Study on the effect of transition process on rotor hovering simulation

转捩过程对旋翼悬停模拟的影响分析与研究

School of Aeronautics, Northwestern Polytechnical University, Xi’an 710072, China

Received: 2 July 2021

Abstract

Hovering is one of important statuses to evaluate the aerodynamic performance of a rotor. With the development of the computer technology and CFD technique, the numerical methods based on the first principle are usually employed to evaluate the hovering performance of the rotor. The transition process will evidently affect the results from the RANS-based numerical simulations in some steady cases for the fixed wing aircrafts, which should be taken into consideration in the design process. But it's not clear whether the transition process would affect the numerical results for the rotor simulation. To provide the reference in designing and evaluating the rotorcraft, the effect of the transition process in the rotor simulation needs to be discussed further. The PSP rotor proposed by NASA is calculated using the in-house solver based on the overset grid in this paper. Simulations are performed with fully turbulent model as well as the transitional model and the results are compared to the experimental data. The results prove the superior ability to simulate the flow around a hovering rotor of the in-house solver. The relative errors of the numerical results are under 5%. The range of the laminar flow on the blade is proportional to the rotor thrust, which causes a higher Figure of Merit in transition simulation than the fully turbulent simulation. The sectional pressure distribution and torque distribution along the blade apparently suffer from the transition process, which doesn't affect the thrust distribution along the blade and the blade vortex wake flow under the rotor disk. An obvious flow separation on the surface of the blade can be observed in the transition simulation compared to the fully turbulent simulation.

摘要

悬停状态是考察旋翼整体气动性能的重要状态之一。随着计算机技术及CFD技术的发展，基于“第一性原理”的数值模拟方法越来越多地被用于评估旋翼悬停性能。在使用基于RANS方程的数值模拟方法进行固定翼飞行器定常计算时，流动转捩现象对某些特定状态下流场及气动特性会产生巨大影响，因此在进行固定翼设计时要考虑流动转捩现象。然而转捩过程是否同样会影响旋翼非定常气动流场及气动特性，国内研究较少，因此有必要研究转捩过程对旋翼流场数值模拟的影响，为旋翼类飞行器的设计及评估提供参考。采用美国航空航天学会旋翼悬停工作组提出的PSP旋翼标模，利用结构化动态嵌套网格技术，在大拉力悬停和小拉力悬停状态下，分别进行全湍流模拟和转捩模拟计算并与试验结果进行了对比。对比结果显示，文中所采用的数值求解器对旋翼悬停效率的计算误差在5%之内。在考虑流动转捩后，由于桨叶表面存在层流区域，计算所得旋翼悬停效率高于全湍流假设下的预测值，而桨叶表面的层流区域与旋翼拉力大小有关。在流动转捩发生的区域，转捩过程会对桨叶截面压力分布以及桨叶展向扭矩分布产生明显影响，同时桨叶表面出现明显的流动分离现象。对于桨叶展向拉力分布和桨盘下方旋翼尾迹桨尖涡区域，转捩过程均不产生明显影响。

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