• Proceedings of the KSME Conference
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• 2000.11b
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• pp.428-433
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• 2000

The characteristics of unsteady heat transfer and boundary layer flow in the SSME turbine rotor passage are investigated with LRN $k-{\varepsilon}$ turbulence model. The unsteady flow and heat transfer in a rotor blade passage as a result of wake/blade interaction is modeled by the inviscid/boundary-layer flow approach. The relevant governing equations are discretized to a system of finite different equations by means of a BTBCS implicit method. These equations have been solved numerically, for the velocity and temperature fields using TDMA method. Heat flux on the blade surface and flow parameters in the rotor passage are calculated with wake interaction. Numerical results show that velocity, pressure, turbulent kinetic energy and heat flux on the blade surface are varied periodically by wake passing.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.1801-1805
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• 2003

The intermittent characteristics of fluid particle accelerations near the wall are investigated with the higher-order statistics and the probability density functions (PDF) by using a direct numerical simulation of turbulent channel flow. Also, the behaviors of acceleration associated with the coherent structures are discussed. The flatness factor of wall-normal acceleration is extremely high near the wall and it exceeds the previously reported value obtained in isotropic turbulence. The presence of the wall seems to make the accelerations more intermittent and the associated mechanism is explained with the PDFs. The skewness factor of wall-normal acceleration indicates that accelerations are associated with the streamwise vortices.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.475-478
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• 2002

Maximum drag reduction (MDR) in turbulent channel flow by polymer additives is studied by direct numerical simulation. An Oldroyd-B model is adopted to express the polymer stress because it is believed that MDR is closely related to the elasticity of the polymeric liquids. The Reynolds number based on the bulk velocity and the channel height is 40000. MDR in the present study is $44{\%}$ and this is in a good agreement with the Virk's asymptote. Turbulence statistics are also in good agreements with the experimental observation. In the 'large drag reduction', the decrease of turbulent kinetic energy is compensated by the increase of energy transfer from the polymer to the flow. Therefore, MDR is a dynamic equilibrium state of the energy transfer between the polymer and the flow.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.33 no.11
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• pp.1267-1273
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• 2009

Hot press forming can produce high-strength components by rapidly cooling between closed punch and die after hot forming using quenchable boron steel austenized in a furnace. In the hot press forming process, the cooling rate is influenced by the size, position and arrangement of the cooling channel and the file condition of cooling water in the die. Also, mechanical properties of the final components and operation time are related to cooling rate. Therefore, the design of optimized cooling channel is one of the most important works. In this paper, the effect of position and size of the cooling channel on the cooling rate was investigated by using design of experiment and FE analysis in hot press bending process. Therefore the optimum cooling channel ratio was presented in the HPB.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.6
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• pp.628-637
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• 2004

The present study investigates the performance of dynamic mixed model (DMM; Zang et ai.) in a channel with strong wall injection through a Large eddy simulation (LES) technique. The DMM results are compared with those of DNS and the results obtained with popular dynamic Smagorinsky model (DSM). Better agreement is achieved when using the DMM with box filter than DSM and coarse DNS in predicting the first and second order statistics as well as large-scale structures of velocity and temperature fields. Such favorable features of DMM are attributed to the fact that it explicitly calculates the modified Leonard stress term and only models the remaining cross and the SGS Reynolds stress terms and, thus, it reduces the excessive burden put on the model coefficient of DSM. Also it is demonstrated that the DMM can be successfully extended to the prediction of temperature (passive scalar) field where strong streamwise inhomogeneity exists.

• Journal of Power System Engineering
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• v.15 no.2
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• pp.31-36
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• 2011

The flow characteristics on the periodically arranged semi-circular ribs in a rectangular channel for turbulent flow have been investigated numerically. The aspect ratio of the rectangular channel was AR=5, the rib height to hydraulic diameter ratio was 0.07 and rib height to channel height ratio was e/H=0.117. The v2-f turbulence model and SST k-${\omega}$ turbulence model were used to find the flow characteristics of near the wall which are suited for realistic phenomena. The numerical analysis results show turbulent flow characteristics and pressure drop at the near the wall as observed experimentally. The results predict that turbulent kinetic energy(k) is closely relative to the diffusion of recirculation flow, and v2-f turbulence model simulation results have a good agreement with experimental.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.3 s.234
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• pp.314-321
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• 2005

Discrete Wavelet Transform (DWT) has been applied to the Direct Numerical Simulation (DNS) data of turbulent channel flow. DWT splits the turbulent flow into two orthogonal parts, one corresponding to coherent structures and the other to incoherent background flow. The coherent structure is extracted from not vorticity field but velocity's since the channel flow is not isoropic. By comparing DWT's result of channel flow with that of isotropic flow, it is shown that coherent structure maintains the properties of original channel flow. The velocity field of coherent structures can be represented by few wavelet modes and that these modes are sufficient to reproduce the velocity probability density function (PDF) and the energy spectrum over the entire inertial range. The remaining incoherent background flow is homogeneous, has small amplitude, and is uncorrelated. These results are compared with those obtained for the same compression rate using large eddy simulation (LES) filtering. In contrast to the incoherent background flow of DWT, the LES subgrid scales have a much larger amplitude and are correlated, which makes their statistical modeling more difficult.

• Journal of the Society of Naval Architects of Korea
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• v.42 no.6 s.144
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• pp.608-618
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• 2005

A new technique giving significant drag reduction in turbulent shear flows has been proposed by using the buoyancy effect to generate periodic spanwise motion. Such spanwise motion can be obtained by arranging heating and cooling strips periodically aligned in the spanwise direction of a vertical channel, where the streamwise mean flow is perpendicular to the gravity vector The strip size has been changed in order to obtain the optimum size corresponding to the maximum drag reduction. The bulk Reynolds number, $ Re_{m} = U_{m} \delta / \nu \$ is fixed at 2270 while Grashof numbers is changed between $10^{6}$ to $10^{7}$. As Grashof number increases, considerable drag reduction can be obtained, At the highest Grashof number, an optimum strip size of about 250 wail units gives drag reduction of about 35$\%$. The greater the Grashof number, the smaller the strip size attains the maximum drag reduction.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.1813-1818
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• 2004

Discrete Wavelet Transform (DWT) has been applied to the Direct Numerical Simulation (DNS) data of turbulent channel flow. DWT splits the turbulent flow into two orthogonal parts, one corresponding to coherent structures and the other to incoherent background flow. The coherent structure is extracted from not vorticity field but velocity's since the channel flow is not isotropic. By comparing DWT's result of channel flow with that of isotropic flow, it is shown that coherent structure maintains the properties of original channel flow. The velocity field of coherent structures can be represented by few wavelet modes and that these modes are sufficient to reproduce the velocity probability distribution function (PDF) and the energy spectrum over the entire inertial range. The remaining incoherent background flow is homogeneous, has small amplitude, and is uncorrelated. These results are compared with those obtained for the same compression rate using large eddy simulation (LES) filtering. In contrast to the incoherent background flow of DWT, the LES subgrid scales have a much larger amplitude and are correlated, which makes their statistical modeling more difficult.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.17 no.6
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• pp.514-520
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• 2005

Surface heat transfer of a fully developed turbulent air flow in a $45^{\circ}$ inclined ribbed square duct with two and four heating walls was experimentally investigated, at which the experimental works were peformed for Reynolds numbers ranging from 7,600 to 26,000. The pitch-to-rib height ratio, p/e, was kept at 8 and rib-height-to-channel hydraulic diameter ratio, $e/D_h$ was kept at 0.0667. The channel length-to-hydraulic diameter ratio, $L/D_h$ was 60. The heat transfer coefficient values were decreased with the increase in the number of heat-ing walls. Results of this investigation could be used in various applications of internal channel turbulent flow involving roughened walls.