• Title/Summary/Keyword: drag reduction

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Active Control Methods for Drag Reduction in Flow over Bluff Bodies (뭉툭한 물체 주위 유동에서 항력 감소를 위한 능동 제어 방법)

  • Choi Haecheon
    • Proceedings of the KSME Conference
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    • 2002.08a
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    • pp.11-16
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    • 2002
  • In this paper, we present two successful results from active controls of flows over a circular cylinder and a sphere for drag reduction. The Reynolds number range considered for the flow over a circular cylinder is 40-3900 based on the free-stream velocity and cylinder diameter, whereas for the flow over a sphere it is $10^{5}$ based on the free-stream velocity and sphere diameter. The successful active control methods are a distributed (spatially periodic) forcing and a high-frequency (time periodic) forcing. With these control methods, the mean drag and lift fluctuations decrease and vortical structures are significantly modified. For example, the time-periodic forcing at a high frequency (larger than 20 times the vortex shedding frequency) produces $50{\%}$ drag reduction for the flow over a sphere at $Re=10^{5}$. The distributed forcing applied to the flow over a circular cylinder results in a significant drag reduction at all the Reynolds numbers investigated.

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Drag reduction of a disk with an upstream rod

  • Zhang, Panfeng;Gao, Lei;Wang, Jinjun
    • Wind and Structures
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    • v.9 no.3
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    • pp.245-254
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    • 2006
  • The pressure and drag measurements were carried out in the wind tunnel to investigate the drag reduction of the disk by using an interference rod placed upstream. The results indicate that there is a pair of standing vortices in the front stagnation region of the disk induced by the rod. The standing vortices can decrease the pressure on the disk upwind side; hence it can reduce the drag of the disk. With an increasing rod diameter, the standing vortices are strengthened and more drag reduction can be achieved for the disk. With rod diameter d/D = 0.05 (d, D are the diameters of rod and disk, respectively), the total drag of the disk can be reduced by about 9% compared with that of the bare disk.

Turbulent Flow Analysis and Drag Reduction by Riblet Surfaces (리블렛 표면을 이용한 난류 유동해석 및 마찰 저항감소)

  • 윤현식;구본국;전호환
    • Journal of the Society of Naval Architects of Korea
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    • v.41 no.4
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    • pp.59-67
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    • 2004
  • Direct numerical simulations of turbulent flows over riblet-mounted surfaces are performed to educe the mechanism of drag reduction by riblets. Numerical simulations are performed for flow fields with R $e_$\tau$/=180. For riblet ridge angle $\alpha$=60$^{\circ}$, two different riblet spacings of $s^+/=20 and 40 are used in this study. The computed drag on the riblet surfaces is in good agreement with existing computational and experimental data. The mean velocity profiles show upward and downward shifts in the log-law for drag-decreasing and drag-increasing cases, respectively Turbulence statistics above the riblets are computed and compared with those above a flat plate. The purpose of this study is in two categories: first, to understand the drag reduction mechanism on riblet surface, second, to verify our own code by comparison of the present results with those from previous studies.udies.

The Study on the Drag Reduction for Gas/Liquid Two Phase Flow (기-액(氣-液) 2상유동(二相流動)시 항력(抗力)에 관(關)한 연구(硏究))

  • Cha, K.O.;Oh, Y.K.;Kim, J.G.
    • Journal of ILASS-Korea
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    • v.1 no.3
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    • pp.20-28
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    • 1996
  • It is well known that drag reduction in single phase liquid flow is affected by polymer material, molecular weight, polymer concentration, pipe diameter, and flow velocity. Drag reduction in two phase flow can be applied to the transport of crude oil, phase change system such as chemical reactor, pool and boiling flow, and to present cavitation which occurs in pump impellers. But the research of drag reduction in two phase flow is not sufficient. The purpose of the present work is to evaluate the drag reduction by measuring pressure drop, void fraction whether polymer is added in the horizontal two phase system or not. Experiment has been conducted in a test section with 24 m of the inner diameter and 1,500 mm of the length. The used polymer materials are two kinds of polyacrylamide[PAAM] and co-polymer[A611P]. The polymer concentration was varied with 50, 100 and 200 ppm under the same experimental conditions. Experimental results were shown that the drag is higher reduced by co-polymer rather than polyanylamide.

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Drag Reduction Effect by Counter-flow Jet on Conventional Rocket Configuration in Supersonic/Hypersonic Flow

  • Kim, Yongchan;Kim, Duk-Min;Roh, Tae-Seong;Lee, Hyoung Jin
    • Journal of Aerospace System Engineering
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    • v.14 no.4
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    • pp.18-24
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    • 2020
  • The counter-flow jet from a supersonic/hypersonic vehicle causes a structural change in the shock wave generated around the aircraft, which can lead to reduced drag and heat loads. Since the idea is to mount a counter-flow jet device for drag reduction in the aircraft, it is necessary to understand the effect of such a device on the entire aircraft. In this study, the effect of drag reduction due to counter-flow jet on a conventional rocket configuration was analyzed through CFD analysis. The results showed that the drag reduction effect was the largest in the blunt region and that the counter-flow jet also affected the downstream of the aircraft. The analysis indicated that the drag reduction effect by the counter-flow jet was about 10 to 25 % when targeting the entire rocket-shaped area, while the effect was as high as 50% when targeting only blunt objects.

Drag Reduction Induced by Increased Kinematic Viscosity of Nanofluids Containing Carbon Nanotubes in A Horizontal Tube (카본나노튜브 나노유체의 동점성계수 증가로 인한 관내 유동에서의 항력 감소)

  • Yu, Jiwon;Jung, Se Kwon;Choi, Mansoo
    • Particle and aerosol research
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    • v.9 no.4
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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.

A NUMERICAL STUDY ON THE EFFECT OF DOWN-WASH OF A WING-BODY ON ITS AERODYNAMIC CHARACTERISTICS (익형 동체의 하강기류(Down-wash)가 공기역학적 특성에 미치는 영향에 관한 수치해석연구)

  • Yoon, K.H.;Kim, C.H.
    • Journal of computational fluids engineering
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    • v.18 no.3
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    • pp.8-13
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    • 2013
  • Drag reduction of a running vehicle is very important issue for the energy savings and emission reduction of its power train. Especially for a solar powered electric vehicle, the drag reduction and weight lightening are two serious problems to be solved to extend its driving distance under the given energy condition. In this study, the ground effect of an airfoil shaped road vehicle was studied for an optimum body design of an ultra-light solar powered electric vehicle. Clark-Y airfoil type was adopted to the body shape of the model vehicle to reduce aerodynamic drag. From the study, it was found that the drag of the model vehicle was reduced as the height(h) between ground and the lower surface of the model vehicle was decreased. It is due to the reduction of the down-wash decreasing the induced drag of the vehicle. The lift was also decreased as the height decreased. It is due to the turbulent boundary layer developed beneath the vehicle body. The drag is classified into two types; the form and friction drag. The fraction of form drag to friction one is 76 to 24 on the model vehicle. As the height(h) of the model vehicle from the ground surface increases the form drag also increases but the friction drag is in reverse.

Drag Reduction by Polymer and Surfactant in Tubulent Channel and Pipe Flows (난류 유동일때 관과 channel에서 고분자와 계면활성제에 의한 마찰저항 감소에 관한 연구)

  • Park, S.-R.
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.7 no.3
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    • pp.359-365
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    • 1995
  • The drag reduction phenomenon with an additives of surfactant(STAC, stearlytrimethyl ammonium chloride) and polymer(PEO, polyethlene oxide) was investigated in fully developed turbulent pipe and channel flows at various low Reynolds numbers as well as very low additives concentration. A maximum of 70% drag reduction compared with plain water flow was found. This maximum drag reduction percentage obtained with surfactant solution was slightly higher than that of the Virk's asymptote in polymer solution.

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On the energy economics of air lubrication drag reduction

  • Makiharju, Simo A.;Perlin, Marc;Ceccio, Steven L.
    • International Journal of Naval Architecture and Ocean Engineering
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    • v.4 no.4
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    • pp.412-422
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    • 2012
  • Air lubrication techniques for frictional drag reduction on ships have been proposed by numerous researchers since the 19th century. However, these techniques have not been widely adopted as questions persist about their drag reduction performance beyond the laboratory, as well as energy and economic cost-benefit. This paper draws on data from the literature to consider the suitability of air lubrication for large ocean going and U.S. Great Lakes ships, by establishing the basic energy economic calculations and presenting results for a hypothetical air lubricated ship. All the assumptions made in the course of the analysis are clearly stated so that they can be refined when considering application of air lubrication to a specific ship. The analysis suggests that, if successfully implemented, both air layer and partial cavity drag reduction could lead to net energy savings of 10 to 20%, with corresponding reductions in emissions.

Experimental study on the drag reduction of a helmet for paragliding (패러글라이딩 헬멧의 항력 감소에 관한 실험적 연구)

  • Hwang, Jongbin;Park, Jungmok;Song, Jinseok;Kim, Jooha
    • Journal of the Korean Society of Visualization
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    • v.19 no.3
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    • pp.46-53
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    • 2021
  • In the present study, wind tunnel experiments were performed to reduce the drag of a paragliding helmet in the range of Reynolds numbers from 46,000 to 155,000. The drag force of the helmet model with dimples and deflectors installed was measured by varying the dimple depth and the slant angle of the deflector. The dimples were effective in reducing the drag at low Reynolds numbers, but no significant drag reduction was found in the Reynolds number range in which an actual paraglider flight takes place. On the other hand, the deflector installed tangentially to the side outline of the helmet showed an average drag reduction of 7% in the flight Reynolds number range of real paragliding. This was because the deflector shrunk the size of the wake region and moved the wake region downstream of the deflector.