RESEARCH ARTICLE Three-dimensional wake dynamics of a blunt and divergent trailing edge airfoil M. El-Gammal Æ H. Hangan Received: 12 March 2007 / Revised: 3 November 2007 / Accepted: 6 November 2007 / Published online: 2 December 2007 Ó Springer-Verlag 2007 Abstract The wake dynamics of an airfoil with a blunt and divergent trailing edge is investigated experimentally at relatively high Reynolds. The near wake topology is examined versus different levels of free stream turbulence FST and angles of attack, while the downstream wake evolution is characterized at various levels of FST. The FST is found to have a significant effect on the shapes of turbulence profiles and on the downstream location where the flow reaches its quasi-asymptotic behavior. Streamwise vortices (ribs) corresponding to spanwise variations of turbulence quantities are identified in the near wake region. Simultaneous multi-point hot-wire measurements indicate that their spatial arrangement is similar to Williamson’s (Ann Rev Fluid Mech 29:477–539, 1996) mode B laminar wake flow topology. The results suggest that the statistical spanwise distribution of ribs is independent of FST effects and angle of attack as long as the vortex shedding Strouhal number remains approximately similar. 1 Introduction 1.1 General The aerodynamic performance of airfoils can be enhanced by modifying the trailing edge region. Blunt and divergent trailing edge (DTE) airfoils are one of the recent and effective developments introduced to airfoils for both high and low Reynolds number applications, i.e., wind turbine blades (Standish and Van Dam 2003) and supercritical airfoils (Henne 1990), respectively. These airfoils are characterized by divergent upper and lower surfaces over the last portion of the chord forming a blunt trailing edge. This new variation in the airfoil profile results in a pressure difference between the suction and pressure sides in the trailing edge region, which is larger than that of the cor- responding conventional airfoil. The airfoil structural strength and durability is also increased by adding addi- tional thickness to the trailing edge region. Several researchers have investigated the impact of the trailing edge truncation on the airfoil aerodynamic per- formance. Standish and Van Dam (2003) observed a significant enhancement of the aerodynamic performance after modifying their base line airfoil section for wind turbine blades to be DTE one. They attributed these improvements to the downstream movement of the pres- sure recovery region to occur off the surface in the wake, and thus, reducing the adverse pressure gradients on both pressure and suction sides. Thompson and Lotz (1996), using RANS models observed that the lift to drag ratio was about 2.5% larger when the location of the transonic shock embedded in the suction side was moved on a DLBA 243 DTE modified airfoil compared to the parent DLBA 186 supercritical section. Despite the structural and aerodynamic benefits of DTE airfoils, the existence of trailing edge sharp corners induces flow separation and as a result, vortex shedding and large wakes develop (Thompson and Lotz 2002). Vortex shed- ding adversely increases the base drag and leads to unfavorable acoustic noise and aeroelastic problems that could reduce the airfoil life span. M. El-Gammal (&) Rowan Williams Davies & Irwin Inc. (RWDI), Guelph, ON, Canada N1K 1B8 e-mail: Mohamed.El-Gammal@rwdi.com H. Hangan The Boundary Layer Wind Tunnel Laboratory, The University of Western Ontario, London, ON, Canada N6A 5B9 123 Exp Fluids (2008) 44:705–717 DOI 10.1007/s00348-007-0428-6