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최연택(Yeontaek Choi),이창훈(Changhoon Lee),조상규(Sanggyu Jo) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.11
We develop a generalized RPA(random phase and amplitude) formalism in wave turbulence(WT). RPA theory employ the Hamiliton mechanics description, derived directly from the 3-D inviscid and incompressible Navier-Stokes equation through the Clebsch transformation. We expand randomness even to amplitudes besides phases of wave-like motions. The newly derived equation is also a wave kinetic equation(WKE). We exploit the PDF analysis to analyze the WKE and obtain some new good results. Those are based on PDF analysis from the analysis of WT spectrum, the exact solution of the Hamilton equation.
최연택(Yeontaek Choi),이창훈(Changhoon Lee) 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.6
We investigated the intermittent property of helicity of the isotropic turbulence. As well known, helicity is a quadratic invariant on the inviscid Navier-Stokes flows. But unfortunately it has no invariance in the viscous fluids. Furthermore, it has a intermittent fashion through its cascade in the inertial range. We looked for the sources to raise this intermittency, handling with local quantities such as local enstrophy, dissipation, and local coupling angle of velocity and vorticity. It seems to be the preferred angles to determine the direct relation between local velocity and local vorticity, and to be guessed the source of the intermittency.
최연택(Yeontaek Choi),이창훈(Changhoon Lee) 한국유체기계학회 2006 유체기계 연구개발 발표회 논문집 Vol.- No.-
Helicity in isotropic turbulence was well known to have intermittent fashion in their statistics. But its exact explanation about the onset of intermittency of helicity in turbulence did not give clearly yet. Most probable causes comes from the vortical motion of the fluids. Distribution of the angle between fluid velocity and vorticity have alignment tendency. This may be a clue to investigate intermittency of helicity. In this study, we aim to review and establish approaches to reveal the mechanism and the origin of intermittency of helicity in the isotropic turbulence. To do those work, we look for some quantities like helicity, enstrophy, acceleration and its flatness. And also correlations among them are sought.
박용남(Yongnam Park),최연택(Yeontaek Choi),이창훈(Chanhoon Lee) 대한기계학회 2010 대한기계학회 춘추학술대회 Vol.2010 No.11
The geometric nature of particle trajectory is investigated for understanding the Lagrangian nature of turbulence using direct numerical simulation of isotropic turbulence. Probability density functions and autocorrelations along a fluid particle trajectory associated with geometric quantities such as curvature and torsion of the Lagrangian trajectory are provided. We propose the ratio of torsion to curvature as an important parameter to identify the particle trajectory, and it is found to play a crucial role in understanding the geometric shape of particle trajectory. The relationship between Lagrangian helicity and the ratio of torsion to curvature is investigated where Lagrangian helicity is defined as a dot product of velocity and vorticity vectors at the point of a fluid particle. We also found that probability density functions of torsion and torsion normalized by curvature clearly show well-established slope in log?log plots. Lagrangian helicity is intermittently distributed and high Lagrangian helicity is always found, where high acceleration is observed. Regarding the relationship between coherent structure and acceleration, coherent structure can be understood in terms of Lagrangian helicity, curvature, and torsion.