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A Dynamic Globalization Model for Large Eddy Simulation of Complex Turbulent Flow
Haecheon Choi(최해천),Noma Park(박노마),Jinseok Kim(김진석) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.11
A dynamic subgrid-scale model is proposed for large eddy simulation of turbulent flows in complex geometry. The eddy viscosity model by Vreman [Phys. Fluids, 16, 3670 (2004)] is considered as a base model. A priori tests with the original Vreman model show that it predicts the correct profile of subgrid-scale dissipation in turbulent channel flow but the optimal model coefficient is far from universal. Dynamic procedures of determining the model coefficient are proposed based on the 'global equilibrium' between the subgrid-scale dissipation and viscous dissipation. An important feature of the proposed procedures is that the model coefficient determined is globally constant in space but varies only in time. Large eddy simulations with the present dynamic model are conducted for forced isotropic turbulence, turbulent channel flow and flow over a sphere, showing excellent agreements with previous results.
최병귀,최해천,Choi, Byunggui,Choi, Haecheon 대한기계학회 1999 大韓機械學會論文集B Vol.23 No.11
The sliding-belt concept introduced by Bechert et al. (AIAA J., Vol. 34, pp. 1072~1074) is numerically applied to a turbulent boundary layer flow for the skin-friction reduction. The sliding belt is moved by the shear force exerted on the exposed surface of the belt without other dynamic energy input. The boundary condition at the sliding belt is developed from the force balance. Direct numerical simulations are performed for a few cases of belt configuration. In the ideal case where the mechanical losses associated with the belt can be ignored, the belt velocity increases until the integration of the shear stress over the belt surface becomes zero, resulting in zero skin friction on the belt. From practical consideration of losses occurred In the belt device, a few different belt velocities are given to the sliding belt. It is found that the amount of drag reduction is proportional to the belt velocity.
수학적 이론을 이용한 이차원 곡면 덕트의 최적형상 설계
임석현,최해천,Lim, Seokhyun,Choi, Haecheon 대한기계학회 1998 大韓機械學會論文集B Vol.22 No.9
The objectives of the present study are to develop a systematic method rather than a conventional trial-and-error method for an optimal shape design using a mathematical theory, and to apply it to engineering problems. In the present study, an optimal condition for a minimum pressure loss in a two-dimensional curved duct flow is derived and then an optimal shape of the curved duct is designed from the optimal condition. In the design procedure, one needs to solve the adjoint Navier-Stokes equations which are derived from the Navier-Stokes equations and the cost function. Therefore, a computer code of solving both the Navier-Stokes and adjoint Navier-Stokes equations together with an automatic grid generation is developed. In a curved duct flow, flow separation occurs due to an adverse pressure gradient, resulting in an additional pressure loss. Optimal shapes of a curved duct are obtained at three different Reynolds numbers of 100, 300 and 800, respectively. In the optimally shaped curved ducts, the separation region does not exist or is significantly reduced, and thus the pressure loss along the curved duct is significantly reduced.
최진(Jin Choi),전우평(Woo-Pyung Jeon),최해천(Haecheon Choi) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.11
It is an open question why the drag coefficient on golf ball remains nearly constant with increasing Reynolds number after its sharp decrease. In order to investigate this interesting phenomenon, we measure the drag, separation angle, wall pressure and streamwise velocity inside/outside dimples before main separation. When drag reduction occurs, the separation angle measured is nearly constant and the wall pressure distributions outside dimples are nearly the same even with increasing Reynolds number. From the streamwise velocity measurement, it is found that dimples located at the angles of 65˚~90˚ make an important role in changing flow characteristics. Inside one or two rows of dimples located at those angles, a small separation bubble exists and flow becomes quickly transitional and turbulent with reattachment. The main separation occurs further downstream at 110˚ even with increasing Reynolds number, because downstream dimples do not make an important role in changing flow characteristics any more.
헬리컬 기본교란과 축대칭 분수조화교란을 이용한 원형제트에서의 보텍스 병합 및 제트확산
조성권,유정열,최해천,Cho, Sung Kwon,Yoo, Jung Yul,Choi, Haecheon 대한기계학회 1998 大韓機械學會論文集B Vol.22 No.11
An axisymmetric jet is forced with two helical fundamental waves of identical frequency spinning in opposite directions and an additional axisymmetric sub harmonic wave. The subharmonic component rapidly grows downstream from subharmonic resonance with the fundamental, significantly depending on the initial phase difference. The variations of the subharmonic amplitude with the initial phase difference show cusp-like shapes. The amplification of the sub harmonic results in 'vortex pairing of helical modes'. Furthermore, azimuthal variation of the amplification induces an asymmetric jet cross-section. When the initial subharmonics is imposed with an initial phase difference close to a critical value, the jet-cross section evolves into a three-lobed shape. One lobe is generated by the enhanced vortex pairing and the other two lobes are generated by the delayed vortex pairing. Thus, it is confirmed that the initial phase difference between the fundamental and the subharmonic plays an important role in controlling the jet cross-section.
구에 설치한 딤플과 표면 거칠기에 의한 항력 감소 메커니즘
최진(Jin Choi),전우평(Woo-Pyung Jeon),최해천(Haecheon Choi) 한국유체기계학회 2006 유체기계 연구개발 발표회 논문집 Vol.- No.-
In this paper, we present a detailed mechanism of drag reduction by dimples and roughness on a sphere by measuring the streamwise velocity above the dimpled and roughened surfaces, respectively. Dimples cause local flow separation and trigger the shear layer instability along the separating shear layer, resulting in generation of large turbulence intensity. With this increased turbulence, the flow reattaches to the sphere surface with high momentum near the wall and overcomes strong adverse pressure gradient formed in the rear sphere surface. As a result, dimples delay main separation and reduce drag significantly. The present study suggests that generation of a separation bubble, i.e. a closed-loop streamline consisting of separation and reattachment, on a body surface is an important flow-control strategy for drag reduction on a bluff body such as the sphere and cylinder. In the case of roughened sphere, the boundary layer flow is directly triggered by roughness and changes to a turbulent flow. Due to this change, the drag significantly decreases. As the Reynolds number further increases, transition to turbulence occurs earlier on the sphere surface. Because of faster growth of turbulent boundary layer by roughness, earlier transition thickens the boundary layer, resulting in earlier separation and drag increase with increasing Reynolds number.
충돌제트를 갖는 회전원판 위 3차원 경계층의 난류특성 (I) - 평균유동장 -
강형석,유정열,최해천,Kang, Hyung Suk,Yoo, Jung Yul,Choi, Haecheon 대한기계학회 1998 大韓機械學會論文集B Vol.22 No.9
The objective of the present study is to investigate experimentally the mean flow characteristics of the three-dimensional turbulent boundary layer over a rotating disk with an impinging jet at the center of the disk, which may be regarded as one of the simplest models for the flow in turbomachinery. A relatively strong radial outflow (crossflow) generated from the impinging jet is added to the radial outflow (crossflow) induced by the centrifugal force in order to create the three-dimensional boundary layer. A new calibration technique has been introduced to determine the velocity direction and magnitude using an I-wire probe, where the uncertainties are ${\pm}1.5^{\circ}$ and ${\pm}0.35\;m/s$, respectively, in the laminar boundary layer region, compared with the known exact solutions. The flow in the tangential direction is of similar type to that associated with a favorable pressure gradient, considering that no wake region appears in wall coordinate velocity profiles and the Clauser shape factor is between 4.0 and 5.3. The flow angle is significantly changed by the crossflow generated by the impinging jet.