基于Navier-Stokes／Kirchhoff方程的悬停旋翼跨声速气动声学计算

研究了Kirchhoff积分面是否有盖有底,以及是否计及旋翼网格上的流场值,这两个因素对噪声预测结果的影响.发展了一种基于重叠网格的计算悬停旋翼远场噪声的数值方法.数值计算过程分为流场模拟和声场模拟两部分.悬停旋翼流场的数值模拟是在两个相互重叠的网格上进行的:在高质量的旋翼网格上求解Navier-Stokes方程,用于模拟旋翼附近的粘性流动和近场尾涡的捕捉;在远离粘性区域处布置符合悬停流场物理特征的圆柱形背景网格,控制方程为Euler方程,用于远场尾涡的捕捉.计算得到的流场信息插值到用于声场计算的Kirchhoff积分面上.观测点处的噪声可以认为是由这个完全包含桨叶的Kirchhoff积分面上的面元(声源)发声得到.远场声波的传播由Kirchhoff积分公式描述.计算结果表明:采用有盖有底的Kirchhoff积分面并且同时计及旋翼网格流场值时,计算得到的HSI噪声与实验值吻合最好.     

剪刀式尾桨涡流干扰机理和气动特性研究

采用非常规剪刀式尾桨对直升机整体性能有着重要影响,关于其复杂流动干扰机理的研究尚处在发展之中. 为了掌握剪刀式尾桨的流动干扰机理和参数影响规律,建立了适合于悬停状态下剪刀式尾桨干扰涡流场分析的计算流体力学(computational fluid dynamics, CFD) 数值模拟方法. 采用积分形式的Reynolds-averagedNavier-Stokes (RANS) 方程作为旋翼流场求解控制方程,围绕旋翼流场的结构网格采用嵌套网格方法生成. 在CFD 方法验证基础之上,对悬停状态下两种不同构型剪刀式尾桨桨尖涡的涡核位置和强度的演变规律进行了定量分析,并对流场中桨尖涡与桨叶的贴近干扰、碰撞、破碎运动,同时准确捕捉了不同尺度涡之间的相互干扰、融合的过程进行了分析. 进一步研究了剪刀角和轴间距参数对不同构型剪刀式尾桨气动特性的影响规律. 计算结果表明,剪刀式尾桨流场中存在复杂的桨-涡干扰和涡-涡干扰现象,剪刀角和轴间距对剪刀式尾桨的气动特性有重要影响,L 构型剪刀式尾桨气动性能整体优于U 构型剪刀式尾桨.      

基于动态面搭接技术的直升机旋翼流场分析

为了进一步提高直升机旋翼流场计算的精度与效率,本文根据直升机旋翼的运动特点,发展了一种基于结构化网格动态面搭接技术及低速预处理技术的直升机旋翼流场的非定常数值分析方法。采用该方法分别对Caradonna-Tung、UH-60A旋翼模型的悬停流场及NASA某旋翼无升力前飞流场进行了数值分析;将计算所得的旋翼表面压力分布与风洞试验结果进行了对比,对旋翼的尾涡结构进行了相关分析。分析结果表明:本文所发展的方法具有较高的精度且计算速度快,并能够比较准确地捕捉直升机旋翼的尾涡结构,可以用于直升机旋翼流场的分析研究,从而为开展考虑挥舞及变距运动的直升机旋翼前飞状态非定常气动特性的研究奠定了基础。     

基于高阶和高效格式的悬停旋翼可压缩无粘绕流的计算

发展了一种基于高精度和高效格式计算悬停旋翼复杂绕流的隐式有限体积方法。控制方程为Euler方程,其中对流项通量的左右状态量采用五阶加权基本无振荡(WENO)格式重构,时间推进应用隐式LU-SGS算法,为进一步加速收敛,采用三层V循环多重网格松弛方法。考虑到多重网格方法的思想以及五阶WENO格式涉及6个网格单元,建议仅在最细网格上使用WENO格式。计算结果表明本文方法能有效捕捉激波,对尾迹也有较高分辨率,基于隐式LU-SGS算法的多重网格方法能有效提高计算效率。     

基于CFD的民用飞机水上迫降动力学仿真研究

针对民用飞机水上迫降动态冲击问题,采用数值仿真方法进行了研究.基于计算流体力学采用雷诺平均N–S方程求解,结合流体体积模型和全流场运动网格捕捉水?气交界面,求解六自由度方程获得飞机的位置和姿态.采用该方法研究了尾吊?高平尾布局飞机的水上迫降过程.结果表明,该方法可以较好地模拟水上迫降的运动和受力,入水早期飞机受到较大水...     

用隐式方法和WENO格式计算悬停旋翼跨声速无粘流场

寻找一种能够准确计算以涡为主要特征的复杂流场和克服尾迹耗散问题的数值方法,一直是旋翼空气动力学研究的热点和难点。本文发展了一种基于高阶迎风格式计算悬停旋翼无粘流场的隐式数值方法。无粘通量采用Roe通量差分分裂格式,为提高精度,使用五阶WENO格式进行左右状态插值,并与MUSCL插值进行比较。为提高收敛到定常解的效率,时间推进采用LU-SGS隐式方法。用该方法对一跨声速悬停旋翼无粘流场进行了数值计算,数值结果表明WENO-Roe的激波分辨率高于MUSCL-Roe,体现出了格式精度的提高对计算结果的改善,LU-SGS隐式方法的计算效率比5步Runge-Kutta显式方法的高。     

非结构重叠网格方法研究及应用

航空、航天和兵器技术等领域的研究中存在大量包含运动边界的流场。非结构重叠网格方法是一种高效的处理动边界问题的新方法。围绕相对运动的每个物体单独生成非结构网格,在网格重叠区域通过搜索和插值完成网格系之间的信息传递,提出了动态八叉树搜索算法,发展了绝对坐标系和相对坐标系相结合的流场求解方式,采用二阶精度Van Leer/Hanel格式和四阶Runge-Kutta法分别进行空间和时间离散,形成了一种新的非结构重叠网格算法。对三维Riemann问题的求解结果与精确解能很好吻合,证明了本文的重叠网格算法具有较好的时空离散精度和插值精度。对7.62mm步枪射击过程进行了数值模拟,描述了弹丸离开膛口后膛口流场的发展过程,与实验结果体现的发展过程较为吻合,验证了本文提出的非结构网格算法体系具有较好的计算性能,是研究含动边界复杂流场的一种有效手段。     

大展弦比无人机的静气弹问题计算及分析

普遍采用大展弦比机翼的无人机,其气动弹件的问题显得尤为突出.本文主要研究基于非结构弹性网格体系的欧拉方程CFD计算及其在大展弦比无人机静气动弹性问题中的应用.针对三维非结构运动网格技术进行了研究和开发,在此基础上,利用计算流体力学的方法,发展了一套具有一定通用性的、适用于非结构网格的Euler方程流场求解器,并综合上面的技术,进一步通过耦合结构力学方程,对大展弦比无人机的静气动弹性问题进行了,计算和分析.     

65°三角翼亚音速复杂流场计算和数据可视化

提出一种基于非结构混合网格和有限体积法的有效计算策略,对第二期国际涡流试验项目(second international vortex flow experiment,VFE-2)的尖前缘65°三角翼在马赫数0.4,迎角20.3°,雷诺数2×10~6条件下的亚音速复杂流场结构进行数值模拟,重点探讨了基于计算数据进行该类型复杂涡系干扰表面和空间流场关键特征提取和数据可视化问题.通过与相关试验类比,建立了与先进试验流动显示技术相比拟的定性和定量分析方法,为三角翼这类复杂流场结构的精细分析奠定了技术基础.采用上述方法,细致分析了亚音速三角翼的大迎角复杂旋涡流场结构,得到了与试验一致的结论.研究证实:在大迎角条件下,三角翼流动物理复杂,黏性效应耦合严重,只有通过N-S方程计算才能准确地捕捉主涡和二次涡的发展.     

直升机不平衡旋翼动力学建模与轴心运动轨迹分析方法研究

研究了Johnson提出的倾转旋翼不平衡载荷前飞动力学模型,将其桨叶分析方法应用于直升机旋翼系统模态分析。在刚性条件假设下推导了直升机旋翼弹性阻尼和惯性力综合作用时桨叶的挥舞和摆振运动方程,给出了固定和旋转坐标系下对应的运动方程。通过引入均匀入流和线性扭转假设,获得了运动方程的理论解析解。利用叠加原理,得到了桨毂轴心运动方程;采用Newmark法进行振动微分方程求解,最终得到了直升机旋翼的轴心运动轨迹。以某型直升机旋翼系统为例,验证了本研究所提出旋翼桨叶模态分析方法的准确性,给出了兼顾计算精度和效率的最佳求解步长选取方法;预测了典型飞行状态下的桨毂轴心运动轨迹,为直升机旋翼系统设计提供了基础方法和技术参考。     

An unsteady multiblock multigrid scheme for lifting forward flight rotor simulation

Numerical simulation of multi‐bladed lifting rotors in forward flight is considered. The flow‐solver presented is multiblock and unsteady, which is essential for forward flight, and also includes multigrid acceleration to reduce run‐times. A structured multiblock grid generator specifically for rotor blades has also been developed and is presented here. Previous work has shown that hovering lifting rotor flows are particularly expensive to simulate, since the capture of the vortical wake below the disc requires a long numerical integration time; more than 20 revolutions for an unsteady simulation, or more than 40000 time‐steps for a single grid steady simulation. It is demonstrated here that only two or three revolutions are required to obtain a converged solution for forward flight, since the wake is swept downstream. This requires less than 1.5 × the run‐time of a steady hovering simulation, for the same grid density around each blade, even though an unsteady simulation is required and the complete disk must be solved rather than one blade as in hover. It is demonstrated that very fine meshes are required to capture the unsteady tip vortex motion, and the effects on blade loading of blade‐vortex interaction and rotor shaft inclination are also considered. Copyright © 2004 John Wiley & Sons, Ltd.     

Unsteady rotor flow analysis using a diagonally implicit harmonic balance method and an overset mesh topology

The diagonally implicit harmonic balance method is developed in an overset mesh topology and applied to unsteady rotor flows analysis. Its efficiency is by reducing the complexity of a fully implicit harmonic balance method which becomes more flexible in handling the higher harmonics of the flow solutions. Applied to the overset mesh topology, the efficiency of the method becomes greater by reducing the number of solution interpolations required during the entire solution procedure as the method reduces the unsteady computation into periodic steady state. To verify the accuracy and efficiency of the method, both hovering and unsteady forward flight of Caradonna and Tung and AH-1G rotors are solved. Compared with wind-tunnel experiments, the numerical results demonstrate good agreements at computational cost an order of magnitude more efficient than the conventional time-accurate computation method. The proposed method has great potential in other engineering applications, including flapping wing vehicles, turbo-machinery, wind-turbines, etc.     

A RECONSTRUCTION METHOD FOR FINITE VOLUME FLOW FIELD SOLVING BASED ON INCREMENTAL RADIAL BASIS FUNCTIONS

由于流场参数重构中, 用于重构的基网格单元的物理参数波动量相对于均值较小, 径向基函数（RBF） 直接插值方法重构会产生较大的数值振荡, 论文提出了一种增量RBF 插值方法, 并用于有限体积的流场重构步, 明显改善了插值格式的收敛性和稳定性. 算例首先通过简单的一维模型说明该方法的有效性, 当目标函数波动量相对于均值为小量时, 增量RBF 插值能够抑制数值振荡; 进一步通过二维亚音速、跨音速定常无黏算例、静止圆柱绕流非定常算例以及超音速前台阶算例来说明该方法在典型流场数值求解中的通用性和有效性. 研究表明增量RBF 重构方法可陡峭地捕捉激波间断, 可有效改善流场求解的收敛性和稳定性, 数值耗散小, 计算效率高.     

一种基于增量径向基函数插值的流场重构方法

由于流场参数重构中, 用于重构的基网格单元的物理参数波动量相对于均值较小, 径向基函数（RBF） 直接插值方法重构会产生较大的数值振荡, 论文提出了一种增量RBF 插值方法, 并用于有限体积的流场重构步, 明显改善了插值格式的收敛性和稳定性. 算例首先通过简单的一维模型说明该方法的有效性, 当目标函数波动量相对于均值为小量时, 增量RBF 插值能够抑制数值振荡; 进一步通过二维亚音速、跨音速定常无黏算例、静止圆柱绕流非定常算例以及超音速前台阶算例来说明该方法在典型流场数值求解中的通用性和有效性. 研究表明增量RBF 重构方法可陡峭地捕捉激波间断, 可有效改善流场求解的收敛性和稳定性, 数值耗散小, 计算效率高.      

Third‐order Cartesian overset mesh adaptation method for solving steady compressible flows

A third‐order mesh generation and adaptation method is presented for solving the steady compressible Euler equations. For interior points, a third‐order scheme is used on Cartesian and curvilinear meshes. Concerning the mesh adaptation, the method of Meakin is also extended to third order. The accuracy of the new overset mesh adaptation method is demonstrated by a grid convergence study for 2‐D inviscid model problems and results are compared with a second‐order method. Finally, the method is applied to the computation of an inviscid 3‐D flow around a hovering blade of the ONERA 7A helicopter rotor exhibiting an improvement in the wake capture. With a 7 million point mesh, the tip vortex can be followed for more than three rotor revolutions with the third‐order method. The CPU time needed for this calculation is only 3% higher than with a conventional second‐order method. Copyright © 2007 John Wiley & Sons, Ltd.     

Numerical simulation of flapping‐wing insect hovering flight at unsteady flow

A computational fluid dynamics (CFD) analysis was conducted to study the unsteady aerodynamics of a virtual flying bumblebee during hovering flight. The integrated geometry of bumblebee was established to define the shape of a three‐dimensional virtual bumblebee model with beating its wings, accurately mimicking the three‐dimensional movements of wings during hovering flight. The kinematics data of wings documented from the measurement to the bumblebee in normal hovering flight aided by the high‐speed video. The Navier–Stokes equations are solved numerically. The solution provides the flow and pressure fields, from which the aerodynamic forces and vorticity wake structure are obtained. Insights into the unsteady aerodynamic force generation process are gained from the force and flow‐structure information. The CFD analysis has established an overall understanding of the viscous and unsteady flow around the virtual flying bumblebee and of the time course of instantaneous force production, which reveals that hovering flight is dominated by the unsteady aerodynamics of both the instantaneous dynamics and also the past history of the wing. A coherent leading‐edge vortex with axial flow and the attached wingtip vortex and trailing edge vortex were detected. The leading edge vortex, wing tip vortex and trailing edge vortex, which caused by the pressure difference between the upper and the lower surface of wings. The axial flow, which include the spanwise flow and chordwise flow, is derived from the spanwise pressure gradient and chordwise pressure gradient, will stabilize the vortex and gives it a characteristic spiral conical shape. Copyright © 2006 John Wiley & Sons, Ltd.     

An implicit high-order preconditioned flux reconstruction method for low-Mach-number flow simulation with dynamic meshes

A fully implicit high-order preconditioned flux reconstruction/correction procedure via reconstruction (FR/CPR) method is developed to solve the compressible Navier-Stokes equations at low Mach numbers. A dual-time stepping approach with the second-order backward differentiation formula (BDF2) is employed to ensure temporal accuracy for unsteady flow simulation. When dynamic meshes are used to handle moving/deforming domains, the geometric conservation law is implicitly enforced to eliminate errors due to the resolution discrepancy between BDF2 and the spatial FR/CPR discretization. The large linear system resulted from the spatial and temporal discretizations is tackled with the restarted generalized minimal residual solver in the PETSc (portable, extensible toolkit for scientific computation) library. Through several benchmark steady and unsteady numerical tests, the preconditioned FR/CPR methods have demonstrated good convergence and accuracy for simulating flows at low Mach numbers. The new flow solver is then used to study the effects of Mach number on unsteady force generation over a plunging airfoil when operating in low-Mach-number flows. It is observed that weak compressibility has a significant impact on thrust generation but has a negligible effect on lift generation of an oscillating airfoil.     

一种三阶有限体积法及其在欠膨胀射流激波结构数值模拟中的应用

以喷管出口欠膨胀射流为研究对象,在Lagrange坐标系下建立欠膨胀射流二维积分形式的流动方程。通过在单元交接面处进行三阶ENO(essentially nonoscillatory)格式插值,构造得到一种适用于求解该方程的三阶ENO有限体积法。采用该格式对一维Sod激波管算例和喷管出口欠膨胀射流进行数值计算。计算结果表明,该方法具有高精度、基本无振荡的特点,能很好地捕捉包含激波、滑移线以及三波交点等复杂流场波系结构。计算得到的波系结构中马赫盘的位置与实验结果吻合很好,相对误差小于1.1%。     

A new coupled level set and volume-of-fluid method to capture free surface on an overset grid system

We present a new coupled level set and volume-of-fluid (CLSVOF) method for free surface flow simulations on an overset grid system. The coupled method takes advantages of the strengths of the level set (LS) method and the volume-of-fluid (VOF) method, and is superior to either single method. The novelty of the present method lies in that we develop the methodology for an overset grid system of embedding, overlapping and moving structured grids. The new methodology accurately captures interface and greatly preserves mass on an overset grid system by demonstrating the 3D sphere advection test. The method is coupled to a well validated Reynolds-Averaged Navier–Stokes incompressible flow solver. The method is validated with the dam-breaking flow interacting with a 3D obstacle (square structure/circular cylinder) by comparing the numerical results with available experimental and numerical studies. The water impact of a sphere case is further performed to demonstrate the capabilities of the new method on a complicated moving overset grid system.