Turbulent boundary layers under the influence of a sudden change in wall roughness in the streamwise direction

Turbulent boundary layers (TBLs) travelling over heterogeneous rough surfaces where surface roughness changes in the streamwise direction are commonly encountered in nature and in a wide range of engineering applications. Due to such surface transitions, a thin shear layer of disturbed flow is generated over the downstream wall. This new shear layer is called internal boundary layer (IBL), and it grows in the wall-normal direction, i.e. becomes thicker with the streamwise distance, as the flow within the IBL adjusts to the new wall condition. Although there has been considerable research on predicting the growth of IBLs based on mean flow properties since the seminal work of Elliott (1958), instantaneous behaviour of IBLs and the overall structure of TBLs under the influence of IBLs have remained under explored. Thus, this study aims at providing detailed analysis on the structure of a TBL that is subjected to an abrupt surface transition from a P60 to P24 grit sandpaper. We particularly focus on the behaviour of turbulent/non-turbulent interfaces, uniform momentum zones and their interfaces under such an abrupt surface transition, and compare these interfaces with the edges of instantaneously detected IBLs.