• 대한기계학회논문집B
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• 제27권3호
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• pp.347-356
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• 2003

The characteristics of near wakes of circular cylinders with serrated fins are investigated experimentally using a hot-wire anemometer for various freestream velocities. Near wake structures of the fin tubes are observed using a phase average technique. With increasing fin height and decreasing fin pitch. oscillation of streamwise velocity increases. It file oscillation of lateral velocity decreases. The time averaged V-component velocity distribution of the finned tube is contrary to that of the circular cylinder due to the different strength of entrainment flow. This strength is affected by the distance of (equation omitted) = 1.0 contour lines. (equation omitted) = 1.0 contour line approaches to the wake center line when the fin density is increased. When the distance between (equation omitted) = 1.0 contour lines comes close the shear force should be increased and the flow toward the wake center line can be more strengthened because of the shear force. Factors related to the velocity recovery in the near wake of the finned tube are attributed to tile turbulent intensity, the boundary layer thickness. the position and strength of entrainment process.

• Journal of Ship and Ocean Technology
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• 제4권3호
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• pp.1-12
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• 2000

A number of numerical methods like Computational Fluid Dynamics(CFD) have been developed to predict the flow fields of a vessel but the present study is developed to infer the wake fields on propeller plane by Statistical Fluid Dynamics(SFD) approach which is emerging as a new technique over a wide range of industrial fields nowadays. Neural network is well known as one prospective representative of the SFD tool and is widely applied even in the engineering fields. Further to its stable and effective system structure, generalization of input training patterns into different classification or categorization in training can offer more systematic treatments of input part and more reliable result. Because neural network has an ability to learn the knowledge through the external information, it is not necessary to use logical programming and it can flexibly handle the incomplete information which is not easy to make a definition clear. Three dimensional stern hull forms and nominal wake values from a model test are structured as processing elements of input and output layer respectively and a neural network is trained by the back-propagation method. The inferred results show similar figures to the experimental wake distribution.

• 수산해양기술연구
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• 제50권4호
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• pp.595-603
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• 2014

This study is aimed to analyze the hydrodynamic characteristics of the cage net in the circulating water channel. It visualized wake flows using a PIV (paricle imaging velocimetry) and analyzed the flow velocity distribution. In addition, the vorticity and turbulence intensity were analyzed from the wake flow distribution and compared changes by flow velocity. Results showed that the average turbulence intensity in the circulating water channel was very stable showing less than 1% in the range between 0.2 and 0.8 m/s. The drag coefficient affecting to the netting was estimated to be 1.35. The flow decreasing rate of the wake in the middle of the netting was 2.1% at the range of 0.2 m/s and it was constant at 6.6-6.9% over the range of 0.4 m/s irrespective of velocity increases. Finally, the change of turbulence intensity by netting and knot mesh could be confirmed. These results can be utilized as a basic information for the future research of flow characteristics by fishing nets and meshes.

• Journal of Ship and Ocean Technology
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• 제1권2호
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• pp.19-30
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• 1997

The accurate prediction of the flow and the pressure distribution near the tip of the blade is crucial in determining the tip vortex cavitation inception which usually occurs on the blade tip or inside the core of the tip vortex just downstream of the blade tip. An improved boundary element method is applied to the prediction of the flow around propeller blades, with emphasis at the tip region. In the method, the Blow adapted grid and a higher order panel method, which combines a hyperboloidal panel geometry with a hi-quadratic dipole distribution, are used in order to accurately model the trailing wake geometry and the highly rolled-up regions in the wake. The method is applied to several propeller geometries and the results have been found to agree well to the existing experimental data. Inviscid flow methods are able to predict the pressures at the tip as well as the shape of the trailing wake. On the other hand, they are unable to determine the flow inside the viscous core of the tip vortex, where cavitation inception often occurs. Thus, a method is presented that treats the flow inside the viscous core. The inner flow is treated with a 2-D Clavier-stokes solution without making any assumptions for axisymmetric flow and conicity of the flow along the tip trajectory. The method can thus allow the treatment of general propeller blade configurations. The velocity and pressure distributions inside the core are shown and compared to those from other numerical methods.

• 대한조선학회논문집
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• 제41권4호
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• pp.30-37
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• 2004

Flow characteristics of the inflow ahead of a rotating propeller attached to a container ship model were investigated using a two-frame PIV (Particle Image Velocimetry) technique. Ensemble-averaged mean velocity fields were measured at four different blade phases. The mean velocity fields show the acceleration of inflow due to the rotating propeller and the velocity deficit in the near-wake region. The axial velocity distribution of inflow in the upper plane of propeller is quite different from that in the lower plane due to the thick hull boundary layer. The propeller inflow also shows asymmetric axial velocity distribution in the port and starboard side. As the inflow moves toward the propeller, the effect of phase angle variation of propeller blade on the inflow becomes dominant. In the upper plane above the propeller axis the inflow has very low axial velocity and large turbulent kinetic energy, compared with the lower plane. The boundary layer developed along the bottom surface of stern hull forms a strong shear layer affecting vortex structure of the propeller near-wake.

• 한국전산유체공학회지
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• 제17권2호
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• pp.71-77
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• 2012

Numerical simulations are carried out for flow over a circular cylinder controlled by the momentum forcing which is generated by a pair of plasma actuators symmetrically mounted on the cylinder surface. A popular and empirical plasma model is used for the spatial distribution of momentum forcing. In this study, we consider two different types of actuation, i.e., steady and unsteady (or pulsed) actuation. In the unsteady actuation, the actuation is turned on and off periodically, its frequency being a control parameter. The objective of this study is to investigate the effects of actuator location and actuation frequency on the flow structures and the forces on the cylinder. Results show that the cylinder wake can be effectively controlled by proper actuator location. For example, when the actuators are located at $120^{\circ}$ from the stagnation point, vortex shedding is completely suppressed with the boundary layer almost fully attached to the surface, resulting in drag reduction and lift elimination.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.349-352
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• 2002

The time-development of the wake vortices of the unsteady viscous flow past a semicircular cylinder is simulated using the vortex particle methods for direct numerical simulations(DNS). The early wake behaviour of the flow behind an impulsively started a semicircualr cylinder is evaluated for a range of Reynolds numbers between 60 and 200 with opposite body configurations respectively. The diffusion scheme based on the particle strength exchange(PSE) is used to account far the viscous effect accurately. And the vorticity generation algorithm to enforce the no-slip boundary conditions is employed. In order to redistribute particles efficiently on the distorted Lagrangian grid the particle distribution technique is adaptively revised, while maintaining the uniform resolution. The results of the simulations are compared to other experimental results.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2000년도 춘계학술대회논문집
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• pp.371-379
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• 2000

A noise prediction method of axial flow fan is developed by incorporating through-flow method and vortex shedding noise model. Fan noise is assumed to be generated due to the pressure fluctuation induced by wake vortices of fan blades and radiate as diploe distribution. The wake vortices are analyzed by combining Karman vortex street model and through-flow analysis results, and the vortex-induced fluctuating pressure on blade surface is calculated by thin airfoil theory. The predicted sound pressure levels and directivity patterns of fan noise by the present method are favorably compared with fan noise test data. Furthermore, the present method is shown to be very useful for predicting the aero-acoustic performance map of the fan operated at off-design point.

• 한국자동차공학회논문집
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• 제11권2호
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• pp.126-133
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• 2003

The aerodynamic characteristics of automobiles have received substantial interest recently. Detailed knowledge of the vehicle aerodynamics is essential to improve fuel efficiency and enhance stability at high-speed cruising. In this study, a numerical simulation has been carried out for three-dimensional turbulent flows around a commercial bus body. Also, the effect of rear-spoiler attached at rear end of bus body was investigated. The Wavier-Stokes equation is solved with SIMPLE method in general curvilinear coordinates system. RNG $k-\varepsilon$ turbulence model with the MARS scheme was used for the evaluating aerodynamic forces, velocity and pressure distribution. The results showed details of the three-dimensional wake flow in the immediate rear of bus body and the effect of rear-spoiler on the wake structure. A maximum of 14% reduction in drag coefficient was achieved for a model with a rear-spoiler.

• 대한기계학회논문집
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• 제19권9호
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• pp.2261-2270
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• 1995

Flow patterns were measured in an unshrouded centrifugal impeller. The flow rate in measurements was fixed at the value corresponding to a nearly zero incidence at the blade inlet. By using a single slanted hot-wire probe and a Kiel probe mounted on the impeller hub disk, the 3-D relative velocities and the rotary stagnation pressures were measured in seven circumferential planes from the inlet to the outlet of the impeller rotating at 700rpm, which diameter is 0.39 meter, and the static pressures and the slip factor at the impeller outlet were calculated from the measured values. From the measured data, the primary/secondary flows, the leakage flows, the wake-jet flows, static pressure distribution on blade surfaces and the wake production mechanism in the impeller passage were investigated.