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An important choice needs to be taken when setting up Direct Numerical Simulations (DNS) of turbulent channel flows, regarding how the flow is driven through the channel. Two classic possibilities are to drive the flow at Constant Flow Rate (CFR) or at Constant Pressure Gradient (CPG). While the different choices similar turbulent statistics for canonical flows, they have significant implications on statistics of drag-reduced turbulent flows. For instance, at CFR drag reduction manifests as a reduction of friction but as an increase of bulk velocity at CPG. In neither case, the power transferred to the flow remains constant upon application of drag-reducing control and nor does the rate of production and dissipation of turbulent kinetic energy. Since the uncontrolled and drag-reduced flows differ energetically, it is difficult to address the physics of drag reduction techniques from the energetic standpoint. In the present work we leverage the recently-proposed Constant Power Input (CPI) approach, in which the power transferred to the flow – through pumping and imposition of control – is kept constant, to understand how turbulent drag reduction obtained via several conceptually different strategies modifies the energetic properties of channel flows. Two questions are at the root of this research: i) whether a drag-reduced turbulent flow fundamentally differs from a “natural” turbulent flow; and ii) whether similarities exist in how different control strategies modify the flow from the energetic standpoint. Both questions are important for understanding universal mechanisms related with drag reduction and for developing turbulence models for drag-reduced flows.
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