Investigation of vortex packet recovery in perturbed turbulent boundary layers

Abstract

Turbulent boundary layers (Reτ = 2500) were perturbed by a spanwise array of cylinders with spacing of S= 0.2δ, where δ is the unperturbed boundary layer thickness. Two cylinder heights were considered, H = 0.2δ (H = 500) and H = δ. Although visualizations suggest that disrupted packet signatures downstream of the H = δ array re-organized from bottom up, autocorrelation magnitudes along the streamwise direction for the flow at z = 125 were reduced relative to unperturbed values up to 7δ downstream of the array suggesting that flow features unrelated to packets remained altered there. On the other hand, spectral energy in large spanwise scales recovered substantially at the same location. For the H = 0.2δ case, previous results showed disrupted packet signatures re-appearing beginning 2δ downstream at z = 300, while packet signatures at z = 500 persisted through the array, supporting a top-down reorganization. However, spanwise characteristics of flow features closer to the wall (z = 125) remained altered relative to the unperturbed flow up to 7δ downstream yet streamwise length scales from autocorrelations at z = 125 and x = 7δ matched unperturbed values. Interestingly, the shape of the spanwise energy spectrum at this location resembled shapes of spectra in planes above, suggesting outer-layer influence.