A driving mechanism of near-wall turbulence subject to adverse pressure gradient

A turbulent Couette flow is considered for several values of constant adverse pressure gradient (APG) in the lower wall region to study the APG effect on near-wall turbulence. A DNS in a large computational box shows that near the lower wall at strong APG the flow remains turbulent, while the near-wall streaks disappear and the cross-flow fluctuations intensify. A linear analysis shows that APG inhibits the generation of near-wall streaks due to the significant reduction of the mean shear in the near lower wall region. According to a DNS in a minimal flow unit, at strong APG the near-wall self-sustaining process is strongly weakened or destroyed, and the turbulent fluctuations become more isotropic and localized. Using a conditional averaging analysis, a new near-wall turbulent production mechanism, initiated by the wall-normal nonlinear transport of a wall-normal velocity fluctuation to the near-wall region under strong APG is uncovered. The transported wall-normal velocity fluctuation is amplified by the Orr mechanism, leading to the non-zero turbulent production with spatially localized vortical structures. This mechanism is confirmed by the strong correlation between the wall-normal nonlinear transport and turbulent production observed in a DNS with large computational box.