Dynamic mode decomposition of H-type transition to turbulence

For H-type transition to turbulence in a flat-plate boundary layer, we identify dynamically important features resulting from direct numerical and large eddy simulations (DNS and LES, respectively). Even though LES coupled with a dynamic subgrid-scale model provides an accurate prediction of the transition location, it fails to predict initial overshoot and subsequent turbulent skin friction. Dynamic Mode Decomposition (DMD) is used to quantify the contribution of coherent structures to the total Reynolds shear stress. We show that low-frequency modes, corresponding to the legs of hairpin vortices, account for most of the total Reynolds shear stress gradient. We conclude that LES underpredicts the skin friction because the dynamically important low frequency modes do not attain sufficient amplitude.