旋翼桨尖涡生成及演化机理的高精度数值研究

为了细致捕捉直升机旋翼桨尖涡的生成和演化过程,建立了一个基于高精度网格和高阶通量计算格式的旋翼桨尖涡计算流体力学(CFD)求解方法。在该方法中,流场求解选取旋转坐标系下的Navier-Stokes方程为控制方程;空间离散采用迎风Roe格式,并采用低耗散的5阶WENO(Weighted Essentially Non-Osciltatory)格式进行对流通量的计算;时间推进则采用双时间法,在伪时间步上使用隐式LU-SGS(Lower-Upper Symmetric Gauss-Seidel)推进格式;应用嵌套网格方法实现桨叶网格和背景网格的数据交换。应用所建立的方法对悬停状态的旋翼桨尖涡流场进行了高精度模拟,在桨叶网格上精细地捕捉到了桨尖涡的具体生成过程,在背景网格上捕捉到了更多圈数的桨尖涡尾迹,并对桨尖涡的演化机理进行了深入研究。结果表明:建立的高精度数值方法能够有效地对旋翼桨尖涡的生成和演化过程进行细致模拟;悬停状态下旋翼桨尖气流在上下表面压力梯度的作用下经历了边界层增厚、逐渐卷起形成涡核以及最终脱离桨叶形成桨尖涡的过程。

High-resolution numerical research on formation and evolution mechanism of rotor blade tip vortex

To capture the formation and evolution process of the helicopter blade tip vortex in detail,a computational fluid dynamics (CFD) method based on refined grids and a high-order interpolation scheme is presented.In this method,the Navier-Stokes equation in the rotating coordinate system is selected as the governing equation.For spatial discretization,the upwind Roe scheme together with a fifth-order WENO (Weighted Essentially Non-Oscillatory) scheme is employed to calculate the convective variables.A dual-time method is utilized for time marching,and the implicit LU-SGS (Lower-Upper Symmetric Gauss-Seidel) scheme is adopted for every pseudo time step.The information between blade grid and the background grid is exchanged by using the overset grid method.With the method developed,the tip vortex flowfield of a hovering rotor is simulated accurately,and the detailed formation process and the evolution of the blade tip vortex are captured in the blade and background grids,respectively.The formation and evolution mechanisms of the blade tip vortex are then discussed.It is demonstrated that the current high-accuracy method is effective in simulating the formation and evolution of the blade tip vortex in hover condition.The cross flow around the blade tip is firstly attached to the blade,and then the boundary layer grows due to the pressure gradient.With advancing downstream,the vortex core is gradually identifiable,and is detached from the tip blade ultimately.