A study of long separation bubble on thick airfoils and its consequent effects Amanullah Choudhry ⇑ , Maziar Arjomandi, Richard Kelso School of Mechanical Engineering, The University of Adelaide, South Australia 5005, Australia article info Article history: Received 5 April 2014 Received in revised form 7 October 2014 Accepted 1 December 2014 Keywords: Low Reynolds number Camber effect Airfoil transition modelling Separation induced transition Separation bubble MAV abstract A parametric study has been performed to analyse the flow around the thick-symmetric NACA 0021 air- foil in order to better understand the characteristics and effects of long separation bubbles (LoSBs) that exist on such airfoils at low Reynolds numbers and turbulence intensities. In the article, the prediction capabilities of two recently-developed transition models, the correlation-based c–Re h model and the lam- inar-kinetic-energy-based j–j L –x model are assessed. Two-dimensional steady-state simulations indi- cated that the j–j L –x model predicted the separation and reattachment process accurately when compared with published experimental work. The model was then used to study the attributes and the effects of LoSBs as a function of the angle of attack, freestream turbulence intensity and Reynolds number. It was observed that LoSBs considerably degrade the aerodynamic performance of airfoils and lead to abrupt stall behaviour. It is, furthermore, illustrated that the presence of the LoSB leads to an induced camber effect on the airfoil that increases as the airfoil angle of attack increases due to the upstream migration of the bubble. An increase in the Reynolds number or turbulence levels leads to a reduction in the bubble extent, considerably improving the airfoil performance and leading to a progres- sive trailing-edge stall. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction Separation bubbles are generated primarily in applications involving low Reynolds number flows with large pressure gradi- ents such as compressor blades in turbo-machines, high-altitude unmanned-air-vehicles, micro-air-vehicles and wind turbines (Lin and Pauley, 1996). The presence of the separation bubble is gener- ally considered undesirable since it can impact the aerodynamic efficiency and stall behaviour of airfoils (Nakano et al., 2007; Zhang et al., 2008). The bubble can alter the flow at low Reynolds numbers and can consequently have adverse effects on the perfor- mance of the machine. Difficulties can also arise during airfoil test- ing in wind tunnels for applications involving high Reynolds number flows due to undesirable scale effects since most experi- mental wind tunnels operate in low Reynolds number regimes (Lissaman, 1983; Ol et al., 2005). The traditional methods to avoid these scale effects such as the addition of roughness strips and trip wires on airfoils or the addition of freestream turbulence also add a degree of complication and uncertainty to the process. Therefore, the characteristics of the separation bubble and its effects need to be understood well to improve the design methodology of airfoils. The most prevalent type of transition observed on airfoils and wings at low Reynolds numbers is the separation-induced transi- tion. Separation-induced transition primarily occurs when a lami- nar boundary layer is exposed to large adverse pressure gradients, such as those near the leading edge of airfoils, resulting in its sep- aration. The separated shear layer then undergoes transition due to amplification of velocity disturbances in the flow (Alam and Sandham, 2000a). The resulting turbulent shear layer reattaches some distance downstream resulting in the formation of an enclosed region commonly referred to as a separation bubble. The primary aspects of separation-induced transition, adapted from Horton (Horton, 1968), are illustrated in Fig. 1. The location and size of the separation bubble is a function of the airfoil profile, freestream Reynolds number, turbulence inten- sity and the angle of attack (Tani, 1969; Swift, 2009). Separation bubbles can be classified either as short or long based on their chordwise extent and consequent effects on an airfoil pressure and velocity distributions. A short separation bubble (SSB) encom- passes a chordwise extent of less than one percent and therefore does not influence the pressure distribution around the airfoil to a large degree (Tani, 1961). After transition occurs in the separated http://dx.doi.org/10.1016/j.ijheatfluidflow.2014.12.001 0142-727X/Ó 2014 Elsevier Inc. All rights reserved. ⇑ Corresponding author at: School of Mechanical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia. Tel.: +61 413032885. E-mail address: amanullah.choudhry@adelaide.edu.au (A. Choudhry). International Journal of Heat and Fluid Flow 52 (2015) 84–96 Contents lists available at ScienceDirect International Journal of Heat and Fluid Flow journal homepage: www.elsevier.com/locate/ijhff