大振幅振荡来流条件下非定常气动力模型计算验证与弱可压缩性修正

CFD verification and weak compressibility correction of unsteady aerodynamic force models applied to high-amplitude oscillating incoming flows

针对大振幅振荡来流条件下薄翼受到的非定常气动力,Isaccs和Greenberg分别发展了非定常气动力模型。这两种模型可以用于直升飞机桨叶与风力发电叶片的气动力分析,模型在不可压缩无黏来流条件下建立,但实际流动不可避免粘性和弱可压缩性的影响,需要检验两种模型的适用性。针对粘性效应的影响,2014年Strangfeld对于NACA0018翼型,通过风洞实验验证了在Reynolds数25万时,Isaccs和Greenberg的模型仍适用,实验的Mach数为0.0326,流动近似不可压缩流动。针对可压缩性的影响,通过数值模拟方法进行了研究。首先重复了实验在Mach数为0.0326时的结果,并进一步考察了当Mach数提高为0.1、0.2和0.3时非定常气动力的变化。结果表明随着Mach数的提高,升力系数的最高点逐渐高于模型,并且相位逐渐落后,在Mach数为0.3时差别最明显,非定常升力系数最高点计算与模型相差50%。此即表明弱可压缩性对模型的预测结果影响不可忽略。为了扩展模型在Mach数变化时的适用范围,对模型进行了弱可压缩性修正。通过考虑速度变化引起均匀来流中密度的变化,修正了翼型附近流体密度,使其跟随来流Mach数变化。采用此方法,将计算与模型的幅值差别减小到5%左右。

The two-dimensional airfoil theories of Isaacs and Greenberg for unsteadyaerodynamic forces are widely adopted to estimate the aerody namic performance of wind-and helicopter blade loads.The models are established under the assumption that is incompressible and without viscosity.However,the viscosity and compressibility are inevitableand the applicability of the model to predict the aerodynamic force in real flows needs to bechecked.For the viscous effects,Strangfeld et al.verified the models experimentally using thedata of NACA0018from wind tunnel at Reynolds number0.25million in2014.The Machnumber of the experiment is near0.0326,which makes the flow almost incompressible.To checkthe effects of compressibility,a numerical simulation of NACA0018is conducted.Forverification,the result of Strangfeld et al.at Mach number0.0326is repeated using CFD.Thesimulation further extends to the Mach number0.1?0.2and0.3cases to investigate performanceof the model at higher Mach numbers.The results show that maximum lift coefficient increases and the phase lags with Mach number increasing.Maximum overshot is at Mach number0.3,whichis over50%more than the prediction of the models.The results show that the compressibility is nonnegligibleand the Mach number is needed to be taken into account as a sensitive factor.application range of Mach number,a correction is added to the models to account the influence ofweak compressibility.The correction relates the variation of density of the incoming flow to theMach number changing,which changes the density near the airfoil.As the aerodynamic force areproportional to the density,the correction would increase the aerodynamic force as Mach numberincreases.Using the correction,the differences of the models and simulation is within5%.