INFLUENCE OF LARGE-SCALE LOW- AND HIGH-SPEED STRUCTURES ON A TURBULENT BOUNDARY LAYER

Direct numerical simulation data of a turbulent boundary layer (Reτ ≈ 1000) are used to explore the influences of large-scale structures on the near-wall vortical motions. The large-scale streamwise velocity fluctuations (ul) are extracted by employing a spanwise wavelength filter (λz/δ > 0.5). The r.m.s. of the streamwise swirling strength (λx) is conditionally sampled as a function of the strength of ul. The streamwise swirling strength is attenuated or amplified under the negativeor positive-ul events in the nearwall region, respectively. The asymmetric influence of the large scales on the near-wall region is due to the associated spanwise motions within the footprints of ul, i.e., the congregative and dispersive motions induced by the outer large-scale low(ul < 0) and high-speed structures, respectively (Hwang et al. 2016). The dispersive motions are more intense than the congregative motions because the positive-ul motions toward the wall (sweep) lead to an enhancement of the spanwise momentum. Conditionally averaged velocity fields associated with the vortical structures under the footprints show that the modulated swirling motions lie within the congregative and dispersive motions. The intense dispersive motions under the positive-ul event strengthen the smallscale spanwise velocity fluctuations (ws) close to the wall compared to ws associated with the attenuated vortical motions under the congregative motion. In addition, the wall-normal velocity components around the near-wall swirling motions are attenuated or amplified, which is attributed to the modulation of λx on the large scales. We quantify the contribution of the modulated vortical motions to the skin friction by employing the decomposition method of Yoon et al. (2016). The velocity-vorticity correlation