Abstract
Following the Lagrangian theory of unsteady flow separation on slip boundaries proposed by Lekien and Haller [“Unsteady flow separation on slip boundaries,” Phys. Fluids 20, 097101 (2008)], we use finite time Lagrangian analysis in order to educe large scale, unsteady flow separation downstream a near wall obstacle at a significant Reynolds number. By large scale flow separation, we mean here the ejection of fluid and vorticity outside a neighborhood of the wall at the scale of the obstacle. Indeed, while the separation point at the wall is not spatially resolved by the high speed particle image velocimetry measurements, free-slip boundary conditions are applied before educing unstable manifolds in the near wall region using finite time Lyapunov exponents. For this turbulent flow, conditional statistics are presented in order to discuss the relative contributions of the unsteady aerodynamics and of the turbulence in the separation region. The dynamics of the corresponding separation point has a very clear link with the fluctuating wall pressure induced by this unsteady turbulent flow.
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