Backflow phenomenon in converging and diverging channels

To date, a backflow phenomenon that occurs in subsonic zero-pressure-gradient and adverse-pressure-gradient turbulent boundary layers has been well studied and its existence in such flow types is beyond doubt. In the present investigation, focusing on a turbulent air flow in a duct with adjustable cross section, for the first time we provide experimental evidence for the existence of backflow events in a converging channel and demonstrate the possibility of their occurrence in a favorable-pressure-gradient turbulent boundary layer. Taking into account that the backflow events are characterized by the low probability of their occurrence and spatio-temporal extent at the level of the Kolmogorov scale, we pay special attention to the use of high-speed particle image velocimetry with high spatio-temporal resolution. The measurement results revealed a general tendency in the backflow formation mechanism in the considered flow regimes and channel configurations. In conjunction with the findings of other studies, we suggest that the backflow events are induced by transversely oriented vortices located in the buffer region of the turbulent boundary layer and formed as a result of the interaction of large-scale high-momentum and low-momentum flow regions.