The presence of inner layer streaks in large-defect APG TBLS

Turbulent boundary layers (TBLs) develop a defect in their mean velocity profile when they are exposed to an intense or prolonged adverse pressure gradient (APG). The changes in the mean velocity profile affect the turbulent activity significantly. In large-defect TBLs, the outer-layer turbulent activity dominates the flow and the inner peak of the 2>-spectra, which is also the signature of the inner-layer streaks, disappears. However, it is not clear if the streaks disappear or become hidden due to the imprints of energetic outer-layer structures in the near-wall region. In this study, our aim is to determine the presence of inner-layer streaks in large-defect TBLs. For this, we have generated a non-equilibrium APG TBL database using direct numerical simulation, based on a previous APG TBL, where the outer-layer turbulence is artificially eliminated to examine the inner layer without the effect of the outer layer. We investigate the two-point correlations and the spectral distributions in the inner layer and the preliminary results suggest that the inner-layer streaks are present in large-defect TBLs even when the mean shear is very low. Together with the variations of the streaks characteristics, we are currently determining the proportions of streaks generated locally and those convected from a position upstream where the shear is still not that small.