The nonlinear lift coefficient characteristics and active flow control of a symmetrical airfoil at a low Reynolds number

Abstract

In this paper, the flow characteristics of a symmetrical airfoil at Re = 40 000 near a 0° angle of attack are investigated numerically, and the nonlinear lift coefficient phenomenon and different types of laminar separation bubble (LSB) structures are clearly observed. It is confirmed that the nonlinear characteristics of the lift coefficient originate from the differently shaped pressure distributions in the LSB. The causes of the different characteristics of the pressure distributions in different types of LSB are revealed by deriving the Reynolds-averaged pressure gradient equation. It is determined that the viscous stress distribution near the wall is the key to the formation of different pressure distributions. Moreover, in order to suppress the disadvantages associated with the nonlinear lift coefficient of a symmetrical airfoil, an active flow control method based on local oscillation is adopted. By introducing an oscillation disturbance upstream of the separation bubble, the effect of Reynolds stress and convection on the wall is enhanced, the reattachment of the separation flow is promoted, and the formation of an LSB at the trailing edge is suppressed. Thus, the nonlinear characteristics of the lift coefficient due to the switching of the LSB structure are eliminated.