The turbulence structure of 3D separation (Stall Cells) over an airfoil

Abstract

The flow over airfoils that experience separation of the trailing edge type becomes three-dimensional at angles of attack around maximum lift and Stall Cells (SCs) form. SCs are large scale coherent structures of separated flow that consist of two counter-rotating vortices. In the present study the turbulence structure of a SC over a rectangular wing is investigated using Stereo PIV measurements. It is found that the turbulence characteristics of the flow are highly anisotropic and that the Boussinesq approximation is invalid. High values of normal Reynolds stresses in the SC vortices and the separation shear layer region indicates a wandering motion of the former and a flapping motion of the latter. Based on the available data the relation between Reynolds stresses, their production terms and the mean flow gradients is examined. It is found that at the centre of the SC, between the two vortices, the flow characteristics resemble those of a double shear layer while at the vortex region the effect of the vortices leads to double peaks in production terms.