The Progress of Aerodynamic Mechanisms Based on Avian Leading-Edge Alula and Future Study Recommendations

Birds in nature have many unique devices to help them acquire excellent flight abilities under various complex flight conditions. One of the unique devices is the leading-edge alula, located at the junction of the arm wing and the hand wing of most birds. It often spreads out during takeoff and landing, probably playing a similar role to high-lift devices in fixed-wing aircraft. This paper analyzed and reviewed the results of current research on leading-edge alula, finding some important factors, such as the complex flapping motions, flexibility, and the plane and section shape of the wing, that have been ignored in current research to a certain extent. These would greatly affect the conclusions obtained. Hence, for a deeper understanding of the aerodynamic mechanisms and functions of the alula, some new study predictions for future research are presented. In addition, the feasible models and methods for further research based on these predictions are discussed and proposed. For example, the higher-accuracy LES or hybrid LES/RANS method and the combinations of these methods with wind-tunnel experiments using PIV technology are recommended.

The study of typical water header flow structure by LES and RANS

Water header is the most common structure in the design of flow system for energy and power system. The complex flow structure could result in some problems when Computational Fluid Dynamic (CFD) simulation is applied in the whole system analysis. The rapid change in velocity distribution of the flow field leads to difficulties to create suitable boundary layer mesh, and the complex flow structure will also make residuals hard to reach convergence criteria. Large Eddy Simulation (LES) is promising to promote these studies, it is more accurate than RANS method and can capture many non-steady state characteristics those RANS method can?t obtain. In this study a typical water header flow structure is investigated by RANS and LES methods. By comparing the detailed flow structures in the results of two methods, the deficiency of RANS method was found. The results of LES can be used to guide the establishment of meshes and the application of time-averaged turbulence models to improve efficiency in engineering. And the asymmetric Reynolds stresses may induce asymmetric flow field in symmetric geometry.