Aerospace Science and Technology 13 (2009) 267–276 Contents lists available at ScienceDirect Aerospace Science and Technology www.elsevier.com/locate/aescte Numerical and experimental study of flow field characteristics of an iced airfoil Masoud Mirzaei a,∗,1 , Mohammad A. Ardekani b,2 , Mehdi Doosttalab c,3 a Department of Aerospace, K.N. Toosi University of Technology, 322 Mirdamad Ave. West, 19697, Tehran, Iran b Iranian Organization of Science and Technology, No. 71, Forsat St, Ferdousi Sq, Tehran, Iran c Department of Aerospace, K.N. Toosi University of Technology, Iran article info abstract Article history: Received 6 November 2007 Received in revised form 3 May 2009 Accepted 13 May 2009 Available online 19 May 2009 Keywords: Iced-airfoil Numerical and wind tunnel experiments Unsteady flow Hot-wire In this paper, characteristics of separated bubbles and unsteady features of flow fields around a glaze-iced airfoil are investigated. The research was performed using both experimental and numerical approaches. The airfoil was a natural laminar airfoil (NLF-0414) and the ice is considered as the glaze accretion. The experimental measurements were carried out using hot-wire anemometry at Reynolds number of 0.5 × 10 6 and angle of attack ranging from 0 ◦ to 6 ◦ . In numerical calculations, N–S equations were adopted as governing equations and finite-volume technique was employed to solve the equations. Numerical calculations were performed at Reynolds numbers of 0.5 × 10 6 and 1.8 × 10 6 . CFD results and experimental data indicated increasing in the bubble length with increasing in airfoil angle-of-attack. The results showed two separated bubbles in different sizes and also unsteadiness behavior of the flow field which led to low frequency oscillation in lift coefficient with the order of 10 Hz. The frequency of the vortex structures observed in the shear layer measured with hot-wire and it is found that the frequency was in the range of 100 Hz. This frequency was reduced with increasing the angle-of-attack. Vortex shedding was also observed at the downstream of the reattachment location. © 2009 Elsevier Masson SAS. All rights reserved. 1. Introduction Ice accretion on the leading edge of an aircraft wings changes the shape of the wings surface and consequently changes the flow field. This phenomenon influences the pressure distribution and aerodynamic characteristics of the wings and causes increment in drag, reduction in maximum lift, premature stall, and vibration of the wings and finally losing the control of the aircraft. Since degra- dation of these aerodynamic parameters plays a vital role in flight safety, understanding the flow field and prediction of these penal- ties are very important. Glaze is a wet growth ice formed at a temperature around 0 ◦ C and high liquid water content. It occurs when only a fraction of the water droplets freezes upon impact while the remainder droplets run back along the surface or along and freeze downstream. Glaze- ice grows in both upper and lower surfaces of the wing, near leading edge and has backward step geometry. Glaze-ice accre- tion dangerously affects and alters the shape of the original wing surface producing aerodynamic penalties much more severe than * Corresponding author. Tel.: +98 21 77791044; fax: +98 21 77791045. E-mail address: mirzaei@kntu.ac.ir (M. Mirzaei). 1 Associate professor. 2 Assistant professor. 3 Graduate student. rime ice accretion can cause. Schematic of the flow field on the upper surface of a NLF-0414 airfoil with leading edge glaze-ice accretion is presented in Fig. 1(a). This flow field is quite differ- ent from that of the clean airfoil and this leads to degradation of the aerodynamic performance of a wing. Fig. 1(b) represents lift coefficient-angle of attack curves of a clean and an iced NLF-0414 airfoil. It can be seen that the ice accretion has caused premature stall and reduction in slope of the lift curve. Bragg [2] described that, the flow separates off the edge of the glaze ice due to strong adverse pressure gradient and a bubble forms aft of the ice accretion. A shear layer forms between this re- gion and the inviscid flow above the bubble. Vortex motion in the shear layer improves mixing between the separated bubble and the inviscid flow. This phenomenon strengthens the pressure re- covery and allows the flow to reattach to the airfoil surface down- stream of the separation bubble at low angles-of-attack. At higher angles-of-attack the pressure recovery may not be achieved and stall may occur. The shape and size of the separation bubbles and reattachment point vary with time due to the flow unsteadiness. Moreover vortex shedding may happen along the downstream of the reattachment point. Gurbacki and Bragg [6,7] performed some investigations on unsteady surface–pressure distribution for NACA 0012 airfoil with leading-edge, glaze-ice accretions. They used oil- flow visualization technique, and Particle Image Velocimetry (PIV). They showed that the mean reattachment length increases as the 1270-9638/$ – see front matter © 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ast.2009.05.002