• Journal of the Society of Naval Architects of Korea
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• v.42 no.5 s.143
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• pp.427-434
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• 2005

The free surface influenced the wake behind a rotating propeller and its effects were investigated experimentally in a circulating water channel with the variation of water depth. Instantaneous velocity fields were measured using two-frame PIV technique and ensemble-averaged to study the phase-averaged flow structure in the wake region. For an isolated propeller, the flow behind the propeller is affected only by the propeller rotation speed, the leading on the blades and the proximity of the propeller to the free surface. The phase-averaged mean velocity fields show that the potential wake and the viscous wake developed on the blade surfaces. The interaction between the tip vortices and the slipstream causes the oscillating trajectory of tip vortices. The presence of the free surface greatly affected the wake structure, especially for propeller immersion depth of 0.6D. At small immersion depths, the free surface modified the tip and trailing vortices and the slipstream flow structure downstream of X/D = 0.3 in the propeller wake.

• Journal of Ocean Engineering and Technology
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• v.20 no.6 s.73
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• pp.41-53
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• 2006

This experimental study investigated the flow characteristics for regular waves passing a rectangular floating structure in a two-dimensional wave tank. The particle image velocimetry (PIV) was employed to obtain the velocity field in the vicinity of the structure. The phase average was used to extract the mean flow and turbulence property from repeated instantaneous PIV velocity profiles. The mean velocity field represented the vortex generation and evolution on both sides of the structure. The turbulence properties, including the turbulence length scale and the turbulent kinetic energy budget were investigated to characterize the flow interaction between the regular wave and the structure. The results shaw the vortex generated near the structure corners, which are known as the eddy-making damping or viscous damping. However, the vortex induced by the wave is longer than the roll natural period of the structure, which presents the phenomena opposing the roll damping effect; that is, the vortex may increase the roll motion under the wave condition longer than the roll natural period.

• Journal of the Korean Society of Visualization
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• v.9 no.4
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• pp.63-68
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• 2011

The flow-field of a liquid-metal system is very important for the safety analysis and the design of the steam generator of liquid-metal fast breeder reactor. Dynamic neutron radiography (DNR) is suitable for a visualization and measurement of a liquid metal flow and a two-phase flow in a metallic duct. However, the three dimensional DNR techniques is not enough to obtain the velocity information in the wide channel up to now. In this research, a high speed DNR technique was applied to visualize the heavy liquid-metal flow field in the narrow channel with the HANARO-beam facility. The images were taken with a high frame-rate neutron radiography at 250 fps and analyzed with a Particle Image Velocimetry(PIV) method. The images were compared with the results of the commercial CFX code to study the feasibility of DNR technique for the measuring the heavy liquid-metal flow field. The PIV images could discern the turbulent vortex flow in the two-dimensional narrow channel.

• Journal of the Korean Society of Visualization
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• v.11 no.2
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• pp.12-17
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• 2013

A flow visualization technique was developed to analyze flow distribution inside a dishwasher in this study. Then, a new design plan was proposed to improve the efficiency of the dishwasher by the analyzed results and it was confirmed experimentally by the developed technique. Gas flow fields inside a drying duct of a tub for a drying process was investigated by a developed PIV (Particle Image Velocimetry) and a CFD (Computational Fluid Dynamics) method. Also, the flow visualization technique was developed for the liquid flow field of a rotor-arm system to propose the improved design idea. Also, interactions between liquid and gas were observed around the rotor-arm system. The two-phase flow was modified to the liquid flow field because laser sheets are refracted when pass through the two-phase flow. Thus, the flow visualization techniques was developed in this study to measure the instantaneous flow velocities in the liquid quantitatively.

• Journal of Navigation and Port Research
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• v.32 no.10
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• pp.777-783
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• 2008

Nowadays wavelet transforms are widely used for the analyses of PIV velocity vector fields. This is bemuse the wavelet provides not only spatial information of the velocity vectors but also of time and frequency domains. In this study, a discrete wavelet trC1f1$form has been applied to real PIV images of bubbly flows. The vector fields obtained by a self-made cross-correlation PIV algorithm were used for the discrete wavelet transform The performances of the discrete wavelet transform is investigated by changing the level of power of discretization. The decomposed images by the wavelet multiresolution showed conspicuous characteristics of the bubbly flows according to the level changes. The high spatial bubble concentrated area could be evaluated by the constructed discrete wavelet transform algorithm, at which high leveled wavelets could play a dominant roles to reveal the flow characteristics.

• Journal of the Korean Society of Visualization
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• v.9 no.4
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• pp.47-53
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• 2011

A water driven ejector loop was designed and constructed for air absorption. The used ejector was horizontally installed in the loop and annular water jet at the throat entrained air through the circular pipe placed at the center of the ejector. Wide range of water flow rate was provided using two kinds of pumps in the loop. The tested range of water flow rate was 100${\ell}$ /min to 1,000 ${\ell}$/min. Two-phase flow inside the ejector loop was simulated by CFD analysis. Homogeneous particle model was used for void fraction prediction. Water and air flow rates and pressure drop through the ejector were automatically recorded by using the LabView based data acquisition system. Flow characteristics and air bubble velocity field downstream of the ejector were investigated by two-phase flow visualization and PIV measurement based on bubble shadow images. Overall performance of the two-phase ejector predicted by the CFD simulation agrees well with that of the experiment.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.43 no.9
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• pp.765-772
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• 2015

Aerodynamic force measurements and phase-locked PIV study were carried out to check the bio-mimetic MAV applicability of a swift flight. Two-rotational DOF robotic wing model and blowing-type wind tunnel were employed. The amplitude of twist angle were ${\pm}0$, ${\pm}5$, ${\pm}10$, and ${\pm}20$ deg. and stroke angles were manipulated by simple harmonic function with out-of-phase in regards to the stroke motion. It is acknowledged that the time-varying lift coefficients in accordance with the change of the twist angle did not result in any noticeable differences, just the small decrease and delay. However, the drag exhibited that the small change of the twist angle can produce large thrust. These findings imply why a swift uses small twist angle during flight. The PIV results displayed that the delay of aerodynamic forces is highly associated with the vortical structures around the wing. It is therefore indicated that a process of designing a swift-based Micro Air Vehicle should take the twist angle into consideration, as the essential parameter.

• Journal of the Korean Society of Visualization
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• v.9 no.2
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• pp.54-60
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• 2011

This paper is a basic study on volatile organic compounds(VOC) recovery system in a crude oil carrier. VOC is easily evaporated in cargo tankers during loading and transportation of crude oil, causes serious environmental contamination and a huge economic loss. An ejector system is designed to mix VOC gas into crude oil flow to reduce VOC concentration. Detail two-phase flow inside the ejector is simulated using a commercial CFD code. To verify the numerical prediction, a scale-down experiment is conducted. Instead of crude oil and VOC, water and air are used as the working fluids. Flow characteristics and main parameters are obtained by two-phase flow visualization and PIV measurements. Air volume flow rate induced by the ejector is compared with respect to the volume flow rate of water using experimental and numerical results. Overall performance of the two-phase ejector predicted by the CFD simulation agrees well with that of the experiment.

• International Journal of Air-Conditioning and Refrigeration
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• v.13 no.1
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• pp.44-50
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• 2005

In the present study, the characteristics of upward bubble flow were experimentally investigated in a liquid bath. An electro-conductivity probe was used to measure local volume fraction and bubble frequency. Since the gas was concentrated at the near the nozzle, the flow parameters were high near the nozzle. In general their axial and radial values tended to decrease with increasing distance. For visualization of flow characteristics, a Particle Image Velocimetry (PIV) and a thermo-vision camera were used in the present study. The experimental results showed that heat transfer from bubble surface to water was largely completed within z = 10 mm from the nozzle, and then the temperature of bubble surface reached that of water rapidly. Due to the centrifugal force, the flow was more developed near the wall than at bubble-water plume. Vortex flow in the bottom region was relatively weaker than that in the upper region.

• International Journal of Fluid Machinery and Systems
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• v.7 no.2
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• pp.54-59
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• 2014

Particle Image Velocimetry combined with developed image processing method is adopted to study the liquid-solid two phase flow in the centrifugal pump impeller with crystallization phenomenon. The tracer particle is used to follow the liquid phase, which has the diameter between 8 to $12{\mu}m$. The crystal particle precipitates from the sodium sulfate solution does change the wavelength of the laser, and which has great laser scattering characteristics. The diameter of the crystal particle is larger than $20{\mu}m$. Through calculating the diameter of the particles in the image, the tracer particle and the crystal particle can be distinguished. By analyzing the experimental result, the following conclusion has been obtained. During the delay period, there is not any crystal particle and the pump performance has not been changed. As the crystallization process begins, the crystal nuclei appears from the supersaturation solution and grows larger with temperature decreasing, which has the tendency of moving towards the pressure side. The characteristics of liquid-solid two phase flow with crystallization phenomenon in the pump are obtained according to analysis of experimental results, and some guiding advices are presented to mitigate the crystallization phenomenon in pump impeller.