• Title/Summary/Keyword: Air drag reduction

검색결과 76건 처리시간 0.021초

Bioinspired Nanoengineering of Multifunctional Superhydrophobic Surfaces

  • Choi, Chang-Hwan
    • 한국표면공학회:학술대회논문집
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    • 한국표면공학회 2015년도 추계학술대회 논문집
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    • pp.102-133
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    • 2015
  • Nature, such as plants, insects, and marine animals, uses micro/nano-textured surfaces in their components (e.g., leaves, wings, eyes, legs, and skins) for multiple purposes, such as water-repellency, anti-adhesiveness, and self-cleanness. Such multifunctional surface properties are attributed to three-dimensional surface structures with modulated surface wettability. Especially, hydrophobic surface structures create a composite interface with liquid by retaining air between the structures, minimizing the contact area with liquid. Such non-wetting surface property, so-called superhydrophobicity, can offer numerous application potentials, such as hydrodynamic drag reduction, anti-biofouling, anti-corrosion, anti-fogging, anti-frosting, and anti-icing. Over the last couple of decades, we have witnessed a significant advancement in the understanding of surface superhydrophobicity as well as the design, fabrication, and applications of superhydrophobic coatings/surfaces/materials. In this talk, the designs, fabrications, and applications of superhydrophobic surfaces for multifunctionalities will be presented, including hydrodynamic friction reduction, anti-biofouling, anti-corrosion, and anti-icing.

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하드 디스크 드라이브용 패드 슬라이더의 트라이볼로지 특성에 관한 실험적 연구 (Experimental Analysis of Tribological Performances of Padder Slider in HDD)

  • 홍수열;좌성훈;고정석;이형재
    • Tribology and Lubricants
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    • 제17권4호
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    • pp.312-320
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    • 2001
  • In magnetic hard disk drives, higher areal recording density requires reduction of head-disk spacing. To overcome the increase of stiction associated with reduction of head-disk spacing, a padder slider, which adds pads to slider's air bearing surface, can be one of the practical solution for sub 20 nm flying height, and even for near contact recording. This study investigated the tribological characteristics of a padder slider. A padder slider took off slowly but showed less friction force than a normal slider. The hot/dry CSS test and drag test indicated that pad wear of a padder slider was negligible. The tribological performance of disk is an important factor to be considered. In particular, less carbon overcoat layer of the disk will result in higher stiction and wear in slider/disk interface. In conclusion, a padder slider shows encouraging tribological performances for practical use in HDD.

고분자물질 및 계면활성제의 유동마찰 저감 특성 비교 연구 (A Comparison Study on Flow-Friction Characteristic of Polymer Solution and Surfactant as Drag Reduction Additive)

  • 하재순;유재성;김성수;조성환;윤석만;엄재식
    • 대한설비공학회:학술대회논문집
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    • 대한설비공학회 2009년도 하계학술발표대회 논문집
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    • pp.863-868
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    • 2009
  • The drag reduction(DR) for Betaine+Amin and Xantan Gum as kinds of surfactant and Polyacrylamide as kinds of polymer solution according to the fluid velocity, temperature and surfactant concentration were compared experimentally. For this study, two kinds of experimental apparatus for short time and long time measurement were established. Each experimental appratus was equipped with hot water storage tanks, pumps, testing pipe network, flowmeter, two pressure gauges and data logging system was built for them. Results showed that Betaine+Amin and Xanthan Gum as kinds of surfactant had appeared optimal DR around 200-500 ppm and their DR tended to be decreased when flow velocity increased but Polyacrylamide as kinds of polymer solution showed the opposite trend to be increased when flow velocity increased. The both of them showed above 40% DR in the case of better condition by the short term measurement. But Polyacrylamide as kinds of polymer solution showed more degradation than Betaine+Amin and Xanthan Gum as kinds of surfactant by the long term measurement. As a result, Betaine+Amin and Xanthan Gum as kinds of surfactant showed better materials to use to the district heating system.

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수목의 초미세먼지(PM2.5) 저감 효과에 대한 CFD 수치 모의: 부산 감만동 지역을 대상으로 (CFD Simulations of the Trees' Effects on the Reduction of Fine Particles (PM2.5): Targeted at the Gammandong Area in Busan)

  • 한상철;박수진;최원식;김재진
    • 대한원격탐사학회지
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    • 제38권5_3호
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    • pp.851-861
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    • 2022
  • 본 연구는 전산유체역학(computational fluid dynamics, CFD) 모델을 이용하여 도시 지역에서 수목이 PM2.5 저감에 미치는 영향을 조사하였다. 현실적인 수치 모의를 위해, 기상청에서 현업으로 운영 중인 국지예보시스템(LDAPS)이 예측한 기상 자료를 CFD 모델의 초기·경계 자료로 사용하였다. CFD 모델 성능 검증은 연구 대상지 내에 구축된 6개의 센서에서 측정한 PM2.5 농도를 이용하였다. 본 연구에서는 수목이 PM2.5 농도 분포에 미치는 영향을 분석하기 위하여, 수목이 식재 되지 않았다고 가정한 경우, 수목이 식재되어 있지만 바람에 대한 항력 효과만 존재한다고 가정한 경우, 수목의 항력 효과와 침적 효과가 모두 존재한다고 가정한 경우에 대한 수치 실험을 수행하였다. 분석대상 기간 동안 PM2.5 저감 효과가 뚜렷하게 나타난 세 가지 영역 중 군부대 내의 PM2.5 평균 농도를 비교한 결과, 수목이 식재되지 않은 경우는 12.8 ㎍ m-3, 수목의 항력 효과만 고려한 경우는 12.5 ㎍ m-3이 나타났고, 수목의 항력 효과와 침적 효과가 모두 고려한 경우는 6.8 ㎍ m-3가 나타났다. 수목에 의한 건성 침적이 PM2.5 농도를 감소시키는 효과가 있는 것으로 확인되었다.

실린더 형상 변화가 실린더 주위 강제대류에 미치는 영향에 관한 수치적 연구 (A Numerical Study on the Effect of the Cylinder Shape Modification on the Forced Convection Around a Circular Cylinder)

  • 김민호;하만영;윤현식;이진욱
    • 설비공학논문집
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    • 제23권10호
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    • pp.670-677
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    • 2011
  • Direct numerical simulation are performed in order to investigate the effect of the circular cylinder shape on the forced convection around a circular cylinder at the Reynolds number of 300 and Prandtl number of 0.71. Three-dimensional characteristics of fluid flow and heat transfer around the smooth, wavy and torsional cylinders are investigated. A wavy cylinder has the sinusoidal variation in the cross sectional area along the spanwise direction with the wave length of ${\pi}/3$ and wavy amplitude of 0.1. A torsional cylinder has the twisted elliptic cross section with a torsional period of ${\pi}/2$ and an axis ratio of 1.35 corresponding to the major axis of 1.15 and the minor axis of 0.85. The value of time-and surface-averaged drag coefficient for the smooth cylinder is similar to that for the wavy cylinder, but larger than that for the torsional cylinder. The time and surface-averaged lift coefficient for the smooth cylinder is larger than that for the wavy and torsional cylinders. The time-averaged local heat transfer rate for the wavy and torsional cylinders shows different distribution along the circumferential direction, compared to that for the smooth cylinder because of the shape change in the spanwise direction for the cases of the wavy and torsional cylinders.

국소적 초음파 가진이 난류경계층에 미치는 영향 (Influence of Local Ultrasonic Forcing on a Turbulent Boundary layer)

  • 박영수;성형진
    • 한국가시화정보학회:학술대회논문집
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    • 한국가시화정보학회 2005년도 추계학술대회 논문집
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    • pp.17-22
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    • 2005
  • An experimental study was carried out to investigate the effect of local ultrasonic forcing on a turbulent boundary layer. Stereoscopic particle image velocimetry (SPIV) was used to probe the characteristics of the flow. A ultrasonic forcing system was made by adhering six ultrasonic transducers to the local flat plate. Cavitation which generates uncountable minute air-bubbles having fast wall normal velocity occurs when ultrasonic was projected into water. The SPIV results showed that the wall normal mean velocity is increased in a boundary layer dramatically and the streamwise mean velocity is reduced. The skin friction coefficient ($C_{f}$) decreases $60\%$ and gradually recovers at the downstream. The ultrasonic forcing reduces wall-region streamwise turbulent intensity, however, streamwise turbulent intensity is increased away from the wall. Wall-normal turbulent intensity is almost the same near the wall but it increases away from the wall, In tile vicinity of the wall, Reynold shear stress, sweep strength and production of turbulent kinetic energy were decreased. This suggests that the streamwise vortical structures are lifted by ultrasonic forcing and then skin friction is reduced.

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원관내 수직상향 2상유동에서 고분자물질이 유동양식에 미치는 영향 (The Effect of Flow Patterns with Polymer Additivies From Two Phase Flow at Vertical up Ward in Circular Tube)

  • 김재근
    • Journal of Advanced Marine Engineering and Technology
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    • 제22권4호
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    • pp.505-514
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    • 1998
  • Flow pattern of air-water two phase flow depends on the conditions of pressure void fraction and channel geometry. We classify the flow pattern by measuring the output signal of the conductivity probe. under the classified flow pattern we mount a visualization equipment on the test section and take pictures. We vary the concentration of pure solvent and polymer to measure local void fraction. We know that the maximum point position of local void fraction distribution move from the center of the pipe to the wall of the pipe as JSL increase when JSA is constant in two phase flow. But we find that the maximum point position of local void friction move from the wal of the pipe to the center of the pipe when polymer concentration increase.

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리블렛 표면에서 유적의 젖음성에 대한 수치 해석 (Wettability Simulation of Oil Droplet on Riblet Surface)

  • 김태완
    • Tribology and Lubricants
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    • 제35권2호
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    • pp.94-98
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    • 2019
  • The riblet structure like shark skin has been widely studied owing to its drag reduction and anti-fouling properties. In this study we simulated the wettability of an oil droplet on a riblet surface. We developed a numerical analysis method using the Wenzel equation and Cassie-Baxter equation that can estimate the contact angle with a penetrated depth of the droplet on rough surfaces. Riblet surfaces with nine scales composed of five hemi-elliptical ribs are generated numerically. The variation of contact angles with fractional depth of penetration for the generated riblet surfaces with and without coatings is demonstrated in the condition of solid-air-oil and solid-water-oil interfaces. The contact angle for the uncoated surface decreases with increasing fractional depth of penetration more drastically than that for the coated surface. For the effect of surface roughness on the contact angle of the droplet, the oleophilic surface gives lower contact angle when the surface is rougher, whereas the oleoophobic surface gives higher contact angle with higher roughness To verify the analysis results, the wetting angle was measured in the solid-air-oil interface and solid-water-oil interface for the shark-skin template and shark-skin replica. The effects of teflon coating were also evaluated. It is shown that the simulation results cover the experimental ones.

선저부 공기공동을 이용한 실선선형의 저항성능 개선을 위한 선저형상 개량연구 (On the Suitable Shape of Bottom for the Application of Air Cavity on Hull Bottom to the Practical Hull Form)

  • 고석천;김효철
    • 대한조선학회논문집
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    • 제36권3호
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    • pp.1-7
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    • 1999
  • 본 논문에서는 충주호에서 유람선으로 사용되고 있는 광폭천흘수형 선박을 대상으로 선정하고, 공기를 공급하여 선저에 공동을 형성시켜 마찰저항을 감소시킬 수 있도록 선박의 선저형상을 개량하였다. 2차원 공동문제로 이상화하여 선저에 부착되는 공동을 이론적 방법으로 수치해석하여 공기공동의 형상과 내부압력을 추정하였고 공동현상을 지배하는 무차원수를 살펴보았다. 예인수조에서 모형선실험을 시행하고 단의 높이변화, 선측의 공기유출막이벽 등에 의한 공기공동의 형성과 저항감소효과를 조사하였다. 또한 투명한 아크릴로 제작된 선저부를 통해, 선저에 부착되는 공기공동의 발달과정을 관측하였다. 이러한 연구를 통하여, 실선선형의 선저부 형상을 적절히 개량하고 공기를 공급함으로써 설계속도 부근에서 원래선형의 전저항의 25%정도를 줄일 수 있음을 모형선에서 확인하였다.

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고속 어뢰의 인공 초공동 특성에 대한 실험 연구 (Experimental Study on Artificial Supercavitation of the High Speed Torpedo)

  • 안병권;정소원;김지혜;정영래;김선범
    • 한국군사과학기술학회지
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    • 제18권3호
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    • pp.300-308
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    • 2015
  • Recently supercavitating underwater torpedo moving at high speed (over 200 knots) has been interested for their practical advantage of the dramatic drag reduction. Cavitator located in front of the torpedo plays an important role to generate a natural supercavity and control the motion of the object. Supercavity can be created artificially by injection of compressed gas from the rear of the cavitator at a relatively low speed. In this paper, we investigated physical characteristics of artificial supercavities through cavitation tunnel experiments. One of the main focuses of the study was to measure pressure inside the cavity, and examined variation of the gravity effects appearing according to different amount of injected air. It was also found that a stable supercavity could be sustained at injection rates less than that required to form the stable supercavity because of hysteresis effect.