Abstract: The turbulent/non-turbulent interface (TNTI) in an adverse pressure gradient (APG, β = 1.45) turbulent boundary layer (TBL) is explored here by using direct numerical simulation (DNS) data; β is the Clauser pressure gradient parameter. For comparison, the DNS data for a zero pressure gradient (ZPG) TBL is included. The interface is extracted with an approach based on enstrophy criteria. Depending on the enstrophy, the outer boundary layer flow can be classified into the free stream, boundary layer wake, and intermittent flow regimes. The fractal dimension of the interface is obtained by using the box-counting algorithm, and was found to be constant over a long range of box sizes. The TNTI shows a monofractal behavior. The geometric complexity of a TNTI can be determined in terms of the genus, which is defined as the number of handles in a geometric object. We examine the volume and projection area of the genus of the TNTI to analyze the entrainment process. The geometric complexity of the APG TBL interface and the local entrainment are greater than those of the ZPG TBL, as is evident in the increases in the genus near the interface. The local entrainment velocity is dominantly affected by the viscous diffusion at the interface.

Curriculum Vitae: Dr. Hyung Jin Sung is a professor at KAIST. In 1984, he joined the faculty at KAIST as an assistant professor, and was promoted as an associate professor in 1990 and a full professor in 1994. He is currently serving as the Director of NSCN (National Science Challenges Support & Networks) under the Ministry of Science and Technology of Korea. Professor Sung has devoted his research career to a variety of research fields in fluid mechanics including turbulence, fluid-structure interaction and acoustofluidics, and flow visualization. He has authored more than 478 SCI journal papers published in prestigious journals. His papers have been cited in more than 20,470 journal papers according to Google Scholar (h-index, 71). His h-index in Web of Science is 67. Based on outstanding research achievements, he was nominated as an American Physics Society (APS) fellow in 2013, a Korea Telecommunication (KT) endowed chair professor in 2015 and received many awards including Sudang Award in 2014, KAIST Grand-Prix Academic Award in 2009.