for DMM and DEE are represented 10.33 and 3.41% decreasing rates respectively. It was found that changes in spray characteristics of oxygenated fuel were easily large enough to impact pollutant emissions. It was clear from this study that spray characteristics of oxygenated fuel is one of possible cause of reducing pollutant emissions. It was clear from this study that spray characteristics of oxygenated fuel is one of possible cause of reducing pollutant emissions from diesel engines when oxygenated fuels is applied.