EXPERIMENTAL INVESTIGATION OF SHEAR INFLOW OVER AN AIRFOIL

Abstract

Flow over an airfoil at low Reynolds is a challenging research area which has many engineering applications that attracts the interests of the scientific community. There is shear layer transition and wake transitions involved in this regime which can directly effects the aerodynamic characteristics of the airfoil. Study of these transition phenomena may help in controlling the flow better as well as it will aide to the understanding of the subject. Apart from this, the uniform free stream flow over an airfoil has been extensively studied in the literature. Effects of uniform shear flow on the load characteristics of airfoil and wake structure of flow over the airfoil is not well known yet since the literature is scarce in this field of research. There are many parts of the aircraft which are subjected to shear flow like airfoil area near the propellers and jet engines and root the wings. Moreover, while landing and take-off, the aircraft wings have to generate massive lift force and minimum drag force under the shear flow and even during cruising, the aircraft has to face turbulence caused by shear flow near mountains. The present study takes this research problem and tries to reveal the flow physics involved in the low Reynolds regime comparing the flow kinematics and dynamics of uniform inflow and uniform shear inflow to a NACA0012 airfoil. Hotwire anemometer setup (Constant temperature type) is used in the present study to characterize the flow over the airfoil. The experiments are performed in a low turbulence level subsonic wind tunnel. The study reveal that the uniform shear inflow enhances the momentum flux of the boundary layer thus makes it able to bear adverse pressure gradients at High angles of attack. The separation is delayed in the case of uniform shear inflow over the airfoil with respect to uniform free stream inflow. The vortex shedding is observed for high angles of attack in uniform free stream inflow case whereas, for uniform shear inflow case it is observed for only 12º angle of attack. The time-averaged drag coefficient calculation using momentum balance method reveals that airfoil bears fewer drag in the case of uniform shear inflow than uniform freestream inflow for nearly all angles of attack and for both the Reynolds numbers used in the study.