Numerical and experimental analyses of the aerodynamic performance of a triangular airfoil in low-Reynoldsnumber compressible flow are performed. This airfoil is one of the candidates for propeller blades on a possible future Martian air vehicle design. Based on past experimental studies conducted in the Mars Wind Tunnel at Tohoku University, this airfoil is known to exhibit nonlinear lift behavior. In the present study, direct numerical simulations of low-Reynolds-number compressible flow over a spanwise periodic triangular airfoil are conducted to identify the source of nonlinear lift. The numerical results reveal that the source of the nonlinear aerodynamic behavior is the lift enhancement provided by the large leading-edge vortex generated. For compressible low-Reynolds-number flow, the wake structure becomes elongated, causing the nonlinear lift enhancement to appear at higher angles of attack compared to the case of incompressible flow.

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Aerodynamic performances of four kinds of airfoils are examined in flows with low-Reynolds number. The aim of the present study is to find the airfoil shape, which provides a large value of a maximum lift-drag ratio in order to develop a high efficiency propeller for the Mars exploration airplane. A triangle airfoil is a base contour of the test airfoils and effects of blunt trailing edges on airfoil characteristics are examined using numerical simulations. Furthermore, a variation of aerodynamic characteristics of one airfoil with a blunt trailing edge due to decreased thickness and increased camber is also examined. The numerical results are verified by experiments. As the trailing edge thickness is increased, the lift-drag ratio becomes larger and values of the lift and the drag forces become smaller at the angle of attack where the lift-drag ratio becomes maximum. By increasing the trailing edge thickness, the boundary layer separation is suppressed and the aerodynamic forces are reduced. The increased camber and decreased thickness yield a significant lift increase of the airfoil with the blunt trailing edge and a small amount of drag increase. As a result, the airfoil with the blunt trailing edge yields large value of the lift-drag ratio in the flows with low-Reynolds numbers.