• Journal of Mechanical Science and Technology
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• v.15 no.11
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• pp.1580-1587
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• 2001

In Part I, we extended the capability of the Goore Scheme for application to multi-dimensional problems and improved convergence performance. In this paper, we apply the improved Goore Scheme to th e control of turbulence for drag reduction. Direct numerical simulations combined with the control scheme are carried out to simulate a controlled turbulent channel flow at low Reynolds number. The wall blowing and suction is applied through the Goore algorithm using the total drag as feedback. An optimum distribution of the wall blowing and suction in terms of the wall-shear stresses in the spanwise and streamwise directions is sought. The best case reduces drag by more than 20 %.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.8 no.4
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• pp.519-526
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• 1996

A 2D-LDV system was employed to investigate the flow field characteristics in fully developed drag reducing turbulent channel flows. The additive used in this study was Habon-G which showed splendid drag reduction effect and minimum mechanical degradation trend in the closed flow circulation loop. In order to have better understanding of the drag reduction mechanism, the instantaneous velocities were carefully measured under various experimental conditions and the flow characteristics including time-averaged velocity, turbulent intensity and Reynolds shear stresses were carefully assessed. The time-averaged velocity profiles of surfactant flows showed more parabolic shape(typically shown in a laminar flow) together with significant suppression of turbulent production, yielding the shear induced micelle structure orienting in the flow direction due to its isotropic characteristics. Especially it was observed that the maximum intensity for drag reducing flows was shifted away from the wall and that the streamwise and normal turbulent intensities were strongly altered. This phenomenon strongly suggests that the viscous sublayer becomes thicker with addition of surfactant. Turbulent momentum transport was drastically suppressed across the whole drag reducing channel flow.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.9
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• pp.1290-1306
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• 1998

An experimental study has been performed on a three-dimensional boundary layer over a rotating disk with an impinging jet at the center of the disk. The objective of the present study is to investigate the turbulence statistics of the three-dimensional turbulent boundary layer, which may be regarded as one of the simplest models for the flow in turbomachinery. Six components of the Reynolds stresses and ten triple products are measured by aligning the miniature X-wire probe to the mean velocity direction. The ratio of the wall-parallel shear stress magnitude to twice the turbulent kinetic energy in the near-wall region is strongly decreased by the impinging jet. In the case of the free rotating disk flow the shear stress vector lags behind the mean velocity gradient vector in the whole boundary layer, while the lag is weakened as the impinging jet speed increases.

• 한국가시화정보학회:학술대회논문집
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• 2001.12a
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• pp.86-99
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• 2001

Vortex lock-on or resonance in the flow behind a circular cylinder is investigated from a time-resolved PIV when a single frequency oscillation is superimposed on the mean incident velocity. Measurements are made of the $K\acute{a}rm\acute{a}n$ and streamwise vortices in the wake-transition regime at the Reynolds number 360. Streamwise vortices at the lock-on and natural shedding states are observed, as well as the changes in the wake region with the change of the shedding frequency of lock-on state. When lock-on occurs, the vortex shedding frequency is found to be half the oscillation frequency as expected from previous experiments. At the lock-on state, the $K\acute{a}rm\acute{a}n$ vortices are observed to be more disordered by the increased strength and spanwise wavelength of the streamwise vortices, which leads to a strong three-dimensional motion. Recirculation and vortex formation region at the lock-on state is reduced as the oscillating frequency is increased. By comparing the Reynolds stresses at the lock-on and natural shedding states, $\bar{u'u'}\;and \;\bar{u'u'}$ at the lock-on state are concentrated on the shear layer around the cylinder. The $\bar{u'u'}\;at\;f_o/f_n=2.0$ has a large value near the centerline, compared with that of other cases. Considering the traces of maximum of u', in the wake region near the cylinder, wake width at the lock-on state is wider than that at the natural shedding state.

• Journal of Korea Water Resources Association
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• v.55 no.2
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• pp.147-157
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• 2022

The backward-facing step (BFS) is a benchmark geometry for analyzing flow separation occurred at the edge and resulting development of shear layer and recirculation zone that are occupied by turbulent flow. It is important to accurately reproduce and analyze the mean flow and turbulence statistics of such flows to design physically stable and performance assurance structure. We carried out 3D RANS computations with widely used, two representative turbulence models, k-ω SST and RNG k-ε, to reproduce BFS flow at the Reynolds number of 23,000 and the Froude number of 0.22. The performance of RANS computations is evaluated by comparing numerical results with an experimental measurement. Both RANS computations with two turbulence models appear to reasonably well reproduce mean flow in the shear layer and recirculation zone, while RNG k-ε computation results in about 5% larger velocity between the outer edge of boundary layer and the free surface above the recirculation zone than k-ω SST computation and experiment. Both turbulence models underestimate the shear stress distribution experimentally observed just downstream of the sharp edge of BFS, while shear stresses computed in the boundary layer downstream of reattachment point are agree reasonably well with experimental measurement. RNG k-ε modeling reproduces better shear stress distribution along the bottom boundary layer, but overestimates shear shear stress in the approaching boundary layer and above the bottom boundary layer downstream of the BFS.

• 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).

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.8 s.227
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• pp.953-960
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• 2004

In this study, the effects of freestream turbulence intensity on laminar-turbulent transition of separated shear layers in the wake of a circular cylinder are investigated using an immersed boundary method and LES. It is shown that the present numerical results without freestream turbulence for Re=3,900 based on bulk mean velocity and the cylinder diameter are in good agreement with other authors' experimental observations and numerical results, verifying our numerical methodology. Then a 'prescribed power spectrum' method is imposed to generate isotropic turbulence at the inlet of the computational domain at each time step. The principal effects of freestream turbulence intensity on flow statistics are investigated for Re=3,900. Statistical study reveals that the Reynolds stresses in the near-wake region gradually increase, and transition occurs further upstream, as the turbulence intensity increases. On the other hand, the bubble size behind the cylinder decreases as the turbulence intensity increases, which indicates that the freestream turbulence helps mean velocity be quickly recovered.

• Journal of Power System Engineering
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• v.4 no.3
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• pp.25-33
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• 2000

This study represents three-dimensional turbulent flow characteristics around an axial fan measured at the operating point ${\varphi}=0.32$, which is equivalent to the maximum flowrate region, by using three-dimensional fiber-optic type LDA system. This LDA system is composed of a 5 W Argon-ion laser, two optics in back-scatter mode, three BSA's, a PC, and a three-dimensional automatic traversing system. A kind of paraffin fog is used for laser particles in this study. Mean velocity profiles around an axial fan along the downstream radial distance show that the streamwise and the tangential components exist as a predominant velocity and have the maximum value at the radial distance ratio 0.8, while the radial component has a small scale distribution and its flow direction is inward except a part of blade tip. The turbulent intensity profiles show that the radial component exists the most greatly. And also the turbulent kinetic energy shows about 60% as a maximum value at the radial distance ratio 0.9. Moreover, the Reynolds shear stresses do not exist at upstream flow, but the streamwise and the radial components of them show about 20% as a maximum value at the radial distance ratio 0.9 at downstream flow.

• Journal of Mechanical Science and Technology
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• v.15 no.7
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• pp.895-905
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• 2001

This paper presents vector fields, three dimensional mean velocities, turbulent intensities, turbulent kinetic energy and Reynolds shear stresses measured in the X-Y plane of the gas swirl burner with a cone type baffle plate by using an X-type hot-wire probe. This experiment is carried out at the flow rates of 350 and 450ℓ/min which are equivalent to the combustion air flow rate necessary to release 15,000 kcal/hr in a gas furnace. The results show that the maximum axial mean velocity component exists around the narrow slits situated radially on the edge of a burner. Therefore, there is some entrainment of ambient air in the outer region of a burner. The maximum values of turbulent intensities occur around the narrow slits and in front of a burner up to X/R=1.5. Moreover, the turbulent intensity components show a relatively large value in the inner region due to the flow diffusion and mixing processes between the inclined baffle plate and the swirl vane. Consequently, the combustion reaction is expected to occur actively near these regions.

• Journal of Korea Water Resources Association
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• v.33 no.2
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• pp.181-193
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• 2000

This paper presents the flow and the suspended sediment movement over ripples for oscillatory flows. A new numerical model system is developed, and applied to a laboratory experimental condition of regular waves and a fictitious condition of irregular waves. The flow field is obtained from a programme proposed by Kim et. al.(1994), which is a modified version of SOLA based on SMAC scheme. The sub-model solves the continuity and Reynolds momentum equations in the x-z plane. The wave orbital velocities, shear stresses, and pressure are all reasonably reproduced by the model. The model results on the vertical velocity component show good agreement with the measurements. The suspended sediment transport sub-model is newly set up to solve the advection-diffusion equation of suspended sediment using a split method, and involving a special shear entrainment from the whole ripple surface. The calculated suspended sediment concentrations for regular waves show reasonable agreement with measurements at Deltaflume. The model results for random waves show that the suspended sediment concentration is higher than those for regular waves and that the sediment diffuses higher than for regular waves with the significant wave height and the peak wave period of the irregular waves.