Olivier Doche

Active control effectiveness and synchronization of wall turbulence under localized imposed unsteadiness

The effect of a spatially localized time-periodic perturbation on the efficiency of suboptimal control of the wall turbulence is analyzed. It is shown that the imposed unsteadiness with a frequency in the median production range doubles the turbulent drag reduction under suboptimal control strategy. It is further observed that the spatially averaged turbulent wall shear is synchronized in time with the imposed perturbation waveform. This is related to the synchronization of the unstable periodic orbits present in the near-wall turbulence in connection with the regeneration cycle of turbulence producing coherent structures.

Turbulent Passive Scalar Transport under Localized Blowing

The effect of blowing through a localized slot on the wall turbulence dynamics and heat transfer process is analyzed by direct numerical simulations in a fully developed turbulent channel flow. The severity parameter is mild and there is no flow separation induced by the blowing. The shear stress transport and temperature energy budget is discussed in detail. The wall shear and flux decreases immediately downstream the slot in a similar manner but the Reynolds analogy does not hold over the slot. The physical process is governed by the production and pressure redistribution over the slot in a complex manner. The turbulent transport and especially the advection play an essential role in the heat transfer mechanism.

Enhancement of Suboptimal Controllability in Wall Turbulence

The wall turbulence is forced to a predictable state through localized time-periodical blowing. It is shown through experiments and direct numerical simulations that the temporal waveform of the localized blowing plays a crucial role in the response of turbulent wall drag. Imposed unsteadiness increases the capacity of the controllability significantly. Results on the optimum periodical temporal waveform of localized blowing are also discussed. We use the characteristics of the cyclostationnarity to achieve this particular goal.