Influence of Large-Scale Low- and High-Speed Structures on a Turbulent Boundary Layer

Abstract

Direct numerical simulation data of a turbulent boundary layer (Ret≈1000) are used to explore the influences of large-scale structures on the near-wall vortical motions. The streamwise swirling strengths (λx) are conditionally sampled in terms of the large-scale streamwise velocity fluctuations (ul). In the near-wall region, the amplitudes of λx diminish under negative ul rather than under positive ul. This behaviour is due to the near-wall spanwise motions within the footprints of largescale structures, which are the congregative and dispersive motions induced by the outer large-scale low-speed and high-speed structures. The intense dispersive motions under the footprint of positive ul strengthen the smallscale spanwise velocity fluctuations (ws) close to the wall compared to ws affected by the congregative motions of negative ul. The wall-normal velocity components were attenuated or amplified around the modulated vortical motions, which leads to the dependence of λx on the large scales. To quantify the contribution of the modulated vortical motions, we use the decomposition method of Yoon et al. (2016), which is the relationship between the velocity-vorticity correlations and the skin-friction coefficient (Cf). In the near-wall region, intense values of [removed] wλy, related to change-of-scale effect due to the vortex stretching force, are observed for the positive-ul event. The skin friction induced by the amplified vortical motions within ul+ > 2 carries 15% of the total skin friction generated by the change-of-scale effect.