Turbulent/turbulent Entrainment: a detailed examination of entrainment, and the behaviour of the outer interface as affected by background turbulence

This thesis presents an experimental investigation into the effects of free-stream turbulence on entrainment and examines the outer boundaries of fluidic regions with turbulence present on both sides of the interface. Simultaneous particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) experiments are conducted in the wake of a circular cylinder subjected to grid generated turbulence. The turbulent/turbulent interface (TTI) is identified for the first time. Analogous to the classical case of a turbulent/non-turbulent interface (TNTI), it facilitates the change in the enstrophy level between one region to the other. The TTI is seen to persist even at the most extreme background turbulence conditions. In the far wake of a circular cylinder, free-stream turbulence is seen to reduce the net entrainment into the wake relative to a non-turbulent background. This behaviour is driven by infrequent yet powerful, intermittent detrainment events. It is found that the intensity of the background turbulence is most important in determining this behaviour, whereas the length scale is seen to have, only a second order effect. The mechanism of this entrainment process is investigated and it is observed that role of viscosity in this process is insignificant when turbulence is present on both sides of the fluidic boundary. This is possible, since the availability of turbulent strain (and vorticity) on both sides of the interface in a TTI, allows the inertial term to take over the responsibilities of enstrophy production. Finally, in the near-field of a cylinder wake, the influence of length scale is more important. Unlike in the far-field, coherent structures play a vital role in near wake development. The traditional increase in a bluff body wake width in the near field is reproduced and the effects of both wake meandering as well as wake growth through entrainment are investigated.Open Acces