Nature of sweep and ejection events in transitional and turbulent boundary layers

This paper represents an experimental investigation of the physical nature of the turbulence production mechanisms in boundary layers associated with three fundamental phenomena known from previous studies: (i) sweep events, (ii) ejection events, and (iii) coherent vortical structures. The main goal of this study is to clarify connections between all these phenomena in transitional and turbulent flows. The experimental approach is based on a modified version of the hydrogen-bubble (HB) visualization technique called the synchronous visualization method, which is combined with quantitative processing of video images in a PIV-like manner. The results of investigation of sweep/ejection events and their connection with vortical structures are obtained at late and super-late stages of boundary-layer transition, as well as in fully turbulent boundary layers. The developed approach gives us the possibility to obtain detailed synchronous information about instantaneous flow characteristics in a great range of spatial coordinates and in two projections (plane and side views). It is assumed that the results obtained in transitional flow are very useful for analysis of the turbulent production mechanisms occurring in developed turbulent boundary layers due to the existence of the well-known similarity of these mechanisms found in previous investigations. As a result, two types of ejection events and three types of sweep events are found. It is shown that 'sweep 1' and 'ejection 1' events are produced by well-known Lambda- ('horseshoe') vortices, while 'sweep 2' and 'ejection 2' events are generated by 'ring-like vortices' ('typical eddies') also found previously. The latter turned out to be significantly stronger than the former. The strongest sweep event (called 'sweep 3') is observed when 'sweep 1' and 'sweep 2' events become superimposed in space (in the (x, z)-plane). This superposition gives rise to the appearance (in the visualized plane view) of some very characteristic patterns associated with 'dark spots' ('pockets') observed previously in the wall regions in the developed turbulent boundary layers.