Direct numerical simulation of out-scale-actuated spanwise wall oscillation in turbulent boundary layers

Abstract

Spanwise wall oscillation (SWO) of turbulent boundary layers (TBLs) is investigated via direct numerical simulations over an extended actuation region with oscillation periods up to T_{sc}^+=600, scaled by the uncontrolled friction velocity u_{τ0} at the onset of SWO (i.e. Re_θ=344). For low periods (T_{sc}^+<200), drag reduction (DR) decreases with increasing Re_θ, consistent with conventional inner-scaled control strategies targeting near-wall turbulence. In sharp contrast, for large periods, DR increases with Re_θ. For example, at T_{sc}^+=600, DR rises from 1.3% at Re_θ=713 to 7.0% at Re_θ=2340. This unexpected growth is partly explained by the streamwise evolution of the effective oscillation parameter: as TBL develops, u_{τ0} decreases downstream, reducing the local-scaled period T^+ and thereby enhancing suppression of near-wall turbulence. Interestingly, even the results are compared at approximately fixed T^+, DR for T^+>350 still exhibits a weak positive dependence on Re_θ, consistent with recent experiments by Marusic et al. Nat. Commun., vol. 12, 2021, 5805. We further develop a new analytical relationship that links DR to the upward shift of mean velocity in the wake region. Unlike previous formulations, the relationship avoids logarithmic-region fitting and does not rely on an invariant Karman constant under SWO, while maintaining good agreement with DNS data. Flow diagnostics -- including Reynolds stresses, skin-friction decomposition, and energy spectra -- demonstrate that the observed variation of DR with Reynolds number (Re) arises from period-dependent modulation of near-wall turbulence. Overall, these findings challenge the conventional view that DR inevitably deteriorates with Re and demonstrate that out-scaled actuation can instead enhance DR performance -- offering new physical insights for high-Re control strategies.

Figures (24)

• Figure 1: Contours of $c_f$ (a) and $DR$ (b) as functions of $Re_\theta$ and $T_{sc}^+$.
• Figure 2: $c_f$ (a) and $DR$ (b) as a function of $x$ for cases W12T100, W12T200, W12T600, and WS12T600. The shading represents the actuation region, i.e. $250\delta_0^* \le x \le 2500\delta_0^*$.
• Figure 3: Contours of non-dimensional periods $T^+$ scaled by local uncontrolled friction velocity $u_{\tau 0}$ as functions of $Re_\theta$ and $T_{sc}^+$.
• Figure 4: Contours of $DR$ as functions of $Re_\theta$ and $T^+$ (scaled by local uncontrolled $u_{\tau0}$). The two horizontal dashed lines represent $T^+=100$ and $350$.
• Figure 5: $DR$ as a function of $T^+$ at $Re_{\tau}=200$ (magenta), $Re_{\tau}=500$ (blue), and $Re_{\tau}=800$ (red). Previous channel data with hollow markers are from Gatti_2013_Performance with $Re_\tau=200$ (magenta) and $Re_\tau=1000$ (red), Hurst_2014_Effectb with $Re_\tau=400$ (blue) and $Re_\tau=800$ (red), and Yao_2019_Reynolds with $Re_\tau=200$ (magenta) and $Re_\tau=500$ (blue). Previous TBL data with solid markers are from Skote_2012_TemporalSkote_2022_Drag, and Skote_2015_DragSkote_2019_Wall.