A crucial problem of designing thick airfoils is balancing structural and aerodynamic requirements. This paper documented a new idea to deal with the thick airfoil's design. Firstly, the relative thickness of the original airfoil was increased to enhance its structural property. Then the overall aerodynamic performance was improved by the optimization design method. Specifically, this paper put forward a mathematical model of the overall optimization employing airfoil's performance evaluation indicators which represent modern rotor blades' aerodynamic requirements of “high efficiency, low extreme load, wide range of operating angle of attack and stability with varying operating conditions”. Based on this model, an integrated optimization platform for thick airfoils' overall design was established. Through an optimization experiment, a new 35-percent relative thickness airfoil was obtained. The new airfoil was predicted with high design lift coefficient, acceptable maximum lift to drag ratio, moderate stall parameter, and desirable stability parameters. These characteristics contribute to a high overall performance which could be competent with commonly used thick DU airfoils. Lift characteristics of the new airfoil have been validated by tests. These results confirmed the proposed method has effectively balanced airfoil's complicated requirements and successfully improved the new airfoil's overall performance.
The design of thick airfoils used in the middle span of blades is a typical problem of balancing structural and aerodynamic requirements. Prior work has already employed numerical optimization method to deal with this multi-objective problem with objective parameters as l/dmax and second cross moment of area. However, many other crucial aerodynamic parameters denoting design performance, off-design performance, stall characteristics and performance stability were not directly taken into consideration as objectives nor constraints during the optimization process, which may not satisfy blades' comprehensive performance requirements. Therefore, based on the airfoil's performance indicators satisfying modern rotor blades' complicated aerodynamic requirements of “high efficiency, low extreme load, wide range of operating a and stability with varying operating conditions”, this paper documented a new idea to deal with the thick airfoil's design. We firstly increased the relative thickness of the original airfoil to enhance its structural property and then focused on the improvement of overall aerodynamic performance using the optimization design method. Through a design experiment, we obtained a new 35-percent relative thickness airfoil with high cl,design, acceptable l/dmax, moderate stall parameter and desirable stability parameters, contributing to a high overall performance which could be competent with commonly used thick DU airfoils. The lift characteristics of the new airfoil were validated by tests and compared with experimental data of target DU airfoils. The results confirmed that compared with the traditional inverse design method, the numerical optimization method is more effective to deal with multi-objective problems. More notably, the employment of defined performance indicators as target parameters compromising the overall objective functions has effectively balanced airfoil's complex and contradict requirements and successfully improved the new airfoil's overall performance. To our knowledge based on the published references, the method and findings in this paper extend previous work on thick airfoil's design. We expect this method could offer a new and more effective way to solve the discrepancy of load, structure, and technics during the three dimensional design and flow control of blades by fundamental design of airfoils. However, some limitations are worth noting. The lift characteristics of the new airfoil have been validated with experimental result. But it's not complete. Because other performance (especially the drag characteristic) was not obtained during the experiments due to limitations of the wind tunnel. Therefore, wind tunnel test still needs to be performed to further validate it.
The results have been published on Energy 116 (2016) 202-213.
Fig. 1. The framework and workflow of the airfoil's optimization design platform.