Published online by Cambridge University Press: 26 September 2017
Direct numerical simulation data of a turbulent boundary layer ($Re_{\unicode[STIX]{x1D70F}}=1000$) were used to investigate the large-scale influences on the vortical structures that contribute to the local skin friction. The amplitudes of the streamwise and wall-normal swirling strengths ($\unicode[STIX]{x1D706}_{x}$ and $\unicode[STIX]{x1D706}_{y}$) were conditionally sampled by measuring the large-scale streamwise velocity fluctuations ($u_{l}$). In the near-wall region, the amplitudes of $\unicode[STIX]{x1D706}_{x}$ and $\unicode[STIX]{x1D706}_{y}$ decreased under negative $u_{l}$ rather than under positive $u_{l}$. This behaviour arose from the spanwise motions within the footprints of the large-scale low-speed ($u_{l}<0$) and high-speed structures ($u_{l}>0$). The intense spanwise motions under the footprint of positive $u_{l}$ noticeably strengthened the small-scale spanwise velocity fluctuations ($w_{s}$) below the centre of the near-wall vortical structures as compared to $w_{s}$ within the footprint of negative $u_{l}$. The streamwise and wall-normal components were attenuated or amplified around the modulated vortical motions, which in turn led to the dependence of the swirling strength on the $u_{l}$ event. We quantified the contribution of the modulated vortical motions $\langle -w\unicode[STIX]{x1D714}_{y}\rangle$, which were related to a change-of-scale effect due to the vortex-stretching force, to the local skin friction. In the near-wall region, intense values of $\langle -w\unicode[STIX]{x1D714}_{y}\rangle$ were observed for positive $u_{l}$. By contrast, these values were low for negative $u_{l}$, in connection with the amplification of $w_{s}$ and $\unicode[STIX]{x1D706}_{y}$ by the strong spanwise motions of the positive $u_{l}$. The resultant skin friction induced by the amplified vortical motions within $u_{l}^{+}>2$ was responsible for 15 % of the total skin friction generated by the change-of-scale effect. Finally, we applied this analysis to a drag-reduced flow and found that the amplified vortical motions within the footprint of positive $u_{l}$ were markedly diminished, which ultimately contributed to the total drag reduction.