Abstract: Small-scale vortex tubes and shear layers within the turbulent/non-turbulent interface (TNTI) layer play a pivotal role in the process of entrainment. Shear layers in turbulence are known to be unstable against perturbations with wavelengths approximately 30 times the Kolmogorov scale. This study conducts numerical experiments aimed at investigating the potential for enhancing turbulent mixing through the excitation of small-scale shear instability. Direct numerical simulations are conducted for a turbulent front with a passive scalar transfer evolving in the absence of mean shear, where solenoidal velocity perturbations of constant wavelength are introduced outside the turbulent region. These perturbations are found to enhance the entrainment rate significantly when their wavelength coincides with the unstable mode of shear layers. The prompted instability also causes the amplification of the mean scalar dissipation rate, facilitating faster mixing at small scales. The present results prove the potential of small-scale shear instability to efficiently enhance passive scalar mixing in turbulent flows.

Curriculum Vitae: Tomoaki Watanabe, Ph.D., is currently an Associate Professor in the Department of Mechanical Engineering and Science at Kyoto University. After earning my Ph.D. in Mechanical Engineering from Nagoya University in 2014, I began my academic career as a JSPS Research Fellow at the same institution, subsequently serving as a visiting researcher at University of Washington and Instituto Superior Técnico. Later, I joined Nagoya University as an Assistant Professor. My recent research focuses on the experimental and numerical investigation of small-scale shearing motions in various turbulent flows, including compressible and density-stratified flows.