Aerodynamic and Flow Characteristics of a Bidirectional S-Shaped Airfoil

Abstract

Bidirectional axial-flow turbomachines are used in a wide range of applications. Despite this, little research has been conducted to understand the flow characteristics of bidirectional airfoils, particularly S-shaped airfoils, which are likely the most commonly used type of bidirectional airfoil. This paper presents a direct description of the flow features of a typical S-shaped airfoil, which is based on a combination of the two front parts of the standard Clark-Y airfoil. A comparison between the flow fields for the S-shaped airfoil and those of the Clark-Y airfoil reveals why the aerodynamic performance of S-shaped airfoils differs from that of the standard airfoils. It is shown that S-shaped airfoils can improve the aerodynamic performance under high angles of attack (AoAs), while they degrade the aerodynamic performance under low AoAs. The decline in performance for S-shaped airfoils should be caused by a reduction of the effective AoA or incidence at the leading edge and the outward pressure gradient caused by the curved streamlines along the bulge of the rear half of the pressure surface. The reason for the performance improvement at high AoAs should be that the upwarp in the rear half of S-shaped airfoils alleviates the local flow deceleration, and thus diminishes the flow separation on the suction surface. This study can help deepen our understanding of the flow behaviors of bidirectional S-shaped airfoils, and hence offer guidance for enhancing the performance of bidirectional turbomachines.