Perturbations evolution and streaks formation in turbulent channel flow.

etc15 Tracking Number 159

Evolution of initially small perturbations superimposed on turbulent channel flow is investigated via DNS. On the linear stage perturbations grow exponentially in average as $\sim \exp(\lambda t)$ with $\lambda^+\approx0.021$ in accordance with previous findings \cite{NV1,NV2}. Maximum perturbation amplitude is located at distance $y^+=12$ from the wall, thus, the perturbation growth may be attributed to streak instability. However, in contrast to existing models of streak instability the growing patterns appear and disappear occasionally as localized spots in $x-z$ plane. Streaks in perturbation field appear on the nonlinear stage of evolution. Perturbation amplitude saturates finally on the level of $\sqrt{2}$ of that of turbulence fluctuations in underlying turbulent flow indicating that disturbed flow is uncorrelated with original undisturbed flow field.