Byung-Gu Kim

On the near-wall characteristics of acceleration in turbulence

The behaviour of fluid-particle acceleration in near-wall turbulent flows is investigated in numerically simulated turbulent channel flows at low to moderate Reynolds numbers, Re τ = 180 ~ 600. The acceleration is decomposed into pressure-gradient (irrotational) and viscous contributions (solenoidal acceleration) and the statistics of each component are analysed. In near-wall turbulent flows, the probability density function of acceleration is strongly dependent on the distance from the wall. Unexpectedly, the intermittency of acceleration is strongest in the viscous sublayer, where the acceleration flatness factor of O(100) is observed. It is shown that the centripetal acceleration around coherent vortical structures is an important source of the acceleration intermittency. We found sheet-like structures of strong solenoidal accelerations near the wall, which are associated with the background shear modified by the interaction between a streamwise vortex and the wall. We found that the acceleration Kolmogorov constant is a linear function of y + in the log layer. The Reynolds number dependence of the acceleration statistics is investigated.

Particle Dispersion and Effect of Spin in the Turbulent Boundary Layer Flow

In this paper, we develope a dispersion model based on the Generalized Langevin Model. Thomson`s well-mixed condition is the well known criterion to determine particle dispersion. But, it has `non-uniqueness problem`. To resolve this, we adopt a turbulent model which is a new approach in this field of study. Our model was greatly simplified under the self-similarity condition, leaving model only two model constants and that control the dispersion and spin which measures rotational property of the Lagrangian particle trajectory. We investigated the sign of spin as well as magnitude by using the Direct Numerical Simulation. Model calculations were performed on the neutrally stable boundary layer flow. We found that spin has weak effect on the particle dispersion but it shows the significant effect on the horizontal flux compared to the zero-spin model.l.愯半佈䍉⼲〰㤯瘴㉮㘟S䩏䡃䥟㈰〹彶㐲渶彴桵浢湡楬⹪灧