• 한국가시화정보학회지
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• 제15권3호
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• pp.27-33
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• 2017

Direct numerical simulations of turbulent channel flows with moving wall conditions on the top wall are performed to examine the effects of the moving wall on the turbulent characteristics. The moving wall velocity only applied to the top wall with the opposite direction to the main flow is systematically varied to reveal the sustained-mechanism for turbulence. The turbulence statistics for the Couette-Poiseuille flow, such as mean velocity, root mean square of the velocity fluctuations, Reynolds shear stress and pre-multiplied energy spectra of the velocity fluctuations, are compared with those of canonical turbulent channel flows. The comparison suggests that although the turbulent activity on the top wall increases with increasing the Reynolds number, that on the bottom wall decreases, contrary to the previous finding for the canonical turbulent channel flows. The increase of the turbulent energy on the top wall is attributed to not only the increase of the Reynolds number but also elongation of the logarithmic layer due to increase of the wall layer on the top wall. However, because the logarithmic layer is shortened on the bottom wall due to the decrease of the wall layer, the turbulence energy on the bottom wall decreases despite of the increase of the Reynolds number.

• 한국자동차공학회논문집
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• 제4권5호
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• pp.26-36
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• 1996

The fully developed turbulent momentum and heat transfer induced by the square-ribbed roughness elements on both the inner and outer wall surfaces in concentric annuli are studied analytically based on a modified turbulence model. The analytical results of the fuid flow are verified by experiment. The experiment is done with a pitot tube and a X-type hot wire anemometer to measure the time mean velocity profiles, zero shear stress positions, maximum velocity positions and friction factors, and etc. shown in Fig.1. The resulting momentum and heat transfer are discussed in terms of various parameters, such as the radius ratio, the relative roughness, the roughness density, Reynolds number, Nusselt bumber and Prand시 number. The study demonstrates that certain artificial roughness elements may be used to enhance heat transfer rates with advantage from the overall efficiency point of view by investigating turbulent flows and heat transfer in Fig.1.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2003년도 The Fifth Asian Computational Fluid Dynamics Conference
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• pp.46-47
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• 2003

Direct numerical simulations are performed for a turbulent flow subjected to a sudden change in pressure gradient. The calculations are started from a fully-developed turbulent channel flow at $Re_{\tau}=180$. The pressure gradient of the channel flow is then changed abruptly. The responses of the turbulence quantities (e.g., turbulence intensities, Reynolds shear stress, and vorticity fluctuations) and the near-wall turbulence structure to the pressure gradient change are investigated. It is found that there are two different relaxations: a fast relaxation at the early stage and a slow one at the later stage. The early response of the velocity fluctuations shows an anisotropic response of the near-wall turbulence.

• 상하수도학회지
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• 제24권1호
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• pp.103-108
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• 2010

Until now, research reports that it is difficult for brokenup floc after coagulation to reaggregate and settling efficiency of reaggregated floc is relatively low have dominated in water treatment process. In contrast, from recent study conducted by the French researcher, because the density of the reaggregated floc was higher than the previous floc, the settling efficiency of reaggregated floc increased. In this study, 15 times wet test were carried out and the removal efficiency of reagrregated floc was considerably increased. Moreover, this result was explained using the turbulent model for the flow occurred around the floc. Consequently, in the case of suitable hydrodynamic condition for the reaggregation, the characteristics of the reaggregated floc was changed into the favorable condition for improvement of settling efficiency. Also, the most important factor for reaggregation of floc was governed by hydrodynamic shear stress.

• International Journal of Aeronautical and Space Sciences
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• 제18권1호
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• pp.48-55
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• 2017

Aeroacoustic computation of a fully-developed turbulent pipe flow at $Re_{\tau}=175$ and M = 0.1 is conducted by LES/LPCE hybrid method. The generation and propagation of acoustic waves are computed by solving the linearized perturbed compressible equations (LPCE), with acoustic source DP(x,t)/Dt attained by the incompressible large eddy simulation (LES). The computed acoustic power spectral density is closely compared with the wall shear-stress dipole source of a turbulent channel flow at $Re_{\tau}=175$. A constant decaying rate of the acoustic power spectrum, $f^{-8/5}$ is found to be related to the turbulent bursts of the correlated longitudinal structures such as hairpin vortex and their merged structures (or hairpin packets). The power spectra of the streamwise velocity fluctuations across the turbulent boundary layer indicate that the most intensive noise at ${\omega}^+$ < 0.1 is produced in the buffer layer with fluctuations of the longitudinal structures ($k_zR$ < 1.5).

• 대한기계학회논문집B
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• 제25권7호
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• pp.967-974
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• 2001

Experimental investigations on the flow characteristics of downstream region of a butterfly valve, which is used in SI engine, have been conducted according to Reynolds number and valve angle. Measurement programs of the flowfield using x-type of hotwire anemometry include the mean and fluctuating velocity, turbulnet intensity, shear stress, power spectrum and pressure loss coefficient. Experimental results show that flow characteristics and independent of relatively high Reynolds number; 60,000 and 80,000. It is also seen that streamwise mean velocities have relatively large velocity gradient around the butterfly valve with increasing the valve opening angle and this trend appears even in the far downstream region. The distributions of turbulent intensity and shear stress show irregular behavior regardless of the valve opening angle and those of the case of the valve opening angle of 45°are the largest. The pressure loss coefficient of the body surface of the throttle valve increases mildly with the increase of Reynolds number and increases rapidly with the reduction of the valve opening angle.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.811-814
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• 2002

Using a mathematical theory, we show that the optimality condition of a turbulent diffuser with maximum pressure recovery at the exit is zero shear stress along the wall. The optimal diffuser shape is designed through iterative procedures by using the $k-{\varepsilon}-{\nu}^{2}-f$ turbulence model for flow simulation. The Reynolds number based on the bulk mean velocity and the channel height at the diffuser entrance is 18,000. We also perform large eddy simulation to validate the shape design results and investigate the flow characteristics near the zero-skin friction wall. Results from large eddy simulation show that the skin friction is slightly higher than zero but is still very small as compared to that of the flat plate boundary layer flow Although the time-averaged wall shear stress is slightly above zero along the diffuser wall, instantaneous flow reversals occur intermittently. The streamwise mein velocity shows an asymptotic behavior of the half-power-law near the wall where the skin friction is close to zero.

• 대한기계학회논문집B
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• 제22권9호
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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.

• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2004년도 학술발표회
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• pp.429-433
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• 2004

According to researchers, the influential factors of scouring are generally divided into three factors: the flow conditions, the type and position of structures, and the characteristics of bed materials. In addition, scouring is affected by the 3-dimensional turbulent boundaries, the unsteady flow, the movement of sediment in the scour-hole area, the approach flow velocity and depth, the width of bridge foundation/pier, and the particle size of bed materials. However, it is difficult to estimate the scour depth near bridge piers when all conditions are factored in at once. Therefore, for reasonably accurate estimates of scour depth, it is essential to consider sufficiently the flow force and resisting force for scour. That is, to determine the shear stress concerning the bed material distribution is needed. In this study, the experiments were performed under the condition of a steady state of flow. As a result, scouring occurred at velocity ratios of 0.476,$(V/V_c=0.476)$, and the scour depth was increased linearly as the velocity ratio increased. in addition, the average values of shear stress ratio at zero scouring depth in both rectangular and circular piers were approximately 7$(\tau_c/\tau_{approach})$ and in the case for same size bed particle material. The results of this study can be used for the fundamental material for estimating the scour depth of bed materials.

• 대한기계학회논문집B
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• 제28권1호
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• pp.41-51
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• 2004

A direct numerical simulation is performed to analyze the effects of a localized time-periodic blowing on a turbulent boundary layer flow at R $e_{+}$=300. Main emphasis is placed on the blowing frequency effect on near-wall turbulent flow structures at downstream. Wall-normal velocity on a spanwise slot is varied periodically at different frequencies (0.004$\leq$ $f^{＋}$$\leq$0.080). The amplitude of periodic blowing is $A^{＋}$=0.5 in wall nit, which corresponds to the value of $v_{rms}$ at $y^{＋}$=15 without blowing. The frequency responses are scrutinized by examining the phase or time-averaged turbulent statistics. The optimal frequency ( $f^{＋}$=0.03) is observed, where maximum increase in Reynolds shear stress, streamwise vorticity fluctuations and energy redistribution occurs. The phase-averaged stretching and tilting term are investigated to analyze the increase of streamwise vorticity fluctuations which are closely related to turbulent coherent structures. It is found that the difference between PB and SB at a high blowing frequencies is negligible.e.e.