Influence of large-scale low- and high-speed structures on the near-wall vortical motions in turbulent boundary layer*

Direct numerical simulation data of turbulent boundary layer ($Re_{\tau } =1000)$ are used to investigate the large-scale influences on the vortical structures in the near-wall region. The streamwise swirling strength ($\lambda_{x})$ depends on the strength of large-scale streamwise velocity fluctuations ($u_{l})$. The amplitude of $\lambda_{x}$ decreases under the negative $u_{l}$ rather than the positive $u_{l,}$ analogous to the amplitude modulation of the velocity fluctuations. The dependence of $\lambda_{x\thinspace }$on $u_{l}$ is due to the opposite spanwise motions in the footprints of low- and high-speed structures, which are congregative and dispersive, respectively. Conditionally averaged fields conditioned on $\lambda_{x}$ under the negative- and positive-$u_{l\thinspace }$events show that the swirling motions lie within the congregative and dispersive motions. The dispersive motion is more intense than the congregative motion because the sweep of high-momentum fluid under $u_{l} >0$ splats on the wall while the spanwise motions under $u_{l} <0$ decrease as the flows come close to each other. Due to the strong dispersive motion, the small-scale spanwise velocity fluctuations ($w_{s})$ are strengthened, whereas $w_{s}$ are relatively weak (attenuated) under $u_{l} <0$. As a result, the wall-normal components are also enhanced under $u_{l} >0$, which contributes to the amplification of $\lambda_{x}$.