• Journal of the Society of Naval Architects of Korea
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• v.49 no.3
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• pp.213-221
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• 2012

The present study numerically investigates the interaction between a free-surface and flow around a circular cylinder over a moving wall. In order to simulate the flow past the circular cylinder over a moving wall near a free-surface, this study has adopted the direct-forcing/fictitious domain (DF/FD) method with the level set method in the Cartesian coordinates. Numerical simulation is performed for a Reynolds numbers of 100 in the range of $0.25{\leq}g/D{\leq}2.00$ and $0.5{\leq}h/D{\leq}2.00$, where g/D and h/D are the gaps between the cylinder and a moving wall and the cylinder and a free-surface normalized by cylinder diameter D, respectively. According to g/D and h/D, the vortex structures have been classified into three patterns of the two-row, one-row, steady elongation. In general, both of g/D and h/D have the large values which mean the cylinder is far away from the wall and the free-surface, two-row vortex structure forms in the wake. When g/D decreases, the two-row vortex structure gradually transfers into the one-row vortex structure. When the g/D reveals the critical value below which the flow becomes steady state, resulting in the steady elongation vortex.

• Journal of the Society of Naval Architects of Korea
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• v.54 no.6
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• pp.492-500
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• 2017

In the study, numerical simulation on the greenwater impact load of free surface offshore structure in the regular waves using fixed cartesian grid system and Modified Marker-Density (MMD) method were carried out and the results were reviewed. In order to compare numerical simulation and experimental results, the FPSO with the scale ratio of 1/100 model ship with fixed rectangular deck was selected and turbulence characteristic of the flow was considered by applying the Sub-Grid Scale (SGS) in laminar flow. As a result, it is reviewed how the greenwater impact load inflowed from bow in regular headsea wave influence the flow on the deck and the flow characteristic by numerical simulation and the experiment results were compared and reviewed. Based on this study, it would be useful to numerically study the effect of greenwater on offshore structure.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.5
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• pp.1319-1332
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• 1995

Numerical analysis was performed to characterize the particle deposition behavior on a horizontal free-standing wafer with thermophoretic effect under the turbulent flow field. A low Reynolds number k-.epsilon. turbulence model was used to analyze the turbulent flow field around the wafer, and the temperature field for the calculation of the thermophoretic effect was predicted from the energy equation introducing the eddy diffusivity concept. The deposition mechanisms considered were convection, diffusion, sedimentation, turbulence and thermophoresis. For both the upper and lower surfaces of the wafer, the averaged particle deposition velocities and their radial distributions were calculated and compared with the laminar flow results and available experimental data. It was shown by the calculated averaged particle deposition velocities on the upper surface of the wafer that the deposition-free zone, where the deposition velocite is lower than 10$^{-5}$ cm/s, exists between 0.096 .mu.m and 1.6 .mu.m through the influence of thermophoresis with positive temperature difference of 10 K between the wafer and the ambient air. As for the calsulated local deposition velocities, for small particle sizes d$_{p}$<0.05 .mu.m, the deposition velocity is higher at the center of the wafer than at the wafer edge, whereas for particle size of d$_{p}$ = 2.0 .mu.m the deposition takes place mainly on the inside area of the wafer. Finally, an approximate model for calculating the deposition velocities was recommended and the calculated deposition velocity results were compared with the present numerical solutions, those of Schmidt et al.'s model and the experimental data of Opiolka et al.. It is shown by the comparison that the results of the recommended model agree better with the numerical solutions and Opiolka et al.'s data than those of Schmidt's simple model.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.9
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• pp.2944-2952
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• 1996

In this paper the CSCM type upwind flux difference splitting Navier-Stokes method has been applied to study the ARL-SL19 transoni $c^ersonic compressor cascade flow. First, the general characteristics of baseline cascade flow were analyzed. At freestream Mach n.1.612 and exit/inlet pressure ratio 2.15, the results from current laminar flow were compared well in suction surface with the experiment; however, not well in pressure surface. Second, numerical study of the transoni $c^ersonic compressor cascade flow demonstrated the effectiveness of a passive control by the various size cavities. A cavity under the shock foot point at the suction surface of the blades was used as a passive control. The passive control of shock-boundary layer interaction by a cavity reduced total pressure losses. The effect of cavity length and depth was studied. The total pressure loss was reduced by about 10% and the isentropic efficiency was improved slightly. The effect of cavity depth in current study(d/l = 0.05, 0.02) was not found strong. Further adequate turbulence modeling and TVD schemes would help to capture the shock more accurately and increase the effectiveness of the current shock-boundary layer interaction study using upwind flux difference splitting computational methods.thods.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.10
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• pp.795-803
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• 2015

The boundary layer developing over the suction surface of a first-stage turbine blade for power generation has been investigated in this study. For three locations selected in the region where local thermal load changes dramatically, mean velocity, turbulence intensity, and one-dimensional energy spectrum are measured with a hot-wire anemometer. The results show that the suction-surface boundary layer suffers a transition from a laminar flow to a turbulent one. This transition is confirmed to be a "separated-flow transition", which usually occurs in the shear layer over a separation bubble. The local minimum thermal load on the suction surface is found at the initiation point of the transition, whereas the local maximum thermal load is observed at the location of very high near-wall turbulence intensity after the transition process. Frequency characteristics of turbulent kinetic energy before and after the transition are understood clearly from the energy spectrum data.

• The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY
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• v.3 no.2
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• pp.53-58
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• 1998

Ekman layer equation with rough-bottom boundary condition has been solved to determine the effect of roughness on the magnitude of Ekman veering. The bottom boundary condition and the flow field were expanded in a power of roughness (h) which is always smaller than the Ekman layer thickness (${\delta}_E$). By changing the magnitude of roughness parameter (h/${\delta}_E$), the magnitude of the veering, which rotates counterclockwise from the interior geostrophic flow, has been computed. At a fixed depth within the Ekman layer, the magnitude of veering increases as the roughness parameter increases. However, the cross-isobar flux turns out to decrease with increasing roughness. To verify the analytic solution, laboratory experiments were carried out. Rough-bottom cylinderical container filled with homogeneous fluid was sit on a rotating table. The flow pattern during the period of steady spin-up shows that the degree of veering coincides well with the analytic results for various roughness parameters.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.37 no.11
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• pp.999-1007
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• 2013

The present work reports numerical results of the pressure drop and heat transfer characteristics of pipes with various shapes such as circular, elliptical, circumferential wavy and twisted using a three-dimensional simulation. Numerical simulations are calculated for laminar to turbulent flows. The fully developed flow in pipes was modeled using steady incompressible Reynolds-averaged Navier-Stokes (RANS) equations. The friction and Colburn factor of each pipe are compared with those of a circular tube. The overall flow and heat transfer calculations are evaluated by the volume and area goodness factor. Finally, the objective of the investigation is to find a pipe shape that decreases the pressure loss and increases the heat transfer coefficient.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.2 no.1
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• pp.37-48
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• 1990

The drag and heat transfer reduction phenomena and degradation effects of drag reducing polymer solutions which are known as the viscoelastic fluids are investigated experimentally for the turbulent circular tube flows. Two stainless steel tubes are used for the experimental flow loops. Aqueous solutions of Polyacrylamide Separan AP-273 with concentrations from 300 to 1000 wppm are used as working fluids. Flow loops are set up to measure the friction factors and heat transfer coefficients of test tubes in the once-through system and the recirculating flow system. Test tubes are heated by power supply directly to apply constant heat flux boundary conditions on the wall. Capillary tube viscometer and falling ball viscometer are used to measure the viscous characteristics of fluids and the characteristic relaxation time of a fluid is determined by the Powell-Eyring model. The order of magnidude of the thermal entrance length of a drag reducing polymer solution is close to the order of magnitude of the laminar entrance length of Newtonian fluids. Dimensionless heat transfer coefficients of the viscoelastic non-Newtonian fluids may be represented as a function of flow behavior index n and newly defined viscoelastic Graetz number. As degradation continues viscosity and the characteristic relaxation time of the testing fluids decrease and heat transfer coefficients increase. The characteristic relaxation time is used to define the Weissenberg number and variations of friction factors and heat transfer coefficients due to degradation are presented in terms of the Weissenberg number.

• Journal of the Korean Society for Precision Engineering
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• v.29 no.2
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• pp.214-221
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• 2012

Recently, the biochip which is a prime representation of NT, IT, BT, as an example of convergence technology, has been frequently mentioned. With the recent rapid advance in biotechnology, these compact devices, such as lab-on-a-chip or u-TAS, have been developed, and more research is needed. These compact devices typically use the micro-channel in order to shed or detach and mix a variety of materials. Specially, in micro-fluidic systems, a mixer is necessary to produce a mixture because only laminar flow occurs at a low-Reynolds number. To solve this problem, HVM a micromixer that induces a horizontal and vertical multi-mixing flow motion, is proposed. The mixing performance was analyzed and verified by optimizing the shape through the CFD analysis and evaluating the structural analysis and the soundness with material properties that are obtained through the basic experiment.

• Journal of the Society of Naval Architects of Korea
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• v.59 no.1
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• pp.18-28
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• 2022

The turbulence stimulation effect of studs for boundary layer over a flat plate was investigated through the flow measurement in KRISO cavitation tunnel. For the test, Laser Doppler Velocimetry (LDV) and three flat plate models were used: (1) flat plate without studs; (2) flat plate with one stud row; (3) flat plate with two stud rows. The dimension and location of stud rows and the inflow speed were selected considering test conditions for standard-sized model ships in KRISO towing tank. The boundary layer characteristics of test models were analyzed and compared in terms of mean velocity profiles, turbulence intensity profiles, boundary layer thickness, and shape factor. In the case of the flat plate without studs, transition from laminar to turbulent flow occurred around Re_x=3.83 ~ 5.19 × 10⁵. In the case of flat plates with stud rows, the flow rapidly changed into turbulent flow right after passing the first stud row. In the state where turbulence was already developed, the second stud row slightly increased the turbulence intensity near the top of the stud, but did not significantly affect the boundary layer characteristics such as mean velocity distribution, boundary layer thickness, and shape factor.