• Nuclear Engineering and Technology
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• 제54권4호
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• pp.1495-1507
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• 2022

The improvement of thermal-hydraulic analysis techniques is essential to ensure the safety and reliability of nuclear power plants. The one-dimensional two-fluid model has been adopted in state-of-the-art thermal-hydraulic system codes. Current constitutive equations used in the system codes reach a mature level. Some exceptions are the partition method of wall friction in the momentum equation of the two-fluid model and the interfacial drag force model for a horizontal two-phase flow. This study is focused on deriving the partition method of wall friction in the momentum equation of the two-fluid model and modeling the interfacial drag force model for a horizontal bubbly flow. The one-dimensional momentum equation in the two-fluid model is derived from the local momentum equation. The derived one-dimensional momentum equation demonstrates that total wall friction should be apportioned to gas and liquid phases based on the phasic volume fraction, which is the same as that used in the SPACE code. The constitutive equations for the interfacial drag force are also identified. Based on the assessments, the Rassame-Hibiki correlation, Hibiki-Ishii correlation, Ishii-Zuber correlation, and Rassame-Hibiki correlation are recommended for computing the distribution parameter, interfacial area concentration, drag coefficient, and relative velocity covariance of a horizontal bubbly flow, respectively.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.479-482
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• 2002

Riblets with longitudinal grooves along the streamwise direction have been used as an effective flow control technique for drag reduction. A flexible micro-riblet with v-grooves of peak-to-peak spacing of $300{\mu}m$ was made using a MEMS fabrication process of PDMS replica. The flexible micro-riblet was attached on the whole surface of a NACA0012 airfoil with which grooves are aligned with the streamwise direction. The riblet surface reduces drag coefficient about $7.9{\%}\;at\;U_o=3.3m/s$, however, it increases drag about $8{\%}\;at\;U_o=7.0m/s$, compared with the smooth airfoil without riblets. The near wake has been investigated experimentally far the cases of drag reduction ($U_o\;=\;3.3 m/s$) and drag increase ($U_o\;=\;7 m/s$). Five hundred instantaneous velocity fields were measured for each experimental condition using the cross-correlation PIV velocity field measurement technique. The instantaneous velocity fields were ensemble averaged to get spatial distribution of turbulent statistics such as turbulent kinetic energy. The experimental results were compared with those of a smooth airfoil under the same flow condition. The micro-riblet surface influences the near wake flow structure largely, especially in the region near the body surface

• Wind and Structures
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• 제15권5호
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• pp.423-439
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• 2012

Three common medium- rise building forms were physically tested to study their overall wind induced structural response. Emphasis was placed on the torsional response and its correlation with other peak responses. A higher correlation was found between the peak responses than between the general fluctuating parts of the signals. This suggests a common mechanism causing the peak event, and that this mechanism is potentially different to the mechanism causing the general load fluctuations. The measurements show that about 80% of the peak overall torsion occur simultaneously with the peak overall along wind drag for some generic building shapes. However, the peak torsional response occurs simultaneously with only 30%-40% of the peak overall drag for the rectangular model. These results emphasise the importance of load combinations for building design, which are often neglected in the design of medium sized rigid buildings for which the along-wind drag is dominant. Current design wind loading standards from around the world were evaluated against the results to establish their adequacy for building design incorporating wind-induced torsion effects. Although torsion is frequently neglected, for some structural systems it may become more important.

• Wind and Structures
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• 제34권1호
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• pp.137-149
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• 2022

In this paper, the fluctuating lift and drag forces on 5:1 rectangular cylinders with two different geometric scales in three turbulent flow-fields are investigated. The study is particularly focused on understanding the influence of the ratio of turbulence integral length scale to structure characteristic dimension (the length scale ratio). The results show that both fluctuating lift and drag forces are influenced by the length scale ratio. For the model with the larger length scale ratio, the corresponding fluctuating force coefficient is larger, while the spanwise correlation is weaker. However, the degree of influence of the length scale ratio on the two fluctuating forces are different. Compared to the fluctuating drag, the fluctuating lift is more sensitive to the variation of the length scale ratio. It is also found through spectral analysis that for the fluctuating lift, the change of length scale ratio mainly leads to the variation in the low frequency part of the loading, while the fluctuating drag generally follows the quasi-steady theory in the low frequency, and the slope of the drag spectrum at high frequencies changes with the length scale ratio. Then based on the experimental data, two empirical formulas considering the influence of length scale ratio are proposed for determining the lift and drag aerodynamic admittances of a 5:1 rectangular cylinder. Furthermore, a simple relationship is established to correlate the turbulence parameter with the fluctuating force coefficient, which could be used to predict the fluctuating force on a 5:1 rectangular cylinder under different parameter conditions.

• 태양에너지
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• 제17권4호
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• pp.13-22
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• 1997

본 연구에서는 회전원판 장치를 사용하여 난류 유동장에서의 고분자에 의해 유도되는 마찰저항 감소효과에 대하여 조사하였다. 해양온도차 발전에서 해수를 이동시키는 유동장은 난류상태로 이러한 난류계에 대하여 마찰저항 감소는 충분한 적용가치가 있다. 네가지의 분자량이 다른 PEO를 마찰저항 첨가제로, 실험실에서 제조한 인공해수를 용매로 사용하여 고분자의 분자량, 고분자의 농도와 원판의 회전속도와 괌이 마찰저항 감소효과에 영향을 줄 수 있는 여러 인자들에 대해서 살펴보았다. 마찰저항 감소의 농도의존성은 Virk의 Universal correlation를 따르는 것을 확인하였다. 해수에서도 PEO와 용매간의 Universal 곡선이 증류수를 용매로 하였을 때와 동일함을 확인하였다.

• 대한기계학회논문집B
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• 제33권6호
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• pp.389-395
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• 2009

A specially designed flat plate was mounted vertically over the axial line in the wind tunnel of the Pusan National University. Strain balances were mounted in the trailing part of the plate to measure the skin friction drag over removable insertions of $0.55{\times}0.25m^2$ size. A set of the insertions was designed and manufactured: 3 mm thick polished metal surface and three compliant surfaces. The compliant surfaces were manufactured of a silicone rubber Silastic$^{(R)}$ S2 (Dow Corning company). To modify the viscoelastic properties of the rubber, its composition was varied: 90% rubber + 10% catalyst (standard), 92.5% + 7.5% (weak), 85% + 15% (strong). Modulus of elasticity and the loss factor were measured accurately for these materials in the frequency range from 40 Hz to 3 kHz. The aging of the materials (variation of their properties) for the period of one year was documented as well. Along with the drag measurement using the strain balance, velocity and pressure were measured for different coating. The strong compliant coating achieved 5% drag reduction within a velocity range $20{\sim}40$ m/s while standard and weak coatings increased drag reduction.

• 한국전산유체공학회지
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• 제19권2호
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• pp.66-71
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• 2014

Aerodynamic design of a vehicle has very important meaning on the fuel economy, dynamic stability and the noise & vibration of a moving vehicle. In this study, the correlation of aerodynamic effect between two model vehicles moving inline on a road was studied with the basic SAE model vehicle. Drag and lift are two main physical forces acting on the vehicle and both of them directly effect on the fuel economy and driving stability of the vehicle. For the research, the distance between two vehicles is varied from 5m to 30m at the fixed vehicle speed, 100km/h and the side-wind was assumed to be zero. The main issue for this numerical research is on the understanding of the interaction forces; lift and drag between two vehicles formed inline. From the study, it was found that as the distance between two vehicles is closer, the drag force acting on both the front and rear vehicle decreases and the lift force has same trend for both vehicle. As the distance(D) is 5m, the drag of the front vehicle reduced 7.4% but 28.5% for the rear-side vehicle. As the distance is 30m, the drag of the rear vehicle is still reduced to 22% compared to the single driving.

• 대한기계학회논문집B
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• 제41권3호
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• pp.171-181
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• 2017

2상 횡 유동은 응축기, 증발기와 원자력의 증기 발생기와 같은 열교환기의 튜브와 셀 사이에 존재한다. 공기/물의 2상 유동에 놓인 관군에 작용하는 항력을 실험적으로 평가하였다. 2상 유동에 놓인 관군은 정사각형 배열이다. 피치 직경 비는 1.35이었고, 실린더의 직경은 18 mm이다. 관군에 유동방향으로 작용하는 항력을 측정하여 항력계수와 2상 유동 감쇠비를 계산하였다. 2상 유동 감쇠비는 균질 2상 유동의 이론식을 사용하여 구하여 실험의 결과와 비교하였다. 압력과 항력의 상관계수를 실험결과를 고려하여 평가하였다. 상관계수는 이론적으로 항력을 계산할 때에 사용된다. 질량유량을 증가할수록 측정된 항력으로부터 구한 항력계수와 감쇠비가 균질 유동의 이론적 결과와 잘 일치함을 보이고 있다. 결과적으로 충분히 큰 질량 유량의 기포 유동인 경우에는 감쇠비를 균질 유동에 근거한 이론식으로 계산할 수 있다.

• 대한기계학회논문집B
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• 제34권8호
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• pp.799-807
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• 2010

폴리머 첨가제에 의한 항력감소 난류 채널 유동에 대한 직접수치모사를 스펙트럴 기법을 통해 수치적으로 해석하였다. 마찰속도 및 채널 높이의 절반으로 무차원화한 레이놀즈수는 395 이며, 폴리머 첨가제에 의해 발생하는 폴리머 응력은 FENE-P 모델을 통해 고려하였다. 폴리머 분자의 이완 시간 및 최대 연신 한계와 같은 FENE-P 모델 인자는 항력감소율에 큰 영향을 끼치는 것으로 나타났다. 항력감소율이 낮은 유동과 높은 유동에 대해 항력감소에 따른 난류 통계량의 변화를 조사하였다. 또한, 동일한 항력감소율을 갖는 유동에 대해, 서로 다른 FENE-P 모델 인자가 난류 통계량의 변화에 미치는 영향도 조사하였다. 최종적으로, Li 등(2006) 이 제시한 유변학 인자들과 항력감소율과의 상관관계식을 본 수치해석 결과를 통해 확인하였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회B
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• pp.2753-2756
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• 2007

In the present study, we reduce the drag and lift fluctuations of the sphere by providing a linear proportional control. For this purpose, we measure the radial velocity along the centerline in the wake and provide blowing and suction at a part of sphere surface based on the measured velocity. Zero-net mass flow rate is satisfied during the control. This control is applied to the flow over a sphere at Re=300 and 425. We vary the sensing location at $0.8d{\leq}X_s{\leq}1.3d$ and find that the most effective sensing region coincides with the location at which minimum correlation between the lift and sensing-velocity directions occurs. As a result, the lift and drag fluctuations are significantly reduced.