• 유체기계공업학회:학술대회논문집
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• 2006.08a
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• pp.191-194
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• 2006

In this paper, we present a detailed mechanism of drag reduction by dimples and roughness on a sphere by measuring the streamwise velocity above the dimpled and roughened surfaces, respectively. Dimples cause local flow separation and trigger the shear layer instability along the separating shear layer, resulting in generation of large turbulence intensity. With this increased turbulence, the flow reattaches to the sphere surface with high momentum near the wall and overcomes strong adverse pressure gradient formed in the rear sphere surface. As a result, dimples delay main separation and reduce drag significantly. The present study suggests that generation of a separation bubble, i.e. a closed-loop streamline consisting of separation and reattachment, on a body surface is an important flow-control strategy for drag reduction on a bluff body such as the sphere and cylinder. In the case of roughened sphere, the boundary layer flow is directly triggered by roughness and changes to a turbulent flow. Due to this change, the drag significantly decreases. As the Reynolds number further increases, transition to turbulence occurs earlier on the sphere surface. Because of faster growth of turbulent boundary layer by roughness, earlier transition thickens the boundary layer, resulting in earlier separation and drag increase with increasing Reynolds number

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.479-482
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• 2002

Riblets with longitudinal grooves along the streamwise direction have been used as an effective flow control technique for drag reduction. A flexible micro-riblet with v-grooves of peak-to-peak spacing of $300{\mu}m$ was made using a MEMS fabrication process of PDMS replica. The flexible micro-riblet was attached on the whole surface of a NACA0012 airfoil with which grooves are aligned with the streamwise direction. The riblet surface reduces drag coefficient about $7.9{\%}\;at\;U_o=3.3m/s$, however, it increases drag about $8{\%}\;at\;U_o=7.0m/s$, compared with the smooth airfoil without riblets. The near wake has been investigated experimentally far the cases of drag reduction ($U_o\;=\;3.3 m/s$) and drag increase ($U_o\;=\;7 m/s$). Five hundred instantaneous velocity fields were measured for each experimental condition using the cross-correlation PIV velocity field measurement technique. The instantaneous velocity fields were ensemble averaged to get spatial distribution of turbulent statistics such as turbulent kinetic energy. The experimental results were compared with those of a smooth airfoil under the same flow condition. The micro-riblet surface influences the near wake flow structure largely, especially in the region near the body surface

• Transactions of the Korean Society of Mechanical Engineers B
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• v.34 no.8
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• pp.799-807
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• 2010

Direct numerical simulations (DNS) of turbulent channel flow for which the drag is reduced by using polymer additives have been performed by a pseudo-spectral method. The Reynolds number based on the friction velocity and half-channel height is 395, and the polymeric stresses due to the polymer additives are evaluated using the FENE-P (finitely extensible nonlinear elastic-Peterlin) model. The numerical results show that the drag reduction rate is significantly affected by the parameters used in the FENE-P model, such as the maximum extensibility and relaxation time of the polymer molecules. The turbulence data for both low- and high-drag reduction regimes are analyzed. In addition, the effects of FENE-P model parameters on the flow characteristics have been investigated for the same drag reduction rate due to the polymer additives. Finally, the present DNS results have been used to verify the correlation between rheological parameters and the extent of drag reduction, which was suggested by Li et al. (2006).

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.2
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• pp.345-352
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• 2002

A suboptimal control law in turbulent pipe flow is derived and tested. Two sensing variables ∂p/∂$\theta$│$\_$w/ and ∂v$\_$$\theta$/∂γ│$\_$w/ are applied with two actuations ø$\_$$\theta$/ and ø$\_$γ/. To test the suboptimal control law, direct numerical simulations of turbulent pipe flow at Re$\_$$\tau$/=150 are performed. When the control law is applied, a 13∼23% drag reduction is achieved. The most effective drag reduction is made at the pair of ∂v$\_$$\theta$/∂γ│$\_$w/ and ø$\_$γ/. An impenetrable virtual wall concept is useful for analyzing the near-wall suction and blowing. The virtual wall concept is useful for analyzing the near-wall behavior of the controlled flow. Comparison of the present suboptimal control with that of turbulent channel flow reveals that the curvature effect is insignificant.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.8 no.3
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• pp.330-337
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• 1996

The effects of pump and temperature on drag reducing characteristics were investigated with a polymer(PAAM : Polyacrylamide) and three kinds of surfactants(CTAC, STAC, Habon-G) in fully developed turbulent pipe flows with various experimental parameters such as additive concentration(30~500ppm), pipe diameter(4.65mm, 10.85mm), Reynolds number($4{\times}10^4{\sim}10^5$) and working fluid temperature($20{\sim}80^{\circ}C$). The pump effect on PAAM was severe such that the drag reduction rates obtained with pump were decreased upto 30% as compared with those obtained with compressed air in 4.65mm test section. The temperature effect on PAAM was noticeably considerable, that is, the higher temperaute, the less drag reduction rate. On the other hand, no significant pump effect on the surfactants was observed. The drag reducing effectiveness of CTAC was totally lost in the temperature ragne of 60 to $80^{\circ}C$, whereas STAC and Habon-G kept their distinct drag reducing capability at a temperature of $80^{\circ}C$. This study clearly elucidated that for DHC application of drag reducing additives, the pump and temperature effects as well as additive concentration and pipe diameter should be carefully taken into consideration.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.483-486
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• 2002

Injection of microbubbles and/or polymer solution has been known to be a promising method for the reduction of frictional drag of water-borne vehicles. Naval Architects have been interested in friction drag reduction technology, since the friction drag of a commercial ship can be over $70{\%}$ of total resistance. The reduction of friction drag is also important for autonomous underwater vehicles and naval submarines to improve their durability and survivability In this study two sets of experiments were carried out for the friction drag reduction of 2-D channel wall and flat plate in the circulating water channels in Chungnam National University. Preliminary results from the experiments are presented and discussed.

• International Journal of Naval Architecture and Ocean Engineering
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• v.9 no.1
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• pp.35-44
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• 2017

Supercavitation is one of the most attractive technologies to achieve high speed for underwater vehicles. However, the multiphase flow with high-speed around the supercavitating vehicle (SCV) is difficult to simulate accurately. In this paper, we use modified the turbulent viscosity formula in the Standard K-Epsilon (SKE) turbulent model to simulate the supercavitating flow. The numerical results of flow over several typical cavitators are in agreement with the experimental data and theoretical prediction. In the last part, a flying SCV was studied by unsteady numerical simulation. The selected computation setup corresponds to an outdoor supercavitating experiment. Only very limited experimental data was recorded due to the difficulties under the circumstance of high-speed underwater condition. However, the numerical simulation recovers the whole scenario, the results are qualitatively reasonable by comparing to the experimental observations. The drag reduction capacity of supercavitation is evaluated by comparing with a moving vehicle launching at the same speed but without supercavitation. The results show that the supercavitation reduces the drag of the vehicle dramatically.

• Journal of Power System Engineering
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• v.17 no.6
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• pp.95-101
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• 2013

A stereo PIV measurements in a circulating water channel has been performed to investigate the skin friction reduction mechanism of the outer-layer vertical blades first devised by Hutchins. In a recent PIV measurement study, considerable skin friction reduction was achieved as much as 2.73%~7.95% by outer-layer vertical blades array. In the present study, the influence of vertical blades array upon the characteristics of the turbulent coherent structures was analyzed by proper orthogonal decomposition method. It is observed that the vortical structures are cut and deformed by blades array and also the turbulent intensity and the Reynolds stress were weakened by the blades. These phenomena strongly associate the skin-friction drag reduction mechanism in the turbulent boundary layer flow.

• Korea-Australia Rheology Journal
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• v.16 no.2
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• pp.57-62
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• 2004

Turbulent drag reduction (DR) efficiency of water soluble poly(ethylene oxide) (PEO) with two different molecular weights was studied as a function of polymer concentration and temperature in a turbulent flow produced via a rotating disk system. Its mechanical degradation behavior as a function of time in a turbulent flow was also analyzed using both a simple exponential decay function and a fractional exponential decay equation. The fractional exponential decay equation was found to fit the experimental data better than the simple exponential decay function. Its thermal degradation further exhibited that the susceptibility of PEO to degradation increases dramatically with increasing temperature.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.18 no.3
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• pp.545-550
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• 2017

When the systolic blood pressure is high, intermittent turbulence in blood flow appears in the aorta and carotid artery with stenosis during the systolic period. The turbulent blood flow is difficult to analyze using the Newtonian turbulence model due to the viscous characteristics of blood flow. As the shear rate is increased, the blood viscosity decreases by the viscoelastic properties of blood and a drag reduction phenomenon occurs in turbulent blood flow. Therefore, a new non-Newtonian turbulent model is required for viscoelastic fluid and hemodynamics. The main aims of this study were to develop a non-Newtonian turbulence model using the drag reduction phenomenon based on the standard $k-{\varepsilon}$ turbulent model for a general non-Newtonian fluid. This was validated with the experimental data and has a good tendency for non-Newtonian turbulent flow. In addition, the computation time and resources were lower than those of the low Reynolds number turbulent model. A modified turbulent model was used to analyze various turbulent blood flows.