Journal of Fluid Mechanics


Transverse instability and low-frequency flapping in incompressible separated boundary layer flows: an experimental study

Pierre-Yves Passaggiaa1, Thomas Lewekea1 c1 and Uwe Ehrensteina1

a1 IRPHE, UMR 7342 CNRS, Aix–Marseille Université, F-13384 Marseille CEDEX 13, France


The unstable dynamics of a transitional laminar separation bubble behind a two-dimensional bump geometry is investigated experimentally using dye visualizations and particle image velocimetry measurements. For Reynolds numbers above a critical value, the initially two-dimensional recirculation bubble is subject to modulations in the spanwise direction which can trigger vortex shedding. Increasing the Reynolds number further, the unstable behaviour is dominated by a low-frequency flapping motion, well known in transonic flows, and here investigated for the first time experimentally in an incompressible flow regime. These phenomena are characterized by non-intrusive measurements of the spatial structure and the frequencies of the unsteady motion. The results are in excellent agreement with previous numerical and theoretical predictions for the same geometry.

(Received January 10 2012)

(Reviewed March 22 2012)

(Accepted May 09 2012)

(Online publication June 13 2012)

Key Words:

  • boundary layer separation;
  • instability


c1 Email address for correspondence: