On the coherent structures and stability properties of a leading-edge separated aerofoil with turbulent recirculation

• Published in: Journal of fluid mechanics Vol. 683; pp. 395 - 416
• Main Authors: Kitsios, V., Cordier, L., Bonnet, J.-P., Ooi, A., Soria, J.
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 25.09.2011
• Subjects: Aerodynamics; Applied fluid mechanics; Computational fluid dynamics; Exact sciences and technology; Flow control; Fluid dynamics; Fluid flow; Fluid mechanics; Fundamental areas of phenomenology (including applications); Harmonic analysis; Hydrodynamic stability; Instability of shear flows; Marine; Perturbation methods; Physics; Shear layers; Stability; Turbulence; Turbulence simulation and modeling; Turbulent flow; Turbulent flows, convection, and heat transfer; wakes/jets; flow control; absolute/convective instability; Large eddy simulation; Flow separation; Recirculating flow; Digital simulation; Hydrodynamic instability; Vorticity; Flow control; Airfoils; Coherent structures; Modelling; Shear layer; Turbulence structure
• ISSN: 0022-1120; 1469-7645
• DOI: 10.1017/jfm.2011.285

The present study is motivated by a need to produce stability modes to assist in the understanding and control of unsteady separated flows. The flow configuration is a NACA 0015 aerofoil with laminar leading-edge separation and turbulent recirculation. In previous water tunnel experiments, this flow configuration was measured in an unperturbed (uncontrolled) separated state, and a harmonically perturbed (controlled) reattached state. This study presents numerical data of the unperturbed case, and recovers stability modes to describe the evolution of perturbations in this environment. The unperturbed flow is numerically generated using large eddy simulation. Its temporal properties are quantified via a Fourier analysis of the velocity time history at selected points in space. The leading-edge shear layer instability is characterized by instantaneous vortex structures, and the bluff body shedding is illustrated by proper orthogonal decomposition modes. Statistical measures of the velocity field agree well with the water tunnel measurements. Finally a stability analysis is undertaken using a triple decomposition to distinguish between the time averaged field, the unsteady scales of motion, and a coherent wave (perturbation). This analysis identifies that perturbations in the region immediately downstream of the separated shear layer have the highest spatial growth rates. The associated frequency is of the order of the sub-harmonic of the shear layer instability.