Linking Fluctuations to Turbulent drag in Turbulence: towards an understanding of drag reduction – LIFT

Whether in aircraft flight or the transportation of fluids in pipelines, energy dissipation or turbulent drag can severely limit performance and cause undesired energy losses. An understanding of the variation of turbulent drag versus flow properties is still lacking despite a century of intense effort. Advances in the theory of turbulent drag suggest that the key to understand these losses resides in linking the macroscopic flow properties, in particular the turbulent drag, to the statistical properties of turbulent fluctuations. This link is familiar in statistical mechanics, where macroscopic properties are controlled by ?uctuations as in the fluctuation dissipation relation, but has no precedent in turbulence. These advances cannot however be fully tested; their stringent test would need to vary the statistical properties of the underlying turbulent flows but this is not possible in three dimensional (3D) flows. To overcome this bottleneck, we have recently used two dimensional turbulent flows to tune the statistical properties of turbulence and investigate the direct dependence of the macroscopic properties on the nature of the fluctuations. Here, we will focus on how polymer additives affect drag in 2D turbulent flows obtained using soap films. Polymers can reduce turbulent drag, but this fundamental and economically interesting phenomenon is still not well understood. Linking the macroscopic properties of turbulent flows to the small scale properties is a new window into drag reduction of turbulent flows and its outcome may have applicability in the way energy losses can be understood and eventually modified by tuning the statistical properties notably by using additives such as polymers, surfactants, or air bubbles.