Directly Characterising the Interplay between Entrainment and the TNTI in a Turbulent Boundary Layer

A plethora of natural and engineered processes are influenced or driven by turbulent flows. Since the advent of fluid mechanical research, turbulent boundary layers (TBLs) have been studied as a fundamental model through which the characteristics of turbulent transport can be understood. In recent years, a large body of research has focussed on the role of intermittency and the turbulent/non-turbulent interface (TNTI). The entrainment of external flow into the boundary layer is understood to be responsible for shaping the TNTI and thus the transfer of momentum between the near-wall and external flow. However, it is non-trivial to experimentally characterise entrainment and the TNTI itself using conventional PIV while retaining information about the outer flow. To this end, locally seeding the TBL with particles that fluoresce under laser emission while globally seeding the flow with particles that generate only Mie scattering has shown promise in directly characterising entrainment and the TNTI without compromising information about the external flow. Furthermore, it is possible to measure and characterise the flow properties of entrained regions to provide greater insights into the interaction between them and the flow within the boundary layer.