Quantifying engulfment in Shear Free Turbulence, Jets and Wakes

Turbulent entrainment (TE) is present in wakes, jets, mixing layers and boundary layers. It can be described as the process by which irrotational fluid is absorbed into the core turbulent region, by moving across the turbulent/non-turbulent interface (TNTI) layer. TE controls several important flow characteristics, such as the spreading rate of jets, wakes and mixing layers. This process is carried through by two types of phenomena: engulfing and nibbling. Nibbling is associated with small scale vorticity diffusion along the entire TNTI, whereas engulfment is associated with large scale swirling motions intermittently occurring at the TNTI. In the present work direct numerical simulations (DNS) of jets, wakes and shear free turbulence are used to quantify these two processes and their relative importance.