• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집B
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• pp.517-522
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• 2001

The exit edges of a diffuser are smoothly rounded, and a wall is located perpendicularly to a diffuser exit. The fluid is discharged towards the radial direction of a diffuser after impinging against a wall from a diffuser. In this flow path, pressure loss coefficients have been calculated by the variables of Reynolds number at a diffuser inlet, distance between a diffuser exit and a wall, and turbulence models. As a result, it was calculated that $h/D_0$ ratio between $0.35\sim0.4$ has the minimum pressure loss coefficient regardless of Reynolds number and turbulence models. It was also found that in case of the flow with relatively high Reynolds number at a diffuser inlet, the pressure loss coefficients by RNG $k-\varepsilon$ model have a tendency to be near to those by standard $k-\varepsilon$ model at small ratio of $h/D_0$, but to those by RSM at large ratio.

• 대한기계학회논문집B
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• 제22권9호
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• pp.1325-1334
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• 1998

The objectives of the present study are to develop a systematic method rather than a conventional trial-and-error method for an optimal shape design using a mathematical theory, and to apply it to engineering problems. In the present study, an optimal condition for a minimum pressure loss in a two-dimensional curved duct flow is derived and then an optimal shape of the curved duct is designed from the optimal condition. In the design procedure, one needs to solve the adjoint Navier-Stokes equations which are derived from the Navier-Stokes equations and the cost function. Therefore, a computer code of solving both the Navier-Stokes and adjoint Navier-Stokes equations together with an automatic grid generation is developed. In a curved duct flow, flow separation occurs due to an adverse pressure gradient, resulting in an additional pressure loss. Optimal shapes of a curved duct are obtained at three different Reynolds numbers of 100, 300 and 800, respectively. In the optimally shaped curved ducts, the separation region does not exist or is significantly reduced, and thus the pressure loss along the curved duct is significantly reduced.

• Tribology and Lubricants
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• 제32권4호
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• pp.132-139
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• 2016

The piston of a marine diesel engine works under severe conditions, including a combustion pressure of over 180 bar, high thermal load, and high speed. Therefore, the analyses of the fatigue strength, thermal load, clamping (bolting) system and lubrication performance are important in achieving a robust piston design. Designing the surface profile and the skirt ovality carefully is important to prevent severe wear and reduce frictional loss for engine efficiency. This study performs flexible multi-body dynamic and elasto-hydrodynamic (EHD) analyses using AVL/EXCITE/PU are performed to evaluate tribological characteristics. The numerical techniques employed to perform the EHD analysis are as follows: (1) averaged Reynolds equation considering the surface roughness; (2) Greenwood_Tripp model considering the solid_to_solid contact using the statistical values of the summit roughness; and (3) flow factor considering the surface topology. This study also compares two cases of skirt shapes with minimum oil film thickness, peak oil film pressure, asperity contact pressure, wear rate using the Archard model and friction power loss (i.e., frictional loss mean effective pressure (FMEP)). Accordingly, the study compares the calculated wear pattern with the field test result of the piston operating for 12,000h to verify the quantitative integrity of the numerical analysis. The results show that the selected profile and the piston skirt ovality reduce friction power loss and peak oil film pressure by 7% and 57%, respectively. They also increase the minimum oil film thickness by 34%.

• 공업화학
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• 제21권3호
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• pp.258-264
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• 2010

대형 선박과 발전소 및 화학 공장 등을 구성하는 배관 및 계통은 다양한 구성요소들로 이루어져 있다. 밴드, 티, 급격 확대, 급격축소, 오리피스와 같은 이러한 구성요소들은 시스템 전체의 압력강하를 유발한다. 집중변수모델을 사용하여 구성요소들에 의한 압력손실은 계산할 시에는 압력손실계수인 k-factor가 제공되어야 한다. 일반적으로 많은 공학 분야에서 k-factor의 계산에 Idelchik 모델이 사용되어 왔다. 본 연구에서는 전산유체역학 해석을 통하여 압력손실계수를 계산하고 그 결과를 Idelchik이 제안한 압력손실계수와 비교하였다. 이는 복잡한 유동영역의 압력손실계수 계산에 전산유체역학 코드의 활용성을 검증하기 위함이다. 해석결과, 레이놀즈 응력 모델이 압력손실계수를 가장 잘 예측하고 있다. 전산유체역학을 통한 압력손실계수 평가는 사용된 난류모델에 영향을 받지만 압력손실계수를 잘 예측하고 있으므로 압력손실 계산에 전산유체역학 코드를 사용하는 것은 타당하다고 판단된다.

• 한국농공학회논문집
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• 제55권6호
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• pp.145-154
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• 2013

Recently, damage occurrence by wind erosion has been increasing in society. In times past, such problems only took place in desert area ; however, in recent years, the wind erosion problem is spreading out to agricultural land. Wind erosion in agricultural land can cause loss of loam soils, the disturbance of the photosynthesis of the crop fields and serious economic losses. To overcome the mentioned problems, installation of windbreak fence can be recommended which function as disturbing strong wind and wind erosion. However, there is still no proper guideline to install the windbreak fence and the installation used to rely on the intuition of the workers due to the lack of related studies. Therefore, this study measured the aerodynamic resistance of screens of the windbreak fence using the apparatus for testing screens. The apparatus for testing screens was designed to measure pressure loss around the screen. Measured pressure loss by wall friction compensated for pressure loss to calculate the aerodynamic resistance of screens. The result of pressure loss by regression analysis derived the aerodynamic coefficient of Darcy-Forchheimer equation and power law equation. The aerodynamic resistance was constant regardless of the overlapped shape when the screen was overlapped into several layers. Increasing the number of layers of the screen, internal resistance increased significantly more, and pressure loss caused by the screen also increased linearly when the wind speed was certain conditions, but permeability had no tendency. In the future, the results of this study will be applied to the computational fluid dynamics simulation. The simulation models will be also validated in advance by wind tunnel experiments. It will provide standard of a design for constructing windbreak fence.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년도 제31회 추계학술대회논문집
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• pp.145-148
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• 2008

현재 항우연에서 개발 중인 우주발사체 터보펌프에 사용되는 터빈은 단단 충동터빈을 적용하고 있다. 우주발사체의 안정성을 위해 팁 간극을 비교적 많이 허용하고 있는데, 효율적인 면에서는 반동터빈 보다 다소 뒤지지만, 높은 출력비를 가지는 충동터빈이 팁 간극이 커질 때 어떤 효율경향을 나타내는지 알아보는 일은 의미 있는 일이다. 실험은 아음속 조건에서 실험하였고, 팁 간극을 각각 코드기준 1%$\sim$20%일 경우 충동과 반동 각각의 경우에 대하여 하류 전압손실을 측정하였다. 측정결과 팁 간극이 10%보다 더 커지면, 충동터빈이 오히려 전압손실량이 반동터빈보다 더 작아진다. 이것은 팁 간극이 비교적 큰 조건에서 충동터빈이 반동터빈보다 효율이나 출력비 면에서 더 나은 성능을 나타냄을 의미한다.

• 한방비만학회지
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• 제24권1호
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• pp.54-67
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• 2024

Objectives: The purpose of this study was to evaluate changes in body weight and blood pressure and also analyze adverse events after weight loss program using Gamitaeeumjowee-tang in patients with class II or III obesity, considering for metabolic and bariatric surgery. Methods: A retrospective chart review was conducted on class III obesity or class II obesity patients with comorbidities, who participated in 24-week weight loss program using Gamitaeeumjowee-tang (n=54). The analysis included changes in body weight, body mass index, fat mass, skeletal muscle mass, waist-hip ratio and blood pressure before and after the program. And also calculated the proportion of patients who lost more than 5%, 10%, and 15% of their initial weight. Adverse events were assessed by causality, severity and system-organ classes. Results: After the weight loss program, body weight decreased 12.21±6.43 kg (12.77%), and BMI decreased 4.61±2.25 kg/m² (12.75%) on average (P<0.05). The 90.7% of the subjects lost more than 5% of their weight, 68.5% lost more than 10%, and 35.1% lost more than 15% of their weight. Blood pressure significantly decreased 11.04±14.53 mmHg in systolic and 7.28±11.89 mmHg in diastolic on average (P<0.05). The 97% of adverse events were mild, and 75% were evaluated as 'unlikely' in the causality evaluation. Conclusions: The results of this study, the weight loss program using Gamitaeeumjowee-tang showed significant weight loss and blood pressure reduction in patients with class III obesity or class II obesity with comorbidities, without serious adverse events. Well-designed clinical studies are recommended for the future.

• 유체기계공업학회:학술대회논문집
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• 유체기계공업학회 1998년도 유체기계 연구개발 발표회 논문집
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• pp.240-247
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• 1998

Leakage vortices formed near blade tip causes an increase of total pressure loss near casing endwall region and as a result, the efficiency of rotor decreases. The reduction of rotor efficiency is related to the size of tip clearance. In this study, the three-dimensional flowfields in an axial flow rotor were calculated with varying tip clearance under various flow rates, and the numerical results were compared with experimental ones. The effects of tip clearance and attack angle on the leakage vortex and overall performance, and the less distributions were investigated through numerical calculations. In this study, tip leakage flow rate and total pressure loss by tip clearance were evaluated using numerical results and aprroximate equations were presented to evaluate the reduction of rotor efficiency by tip leakage flow.

• 상하수도학회지
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• 제23권1호
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• pp.107-112
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• 2009

This study was conducted for verification and systematization of estimation method about the headloss using CFD(Computational Fluid Dynamics). Head loss which happens between the inlet and outlet of in-line mixer can be a major factor for the design and construction. Also, this Case studies about the sensitivity related to the velocity in the piping system. As result, program's default calculation function was used to get each side's total pressure and the differential of each total pressure could be defined as head loss from in-line mixer. In the case of adopting pipe surface friction factor and geometry loss, Calculation residual can be much more reduced. It was found that residual of value between CFD method and field test ranged about 3 through 18 precent.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2002년도 제25회 KOSCI SYMPOSIUM 논문집
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• pp.231-235
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• 2002

A theoretical and experimental study on the combustion process in a constant volume micro combustor is described. Unlike in a macro scale constant volume combustor, the heat loss to the wall plays a major role in flame propagation in a micro micro combustor. In order to analyze the effect of heat loss on combustion phenomena, pressure transition from ignition was measured. A number of cylindrical micro combustors with different diameter and depth were used for experiment to study the effect of length scales and shape factor. The diameter of combustor ranged from 7.5mm to 22.5 mm and the height of cylinder was from 1mm to 4mm. Initial pressure was also varied for the experiment. The diagnostic methods were severely limited due to the size of the apparatus and uncertainties of certain quantities to be measured in a small-scale environment. An analytical method to derive physical quantities that are essential for performance prediction from the pressure measurements is described.