Aerodynamics of Wings in Tandem at Low Reynolds Numbers

Abstract

The aerodynamic interactions between two wings of NACA0012 section arranged in tandem configuration were experimentally investigated at low Reynolds number of 20,000 and 100,000 in the wind tunnel and water tunnel. The force/moment measurements, wake surveys, and flow visualization results at pre and post-stall angles of attack ranging from -90 to +90 degrees were compared with the data of the isolated wing to determine the aerodynamic interactions. At Re = 20,000, a highly nonlinear lift response was observed without discrete stall and four distinct lift behavior regions. Flow visualization revealed laminar instability waves, vortex shedding, and complex interactions between surface vortices and trailing edge vortices in the wake. At Re = 100,000, conventional lift behavior was exhibited with a linear lift curve at pre-stall angles of attack, abrupt leading-edge stall, and static stall hysteresis. Surface flow visualization showed signatures of laminar separation bubble and progression with changing angle of attack. For the tandem configuration, three test cases consisted of: 1) changing the angle of attack of upstream wing while holding the downstream wing at fixed angles of attack; 2) holding the upstream wing at fixed angles while sweeping the downstream wing; and 3) simultaneously varying the angles of attack of both wings. Results highlighted complex aerodynamic couplings between the wings in tandem configurations in the form of upstream wing wake induced downwash on the downstream wing, modifying local velocity and turbulence intensity, altering the pressure distribution and wake trajectory depending on the relative geometric angle, stagger, and gap. The interactions manifest themselves as boundary layer transition, separation, and vortex shedding for each wing. The combined L/D ratio improved in the post-stall region and a novel “secondary stall” phenomenon was observed in the form of a sudden decrease in lift and drag of the upstream wing. Secondary stall showed dependence on wing spacing and angle of attack but was independent of Reynolds number and aspect ratio. Flow visualization indicated that the downstream wing suppressed vortex shedding from the upstream wing at a critical distance by preventing shear layer interaction that reduced lift and drag simultaneously. Spectral analysis of signals from hotwire and force sensor confirmed the aeroelastic coupling between the wake turbulence and wings for relative positions of the wings.