Abstract:
     Wall turbulence is a ubiquitous phenomenon in nature and engineering applications, yet predicting such turbulence is difficult due to its complexity. High-Reynolds-number turbulence arises in most practical flows, and is particularly complicated because of its wide range of scales. Although the attached-eddy hypothesis postulated by Townsend can be used to predict turbulence intensities and serves as a unified theory for the asymptotic behaviors of turbulence, the presence of coherent structures that contribute to the logarithmic behaviors has not been observed in instantaneous flow fields. Here, we demonstrate the logarithmic region of the turbulent intensity by identifying wall-attached structures of the velocity fluctuations (ui) through the direct numerical simulation of a moderate-Reynolds-number boundary layer (Reτ ≈ 1000). The wall-attached structures are self-similar with respect to their heights (ly), and in particular the population density of the streamwise component (u) scales inversely with ly, reminiscent of the hierarchy of attached eddies. The turbulence intensities contained within the wall-parallel components (u and w) exhibit the logarithmic behavior. The tall attached structures (ly+ > 100) of u are composed of multiple uniform momentum zones (UMZs) with long streamwise extents, whereas those of the cross-stream components (v and w) are relatively short with a comparable width, suggesting the presence of tall vortical structures associated with multiple UMZs. The magnitude of the near-wall peak observed in the streamwise turbulent intensity increases with increasing ly, reflecting the nested hierarchies of the attached u structures. These findings suggest that the identified structures are prime candidates for Townsend’s attached-eddy hypothesis and that they can serve as cornerstones for understanding the multiscale phenomena of high-Reynolds-number boundary layers.

Biography

Position in professional societies and activities:

     APS, Fellow. Fluid Dynamics Research, Associate editor. International Journal of Heat and Fluid Flow, Associate editor. Turbulence and Shear Flow Phenomena, Executive organizing committee member.
Awards and Publications:
   1997, Namhun academic award; 2003, Inter-research technology award from President (Korea); 2003, KSME academic award; 2005, KAIST academic award; 2007, KAIST international cooperation award; 2009, KAIST grand-prix academic award; 2014, Sudang award; 2017, Scientist of the month (November) from the Korean government
     352 international journal papers. Details can be found in http://flow.kaist.ac.kr

Resume_HJSung.pdf