Coherent structures in wall turbulence with drag reduction

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Emanuele Gallorini

In this work the physical mechanisms which appear when a flow undergo a drag variation due to a control technique as oscillating wall or streamwise waves of spanwise velocity are studied. To do that 5 Direct Numerical Simulation (DNS) of a turbulent channel flow at Retau = 200 were used: a first case without forcing, two cases with oscillating wall and two cases with travelling waves.

The first part of the thesis is based on the study of the Coherent Structures (CS) of the flow, in particular of the Quasi-Streamwise Vortices (QSV). The aim is to relate the changes induced by the control on the flow with the variation of the QSV dynamics and subsequently understand how and why the friction is modified. To do that several programs, that allowed us to educe QSV, conditionally average them and extract some relevant physical quantities (as Reynolds stresses), were developed. From that it was possible to confirm the presence of the same Q2 suppression and Q4 enhancement mechanisms underlined in Yakeno et al. [2014] for the vortices under an oscillating wall control, while for the first time their relevance for travelling waves was observed.

The last part of the work is focused on the proposal of a formula which allows to predict, given the wavenumber and the frequency associated to a travelling wave, the friction variation, in order to prove the relevance of the physical phenomena observed during the thesis. To do that we started again from a proposal of Yakeno et al. [2014] which was modified taking into account the GSL acceleration and the QSV-roll phenomenon we observed for the travelling waves. An alternative formulation was proposed, observing the striking similarity between the drag reduction map of Yakeno et al. [2014] and the representation of that quantity in the parameter space as suggested in Duque-Daza et al. [2012].