• 유체기계공업학회:학술대회논문집
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• 2006.08a
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• pp.397-400
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• 2006

In this study, we experimentally investigate the possibility of skin-friction drag reduction by series of transverse cavities in a turbulent boundary layer flow. The effects of cavity depth (d), cavity length (l) and cavity spacing (s) on the skin friction drag are examined in the range of $Re_{\theta}\;=\;4030\;{\sim}\;7360$, $d/{\theta}_0\;=\;0.13\;{\sim}1.03$, l/d = 1 ~ 4 and s/d = 5 ~ 20. We perform experiments for twenty different cavity geometries and directly measure total drag force using in-house force measurement system. In most cases, the skin friction drag is increased. At several cases, however, small drag reduction is obtained. The variation of the skin ftiction drag is more sensitive to the cavity length than to the cavity depth or cavity spacing, and drag is reduced at $s/l\;{\geq}\;10$ and $l/{\theta}_0\;{\leq}\;0.26$ irrespective of the cavity depth. At $l/\bar{\theta}_0\;=\;0.13$ and s/l = 10, maximum 2% drag reduction is achieved. When the skin friction drag is reduced, there is little interaction between the flows inside and outside cavity, and the flow changed by the cavity is rapidly recovered at the following crest. A stable vortex is formed inside a cavity in the case of drag reduction. This vortex generates negative skin friction drag at the cavity bottom wall. Although there is form drag due to the cavity itself, total drag is reduced due to the negative skin friction drag.

• Journal of the Society of Naval Architects of Korea
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• v.40 no.3
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• pp.1-8
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• 2003

For the experiment of the friction drag reduction by microbubble injection, a drag reduction water tunnel was specifically designed and made. Experimental apparatus and procedures were devised and developed for measuring the change of wall friction drag with microbubble injection. For fully-developed channel flows. the change of friction drag with important parameters of microbubble injection is investigated and the experimental data and results obtained are presented. The amount of friction drag reduction up to 25% is observed in the present study.

• Journal of the Society of Naval Architects of Korea
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• v.40 no.3
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• pp.9-15
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• 2003

An experimental study has been carried out as a basic research for development of the friction drag reduction technology for ships by polymer injection. Experimental apparatus and procedures have been devised and prepared to measure the changes of the wall friction with injection of a polymer solution and basic experimental data on the friction drag reduction are obtained for a turbulent fiat-plate boundary layer and fully-developed channel flows. Variations of the friction drag reduction with some important parameters of polymer injection, such as the concentration of polymer solution, its injection flow rate and the measuring position downstream from the injection slot, are also investigated. Important experimental data and results obtained in the present study are presented. The amount of friction drag reduction up to 50% is observed.

• 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.

• Journal of Ship and Ocean Technology
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• v.8 no.1
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• pp.1-9
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• 2004

Experiments on friction drag reduction by injecting polymer (Polyethylene oxide) solution or micro-bubbles were carried out in the cavitation tunnel of KRISO. Two different drag reduction mechanisms were applied to a slender axi-symmetric body to measure the total drag reduction. And then the amount of friction drag reduction was estimated under the assumption that the reduction mechanisms were effective only to the friction drag component. As the result of the tests, polymer solution drag reduction up to 23％ of the total drag was observed and it corresponds to about 35％ of the estimated friction drag of the axi-symmetric body. This result matched reasonably well to that of the flat plate test "(Kim et al, 2003)". The normalization of the controlling parameters was tried at the end of this paper. Micro-bubble drag reduction was within 1％ of its total drag. This unexpected result was quite different from that of the flat plate case "(Kim et at, 2003)" The possible reasons were discussed in this paper.

• Tribology and Lubricants
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• v.30 no.6
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• pp.337-342
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• 2014

The reduction of drag torque is an important research issue in terms of improving transmission efficiency. Drag torque in a wet clutch occurs because of the viscous drag generated by the transmission fluid in a narrow gap (clearance) between the friction plate and a separate plate. The objective of this paper is to observe the effects of the friction plate area and the clearance on the drag torque using finite element simulation. The two-phase flow of air and oil fluid is considered and modeled for the simulation. The simulation analysis reveals that as the rotational speed increases, the drag torque generally increases to a critical point and then decreases sharply at a high speed regime. The clearance between the two plates plays an important role in controlling drag torque peak. An increase in the clearance causes a decrease in shear stress; thus, the drag torque also decreases according to Newton's law of viscosity. An observation of the effect of the area of contact between transmission fluid and friction plate shows that the drag torque increases with the contact area. The flow vectors inside the flow channel present clear evidence that the velocity of the fluid flows is faster with a larger friction plate, that is, in the case of a larger contact area. Therefore, the optimum size of the friction plate should be determined carefully, considering both the clutch performance and drag reduction. It is expected that the results from this study can be very useful as a database for clutch design and to predict the drag torque for the initial design with respect to various clutch parameters.

• Fashion & Textile Research Journal
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• v.3 no.5
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• pp.473-478
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• 2001

The newly developed cone drum twister is one of the outer surface contacting friction-twisting devices in false-twist texturing. An investigation of the drag angle for the newly developed cone drum twister texturing mechanism is reported. An analysis is given from which equations can be derived that relate to the conical angle of cone drum, wrapping angle, drag angle, and yam helix angle. Theoretical values of drag angle are calculated and discussed. It is shown that, as the helix angle and the projected wrapping angle increases, the drag angle also increases slowly until the helix angle of $40^{\circ}$ but after the helix angle of $40^{\circ}$ the drag angle increases rapidly. Furthermore the higher the projected wrapping angle and conical angle, the higher the drag angle of friction surface.

• Tribology and Lubricants
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• v.33 no.1
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• pp.1-8
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• 2017

Drag torque reduction in a wet clutch pack is a key aspect of the design process of the dual clutch transmission (DCT) system. In order to reduce the drag torque caused by lubricant shear resistance, recently developed wet clutch pack systems of DCT, as well as automatic transmission and other four-wheel drive (4WD) couplings, frequently utilize wavy wet clutch pads. Therefore, wavy shape of friction pad are made on the groove patterns like waffle pattern for the reduction of drag torque. Additionally, the groove patterns are designed with larger channels at several locations on the friction pad to facilitate faster outflow of lubricant. However, channel performance is a function of the waviness of the friction pad at the location of the particular channel. This is because the discharge sectional area varies according to the waviness amplitude at the location of the particular channel. The higher location of the additional channel on the friction pad results in a larger cross-sectional area, which allows for a larger flow discharge rate. This results in reduction of the drag torque caused by the shear resistance of DCTF, because of marginal volume fraction of fluid (VOF) in the space between the friction pad and separator. This study computes the VOF in the space between the friction pad and separator, the hydrodynamic pressure developed, and the shear resistance of friction torque, by using CFD software (FLUENT). In addition, the study investigates the dependence of these parameters on the location and waviness amplitude of the channel pattern on the friction pad. The paper presents design guidelines on the proper location of high waviness amplitude on wavy friction pads.

• 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.

• Nuclear Engineering and Technology
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• v.54 no.4
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• pp.1495-1507
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• 2022

The improvement of thermal-hydraulic analysis techniques is essential to ensure the safety and reliability of nuclear power plants. The one-dimensional two-fluid model has been adopted in state-of-the-art thermal-hydraulic system codes. Current constitutive equations used in the system codes reach a mature level. Some exceptions are the partition method of wall friction in the momentum equation of the two-fluid model and the interfacial drag force model for a horizontal two-phase flow. This study is focused on deriving the partition method of wall friction in the momentum equation of the two-fluid model and modeling the interfacial drag force model for a horizontal bubbly flow. The one-dimensional momentum equation in the two-fluid model is derived from the local momentum equation. The derived one-dimensional momentum equation demonstrates that total wall friction should be apportioned to gas and liquid phases based on the phasic volume fraction, which is the same as that used in the SPACE code. The constitutive equations for the interfacial drag force are also identified. Based on the assessments, the Rassame-Hibiki correlation, Hibiki-Ishii correlation, Ishii-Zuber correlation, and Rassame-Hibiki correlation are recommended for computing the distribution parameter, interfacial area concentration, drag coefficient, and relative velocity covariance of a horizontal bubbly flow, respectively.