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Julien Cerutti
The turbulent flow over an isolated recess is studied, motivated by the increasing interest in the so-called dimples, to shed light on their ability to impose a spanwise motion on the flow and to reduce drag.
The work is based on DNS simulations of a circular recess with a depth-to-diameter ratio of 0.05 placed inside a channel. Turbulent simulations at Retau = 180 have been carried out and compared to the laminar counterpart.
The flow over the recess presents a converging-diverging pattern and creates a spanwise motion, mainly driven by the pressure gradient, similar to the one observed with the spanwise forcing drag reduction methods. The presence of the recess provides a variation of the skin friction, with very similar values for the laminar and turbulent solutions in the upstream half, but with different behavior in the downstream one: the difference in the results seems to be tied to an increase in the Reynolds shear stress, that was pictured using a variation of the FIK decomposition which allowed to split the skin friction into a laminar and turbulent part. Some hints on how to modify the geometry of the recess to possibly achieve drag reduction are provided.