• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.31 no.8
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• pp.1-7
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• 2003

The numerical simulation of flow-filed around a square cylinder near a wall with $\varepsilon$-SST turbulence model is carried out in this study. The newly suggested $\varepsilon$-SST turbulence model that modifies the original SST turbulence model is proved to yield more accurate results than the other 2-equation turbulence models in large separation region around a bluff body. Therefore, $\varepsilon$-SST turbulence model can be effectively applied for predicting the flow-fields with large separation. And it is found that vortex shedding is suppressed below the critical gap height, the Strouhal number is affected by the gap height and the wall boundary layer thickness.

• Wind and Structures
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• v.23 no.1
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• pp.1-18
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• 2016

This work experimentally and numerically analyzes the flow configurations and the dynamic wind loads on panels of rectangular L/h 5:1 cross section mounted on a structural frame of rectangular bars of L/h 0.5:1, corresponding to a radar structure. The fluid dynamic interaction between panels and frame wakes imposes dynamic loads on the panels, with particular frequencies and Strouhal numbers, different from those of isolated elements. The numerical scheme is validated by comparison with mean forces and velocity spectra of a panel wake obtained by wind tunnel tests. The flow configuration is analyzed through images of the numerical simulations. For a large number of panels, as in the radar array, their wakes couple in either phase or counter-phase configurations, changing the resultant forces on each panel. Instantaneous normal and tangential force coefficients are reported; their spectra show two distinct peaks, caused by the interaction of the wakes. Finally, a scaled model of a rectangular structure comprised of panels and frame elements is tested in the boundary layer wind tunnel in order to determine the influence of the velocity variation with height and the three-dimensionality of the bulk flow around the structure. Results show that the unsteady aerodynamic loads, being strongly influenced by the vortex shedding of the supporting elements and by the global 3-D geometry of the array, differ considerably on a panel in this array from loads acting on an isolated panel, not only in magnitude, but also in frequency.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.17 no.2
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• pp.712-718
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• 2016

A heat exchanger tube array in a heat recovery steam generator is exposed to hot exhaust gas flow that can cause flow induced vibrations, which could damage the heat exchanger tube array. The characteristics of flow induced vibration in the tube array need to be established for the structural safe operation of a heat exchanger. Several studies of the flow induced vibrations of typical heat exchangers have been conducted and the nondimensional PSD (Power Spectral Density) function with the Strouhal number, fD/U, had been derived using an experimental method. The present study examined the results of the previous experimental research on the nondimensional PSD characteristics by CFD analysis and the basis for the application of flow induced vibration to the heat recovery steam generator tube array was determined from the present CFD analysis. The present CFD analysis introduced circular cylinder tube array and calculated using unsteady laminar flow for the tube array. The characteristics of lift and drag fluctuations over the cylinder tube array was investigated. The derived nondimensional lift and drag PSD was compared with the results of the previous experimental research and the characteristics of lift and drag PSD for a circular cylinder tube array was established from the present CFD study.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.10
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• pp.6641-6646
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• 2015

Heat exchanger tube array in a heat recovery steam generator is exposed to the hot exhaust gas flow and it could cause the flow induced vibration, which could damage the heat exchanger tube array. It is needed to establish the characteristics of flow induced vibration in the tube array for the structural safe operation of the heat exchanger. Several researches for the flow induced vibration of typical heat exchangers had been conducted and the nondimensional PSD(Power Spectral Density) function with the Strouhal number, fD/U, had been derived by experimental method. The present study examined the results of the previous experimental researches for the nondimensional PSD characteristics by CFD analysis and the basis for the application of flow induced vibration to the heat recovery steam generator tube array would be prepared from the present CFD analysis. For the previous mentioned purpose, the present CFD analysis introduced circular cylinder tube array and calculated with the unsteady laminar flow for the tube array. The characteristics of lift fluctuation over the cylinder tube array was investigated. The derived nondimensional PSD was compared with the results of the previous experimental researches and the characteristics of lift PSD for circular cylinder tube array was established from the present CFD study.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.5
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• pp.645-652
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• 2001

Heat transfer data are presented which describe characteristics of the transitional thermal boundary layers on the NACA0012 airfoil with upstream wakes. The wakes are generated periodically by circular cylindrical rods which rotate around the airfoil like a squirrel cage. The unsteady wakes simulate those produced by the upstream rotating blade rows in axial turbomachines. The pressure or suction side of the airfoil is also simulated according to the rotating direction of circular rods. As the Reynolds number and the number of rotating rods increase, the boundary layer transition occurs earlier and the Nusselt number increases. The difference of heat transfer coefficient is less on the pressure side than on the suction side. At a constant Reynolds number, the Nusselt number is larger and smaller, respectively, before and after transition as the Strouhal number increases.

• Proceedings of the KSME Conference
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• 2001.06e
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• pp.253-258
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• 2001

Control of drag force on a circular cylinder using a detached splitter plate is numerically studied for laminar flow. A splitter plate with the same length as the cylinder diameter(d) is placed horizontally in the wake region. Its position is described by the gap ratio(G/d), where G represents the gap between the cylinder base point and the leading edge of the plate. The drag varies with the gap ratio; it has the minimum value at a certain gap ratio for each Reynolds number. The drag sharply increases past the optimum gap ratio; this seems to be related to the sudden change in the bubble size in the wake region. This trend is consistent with the experimental observation currently available in case of turbulent flow. It is also found that the net drag coefficient significantly depends on the variation of base suction coefficient.

• Journal of computational fluids engineering
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• v.11 no.3 s.34
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• pp.64-70
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• 2006

A Computational study was carried out in order to investigate the moving wall effect of a circular cylinder at a Reynolds number of $2.0{\times}10^4$. The viscous-incompressible Navier-Stokes equations and Spalart-Almaras turbulent model of the commercial CFD code were adopted for this numerical analysis. The moving wall was set parallel with the freestream, and moving speed was equal to the freestream velocity. The gap ratio is defined as the distance ratio between the circular cylinder diameter and the height from the moving wall. The results show that there is vortex shedding over the critical gap ratio and aerodynamic loads including amplitude and the Strouhal number change according to the gap ratio.

• Journal of computational fluids engineering
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• v.20 no.3
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• pp.47-53
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• 2015

This study performed numerical simulation to elucidate the characteristics of flow past a rectangular cylinder with various values of the aspect ratio(AR) of the cylinder. We calculated the flow field, force coefficients and Strouhal number of vortex shedding depending on the Reynolds number(Re) and the aspect ratio. The $AR{\approx}1$ is preferred for drag reduction, and 0.375$AR{\approx}0$ is recommended if suppression of the lift-coefficient fluctuation and the shedding frequency is desirable. Furthermore the criticality of the Hopf bifurcation is also reported for each AR.

• Journal of Advanced Marine Engineering and Technology
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• v.26 no.4
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• pp.485-492
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• 2002

The present study is aimed to reveal macroscopic aerodynamic characteristics of two-dimensional rectangular prisms with three aspect ratios(D/H=1, 2 and 3) and six angles of attack($0^{circ}, 10^{circ}, 13.5^{circ}, 20^{circ}, 30^{circ} and 45^{\circ}$). The Reynolds number is fixed as $1\times10^4$. The SOLA-based revised finite difference method for the conservation form on irregular grid was adopted as a new numerical method. Instantaneous flow patterns at $45^{\circ}$ in case of D/H=2 and D/H=3 show larger asymmetric wake development which is closely related to the sharp decrease of drag coefficients at higher angles of attack range. Vorticity propagation into enlarged wake region is conjectured to be responsible for this phenomenon. The Strouhal number is found to be sensitive to the angle of attack at higher aspect ratios(D/H=2 and 3).

• The KSFM Journal of Fluid Machinery
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• v.13 no.3
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• pp.5-11
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• 2010

As a basic study of the aero-acoustic noise, Large eddy simulations were carried out for a fixed circular cylinder at Renolds number (Re=$9.0\times10^4$) using commercial CFD code, FLUENT. The subgrid-scale turbulent viscosity was modeled by Smagorinsky-Lilly model adapted to structured meshes. The results of analysis showed that time-averaged value, $\bar{C}_D$ is approximately 1.47 which is considerably adjacent with the experimentally measured value of 1.32 in comparison to the values performed by previous researchers. It is observed that there are the very small acoustic pressure fluctuation with the same frequency of the Karman vortex street.