• Title/Summary/Keyword: Zero-skin friction

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Large Eddy Simulation of Turbulent Flow in an Optimal Diffuser (큰에디모사법을 이용한 최적 디퓨져내의 난류유동 해석)

  • Lim Seokhyun;Caoi Haecheon
    • Proceedings of the KSME Conference
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    • 2002.08a
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    • pp.811-814
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    • 2002
  • Using a mathematical theory, we show that the optimality condition of a turbulent diffuser with maximum pressure recovery at the exit is zero shear stress along the wall. The optimal diffuser shape is designed through iterative procedures by using the $k-{\varepsilon}-{\nu}^{2}-f$ turbulence model for flow simulation. The Reynolds number based on the bulk mean velocity and the channel height at the diffuser entrance is 18,000. We also perform large eddy simulation to validate the shape design results and investigate the flow characteristics near the zero-skin friction wall. Results from large eddy simulation show that the skin friction is slightly higher than zero but is still very small as compared to that of the flat plate boundary layer flow Although the time-averaged wall shear stress is slightly above zero along the diffuser wall, instantaneous flow reversals occur intermittently. The streamwise mein velocity shows an asymptotic behavior of the half-power-law near the wall where the skin friction is close to zero.

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A Study on the Distribution of Residual Stress for Drilled Shaft (현장타설말뚝의 잔류응력 분포에 관한 연구)

  • Kim, Won-Cheul;Hwang, Young-Cheol;Ahn, Chang-Yoon
    • Journal of the Korean GEO-environmental Society
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    • v.6 no.1
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    • pp.45-51
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    • 2005
  • The distribution of shaft resistance is measured by the static load test with the strain gauge or stress gauge, so that the long-term load distribution must be considered for the pile design. However, the measurement by strain gauge generally assumes the 'zero reading', which is the reading taken at 'zero time' with 'zero' load and the residual stress, which is the negative skin friction(or the negative shaft resistance) caused by the pile construction, is neglected. Therefore, the measured value by strain gauge is different from the true load-distribution because residual stresses were neglected. In this study, the three drilled shafts were constructed, and the strain measurements were carried out just after shaft construction. As a result of this study, it is shown that the true load-distribution of drilled shaft is quite different with known load distribution and the true load-distribution of drilled shaft changed from the negative skin friction to the positive skin according to the load increment.

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A Study on the Helical Flow of Newtonian and Non-Newtonian Fluid

  • Woo, Nam-Sub;Hwang, Young-Kyu;Kim, Young-Ju
    • International Journal of Air-Conditioning and Refrigeration
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    • v.15 no.1
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    • pp.1-9
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    • 2007
  • This study concerns the characteristics of helical flow in a concentric and eccentric annulus with a diameter ratio of 0.52 and 0.9, whose outer cylinders are stationary and inner ones are rotating. Pressure losses and skin friction coefficients have been measured for fully developed flows of water and 0.2% aqueous of sodium carboxymethyl cellulose (CMC), respectively, when the inner cylinder rotates at the speed of 0-500 rpm. The effect of rotation on the skin friction is significantly dependent on the flow regime. In all flow regimes, the skin friction coefficient is increased by the inner cylinder rotation. The change of skin friction coefficient corresponding to the variation of rotating speed is large for the laminar flow regime, whereas it becomes smaller as Re increases for the transitional flow regime and, then, it gradually approach to zero for the turbulent flow regime.

A Study on the Rotating Flow in an Annulus (환형관내 회전유동에 관한 연구)

  • 김영주;우남섭;황영규
    • Proceedings of the Korea Society for Energy Engineering kosee Conference
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    • 2003.11a
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    • pp.153-158
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    • 2003
  • This study concerns the characteristics of helical flow in a concentric annulus with a diameter ratio of 0.52 and 0.9, whose outer cylinders are stationary and inner ones are rotating. Pressure losses and skin friction coefficients have been measured for fully developed flows of water and 0.2% aqueous of sodium carboxymethyl cellulose(CMC), respectively, when the inner cylinder rotates at the speed of 0∼500rpm. The effect of rotation on the skin friction is significantly dependent on the flow regime. In all flow regimes, the skin friction coefficient is increased by the inner cylinder rotation. The change of skin friction coefficient corresponding to the variation of rotating speed is large for the laminar flow regime, whereas it becomes smaller as Re increases for the transitional flow regime and, then, it gradually approach to zero for the turbulent flow regime.

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Turbulent Drag Reduction Using the Sliding-Belt Device (미끄러지는 벨트 장치를 이용한 난류 항력 감소)

  • Choi, Byunggui;Choi, Haecheon
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.23 no.11
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    • pp.1481-1489
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    • 1999
  • The sliding-belt concept introduced by Bechert et al. (AIAA J., Vol. 34, pp. 1072~1074) is numerically applied to a turbulent boundary layer flow for the skin-friction reduction. The sliding belt is moved by the shear force exerted on the exposed surface of the belt without other dynamic energy input. The boundary condition at the sliding belt is developed from the force balance. Direct numerical simulations are performed for a few cases of belt configuration. In the ideal case where the mechanical losses associated with the belt can be ignored, the belt velocity increases until the integration of the shear stress over the belt surface becomes zero, resulting in zero skin friction on the belt. From practical consideration of losses occurred In the belt device, a few different belt velocities are given to the sliding belt. It is found that the amount of drag reduction is proportional to the belt velocity.

Experimental Study on the Vortex Flow in a Concentric Annulus with a Rotating Inner Cylinder

  • Kim, Young-Ju;Hwang, Young-Kyu
    • Journal of Mechanical Science and Technology
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    • v.17 no.4
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    • pp.562-570
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    • 2003
  • This experimental study concerns the characteristics of vortex flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one is rotating. Pressure losses and skin friction coefficients have been measured for fully developed flows of water and of 0.4% aqueous solution of sodium carboxymethyl cellulose (CMC), respectively, when the inner cylinder rotates at the speed of 0~600 rpm. Also, the visualization of vortex flows has been performed to observe the unstable waves. The results of present study reveal the relation of the bulk flow Reynolds number Re and Rossby number Ro with respect to the skin friction coefficients. In somehow, they show the existence of flow instability mechanism. The effect of rotation on the skin friction coefficient is significantly dependent on the flow regime. The change of skin friction coefficient corresponding to the variation of rotating speed is large for the laminar flow regime, whereas it becomes smaller as Re increases for the transitional flow regime and. then, it gradually approach to zero for the turbulent flow regime. Consequently, the critical (bulk flow) Reynolds number Re$\_$c/ decreases as the rotational speed increases. Thus, the rotation of the inner cylinder promotes the onset of transition due to the excitation of Taylor vortices.

Analysis of the Shaft Resistance of a Pile Embedded in Sand Responding to Ground Deformation by Model Tests of Simulated Ground Heaving (실내모형실험을 통한 지반 융기시 사질토 지반에 매설된 지반 변형 대응형 말뚝의 주면 마찰 저항 분석)

  • Shin, Sehee;Lee, Kicheol
    • Journal of the Korean Geotechnical Society
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    • v.39 no.1
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    • pp.5-14
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    • 2023
  • The pile driving process may lead to ground heaving, causing additional positive skin friction to act on the piles, compromising their stability. This study proposes a new pile foundation type that can reduce positive skin friction. This was investigated by designing and constructing a pile with a hydraulic cylinder which actively responds to ground deformation. The newly proposed pile design was compared against traditional piles in multiple model tests where ground heaving was simulated. In the tests, base load and total shaft resistance were measured during ground heaving and with expansion of the hydraulic cylinder. As a result of the tests, a very small amount of expansion of the hydraulic cylinder member completely reduced the positive skin friction and increased the base load. Excessive expansion of the hydraulic cylinder, however, generates negative skin friction beyond the zero skin friction state. Therefore, it is necessary to estimate the appropriate level of hydraulic cylinder expansion, taking into account the amount of ground heaving and the allowable displacement of the pile.

Effect of Lecithin on Dermal Safety of Nanoemulsion Prepared from Hydrogenated Lecithin and Silicone Oil

  • Bae, Duck-Hwan;Shin, Jae-Sup;Shin, Gwi-Su;Jin, Fan-Long;Park, Soo-Jin
    • Bulletin of the Korean Chemical Society
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    • v.30 no.4
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    • pp.821-824
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    • 2009
  • In this study, a hydrogenated lecithin-containing nanoemulsion was prepared from hydrogenated lecithin and silicone oil. Tween-60 and liquid paraffin, widely known emulsifiers, were used as standard substances, and high shear was produced by utilizing a high shear homogenizer and microfluidizer. The properties of the nanoemulsion prepared with hydrogenated lecithin were evaluated by measuring interfacial tension, dynamic interfacial tension, droplet size, zeta-potential, friction force, skin surface hygrometery, and dermal safety. The interfacial tension of lecinol S10/silicone oil was lower than that of lecinol S10/liquid paraffin. The nanoemulsion prepared from hydrogenated lecithin shows lower zeta-potential, skin surface hygrometery, and friction force compared with a general emulsion. The silicone nanoemulsion prepared from hydrogenated lecithin showed a zero value in the patch test and thus exhibits high dermal safety.

The effects of tripping structure on the development of turbulent boundary layer subjected to adverse pressure gradient (역압력 구배가 존재하는 난류 경계층의 발달에 트리핑 구조물이 미치는 영향에 관한 연구)

  • 임태현;김대성;윤순현
    • Proceedings of the Korean Society of Marine Engineers Conference
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    • 2001.11a
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    • pp.36-44
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    • 2001
  • The effects of various tripping structures on turbulent boundary layer subjected to adverse pressure gradient were examined. The profiles are compared to zero pressure gradient and adverse pressure gradient. The increases of tripping structures of height, k are affects almost flow parameter included velocity fluctuation, skin friction coefficient and turbulent boundary thickness.

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A Study on the Plane Couette Flow Using Micropolar Fluid Theory

  • Kim, Youn-Jea;Kim, Tae-An
    • Journal of Mechanical Science and Technology
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    • v.18 no.3
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    • pp.491-498
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    • 2004
  • An analysis of the plane Couette flow between two parallel plates of a viscous, incompressible, micropolar fluid is presented. Especially, the effects of non-zero values of the micro-gyration boundary condition coefficient and pressure gradient on the flow fields are studied. Numerical results show that the micro polar parameter was found to have much more of an impact on the flow behaviors. It is also observed that the micro-gyration boundary condition coefficient influenced on the coefficients of skin friction and couple stress due to its different effect on the surface stress.