The skewness of the streamwise velocity is an important parameter in turbulent-boundary-layer (TBL) flows and is intrinsically related to the mechanism of quadratic energy transfer between different scales of motion. In this paper, the skewness in both a canonical and actuated zero-pressure-gradient TBL (${\text{Re}}_{\theta }=1770$) is spectrally decomposed via the real part of the bispectrum. This provides the individual triad interactions contributing to the skewness, and their summation is used to obtain partial sums of the skewness as a function of frequency. The canonical TBL investigated has a Reynolds number that is sufficiently low that no naturally occurring energetic large-scale outer structures exist. The skewness spectral decomposition of the canonical TBL is compared to that in which a plasma actuator introduces large-scale vortices into its outer layer. In this manner, actuation is used as a tool to document the influence of imposed outer-layer large-scale structures on the spectral content of skewness. These measurements are compared for a range of representative near-wall ($y^{+}\le 60$) locations. The bispectral results reveal that there is a close correspondence between the dominant triad interaction frequencies and the near-wall structure frequencies, captured using the variable-interval-time-averaging technique. At $y^{+}=15$, the presence of mostly negative peaks within the lower frequency region of the bispectra indicates the existence of ejection-type events active in the near-wall region. The effect of the artificially induced outer large-scale structures is shown to reach the near-wall region. However, the common features in the bispectra and partial bispectral sums that are associated with the streamwise buffer layer vortices remain largely unchanged. This is evidence of their primary role in the skewness at these wall-normal locations. The bispectral results also show that the effect of actuation remains confined around the lines representing the actuation frequency, which is suggestive of the dominance of the linear inner-layer–outer-layer interaction mechanism over nonlinear interaction for the given configuration, Reynolds number, and actuation parameters. The phase-averaged results indicate that, in an actuated TBL, the outer-layer synthetic structures modulate the strength of the near-wall vortices. It is concluded that in both canonical and actuated TBLs, outer-layer structures do not serve as triggers for the near-wall events.

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• Energy cascade
• Structure & turbulence of boundary layers
• Turbulent boundary layers