• Journal of Magnetics
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• 제21권2호
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• pp.244-248
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• 2016

Aiming to improve the shear yield stress of magnetorheological fluid, magnetorheological fluids with different particle characteristics are prepared, and the influence rules of particle mass fraction, particle size, nanoparticles content and application temperature on shear yield stress are investigated. Experimental results indicate that shear yield stress increases approximate linearly with the enhancement of particle mass fraction. Particle size and the nanoparticles within 10% mass fraction can improve the shear yield stress effectively. When the application temperature is higher than $100^{\circ}C$, the shear yield stress decreases rapidly because of thermal expansion and thermal magnetization effect.

• 대한기계학회논문집B
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• 제25권1호
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• pp.18-28
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• 2001

The present study investigates flow dynamics between two dimensional compliant plates under sinusoidal flow conditions in order to understand influence of wall motion, impedance phase angle (time delay between pressure and flow waveforms), and non-Newtonian fluid on wall shear stress using computational fluid dynamics. The results showed that wall motion induced additional terms in the streamwise velocity profile and the pressure gradient. These additional terms due to wall motion reduced the amplitude of wall shear stress and also changed the mean wall shear stress. The trend of the changes was very different depending on the impedance phase angle. As the impedance phase angle was changed to more negative values, the mean wall shear stress decreased while the amplitude of wall shear stress increased. As the phase angle was reduced from 0°to -90°under $\pm$4% wall motion, the mean wall shear stress decreased by 12% and the amplitude of wall shear stress increased by 9%. Therefore, for hypertensive patients who have large negative phase angles, the ratio of amplitude and mean of the wall shear stress is raised resulting in a more vulnerable state to atherosclerosis according to the low and oscillatory shear stress theory. We also found that non-Newtonian characteristics of the blood protect atherosclerosis by decreasing the oscillatory shear index.

• Advances in concrete construction
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• 제13권5호
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• pp.361-365
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• 2022

The objective of this study is to simulate the two-phase flow in pipes with various two-fluid models and determinate the shear stress. A hyperbolic shear deformation theory is used for modelling of the pipe. Two-fluid models are solved by using the conservative shock capturing method. Energy relations are used for deriving the motion equations. When the initial conditions of problem satisfied the Kelvin Helmholtz instability conditions, the free-pressure two-fluid model could accurately predict discontinuities in the solution field. A numerical solution is applied for computing the shear stress. The two-pressure two-fluid model produces more numerical diffusion compared to the free-pressure two-fluid and single-pressure two-fluid models. Results show that with increasing the two-phase percent, the shear stress is reduced.

• Journal of Mechanical Science and Technology
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• 제18권10호
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• pp.1782-1798
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• 2004

A numerical study of a natural convection in a rectangular cavity with the low-Reynolds-number differential stress and flux model is presented. The primary emphasis of the study is placed on the investigation of the accuracy and numerical stability of the low-Reynolds-number differential stress and flux model for a natural convection problem. The turbulence model considered in the study is that developed by Peeters and Henkes (1992) and further refined by Dol and Hanjalic (2001), and this model is applied to the prediction of a natural convection in a rectangular cavity together with the two-layer model, the shear stress transport model and the time-scale bound ν$^2$- f model, all with an algebraic heat flux model. The computed results are compared with the experimental data commonly used for the validation of the turbulence models. It is shown that the low-Reynolds-number differential stress and flux model predicts well the mean velocity and temperature, the vertical velocity fluctuation, the Reynolds shear stress, the horizontal turbulent heat flux, the local Nusselt number and the wall shear stress, but slightly under-predicts the vertical turbulent heat flux. The performance of the ν$^2$- f model is comparable to that of the low-Reynolds-number differential stress and flux model except for the over-prediction of the horizontal turbulent heat flux. The two-layer model predicts poorly the mean vertical velocity component and under-predicts the wall shear stress and the local Nusselt number. The shear stress transport model predicts well the mean velocity, but the general performance of the shear stress transport model is nearly the same as that of the two-layer model, under-predicting the local Nusselt number and the turbulent quantities.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2000년도 추계학술대회 논문집
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• pp.178-183
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• 2000

Electrorheological(ER) effect on the dispersive system of polarizable fine powder/dielectric oil has been investigated. The electrical and rheological properties of zeolite and starch based ER fluid were reported. The ER fluids were constructed by mixing zeolite and starch power with two different dielectric oils. Yield stress of the fluids were measured on the couette cell type rheometer as a function of electric fields, particle concetrations, and temperatures. The electric field is applied by high voltage DC power supply. The outer cup is connected to positive electrode(+) and the bob becomes ground(-). And the temperatures the viscosity(or shear stress) versus shear rates were measured. In this experiment shear rates were increased from 0 to $200s^{-1}$ in 2 minutes.

• 한국산학기술학회논문지
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• 제15권3호
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• pp.1247-1252
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• 2014

본 연구에서는 온도조건에 따른 MR 유체의 물리적 특성을 연구하고자 침전 및 전단응력실험을 수행하였다. MR 댐퍼에 사용되는 MR 유체는 자성 파우더를 포함하고 있어 침전현상이 발생하므로 침강성과 인가전류에 의한 전단응력 변화를 상온과 고온조건에서 실험하였다. 침강특성을 조사하기 위하여 강제 대류형 오븐을 인가전류에 의한 전단응력을 측정하기 위하여 점도계를 사용하였다. 실험결과로부터 MR 유체는 시간과 온도에 따른 침강 특성이 다름을 그리고 교반시간이 큰 경우에 MR 유체의 분산성이 2배 이상 우수하였다. 전단률이 증가할수록 전단응력은 로그함수의 증가분포를 인가전류를 증가할수록 2차 함수의 증가분포를 그리고 MR 유체의 전단응력은 상온보다 고온에서 6-18% 낮은 분포를 보였다.

• 대한기계학회논문집B
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• 제38권12호
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• pp.1027-1034
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• 2014

회전원판 위 표면에 작용하는 유동 전단응력은 표면마모를 발생시키는 주원인이다. 유체는 원심력에 의해 가장자리까지 자유표면을 갖는 박막유동으로 전달된다. 표면마모는 받음각 또는 곡률에 따른 전단응력 정도에 비례하여 차이를 보이게 된다. 전산해석을 통한 속도분포 기울기로 받음각에 기준한 곡률의 전단응력비를 비교하였다. 곡률반경 변화에 따른 전단유동의 응력감소를 모델링하여 표면마모를 효율적으로 줄일 수 있는 최적구간을 결정하는 것이 본 연구의 주제이며, 이 연구결과는 회전무화나 박막코팅과 같은 박막유동을 활용하는 기구의 최적설계에 적용될 수 있다.

• 대한의용생체공학회:의공학회지
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• 제23권4호
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• pp.281-286
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• 2002

심혈관계에서 자주 발생하는 죽상경화증과 혈전의 발생 및 성장에 관한 복잡한 기전을 이해하기 위하여 뇌의 혈액공급을 담당하는 경동맥을 2차원 축대칭으로 모사하여 수치해석하였다. 박동유동 상태에서 경동맥 내에 25%. 50%, 75%의 협착이 형성된 경우에 대하여 혈관내의 속도분포 및 혈류역학적 벽 파라미터들이 고찰되었다. 혈액은 뉴턴유체 및 전단변형률에 따라 점성이 변화하는 비뉴턴유체로 간주되었으며 비뉴턴모델로는 혈액과 유사한 점성치를 나타내는 Carraeu-Yasuda 모델이 적용되었다. 해석 결과 혈관내벽에 작용하는 벽전단응력은 협착이 커질수록 크게 증가하였으며 비뉴턴유체보다 뉴턴유체의 경우에서 벽전단응력이 크게 평가되었다. 벽전단응력 진동지표(OSI)에 의해 시간평균 재부착점이 예측되었는데 비뉴턴유체보다 뉴턴유체의 경우에서 협착 영역으로부터 멀리 떨어진 곳에서 관찰되었다. 시간평균 벽전단응력구배(WSSG)도 협착이 큰 경우에 상당히 크게 나타났는데 비뉴턴유체보다 뉴턴 유체의 경우에 더 큰 값이 나타났다.

• 대한의용생체공학회:의공학회지
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• 제13권3호
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• pp.253-262
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• 1992

The wall shear stress in the vicinity of end-to end anastomoses under steady flow condi- tions was measured using a flush-mounted hot-film anemometer(FMHFA) probe. The experi- mental measurements were in good agreement lith numerical results except In flow with low Reynolds numbers. The wall shear stress increased proximal to the anastomosis in flow from the Penrose tubing (simulating an artery) to the PTFE graft. In flow from the PTFE graft to the Penrose tubing, low wall shear stress was observed distal to the anastomosis. Abnormal distributions of wall shear stress in the vicinity of the anastomosis, resulting from the compli- ance mismatch between the graft and the host artery, might be an important factor of ANFH formation and the graft failure. The present study suggests a correlation between regions of the low wall shear stress and the development of anastomotic neointimal fibrous hyperplasia (ANFH) in end-to-end anastomoses.

• 대한의용생체공학회:의공학회지
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• 제21권3호
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• pp.261-271
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• 2000

벽면운동(wall motion)과 임피던스 페이즈앵글(impedance phase angle; 압력파와 유랑파 기아의 위상차)을 고려하여 맥동유동하에 있는 복부대동맥 분기관모델에서 2차원 전산유체해석을 수행하였다. 해석결과 분기광 근처에서응 전단응력의 크기가 매우 급격한 변화를 보임을 관찰하였고 벽면운동은 전단응력의 진폭을 감소시키는 효과를 가져왔다. 임피던스 페이즈 앵글이 음의 값을 향해 갈수록 시간 평균된 벽면 전단응력(mean wall shear stress)의 값은 감소하였으나 진폭(amplitude of wall shear stress)은 오히려 증가하였다. 페이즈앵글의 영향은 평균 벽면전단응력이 영에 근접하는 외벽(outer wall or lateral wall)의 바같쪽으로 휘어지는 부분(curvature site)에서 상대적으로 크게 나타났는데 $-90^{\circ}$ 페이즈앵글(혈류파가 혈압파를 1/4주기 앞서는 경우)일 경우에 $0^{\circ}$의 경우에 비해 평균은 $50\%$정도 감소하였고 진폭은 $15\%$정도의 상승를 나타내었다. 그러므로 고혈압 환자와 같이 큰 음의 페이즈앵글을 갖는 경우, 벽면전단응력의 평균은 낮아지고 시간에 따라 변화량(진폭)은 증가하므로 low and oscillatory wall shear stress 이론에 의하면 동맥경화에 더 민감하게 된다. 비뉴턴유체로 모델링한 경우에는 뉴턴유체의 경우에 비해 벽면전단응력의 평균값이 증가하므로서 동맥경화에 덜 민감하게 된다.