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

• Published in: Journal of fluid mechanics Vol. 703; pp. 363 - 373
• Main Authors: Passaggia, Pierre-Yves, Leweke, Thomas, Ehrenstein, Uwe
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 25.07.2012; Cambridge University Press (CUP)
• Subjects: Boundary layer; Boundary layers; Bubbles; Computational fluid dynamics; Engineering Sciences; Exact sciences and technology; Flapping; Fluid dynamics; Fluid flow; Fluid mechanics; Fluids mechanics; Fundamental areas of phenomenology (including applications); Incompressible flow; Instrumentation for fluid dynamics; Mechanics; Modulation; Physics; Reynolds number; Rotational flow and vorticity; Separated flows; Two dimensional; boundary layer separation; instability; Boundary layer separation; Test facilities; Flow separation; Separation bubble; Flow visualization; Particle image velocimetry; Incompressible fluid; Secondary flow; Experimental study
• ISSN: 0022-1120; 1469-7645
• DOI: 10.1017/jfm.2012.225

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.