• 한국가시화정보학회:학술대회논문집
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• 한국가시화정보학회 2005년도 추계학술대회 논문집
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• pp.11-16
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• 2005

Saltation is the most important mechanism of wind-blown sand transport. Till now the interaction between wind and sand has not been fully understood. In this study the saltation of sand sample taken from Taklimakan desert was tested in a simulated atmospheric boundary layer. The captured particle images containing both the tracers for wind and saltating sand, were separated by a digital phase mask technique. Both PIV and PTV methods were employed to extract the velocity fields of wind and the dispersed sand particles, respectively. The mean streamwise wind velocity field and turbulent statistics with and without sand transportation were compared, revealing the effect of the moving sand on the wind field. This study is helpful to understand the interaction between wind and blown sand (in saltation), and provide reliable experimental data fur evaluating numerical models.

• 대한조선학회논문집
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• 제53권3호
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• pp.210-216
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• 2016

Direct numerical simulations of a spatially developing turbulent boundary layer on a flat plate have been performed to verify the applicability of OpenFOAM and adapted mesh with prism layers to turbulent numerical simulation with high fidelity as well as provide a guideline on numerical schemes and parameters of OpenFOAM. Reynolds number based on a momentum thickness at inlet and a free-stream velocity was Re_θ=300. Time dependent inflow fields with near-wall turbulent structures were generated by a method of Lund et al. (1998), which was to extract instantaneous velocity fields from an auxiliary simulation with rescaled and recycled velocities at inlet. To ascertain the statistical characteristics of turbulent boundary layer, the mean profiles of streamwise velocity and turbulent intensities obtained from structured and adapted meshes were compared with the previous data.

• 한국전산유체공학회지
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• 제18권2호
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• pp.85-92
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• 2013

In this study, we consider heat transfer enhancement in laminar channel flow by means of an infinite streamwise array of equispaced identical circular cylinders. This flow configuration can be regarded as a model representing a micro channel or an internal heat exchanger with cylindrical vortex generators. A numerical parametric study has been carried out by varying Reynolds number based on the bulk mean velocity and the cylinder diameter, and the gap between the cylinders and the channel wall. An immersed boundary method was employed to facilitate to implement the cylinders on a Cartesian grid system. No-slip condition is employed at all solid boundaries including the cylinders, and the flow is assumed to be periodic in the streamwise direction. Also, the Prandtl number is fixed as 0.7. For thermal boundary conditions on the solid surfaces, it is assumed that heat flux is constant on the channel walls, while the cylinder surfaces remain adiabatic. The presence of the circular cylinders arranged periodically in the streamwise direction causes a significant topological change of the flow, leading to heat transfer enhancement on the channel walls. The Nusselt number averaged on the channel wall is presented for the wide ranges of Reynolds number and the gap. A significant heat transfer enhancement is noticed when the gap is larger than 0.8, while the opposite is the case for smaller gaps. More quantitative results as well as qualitative physical explanations are presented to justify the effectiveness of varying the gap to enhance heat transfer from the channel walls.

• 한국가시화정보학회:학술대회논문집
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• 한국가시화정보학회 2001년도 Proceedings of 2001 Korea-Japan Joint Seminar on Particle Image Velocimetry
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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.

• 한국항공우주학회지
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• 제31권7호
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• pp.15-25
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• 2003

진동하는 에어포일의 근접후류 특성을 조사하기 위한 실험적 연구가 수행되었다. NACA 4412에어포일은 1/4 시위 지점을 중심으로 조화적으로 피칭운동을 하고, 순간받음각이 +6$^{\circ}$에서 -6$^{\circ}$까지 진동하도록 하였다. 진동하는 에어포일의 근접후류에서의 평균속도를 측정하기 위하여 열선풍속계를 사용하였다. 본 연구에서 자유류의 속도는 3.4, 12.4, 26.2 m/s이다. 이러한 자유류 속도에 따른 시위 레이놀즈수는 $R_N$=5.3${\times}10^4$, 1.9${\times}10^5$, 4.1${\times}10^5$이고, 무차원 진동수는 K=0.1이다. 레이놀즈수가 진동하는 에어포일의 근접후류에 미치는 영향을 나타내기 위하여 축방향 위상평균 속도분포를 제시하였다. 본 측정에서 모든 경우에 속도결손은 $R_N$=5.3${\times}10^4$인 경우에 아주 크고, $R_N$=1.9${\times}10^5$과 4.1${\times}10^5$인 경우에는 작다는 것을 관찰 할 수 있었다. 이와 같이 위상평균속도의 커다란 차이는 $R_N$=5.3${\times}10^4$과 1.9${\times}10^5$ 사이에 있다는 것을 관찰하였다. 따라서 본 연구는 진동하는 에어포일의 근접후류에서의 레이놀즈수의 임계값이 5.3${\times}10^4$에서 1.9${\times}10^5$ 범위에 존재한다는 것을 보여준다.

• 한국가시화정보학회지
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• 제18권3호
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• pp.109-115
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• 2020

Five layers in mean flow are proposed by using the direct numerical simulation data of turbulent pipe flow up to Reτ = 3008. Viscous sublayer, buffer layer, mesolayer, log layer and core region are investigated. In the buffer layer, the viscous force is counterbalanced by the turbulent inertia from the streamwise mean momentum balance, and a log law occurs here. The overlap layer is composed of the mesolayer and the log layer. Above the buffer layer, the non-negligible viscous force causes the power law, and this region is the mesolayer, where it is the lower part of the overlap layer. At the upper part of the overlap layer, where the viscous force itself becomes naturally negligible, the log layer will appear due to that the acceleration force of the large-scale motions increases as the Reynolds number increases. In the core region, the velocity-defect form is satisfied with the power-law scaling.

• 대한기계학회논문집B
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• 제25권6호
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• pp.776-785
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• 2001

Hot-wire measurements are performed in boundary layers developing on a NACA0012 airfoil over which wakes pass periodically. The Reynolds number based on chord length of the airfoil is 2$\times$10(sup)5 and the wakes are generated by circular cylinders rotating clockwise and counterclockwise around the airfoil. This paper and its companion Part II describe the phenomena of wake-induced transition of the boundary layers on the airfoil using measured data; phase-and time-averaged streamwise mean velocities, turbulent fluctuations, integral parameters and wall skin frictions. This paper describes the background and facility together with results of time-averaged quantities. Due to the passing wake with mean velocity defects and high turbulence intensities, the laminar boundary layer is periodically disturbed at the upstream station and becomes steady-state transitional boundary layer at the downstream station. The velocity defect in the passing wake changes the local pressure at the leading of the airfoil, significantly affects the time-mean pressure distribution on the airfoil and eventually, has influence on the transition process of the boundary layer.

• 한국항공우주학회지
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• 제37권11호
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• pp.1080-1088
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• 2009

평판 유동에서 유동을 제어하기 위하여 압전 세라믹을 이용한 액추에이터가 설계되었다. 액추에이터가 15Hz의 낮은 가진 주파수로 구동할 때, 설계된 액추에이터에 의해 발생된 유동 교란을 알아보기 위해 경계층 측정이 수행되었다. 경계층에서의 평균 유동속도와 섭동량이 액추에이터 끝단에서 $x/{\delta}^*=31.9$ 떨어진 하류위치에서 1축 열선프로브(55P14)로 측정되었다. 측정 결과, 속도가 느린 영역과 빠른 영역이 각각 액추에이터의 중심부근 및 바깥부분에서 관찰되었으며, 이것으로 서로 마주보며 회전하는 한 쌍의 유동방향 와류의 발생이 예측되었다. 섭동량은 액추에이터의 바깥부분에서 크게 나타났으며, 벽면근처에서 유동방향 속도의 스팬방향 변화에서 변곡점이 관찰되었다. 액추에이터가 낮은 주파수에서 구동하는 경우에는 경계층 불안정성이 액추에이터의 가진 주파수와 T-S 파동에 해당하는 주파수에서 함께 증폭된다.

• 대한기계학회논문집B
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• 제24권12호
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• pp.1670-1677
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• 2000

An experimental investigation on heat transfer characteristics of two-dimensional heated blocks using a confined impinging slot jet has been performed. The effect of jet Reynolds number(Re=3900, 5800, 9700), streamwise block spacing(p/w=0.5, 1, 1.5) and dimensionless nozzle to block distance(H/B=1, 2, 4, 6) have been examined with five isothermally heated blocks. With the measurement of jet mean velocity and turbulence intensity distributions at nozzle exit, initially turbulent regimes, are classified. To clarify local heat transfer characteristics, naphthalene sublimation technique as used. The maximum Nusselt number at the stagnation point for the jet Reynolds number is occurred at H/B=4. Besides, the local and a average heat transfer of heated blocks increase with decreasing streamwise block spacing and increasing jet Reynolds number.

• 대한기계학회논문집B
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• 제24권11호
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• pp.1417-1427
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• 2000

A flow stabilizer, which is made of a honeycomb and three different mesh screens, is located downstream of a butterfly-type valve, for the reduction of flow disturbances behind the valve. Mean flow and turbulence measurements as well as flow visualizations are conducted in the downstream region of the deepens the non-uniformity of the streamwise velocity component and turbulence. The mesh screens considerably reduce the turbulence and enhance the uniformity of mean velocities. The combination of the honeycomb and the three mesh screens results in an efficient reduction in the flow disturbances. In addition, the flow stabilizer proves to have a good performance in the suppression of turbulence at a short distance.