No abstract

No abstract

As the air density changes dramatically from high to low altitude, the matching of high-altitude propeller and electric motor under low-altitude condition is one of the main challenges that aerodynamic design of high-altitude propeller faces. To solve this problem, variable-pitch propeller is the most commonly used and feasible solution. In this paper, the Reynolds-average Navier-Stokes solver and an efficient surrogate-based optimization toolbox are used for the aerodynamic design of a fixed-pitch propeller. Based on the optimal fixed-pitch propeller, a series of pitch angle are employed to obtain the variable-pitch propeller with maximum thrust. Then, the aerodynamic characteristics of fixed-pitch propeller and variable-pitch propeller under typical working conditions are studied and compared. The results show that under the condition of low-altitude and low wind speed, the fixed-pitch propeller reaches the rated torque of the electric motor at a lower rotational speed, resulting in a lower shaft power and thrust. By adjusting the pitch angle, the variable-pitch propeller can achieve higher rotational speed under the same rated torque constraints, thereby absorbing more shaft power and achieving larger thrust. The lower the altitude, the more obvious the advantage of the variable-pitch propeller. Compared with fixed-pitch propeller, the maximum relative increment of the maximum thrust can reach more than 80% for variable-pitch propeller. It is concluded that variable-pitch propeller can significantly improve the matching of propeller and electric motor, bringing wider flight envelope of UAVs.

To increase the efficiency and robustness of stability-based transition prediction in flow simulations, simplified methods are introduced to substitute direct stability analyses for rapid disturbance growth prediction. For low-speed boundary layers, these methods are mainly established based on self-similar assumptions, which are not applicable to non-similar boundary layers in hypersonic flows. The objective of this article is to investigate the application of surrogate models to stability analysis of non-similar flows over blunt cones, focused on parameterization of boundary-layer (BL) profiles. Firstly, correlations between BL edge and profile parameters are analyzed, along with self-similar flow parameters and discrete points on BL profiles, which present four groups of BL characteristic parameters. Secondly, using these parameters as inputs, surrogate models are built for disturbance growth prediction over an MF-1 blunt cone. Results show that, surrogate models using four BL edge parameters and a BL shape factor {Ue, Te, ρe, ηe, H12} for stability analysis can achieve comparable accuracy with those using 16 discrete BL profile parameters, which are more precise than those using merely self-similar parameters or BL edge parameters. Thirdly, the established surrogate models are validated by stability analysis and transition prediction over the MF-1 blunt cone in flight experiments at the instants of t = 17 s ~ 22 s. Compared with direct linear stability analyses, the mean relative error of predicted disturbance growth rates by surrogate models is 8.0% and the maximum relative error of N factor envelopes is 6.6%, which indicates feasible applications of surrogate models to stability analysis and transition prediction of non-similar boundary layers in hypersonic flows.

No abstract