在直升机旋翼流动的数值模拟中,流动求解精度对旋翼气动噪声的预测有着重要影响。采用中心格式有限体积法求解可压缩Euler方程数值模拟旋翼绕流,并采用基于Ffowcs Williams-Hawkings(FW-H)方程的声类比方法计算旋翼气动噪声。为了抑制中心格式的数值耗散,保持机翼翼尖以及旋翼桨尖的尾涡结构,引入了涡限制法。通过对ONERA M6机翼定常绕流、UH-1H和Caradonna旋翼悬停绕流以及AH-1/OLS旋翼前飞绕流的数值模拟,验证了涡限制法可以有效提高粗网格的尾涡捕捉精度,从而提高旋翼气动噪声的计算精度。结果表明,对于不存在尾迹涡的旋翼无升力状态,涡限制法仍然可以提高旋翼气动噪声的预测精度。此外,进一步研究了涡限制系数对数值计算的稳定性和计算结果的影响,给出了临界涡限制系数随网格量、桨尖马赫数、桨距角和展弦比的取值规律。
In the numerical simulation of helicopter rotor in hover flight, the prediction of aeroacoustic noise for helicopter rotors is highly dependent on the capturing accuracy of vortical flow structure. In this paper, the compressible Euler equations are solved by using a finite volume method with a central scheme to simulate the flow around helicopter rotors, and the Ffowcs Williams-Hawkings (FW-H) equations are utilized to predict the aeroacoustic noise. In order to reduce the numerical dissipation and preserve the vortical flow structure on the wing tip or rotor blade tip, a vorticity confinement method is introduced to the original Euler solver. The method is more applied to the flow around ONERA M6 wing, UH-1H, Caradonna and AH-1/OLS helicopter rotors. It is shown that the method is more able to preserve the vortical flow structure on coarse grids and the predicted results of aeroacoustic noise for helicopter rotors are all improved. The results show that the predict accuracy is also improved, even for non-lifting flight. The effect of vorticity confinement coefficient on the stability and numerical results is investigated. Finally, the effect of conditions (such as grid size, blade tip Mach number, pitch angle, and aspect ratio) on critical vorticity confinement coefficient is concluded.
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