• Journal of the Society of Naval Architects of Korea
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• v.58 no.3
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• pp.144-157
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• 2021

Brake Horse Power (BHP) reduction ratios by air injection to the underside of the hull surface in an actual ship are predicted using an unstructured finite-volume CFD solver and compared with the sea trial results. In addition, air lubrication system installed on the existing vessel is investigated to find a good solution for additional drag reduction. As a results, it is found that the thickness of the air layer should be minimized within a stable range while securing the area covered by the air layer as much as possible. Furthermore, the amount of frictional drag reduced by air injection is found to be independent of surface roughness and still effective on rough surface. Based on the results of this study, it is expected that systematic and reliable air lubrication system can be designed and evaluated using the proposed method.

• 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.

• Nuclear Engineering and Technology
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• v.38 no.8
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• pp.763-778
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• 2006

Injection of microbubbles within the turbulent boundary layer has been investigated for several years as a method to achieve drag reduction. However, the physical mechanism of this phenomenon is not yet fully understood. Experiments in a channel flow for single phase (water) and two phase (water and microbubbles) flows with various void fraction values are studied for a Reynolds number of 5128 based on the half height of the channel and bulk velocity. The state-of-the art Particle Tracking Velocimetry (PTV) measurement technique is used to measure the instantaneous full-field velocity components. Comparisons between turbulent statistical quantities with various values of local void fraction are presented to elucidate the influence of the microbubbles presence within the boundary layer. A decrease in the Reynolds stress distribution and turbulence production is obtained with the increase of microbubble concentration. The results obtained indicate a decorrelation of the streamwise and normal fluctuating velocities when microbubbles are injected within the boundary layer.

• Journal of the Society of Naval Architects of Korea
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• v.49 no.4
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• pp.333-339
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• 2012

The main objective of the present study is to investigate the roles of the micro-dimpled surface on the drag reduction. To investigate the effectiveness of the micro-dimpled surface, the flat plates are prepared. The micro-size dimples are directly carved on the metal surface by ultrasonic nano-crystal surface modification (UNSM) method. Momentum of the main flow is increased by the dimple patterns within the turbulent boundary layer (TBL), however, there is no significant change in the turbulence intensity in the TBL. The influence of dimple patterns is examined through the flow field survey near the flat plate trailing edge in terms of the profile drag. The wake flow velocities in the flat plate are measured by PIV technique. The maximum drag reduction rate is 4.6% at the Reynolds number of $10^6{\sim}10^7$. The dimples tend to increase the drag reduction rate consistently even at high Reynolds number range.

• Corrosion Science and Technology
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• v.7 no.5
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• pp.283-287
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• 2008

The drag reduction agency (DRA) for gas pipeline, a novel method used for reducing friction or drag on a gas flowing to increase the transmission efficiency of gas pipeline, is a more flexible and economical technology than internal flow efficient coatings. In this paper, an effective DRA has been developed in Authors' Institute by analyzing the hydrodynamic friction resistance on internal gas pipeline and then studying the work mechanism and molecular structure of DRA. In the meantime, a group of property test for selecting DRA material has been determined, including viscosity, contact angle, volatility, corrosion, slab extending, and flow behavior in horizontal tube. The inhibition efficiency and drag reduction efficiency of the developed DRA have been investigated finally based on the relevant test methods. Results of corrosion test show that the developed DRA has very good inhibition effect on mild steel by brushing a thin layer of DRA on steel specimens, giving inhibition efficiency of 91.2% and 73.1% in 3%NaCl solution and standard salt fog environment respectively. Results of drag-reducing test also show that the Colebrook formula could be used to calculate friction factors on internal pipes with DRA as the Reynolds number is in the range of $0.75\times10^5\sim2.0\times10^5$. By comparing with normal industrial pipes, the friction resistance coefficient of the steel pipe with DRA on internal wall decreases by 13% and the gas flux increases by 7.3% in testing condition with Reynolds number of $2.0\times10^5$.

• Advances in aircraft and spacecraft science
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• v.10 no.2
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• pp.107-125
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• 2023

Drag reduction is significant research in aircraft design due to its effect on the cost of operation and carbon footprint reduction. Aircraft currently use conventional solid winglets to reduce the induced drag, adding extra structural weight. Fluidic on-demand winglets can effectively reduce drag for low-speed flight regimes without adding any extra weight. These utilize the spanwise airflow from the wingtips using hydraulic actuators to create jets that negate tip vortices. This study develops a computational model to investigate fluidic on-demand winglets. The well-validated computational model is applied to investigate the effect of injection velocity and angle on the aerodynamic coefficients of a rectangular wing. Further, the turbulence parameters such as turbulent kinetic energy (TKE) and turbulent dissipation rate are studied in detail at various velocity injections and at an angle of 30°. The results show that the increase in injection velocity shifted the vortex core away from the wing tip and the increase in injection angle shifted the vortex core in the vertical direction. Further, it was found that a 30° injection is efficient among all injection velocities and highly efficient at a velocity ratio of 3. This technology can be adopted in any aircraft, effectively working at various angles of attack. The culmination of this study is that the implementation of fluidic winglets leads to a significant reduction in drag at low speeds for low aspect ratio wings.

• Journal of computational fluids engineering
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• v.4 no.1
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• pp.34-40
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• 1999

Drag reduction on vehicles are the main concern for the body shape designers in order to lower the fuel consumption rate and to aid the driving stability. The drag of bluff bodies like transportation vehicles is mostly pressure drag due to the flow separation, which can be minimized by controlling the location and size of the separation bubble. In the present study, the TURBO-3D code is incorporated with optimal algorithm based on analytical approximation method to obtain an optimal afterbody shape of the MIRA Model corresponding to the lowest drag coefficient. For this purpose three mutually independent afterbody angles are chosen as design variables, while the drag coefficient is chosen as an objective function. It is demonstrated in the present study that an optimal body shape having the lowest drag coefficient which is about 6% lower than that of the original shape has been successfully obtained within number of iterations of tile optimal design loop.

• Solar Energy
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• v.20 no.2
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• pp.85-96
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• 2000

When the liquid with the additive of ppm unit of a polymer flows, the pressure drop can be manifestly decreased compared to that of pure liquid: that's the drag reduction. This method is that a small amount of a polymer which doesn't make the transformation of the properties of the working fluid is dissolved into the working fluid, the links of chains of the polymer do a buffer action to the molecules of the working fluid which come out between near the wall of the pipe and the interface, so that the pressure drop is dramatically decreased. When we transport the fluid, therefore, we can save a lot of pumping power, or we can increase the transportation capacity with using the same transportation equipment. But when a polymer solution is also flowing in the fluid transportation system, the degradation which have a very close relation with the phenomena of the drag reduction occurs necessarily. When adding polymer to reduce the drag in two phase flow system, It is impossible to find some studies. This study is focussing on a searching examination for the experimental study considering the mechanical degradation in the closed tow phase system to find out the conditions which could improve the pump capacity.

• 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.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2010.05a
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• pp.443-449
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• 2010

Recently underwater systems moving at hyper-speed such as a super-cavitating torpedo have been studied for their practical advantage of the dramatic drag reduction. In this study we are focusing our attention on super-cavitating flows around axisymmetric cavitators. A numerical method based on inviscid flow is developed and the results for several shapes of the cavitator are presented. First using a potential based boundary element method, we find the shape of the cavitator yielding a sufficiently large enough cavity to surround the body. Second, numerical predictions of super-cavity are validated by comparing with experimental observations carried out in a high speed cavitation tunnel at Chungnam National University (CNU CT).