Lagrangian Analysis of the Entrainment Process in a High-speed Turbulent Boundary Layer

Direct numerical simulations of zero-pressure-gradient Mach-4.5 turbulent boundary layer (TBL) is preformed to examine the mechanisms of the entrainment process. The two mechanisms by which the outer irrotational flow can be entrained into the turbulent region and their relative contribution to the growth of the spatially developing boundary layer are evaluated: (i) nibbling is the vorticity transport across the turbulent/non-turbulent interface (TNTI), and (ii) engulfment is the entrapment of pockets of irrotational flow inside the TBL prior to finally breaking apart. The analysis involves the temporal advancement (time tracking) of a certain number of fluid particles. At each time step, particle velocity and position vectors are obtained from the three-dimensional DNS data using an interpolating scheme. This approach is more suited to study the entrainment since the turbulence statistics are calculated for the particle trajectories and reflect the actual distance of the fluid particles to the TNTI during the entrainment process. The results highlight the engulfment mechanism as the dominant form of entrainment in a high-speed TBL.