• 한국수산과학회지
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• 제25권1호
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• pp.65-72
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• 1992

겨울철 동해 북부해역은 저층냉수의 형성역으로 간주될 수 있다. 이러한 저층냉수의 형성으로 인한 동해 순환계의 변화를 조사하기 위해 본 연구에서는 실제 해저지형과 흡사한 2층 해양모델을 사용하였다. 먼저 유입-유출만을 고려하여 동한난류의 형성을 살펴보았고, 이후 동해 북부해역에 냉수형성을 고려하여 냉수의 움직임 및 이것이 동한난류에 미치는 영향을 조사하였다 냉수의 형성으로 인한 경계면의 상승은 냉수형성 지역으로부터 내부 Kelvin파의 형태로 전파하여 약 7개월 후에는 동해 전 연안역의 경계면이 상승하게 되어 경압효과를 강화시켜, 점차적으로 상충에는 시계방향의 순환류가, 하층에는 반 시계방향의 순환류가 강화된다. 즉 냉수의 형성으로 인해 상층에는 한국 동해안을 따라 북상하는 동한난류가 강화되며 하층에서는 남향류가 강화되는 결과를 초래한다.

• 대한기계학회논문집
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• 제13권4호
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• pp.780-788
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• 1989

본 연구에서는 재부착 이후의 재발달 난류경계층에서의 난류구조와 비평형 상태에서 평형상태로의 변화 과정을 실험적으로 고찰하고자 한다.

• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2006년도 학술발표회 논문집
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• pp.1275-1280
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• 2006

자연 저항체인 식생은 유체역학적으로 하천의 평균흐름과 난류량을 변화시키며, 이와 관련하여 유사, 영양 물질, 용존 산소, 그리고 서식 동물 등에 많은 영향을 미치는 것으로 알려져 있다. 과거에는 식생된 수로의 흐름을 식생에 의해 항력이 증가된 경계층 흐름으로 단순하게 보아 왔으나, 근래의 실험유체역학적 연구는 식생된 하천 흐름에 대한 미시적인 구조를 규명하였다. 본 연구는 식생수로에서 평균흐름 및 난류량에 관한 수리특성을 분석하고자 하며, 부유사의 퇴적, 유입, 이송 등의 거동에 큰 영향을 미치는 것으로 알려진 고유구조를 파악하였다. 식생된 개수로 흐름의 일반적인 평균흐름 및 난류구조는 기존의 연구결과와 동일하였으며, 본 연구에서 중점을 둔 고유구조의 경우, 식생높이를 기준으로 하여 상 하부의 흐름특성이 서로 상이한 것을 확인하였다. 식생높이 상부에서는 유속이 수면방향을 향하는 분출현상이 지배적으로 나타났으며, 식생높이 하부에서는 이와는 반대로 유속의 변동성분이 하상으로 향하는 쓸기현상이 지배적으로 나타났다. 이는 개수로에 식생이 존재할 경우 부유사의 농도분포가 수심에 따라 일정해지는 것에 대한 물리적인 근거로 파악된다.

• 한국해안해양공학회지
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• 제12권3호
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• pp.130-138
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• 2000

일방향흐름에 의한 해류의 마찰력 계산을 위하여 Prandtl의 혼합길이 이론을 수심 전구간에 적용하고 난류의 세기와 수리조건에 따라 완난류와 전난류로 분류하여 마찰계수를 산정하는 개수로 마찰계수 산정방법을 제시하였다. 파랑과 해류의 합성류에 의한 해저마찰력을 계산하기 위하여 두 유속의 연직분포를 고려한 BYO 모형을 이러한 흐름특성을 반영하여 개선하였다. BYO 모형은 모든 유속의 연직분포가 해저면으로부터의 직선과 접선으로 만나는 점(Bijker point)을 도출하여 파운동 유속과 해류유속의 합성을 이 점에서 시행하여 일주기의 평균갑을 구하는 모형이다. 일방향흐름의 해류나 천해파에 의한 파운동이나 해저면 가까이 경계층흐름은 완난류, 천이난류, 전난류 등 세가지 종류로 대별된다. 그 중 완난류로 대별하여 두 경우에 대한 합성류 마찰력 산정방법을 제시하였다.

• 대한기계학회논문집B
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• 제24권9호
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• pp.1255-1263
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• 2000

In this study, the modification of turbulent boundary layer flow by local wall vibration is investigated. The wall is locally vibrated using a wall deformation actuator, which moves up and down at the frequencies of 100Hz and 50Hz. Simultaneous measurements of the streamwise velocities in the spanwise direction are performed at several wall-normal and streamwise locations using an in-house multi-channel hot wire anemometer and a spanwise hot-wire-probe rake. The mean velocity is reduced in most places due to the wall vibration and its reduced amount becomes small as flow goes downstream. Interestingly, the mean velocity is found to increase very near the wall and near the actuator. This is due to the motion induced by the streamwise vortices which are generated by the downward motion of the actuator. In case of the streamwise velocity fluctuations, their magnitude increases as compared to the unperturbed turbulent boundary layer, and the increased amount becomes small as the flow moves downstream. The modified flow field at the forcing frequency of 50Hz is not much different from that of 100Hz, except the reduced amount of modification.

• 한국가시화정보학회지
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• 제1권1호
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• pp.64-74
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• 2003

An experimental study was carried out to investigate the effect of periodic blowing and suction on a turbulent boundary layer. Particle image velocimetry (PIV) was used to probe the characteristics of the flow. The local forcing was introduced to the boundary layer via a sinusoidally-oscillating jet issuing from a thin spanwise slot. Three forcing frequencies (f$^{+}$=0.044, 0.066 and 0.088) with a fixed forcing amplitude (A$^{+}$=0.6) were employed at $Re_{=690. The effect of the forcing angles ($\alpha$=60$^{\circ}$ , 90$^{\circ}$ and 120$^{\circ}$ ) was investigated under the fixed forcing frequency (f$^{+}$=0.088). The PIV results showed that the wall region velocity decreases on imposition of the local forcing. Inspection of phase-averaged velocity profiles revealed that spanwise large-scale vortices were generated in the downstream of the slot and persist further downstream. The highest reduction in skin friction was achieved at highest forcing frequency (f$^{+}$=0.088) and a forcing angle of $\alpha$=120$^{\circ}$. The spatial fraction of the vortices was examined to analyze the skin friction reduction.

• 한국가시화정보학회지
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• 제14권3호
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• pp.28-31
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• 2016

In this study, we investigate the wake characteristics in two cases which are laminar inflow and turbulent inflow. To solve the flow with wind turbines and its wake, we use large eddy simulation (LES) technique with actuator line method (ALM) and turbulent inflow of Turbsim. Turbulent inflow which contains the characteristic of the stable atmospheric boundary layer is used. We perform the quantitative analysis of velocity deficit and turbulence intensity in two cases. Time series of velocity deficit at the first, the second column in two cases are compared to observe the performance of wind turbine. The performance in the first column in laminar inflow is overestimated compared to that in turbulent inflow. And we observe that wake in the case with turbulent inflow drive to the span-wise direction and wake recovery in turbulent inflow is more effective. In quadrant analysis of Reynolds stress, the ejection and the sweep motion in turbulent inflow case are bigger than those in laminar inflow case.

• 유체기계공업학회:학술대회논문집
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• 유체기계공업학회 2006년 제4회 한국유체공학학술대회 논문집
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• pp.445-448
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• 2006

The effects of surface roughness on a spatially-developing turbulent boundary layer (TBL) were investigated by performing direct numerical simulations of TBLs over rough and smooth walls. The Reynolds number based on the momentum thickness was varied in the range $Re_{\theta}=300{\sim}1400$. The roughness elements used were periodically arranged two-dimensional spanwise rods, and the roughness height was $k=1.5{\theta}_{in}$, which corresponds to $k/{\delta}=0.045{\sim}0.125$. To avoid generating a rough wall inflow, which is prohibitively difficult, a step change from smooth to rough was placed $80{\theta}_{in}$ downstream from the inlet. The spatially-developing characteristics of the rough-wall TBL were examined. Along the streamwise direction, the friction velocity approached a constant value and a self-preserving form of the turbulent stress was obtained. Introduction of the roughness elements affected the turbulent stress not only in the roughness sublayer but also in the outer layer. Despite the roughness-induced increase of the turbulent stress in the outer layer, the roughness had only a relatively small effect on the anisotropic Reynolds stress tensor in the outer layer. Inspection of the triple products of the velocity fluctuations revealed that introducing the roughness elements onto the smooth wall had a marked effect on vertical turbulent transport across the whole TBL. By contrast, good surface similarity in the outer layer was obtained for the third-order moments of the velocity fluctuations.

• 대한기계학회논문집B
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• 제28권1호
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• pp.31-40
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• 2004

Direct numerical simulations were performed to analyze the effects of time-periodical blowing through a spanwise slot on a turbulent boundary layer. The blowing velocity was varied in a cyclic manner from 0 to 2A$^{+}$(A$^{+}$ =0.25, 0.50 and 1.00) at a fixed blowing frequency of f$^{+}$=0.017. The effect of steady blowing (SB) was also examined, and the SB results were compared with those for periodic blowing (PB). PB reduced the skin friction near the slot, although to a slightly lesser extent than SB. PB was found to generate a spanwise vortical structure in the downstream of the slot. This vortex generates a reverse flow near the wall, thereby reducing the wall shear stress. The wall-normal and spanwise turbulence intensities under PB are increased as compared to those under SB, whereas the streamwise turbulent intensity under PB is weaker than that under SB. PB enhances more energy redistribution than SB. The periodic response of the streamwise turbulence intensity to PB is propagated to a lesser extent than that of the other components of the turbulence intensities and the Reynolds shear stress.

• 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.