The College of Engineering (COE) research on “Vortex reconnection in the late transition in channel flow”  from Yue Yang’s group is published in the latest issue of Journal of Fluid Mechanics as Rapids, which is devoted to the rapid publication of short, high-impact papers in fluid mechanics.
Vortex reconnection, as the topological change of vortex lines or surfaces by cutting and connecting in vortex evolution, plays a crucial role in scale cascade in laminar-turbulent transitional flows. Although vortex reconnection has been reported in ideal simple flows such as the head-on collision of vortex rings, its mechanism and significance are still debatable in real shear flows owing to the lack of accurate characterization methods for tracking evolving vortical structures in fluid dynamics.
Based on the Lagrangian framework for transitional channel flows , Zhao et al. report the first quantitative study on vortex reconnection in shear flows. With the aid of the vortex-surface field (VSF) , the incipient vortex reconnection is characterized by the vanishing minimum distance between a pair of vortex surfaces and the reduction of vorticity flux, and multiple mechanisms of vortex reconnection are elucidated (see figure below). Furthermore, the study finds that the surge of the wall-friction coefficient begins at the identified reconnection time, because the rapid reconnection of vortex lines can induce a velocity opposed to the mean flow. This investigation demonstrates the significance of vortex reconnection in transitional channel flows and the potential of the VSF method as a general flow diagnostic tool in transitional flows.
The corresponding author Yue Yang for this study is the tenure-track Assistant Professor in the Department of Mechanics and Engineering Science and the Center of Applied Physics and Technology. The first author Yaomin Zhao is the PhD Candidate in COE. This work has been supported by the National Natural Science Foundation of China and the Thousand Young Talents Program of China.
Recently, Prof. Yue Yang received the Summer Program Fellowship from the Center for Turbulence Research at Stanford University. Yang is going to apply the VSF to the large database of the direct numerical simulation of transitional boundary layers through international collaborations.
 Y. Zhao, Y. Yang*, and S. Chen, J. Fluid Mech., 802, R4, 2016
 Y. Zhao, Y. Yang*, and S. Chen, J. Fluid Mech., 793, 840-876, 2016
 Y. Yang* and D. I. Pullin, J. Fluid Mech., 685, 146-164, 2011