• Journal of the Korean Society of Visualization
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• v.6 no.1
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• pp.41-46
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• 2008

In this paper, the recirculation flow motion and mixing characteristics driven by air bubble stream in a rectangular water tank is studied. The time-resolved PIV technique is adopted for the quantitative visualization and analysis. 488 nm Ar-ion CW laser is used for illumination and orange fluorescent ($\lambda_{ex}=540nm,\;\lambda_{em}=560nm$) particle images are acquired by a PCO 10bit high-speed CCD camera (1280$\times$1024). To obtain clean particle images, 545 nm long pass optical filter and an image intensifier are employed and the flow rates of compressed air is 3 l/min at 0.5 MPa. The recirculation and mixing flow field is further investigated by time-resolved POD analysis technique. It is observed that the large scale recirculation resulting from the interaction between rising bubble stream and side wall is the most dominant flow structure and there are small scale vortex structures moving along with large scale recirculation flow. It is also verified that the sum of 20 modes of velocity field has about 67.4% of total turbulent energy.

• Water Engineering Research
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• v.1 no.1
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• pp.83-92
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• 2000

Jet flow can occur by gate opening at downstream of a hydraulic structure such as weir of drainage gate. If the stream bed is not hard or the bed protection is not sufficient, vortex erosion occurs and a resulting scour hole will be formed due to the high shear stress of the jet flow. Once the scour hole is formed, a vortex occurs in ti and this vortex causes additional erosion. If this erosion continues and reaches to the hydraulic structure, it can undermine the bottom of the hydraulic structure and this will lead to failure of the structure itself. Thus, it is necessary to define the physical features of the vortex structure in the scour hole for the design of the bed protection. This study presents the turbulent vortex structure in the scour hole by the gate opening of the hydraulic structure. Characteristics of vortex motion, circulation, vortex scale and vortex were analyzed through experiments. Experimental results of the vortex velocity were compared with theoretical ones. From these, circulation and vortex scale were obtained with known values of inflow depth, inflow velocity and scale of scour hole

• Water for future
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• v.27 no.2
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• pp.63-72
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• 1994

In this study, hydraulic characteristics for upstream migration of fish in a pool-and-weir fishway were analyzed through experiment. The results showed that streaming flow was preferable to plunging flow for upstream migration of fish and it was not good to make an orifice beneath the septum since it generates turbulent jet and eddies. Protrusions on the side wall of fishway were preferable to grooves since they decelerate flow velocity and make upstream migration easy. A vertically movable septum was necessary for a flushing of deposited bed material, and net installing over fishway was also needed for birds not to approach the fishway and eat fish.

• Nuclear Engineering and Technology
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• v.41 no.10
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• pp.1347-1360
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• 2009

A multi-dimensional component for the thermal-hydraulic system analysis code, MARS, was developed for a more realistic three-dimensional analysis of nuclear systems. A three-dimensional and two-fluid model for a two-phase flow in Cartesian and cylindrical coordinates was employed. The governing equations and physical constitutive relationships were extended from those of a one-dimensional version. The numerical solution method adopted a semi-implicit and finite-difference method based on a staggered-grid mesh and a donor-cell scheme. The relevant length scale was very coarse compared to commercial computational fluid dynamics tools. Thus a simple Prandtl's mixing length turbulence model was applied to interpret the turbulent induced momentum and energy diffusivity. Non drag interfacial forces were not considered as in the general nuclear system codes. Several conceptual cases with analytic solutions were chosen and analyzed to assess the fundamental terms. RPI air-water and UPTF 7 tests were simulated and compared to the experimental data. The simulation results for the RPI air-water two-phase flow experiment showed good agreement with the measured void fraction. The simulation results for the UPTF downcomer test 7 were compared to the experiment data and the results from other multi-dimensional system codes for the ECC delivery flow.

• Journal of Power System Engineering
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• v.4 no.1
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• pp.26-32
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• 2000

Single-phase heat transfer performance and pressure drop for internally grooved tubes with angles were studied. Experiments were carried out in a counter flow heat exchanger with water as a working fluid. Two commercially available internally grooved tubes and smooth tube were tested. The internal diameter of the smooth tube was 16.5mm and the internal diameters of grooved tubes were 15.4mm, 14.9mm, 15.0mm, 16.7mm, respectively. Grooved angles in the tubes were $37^{\circ},\;43^{\circ},\;45^{\circ},\;50^{\circ}$, respectively. An experimental device to measure the friction factor and heat transfer coefficient was constructed. The experimental results were obtained for the fully developed turbulent flow of water in tube on the condition of uniform heat flux. As the increase of flow rate, Reynolds number, numbers of groove and grooved angle led to the increase of pressure drop. Also this paper showed that heat transfer rate increased with increasing numbers of groove and grooved angle. An empirical relation taken from this study represented most of the data within ${\pm}25%$.

• Journal of Korea Water Resources Association
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• v.39 no.9 s.170
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• pp.727-735
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• 2006

It is well known that sediment particles introduced in open-channel turbulent flows change mean velocity profile, since Vanoni suggested the reduction of the Karman constant in 1946. However, how the sediment particles take such a role and what parameters would be changed have been debated up to now. Some researchers, on the other hand, have insisted that the constant would not be changed regardless of introducing sediment particles. The present study is a careful re-evaluation of the previous studies on this issue. The study revealed some questionable approaches or methods in the decision of the previous researches and found the reason why this issue has been debated for a long time. The result indicated that the Karman number is reduced by adding sediment particles, but the amount of reduction is much smaller than the previous researches insisted. Finally, the present study proposes a mechanism of the Karman number reduction due to sediment particles.

• Journal of the Society of Naval Architects of Korea
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• v.58 no.1
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• pp.1-9
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• 2021

In the present study, the model-scale Propeller Open Water (POW) tests for the propeller of 176K bulk carrier and 8600TEU container ship were conducted through Computational Fluid Dynamics (CFD) simulation. In order to solve the incompressible viscous flow field, the Reynolds-averaged Navier-Stokes (RaNS) equations were employed as the governing equations. The γ-Reθ(gamma-Re-theta) transition model combined with the SST k-ωturbulence model was introduced to describe the laminar-turbulence transition considering the low Reynolds number of model-scale. Firstly, the flow simulation developing over a flat plate was performed to verify the transition modeling, in which the wall shear stresses were compared with experiments and other numerical results. Then, to investigate the effect of the model, the CFD simulation for the POW test was performed and the simulated propeller performance was validated through comparison with the experiment conducted at Korea Research Institute of Ships & Ocean Engineering (KRISO).

• Journal of Environmental Science International
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• v.23 no.4
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• pp.561-569
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• 2014

The water resources system of Jeju-do Island entirely depends on groundwater. This study is making a precision observation of baseflow, surface water, water shortage that might be vulnerable to climate change and drought in future. The field observation of baseflow discharges in Akgeuncheon stream has regularly been made with ADCP and ADC and Flowmate every two weeks for twenty-two (22) months (July 8, 2011 to April 27, 2013). This paper represent the results of calculating discharge of a number of hydraulic structures (broad-crested weirs) with comparing and has been calculated more accurate discharges with suitability of different observation methods. The average discharge has been observed 0.851 $m^3/s$, whereas the average ADC and Flowmate is 0.709 $m^3/s$. Meanwhile, stream discharge has been calculated 0.709 $m^3/s$ through the broad-crested weir equation. The discharge has calculated with the weir equation greatly changed according to even a small change in the water level. However, it showed a similar trend to one of the observed discharge. Although, in past there were generating errors caused by observers' strides, vertical and horizontal flow velocity distribution when the average flow velocity had been measured, non-prismatic flow, turbulent flow and others in ADC. This study comes up with the weir equation is more suitable for the characteristics of Jeju-do could be presented through an observations of baseflow discharge.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.18 no.10
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• pp.83-92
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• 2019

A pump is considered to be submersible when a motor and a pump are integrated and operate while submerged in water. Submersible pumps mainly function as rejection pumps to prevent foods in densely populated areas, as cold water circulation pumps in large power plants, as pumps to supply irrigation water, as drainage pumps to prevent flooding of agricultural lands, as water supply intake pumps, and as inflow pumps for sewage treatment. The flow in such turbomachines (submersible pumps) inevitably involves various eddy currents. Since it is almost impossible to accurately grasp the complex three-dimensional flow structure and characteristics of a rotating turbomachine through actual testing, three-dimensional numerical analysis using computational fluid dynamics techniques measuring the flow field, velocity, and the pressure can be accurately predicted. In this study, the shape of the impeller was developed to reduce vibration and noise. This was done by increasing the efficiency of the existing submersible pump and reducing turbulence. In order to evaluate the pump's efficiency and turbulence reduction, we tried to analyze the flow using ANSYS Fluent V15.0, a commercial finite element analysis program. The results show that the efficiency of the pump was improved by 4.24% and the Reynolds number was reduced by 15.6%. The performance of a developed pump with reduced turbulence, vibration, and noise was confirmed.

• Journal of Mechanical Science and Technology
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• v.14 no.10
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• pp.1159-1167
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• 2000

Particles in liquid-solid suspension flow might enhance or suppress the rate of heat transfer and turbulence depending on their size and concentration. The heat transfer characteristics of liquid-solid suspension in turbulent flow are not well understood due to the complexibility of interaction between solid particles and turbulence of the carrier fluid. In this study, the heat transfer coefficients of liquid-solid mixtures are investigated using a double pipe heat exchanger with suspension flows in the inner pipe. Experiments are carried out using spherical fly ash particles with mass median diameter ranging from 4 to $78{\mu}m$. The volume concentration of solids in the slurry ranged from 0 to 50% and Reynolds number ranged from 4,000 to 11,000. The heat transfer coefficient of liquid-solid suspension to water flow is found to increase with decreasing particle diameter. The heat transfer coefficient increases with particle volume concentration exhibiting the highest heat transfer enhancement at the 3% solid volume concentration and then gradually decreases. A correlation for heat transfer to liquid-solid flows in a horizontal pipe is presented.