• Title/Summary/Keyword: Friction Drag

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COARSE GRID LARGE-EDDY SIMULATION OF FLOW OVER A HEAVY VEHICLE (화물차 주위 유동의 성긴 격자 큰에디모사)

  • Lee, S.;Kim, M.;You, D.;Kim, J.J.;Lee, S.J.
    • Journal of computational fluids engineering
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    • v.21 no.1
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    • pp.30-35
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    • 2016
  • In order to investigate effects of grid resolution on large-eddy simulation of flow over a heavy vehicle, large-eddy simulations over the vehicle with coarse grid and fine grid are conducted. In addition, comparison of drag coefficients with the experimental data obtained by a wind tunnel experiment is conducted. Both of the drag coefficients of coarse grid and fine grid large-eddy simulation show good agreement with the experimental data. Flow fields obtained by the coarse and the fine grid large-eddy simulation are compared in the vehicle frontal-face region, the vehicle rear wheel region, and the vehicle base region. Coarse grid large-eddy simulation shows good agreement with the fine grid large-eddy simulation in the vehicle front face region and the vehicle rear wheel region, since the flow over the present vehicle is dominated by flow separation which is geometrically pre-determined, not by the skin friction which is known to be sensitive to grid resolution.

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

  • Ha, Jae-Sun;Ryu, Jae-Sung;Kim, Seong-Su;Cho, Sung-Hwan;Yoon, Seok-Mann;Eom, Jae-Sik
    • Proceedings of the SAREK Conference
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    • 2009.06a
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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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A Study on Characteristics of Drag Reduction Additive under High Temperature Range (고온영역에서 계면활성제의 마찰저감 성능 특성에 관한 연구)

  • Cho, Sung-Hwan;Ryu, Jae-Sung;Jung, Sang-Hoon
    • Journal of Energy Engineering
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    • v.19 no.2
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    • pp.116-120
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    • 2010
  • Overall total length of hydraulic pipe to transport the hot water in the domestic district heating network is above 3,000 Km approximately. This long pipe network requires a lots of the transport pumping power by surface friction of fluid. In this study, the drag reduction(DR) of Amin Oxide $C_{18}$ as non-ionic surfactant according to the fluid velocity, temperature and surfactant concentration under the condition of above $80^{\circ}C$ fluid temperature were investigated experimentally. Results showed that new amin oxide $C_{18}$ surfactant had DR of maximum 30% in fluid temperature of $80^{\circ}C$ and had 15% DR in fluid temperature over $100^{\circ}$ under short time test condition. And amine oxide had 155 hours duration time to keep the DR characteristic in the fluid temperature of $80^{\circ}$ and 1000 ppm concentration. But duration time of DR was decreased when fluid temperature increased.

CFD Analysis of Aerodynamic Characteristics of a BWB UCAV configuration with Transition effect (천이효과를 고려한 BWB UCAV 형상의 공력 특성 전산해석)

  • Jo, Young-Hee;Chang, Kyoungsik;Sheen, Dong-Jin;Park, Soo Hyung
    • Journal of the Korean Society for Aeronautical & Space Sciences
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    • v.42 no.7
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    • pp.535-543
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    • 2014
  • A computational simulation for a nonslender BWB UCAV configuration with rounded leading edge and span of 1.0m was performed to analyze its aerodynamic characteristics. The freestream is 50m/s over -4 to 26 degree A.o.A.s. Reynolds number based on the mean chord length is $1.25{\times}10^6$. 3D multi block hexahedral grids are used which allow good grid quality and ease to capture boundary layer. ${\gamma}-Re_{\theta}$ model as well as $k-{\omega}$ SST model is employed to assess the effect of transition for flow behavior. Drag and lift of the UCAV were well predicted while $C_M$ is under predicted at high angle of attacks and influenced by the turbulence models strongly. After assessing pressure distribution, skin friction lines and velocity field around the UCAV configuration, it was found that transition effect should be considered to enhance the prediction of aerodynamic behavior by a vortical flowfield.

Wall-roughness effects of trapezoidal ribs on the flow of open channel (개수로 흐름에서 사다리꼴 돌출줄눈의 벽면조도 효과)

  • Shin, Seung Sook;Park, Sang Deog;Park, Ho Kook
    • Journal of Korea Water Resources Association
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    • v.52 no.4
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    • pp.255-264
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    • 2019
  • The trapezoidal ribs had been installed in the retaining wall in order to reduce to flood damage in the impingement of mountain rivers. In this study, experiments in open channel with the trapezoidal ribs on sidewall were conducted to evaluate the effect of flow resistance by the trapezoidal shape. The hydraulic flow characteristics according to the flow rates were surveyed where the wall roughness is k-type that dimensionless spacings, ${\lambda}_{nv}$, are 6, 9, and 12. The flow-resistance factors such as roughness and friction coefficients increased generally with increase of the spacing of ribs. In high flow rate the friction coefficient showed the maximum value when ${\lambda}_{nv}$ is 9. Though the trapezoidal ribs has the relatively smaller flow resistance compared to the square ribs, their form drag accounted for mean 62% of the total flow resistance. It was confirmed that the optimal spacing of trapezoidal ribs to maximize the effect of flow resistance as the wall roughness increases are 9 to 12 times of the height of trapezoidal ribs.

Study on the Effects of Surface Roughness and Turbulence Intensity on Dam-break Flows (댐 붕괴 유동에 미치는 표면 거칠기와 난류강도 변화의 영향 연구)

  • Park, Il-Ryong;Jung, Kwang-Hyo
    • Journal of the Society of Naval Architects of Korea
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    • v.49 no.3
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    • pp.247-253
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    • 2012
  • Dam-break flows, a type of very shallow gravity-driven flow, are substantially influenced by resistance forces due to viscous friction and turbulence. Assuming turbulent flow, the main focus of this study is to validate the increase of drag forces caused by surface roughness and especially turbulence intensity. A Reynolds Averaged Navier-Stokes(RANS) approach with the standard k-${\varepsilon}$ turbulence model is used for this study, where the free surface motion is captured by using a volume of fluid(VOF) method. Surface roughness effects are considered through the law of the wall modified for roughness, while the initial turbulence intensity which determines the lowest level of turbulence in the flow domain of interest is used for the variation of turbulence intensity. It has been found that the numerical results at higher turbulence intensities show a reasonably good agreement with the physical aspects shown by two different dam-break experiments without and with the impact of water.

Application of an integro-differential equation to the analysis of geotechnical problems

  • Poorooshasb, H.B.;Alamgir, M.;Miura, N.
    • Structural Engineering and Mechanics
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    • v.4 no.3
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    • pp.227-242
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    • 1996
  • An important class of problems in the field of geotechnical engineering may be analyzed with the aid of a simple integro-differential equation. Behavior of "rigid" piles(say concrete piles), "deformable" piles(say gravel piles), pile groups, pile-raft foundations, heavily reinforced earth, flow within circular silos and down drag on cylindrical structures (for example the crusher unit of a mineral processing complex) are the type of situations that can be handled by this type of equation. The equation under consideration has the form; $$\frac{{\partial}w(r,\;z)}{{\partial}z}+f(z){\int}^z_0g({\xi})(\frac{{\partial}^2w(r,\;{\xi})}{{\partial}r^2}+\frac{1}{r}\frac{{\partial}w(r,\;{\xi})}{{\partial}r})d{\xi}+h(r,\;z)=0$$ where w(r, z) is the vertical displacement of a soil particle expressed as a function of the polar cylindrical space coordinates (r, z) and the symbols f, g and h represent soil properties and the loading conditions. The merit of the analysis is its simplicity (both in concept and in application) and the ease with which it can be expressed in a computer code. In the present paper the analysis is applied to investigate the behavior of a single rigid pile to bedrock. The emphasis, however, is placed on developing the equation, the numerical techique used in its evaluation and validation of the technique, hereafter called the ID technique, against a formal program, CRISP, which uses the FEM.

Controlling the Depth of Microchannels Formed during Rolling-based Surface Texturing

  • Bui, Quang-Thanh;Ro, Seung-Kook;Park, Jong-Kweon
    • Journal of the Korean Society of Manufacturing Technology Engineers
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    • v.25 no.6
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    • pp.410-420
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    • 2016
  • The geometric dimension and shape of microchannels that are formed during surface texturing are widely studied for applications in flow control, and drag and friction reduction. In this research, a new method for controlling the deformation of U channels during micro-rolling-based surface texturing was developed. Since the width of the U channels is almost constant, controlling the depth is essential. A calibration procedure of initial rolling gap, and proportional-integral PI controllers and a linear interpolation have been applied simultaneously to control the depth. The PI controllers drive the position of the pre-U grooved roll as well as the rolling gap. The relationship between the channel depth and rolling gap is linearized to create a feedback signal in the depth control system. The depth of micro channels is studied on A2021 aluminum lamina surfaces. Overall, the experimental results demonstrated the feasibility of the method for controlling the depth of microchannels.

The study of turbulent flow structures in a wavy channel using direct numerical simulation (직접수치모사를 통한 Wavy Channel 내의 난류 유동 구조의 연구)

  • Lee, Dae-Sung;Ha, Man-Yeong;Yoon, Hyun-Sik;Chun, Ho-Hwan;Jeon, Chung-Hwan
    • Proceedings of the KSME Conference
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    • 2004.04a
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    • pp.1807-1812
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    • 2004
  • Sinusoidal wavy channel is one of the most commonly used devices in the industry for achieving mixing and heat transfer. Here we report on results obtained from the DNS of flow inside the wavy channel performed using the finite volume technique. As a primary stage to obtain the optimal design for heat transfer and mixing, this study observed the basic flow structures in a wavy channel. The mass flow rate is kept constant with friction Reynolds number of $Re_{\tau}$ = 140 . Time- and space-averaged and instantaneous flow fields are illustrated to observe the flow structures. Although the direct comparison of results between turbulent wavy and flat channel is somehow difficult due to the different flow phenomena derived from different configuration, here the mean streamwise velocity and RMS of velocities at same $Re_{\tau}$ of two different channels are compared. The basic difference between wavy and flat channel flow is the existence of small scale wall vortices along the walls in a wavy channel. These vortices make flow more complex, which will accompany the increase of heat transfer, pressure drop and drag.

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Experimental investigation of supercavitating flows

  • Ahn, Byoung-Kwon;Lee, Tae-Kwon;Kim, Hyoung-Tae;Lee, Chang-Sup
    • International Journal of Naval Architecture and Ocean Engineering
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    • v.4 no.2
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    • pp.123-131
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    • 2012
  • When the object is traveling in the water at tremendously high speeds, the cavity forms and grows up at a fore part of the object called cavitator, and the object is eventually enveloped by vaporized water, supercavitation. As a result, the only part of the object in direct contact with the water is the cavitator, so skin-friction drag is significantly reduced. This is why recently supercavitating objects have been interested in many applicable fields. In this study we are focused out attention on supercavitating flows around various shapes of two and three dimensional cavitators. First, general features of supercavitation are examined by analyzing results obtained by the previously developed numerical method. Second, experimental observations are carried out at a cavitation tunnel at the Chungnam National University (CNU CT), and supercavity dimensions are scrutinized.