A thermographic method to evaluate laminar bubble phenomena on airfoil operating at low Reynolds number

Abstract

The aim of this research is the study of the laminar boundary layer separation phenomena on aerodynamic bodies by infrared thermography. The presence and the size of laminar bubble are mainly observed. A thermographic method is adjusted to detect the presence and the longitudinal dimension of the laminar bubble. In this region the convective heat transfer coefficient is lower than in the surroundings, because of the recirculating flow. Heating the airfoil surface, the laminar bubble will appear warmer than the other zones and so it is possible to know its presence and position. Laminar separation bubbles are caused by the presence of the adverse streamwise pressure gradient in the boundary layer flow field. When separation occurs the disturbances in the shear layer are amplified and a transition to turbulence may develop. In this condition the shear layer may reattach on the body surface generating a so called Laminar Separation Bubble (LSB), see Fig.1. Separation bubbles are common in several engineering applications, e.g. low Reynolds number airfoils, turbine blades, ecc… where their presence can increase dramatically the body pressure drag. An extensive literature is available to understand the bubble behaviour and many numerical studies have been carried out but a systematic analysis regarding low Reynolds number airfoils is actually limited, owing to the difficulties to experimentally investigate the phenomenon. Three characteristic points distinguish the LSB: separation point, transition point and the turbulent reattachment point. The streamwise position of the separation point may be find by the investigation of the ‘similar’ solution of the laminar boundary layer equation introducing the Falkner-Skan equation [1], or by approximate methods based on the integral approach at the boundary layer problem [2]. Two flow regions are present in the separated flow: a shear layer and a recirculating zone; the transition of the shear layer is normally studied by the Orr-Sommerfeld stability equation, since this equation is based on a linear approach only the first instability point is carried out. The distance between the separation and transition points is based so on some experimentally suggested criterions. Horton [3] and Van Ingen [4] results have been extensively used. Actually the e n criterion [5-6] seems more suitable for airfoil applications. The turbulent reattachment point is generally obtained using the skin-friction and velocity profile formulas of Swafford [7]. More recent studies [8-12] point out the difficulties to study the LSB because of the unsteadiness of the phenomenon and the dependence on the spectral components of the main flow turbulence. In the present work, IR thermography is proposed as a quantitative experimental technique to determine the three characteristic points of the LSB by analyzing the surface heat transfer coefficient with the heated-thin-foil technique [13]. A comparison with numerical and analytical results, obtained by the applications of the previous discussed criteria, are presented for the tested airfoil.