• 한국입자에어로졸학회지
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• 제9권4호
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• pp.271-277
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• 2013

This article reports the drag reduction phenomenon of aqueous suspensions containing carbon nanotubes (CNTs) flowing through horizontal tubes. Stable nanofluids were prepared by using a surfactant. It is found that the drag forces of CNT nanofluids were reduced at specific flow conditions compared to the base fluid. It is found that the friction factor of CNT nanofluids was reduced up to approximately 30 % by using CNT nanofluids. Increased kinematic viscosities of CNT nanofluids are suggested to the key factors that cause the drag reduction phenomenon. In addition, transition from laminar to turbulent flow is observed to be delayed when CNT nanofluids flow in a horizontal tube, meaning that drag reduction occurs at higher flow rates, that is, at higher Reynolds numbers.

• 해양환경안전학회지
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• 제25권6호
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• pp.795-801
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• 2019

층류 경계층 내 반구에 의해 유기되는 말굽와류를 흡입 제어했을 때 후류영역에서의 마찰저항 변화를 측정하였다. 이를 위해 회류수조에서 유동가시화를 실시하여 최적의 자유유속, 반구 크기 및 흡입제어 구멍 크기를 결정하였고, 반구 후류영역에 설치된 평판과 연결된 동력계로 표면 마찰저항 감소를 측정하는 실험을 수행하였다. 평판에 설치된 반구 전방에는 유입 유동에 의해 반구를 감싸는 말굽와류가 생성되며 그 주위 와도 방향에 의해 후류영역으로는 빠른 유속의 유동이 유입되어 머리핀 와류 생성을 촉진시킨다. 따라서 반구 전방에 생성되는 말굽와류 세기를 흡입 제어에 의해 약화시킴으로써 반구 좌우측으로 길게 형성된 유속방향 와류가 후류영역으로 공급하는 에너지는 감소하게 된다. 즉, 반구 전방의 말굽와류를 제어함으로써 후류영역으로부터 생성되는 헤어핀 와류 발생 주파수가 줄어 들게 된다. 염료 주입을 이용한 유동 가시화 영상을 해석한 결과로 머리핀 와류의 발생 빈도가 흡입제어에 의해 36.4 % 감소되었고, 후류영역에서 측정된 표면 마찰저항은 2.3 % 감소되는 것으로 나타났다.

• KSTLE International Journal
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• 제2권2호
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• pp.150-153
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• 2001

The effects of humidity on friction performance of automotive brake materials were studied using a pad-on-disk type friction tester. Three different friction materials based on a simple formulation were investigated by changing the solid lubricant graphite, MoS$_2$, and Sb$_2$S$_3$. Friction materials without solid lubricants were also examined to study the effect of other ingredients in the matrix on humidity. The friction material containing graphite was strongly affected by the humidity showing lower friction coefficient at high humidity level than other conditions. On the other hand, the friction material containing MoS$_2$exhibited higher friction coefficient at initial stage under high humidity level. The friction material without solid lubricant or with Sb$_2$S$_3$ was not affected by humidity conditions. However, the friction material containing barite showed strong speed dependence.

• Tribology and Lubricants
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• 제13권3호
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• pp.10-19
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• 1997

Frictional characteristics of two different types of automotive friction materials were studied. They were non-asbestos organic and semi-metallic friction materials. The two friction materials were tested using an inertial brake dynamometer to investigate friction stability, rooster tailing phenomena, temperature change during drags and stops. Results show that the level of the friction force is strong functions of time, temperature, and speed regardless of the type of friction materials. In particular, rooster tailing effects are pronounced in the case of semi-metallic friction materials compared to non-asbestos organic friction materials. The phenomena appear strongly dependent on raw materials contained in the friction materials.

• 한국윤활학회:학술대회논문집
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• 한국윤활학회 1999년도 제29회 춘계학술대회
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• pp.92-100
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• 1999

Friction characteristics of automotive friction materials according to the types of phenolic resin were investigated by using a pad-on-disk type friction tester. Four different simplified friction materials bound with Bakelite$^{TM}$ and Xylok$^{TM}$ phenolic resin were studied in this work. Two different modes of drag test(constant initial temperature test and constant interval test) were employed to analyze the effects of the binders on friction characteristics. Friction materials containing modified Xylok$^{TM}$ resin showed good heat resistance and friction stability. The results also showed that aramid fiber played important roles in improving friction stability and weard wear

• 한국윤활학회:학술대회논문집
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• 한국윤활학회 1998년도 제28회 추계학술대회
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• pp.323-329
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• 1998

The effect of humidity on friction characteristics of friction materials were studied using a pad-on-disk type friction tester. Three different friction materials containing simplified ingredients were investigated by changing the solid lubricant, graphite, MoS$_2$, and $Sb_2S_3$. A friction material without solid lubricant was also examined to study the effect of ingredients in the matrix on humidity. Friction material containing graphite showed lower friction coefficient at high humidity level than other conditions, however, friction material containing MoS$_2$ indicated higher friction coefficient at initial stage under high humidity level. Friction materials without solid lubricant and with $Sb_2S_3$ were little affected by humidity conditions.

• 한국윤활학회:학술대회논문집
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• 한국윤활학회 1999년도 제29회 춘계학술대회
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• pp.255-264
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• 1999

Friction characteristics of automotive friction materials containing different metallic fibers rubbing against Al-MMC and cast iron disk specimens have been studied. Friction materials containing aluminum, copper or low steel carbon fiber were tested. Friction tests were composed of three different phases to investigate the effect of temperature, pressure, speed, and drag time. The results showed that the friction material containing Al fibers has lower friction force and wear amount than the others with Cu or Steel fiber. On the other hand, the wear of friction material was severe in the case of using Al-MMC rotors. These results showed that the thermal decomposition of solid lubricants (and organic components), formation of transfer layer, and SiC particles in the AI-MMC rotor play crucial roles in determining the friction characteristics.

• 한국항공우주학회지
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• 제48권7호
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• pp.489-495
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• 2020

충격파 터널에서 항력 측정을 위하여 가속도계 기반 시스템을 설계 및 구축하고, 마하 6 유동조건에서 원뿔형 모델의 항력계수를 측정하였다. 항력 측정 시스템의 마찰력을 보정하기 위하여 간단하고 직관적인 교정 방식을 제시하고, 항력 측정 결과를 단순 원뿔형 모델의 전산유체역학 해석 결과와 비교, 분석하였다. 다양한 형상의 지지대를 이용하여 지지대에 의한 항력 측정 간섭 영향을 파악하고 이를 최소화하기 위한 디자인을 제시하였다. 수정된 지지대를 이용하여 항력계수를 측정한 결과, 지지대에 의한 항력계수 오차가 작아지는 것이 확인되었다.

• Journal of Advanced Marine Engineering and Technology
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• 제21권2호
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• pp.126-135
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• 1997

Drag reduction in polymer solutions is the phenomenon where by extremely dilute solutions of high molecular weight polymers exhibit frictional resistance to flow much lower than the pure solvent. This effect, largely unexplained as yet, has attracted the attention of polymer scientists and fluid flow specialists. Although applications are beginning to appear, the principle interest to data has been in attempting to relate the effect to the fluid mechanics of turbulent flow. Drag reduction in two phase flow can be applied to the transport of crude oil, phase change system such as chemical reactor, and pool and boiling flow. But the research on drag reduction in two phase flow is not intensively investigated. Therefore, experimental investigations have been carried out to analyze the drag reduction produced by polymer addition in the single phase and two phase flow system. The objectives of the proposed investigation are primarily in identifying and developing high performance polymer additives for fluid transportations with the benefits of turbulent drag. Also we want to is to evaluate the drag reduction in horizontal flow by measuring pressure drop and mean velocity. Experimental results show higher drag reduction using co - polymer(A611P) then using polyacrylamide (PAAM) and faster degradation using PAAM than using A611P under the same superficial velocity.

• 한국윤활학회:학술대회논문집
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• 한국윤활학회 1998년도 제28회 추계학술대회
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• pp.330-336
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• 1998

The friction and wear characteristics of automotive friction materials reinforced with carbon fibers were studied using a direct drive brake dynamometer. Two types of model friction materials, a low-metallic and an NAO type, were prepared and each of the materials was modified by substituting 5 vol% of carbon fibers with other reinforcing fiber used in the model formulations. Drag tests were carried out to investigate the friction properties of these materials at various braking conditions. Results showed that the modified friction materials were improved in the friction stability and the wear resistance.