An experimental investigation into the influence of trailing-edge separation on an aerofoil's dynamic stall performance
The influence of trailing-edge separation on the dynamic stall characteristics of a typical rotor section is at present unclear. Although previous research has given a fundamental understanding of the unsteady stall process, the variety of aerofoils tested has made it difficult to isolate the effect of trailing-edge separation. Further investigation into this field may be carried out by testing two similar aerofoils which differ only in their trailing-edge separation characteristics. The early part of the work concentrated on the development of a numerical method whereby the theoretical pressure gradient over the trailing-edge upper surface of a given aerofoil may be modified to either enhance or reduce such separation. Since previous work at the University of Glasgow had included a detailed unsteady aerodynamic study of a NACA 23012 aerofoil, this was the appropriate profile for modification. The above technique was applied to this aerofoil with the objective of modifying the geometry in such a manner that would retain the leading-edge pressure distribution whilst forcing an earlier and more gradual trailing-edge separation growth. The subsequently designed aerofoil, designated the NACA 23012(A), was shown to display an enhancement of the trailing-edge separation characteristics via both boundary-layer calculations and oil-flow visualisation tests. On comparison with unsteady data previously collected for the NACA 23012, several systematic methods of estimating the effects of trailing-edge separation on the dynamic stall process are presented. During oscillatory tests the NACA 23012(A) displayed a more stable damping characteristic which was attributed to the enhanced trailing-edge separation producing an earlier pitching-moment break. Based on the analysis of pressure-time histories obtained during ramp tests, it was deduced that a consequence of significant trailing-edge separation was to delay the initiation of the dynamic stall vortex. Detailed analysis of hot-film data led to the conclusion that aerofoils which display a tendency to stall in steady conditions, via separation growth from the trailing-edge, will experience vortex initiation by the breakdown of a thin layer of reversed flow travelling upstream beneath a stable shear layer which remains in close proximity to the aerofoil's surface contour.