Effects of Bioinspired Leading-Edge Tubercles on Flow Separation and Loss in Compressor Cascades with Controlled Diffusion Airfoils

Abstract

Separated flow inside a compressor cascade is a challenging and complicated issue in aero-engines. Severe flow separation is an important cause of rotational stall in compressors. It is well known that humpback whales have excellent underwater maneuverability owing to the presence of unique raised structures, called tubercles, in front of their flippers. Inspired by the foregoing, we introduced the leading-edge tubercles of humpback whales in the stator of a compressor. The effects of this approach on flow separation were investigated, particularly in the corner region. First, a suitable tubercle amplitude and wavelength were selected for the stator. The flow losses and flow characteristics of the baseline and bioinspired airfoil were numerically determined using the steady Reynolds-averaged Navier–Stokes (RANS) method. The feasibility of the numerical model was verified by comparison with available experimental results. The working conditions were then divided into three regions according to the flow characteristics. Typical working conditions representative of different loss characteristics were studied. Finally, the influence mechanism of the tubercle on flow separation at the corner region under three-dimensional flow-separation conditions was investigated. The results revealed that the leading-edge tubercles induced the formation of a pair of counter-rotating streamwise vortices, substantially reducing the flow separation at the front of the pressure side. This delayed the stall at high negative incidence angles by driving the low-momentum flow in the separation area to interact with the high-momentum main flow. The relative loss reduction improved by 9.65% at i=−20°. At high positive incidence angles, owing to blockage in the middle of the linear cascade, the induced vortices formed by each leading-edge tubercle converged into a larger vortex structure with a scale opposite to that of the passage vortex. They interacted with the passage vortex and corner vortex and suppress their development. Therefore, the tubercles effectively reduced corner separation and widened the stall boundary. The relative loss reduction improved by 9.35% at i=+9°. In this study, we evaluated the vortex and flow field structure of a secondary flow with the introduction of tubercles. The physical mechanism was numerically revealed using a linear cascade. The performance of the tubercles illustrated that the introduction of tubercles caused minimal additional losses in the region of low incidence angles. At high incidence angles, the tubercles induced a pair of counter-rotating streamwise vortices, effectively controlling corner separation and improving the aerodynamic performance. The relative loss reduction improved by 9.65% at i=−20° and by 9.35% at i=+9°. Thus, this paper presents a novel concept for the application of tubercles along the leading edge of a stator in a compressor. The tubercles were found to perform well, particularly under conditions with large flow separations. We can consider this as a new passive flow control method, similar to a vortex generator. This method may also be applied to control the flow separation in rotor or turbine blades and has potential applications in the fields of drag and noise reduction, stall suppression, and heat transfer.