Visualization and assessment of turbulent coherent structures in laminarescent boundary layers

Direct numerical simulation (DNS) with high spatial/temporal resolution of spatially developing turbulent boundary layers subject to very strong favorable pressure gradient (FPG) has been performed. The sudden acceleration imposed on the flow provokes a significant attenuation of turbulent intensities (Araya et al. in J Fluid Mech 775:189–200, 2015), particularly due to the dominance of pressure forces over approximately frozen Reynolds stresses, Narasimha and Sreenivasan (Adv Appl Mech 19:221–309, 1979). In this article, visualization of the DNS velocity field is carried out in order to evaluate the effect of very strong FPG on elementary coherent structures, such as low- and high-speed streaks, sweeps and ejections, quasi-streamwise and horseshoe vortices, and large-scale motions. An important symbiosis between quasi-streamwise vortices and low-speed streaks (or regeneration cycle) has been observed in the FPG zone. The destabilizing shear layers upstream of vortex heads in strong accelerated flows appear as very thin zones developing in the buffer region (\(20<y^+< 30\)). It seems that decaying \(uv'\) in strong FPG flows (Araya et al. 2015) is principally due to the annihilation of sweeps, while ejections are responsible for the turbulence residual.