Precursors of backflow events and their relationship with the near-wall self-sustaining process

Abstract

This study examines the precursors and consequences of rare backflow events at the wall using direct numerical simulation of turbulent pipe flow with a high spatiotemporal resolution. The results obtained from conditionally averaged fields reveal that the precursor of a backflow event is the asymmetric collision between a high- and a low-speed streak (LSS) associated with the sinuous mode of the streaks. As the collision occurs, a lifted shear layer with high local azimuthal enstrophy is formed at the trailing end of the LSS. Subsequently, a spanwise or an oblique vortex spontaneously arises. The dominant nonlinear mechanism by which this vortex is engendered is enstrophy intensification due to direct stretching of the lifted vorticity lines in the azimuthal direction. As time progresses, this vortex tilts and orientates towards the streamwise direction and, as its enstrophy increases, it induces the breakdown of the LSS located below it. Subsequently, this vortical structure advects as a quasi-streamwise vortex, as it tilts and stretches with time. As a result, it is shown that reverse flow events at the wall are the signature of the nonlinear mechanism of the self-sustaining process occurring at the near-wall region. Additionally, each backflow event has been tracked in space and time, showing that approximately 50 % of these events are followed by at least one additional vortex generation that gives rise to new backflow events. It is also found that up to a maximum of seven regenerations occur after a backflow event has appeared for the first time.

Backflow structures in turbulent pipe flows at low to moderate Reynolds numbers

“…The observed dynamics of the large-scale structures are consistent with Guerrero et al. (2022), and are presented in the supplementary movie available at . With indication from the size of the high- and low-speed region in figure 5, both the high- and low-speed structures remain in strength as precursors of the backflow until it is formed, and rapidly weaken after the backflow events.…”

“…The high-speed structure is convected downstream naturally faster than the low-speed structure and they ‘collide’ as described in Guerrero et al. (2022) when the backflow events take place underneath the high-speed structure at the tail tip of the low-speed structure where it has become strong enough to overcome the mean shear. The observed dynamics of the large-scale structures are consistent with Guerrero et al.…”

“…The dynamics of the large-scale structures are evaluated by means of spatiotemporal conditional averaging similar to Guerrero et al. (2022), centring the backflow events at where is the time of the events. Figure 5( a , b ) show the time track of the low-speed structure with ejection events and the high-speed structure with sweep events, respectively, and the interaction between the large-scale structures is illustrated in figure 5( c ).…”

“…2018 a ) and pipes (Chin et al. 2018 b , 2020; Jalalabadi & Sung 2018; Guerrero, Lambert & Chin 2020, 2022). The backflow was also investigated in the DNS of fully developed flows in ducts (Zaripov et al.…”

“…The spatiotemporal conditional average fields in Guerrero et al. (2022) unfolded the evolution of the large-scale structures during the formation of backflow. The backflow was found to be a consequence of the ‘collision’ between the large-scale high- and low-speed structures which are the precursors of the backflow events.…”