Simulation of Aerodynamic Performances of Flexible Flapping Wing Airfoils北大核心CSCD

与固定翼相比,在低速、小Reynolds数条件下,扑翼飞行具有显著的气动性能优势,受到越来越多的重视。然而,目前对扑翼翼型的研究以刚性翼型为主,对柔性翼型气动性能认识还不清楚。该文建立了柔性椭圆翼型的流固耦合仿真模型,分析了不同风速、迎角下柔性椭圆翼型的周围流场、变形以及气动性能。仿真结果表明,较刚性翼型,柔性翼型延缓了尾涡脱落时间,有效降低升力扰动振荡频率;柔性翼型显著抑制了尾流流场的扰动,降低升力扰动振荡幅值,合适的弹性模量翼型使得扰动振荡完全消除。研究结果可为软飞行器气动设计提供参考。     

Enhancement of a viscous-inviscid-interaction airfoil analysis code using the parallel direct search algorithm

The coupling mechanism for an existing viscous-inviscid-interaction (VII) code, developed for the analysis of two-dimensional, turbulent, attached flow around airfoils, is enhanced using the parallel direct search (PDS) optimization algorithm. It is demonstrated that this parallel processing implemented optimization scheme leads to faster convergence of the VII code, and therefore, requires less computational time when the number of optimization (or design) variables is low, and a moderate number of processors are available. As the number of design variables increases, more processors are required to maintain this advantage. Results are presented for the NACA-0012 and the RAE-2822 airfoils. The quality of the results obtained is satisfactory and confirms that the enhanced VII code can be an acceptable alternative to reduced Navier-Stokes solvers as an airfoil analysis tool.     

A numerical methods for 2-D flow of flapping airfoil

To study the physics of the flow of flapping flight, a numerical model for the two-dimensional flow around an airfoil undergoing prescribed oscillatory motions in a viscous flow is described. The model is used to examine the flow characteristics and power coefficients of a symmetric airfoil heaving sinusoidally over a range of frequencies and amplitudes. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Maximization of the lift/drag ratio of airfoils with a turbulent boundary layer: Sharp estimates,approximation, and numerical solutions

The lift/drag ratio of an airfoil placed in an incompressible attached flow is maximized taking into account the viscosity in the boundary-layer approximation. An exact solution is constructed. The situation when the resulting solutions are not in the admissible class of univalent flows is discussed. A procedure is proposed for determining physically feasible airfoils (with a univalent flow region) with a high lift/drag ratio. For this purpose, a class of airfoils is constructed that are determined by a twoparameter function approximating the found exact solution to the variational problem. For this class, the ranges of free parameters leading to physically feasible flows are found. The results are verified by computing a turbulent boundary layer using Eppler’s method, and airfoils with a high lift/drag ratio in an attached flow are detected.     

Analysis of a FEM/BEM coupling method for transonic flow computations

A sensitive issue in numerical calculations for exterior flow problems, e.g.around airfoils, is the treatment of the far field boundary conditions on a computational domain which is bounded. In this paper we investigate this problem for two-dimensional transonic potential flows with subsonic far field flow around airfoil profiles. We take the artificial far field boundary in the subsonic flow region. In the far field we approximate the subsonic potential flow by the Prandtl-Glauert linearization. The latter leads via the Green representation theorem to a boundary integral equation on the far field boundary. This defines a nonlocal boundary condition for the interior ring domain. Our approach leads naturally to a coupled finite element/boundary element method for numerical calculations. It is compared with local boundary conditions. The error analysis for the method is given and we prove convergence provided the solution to the analytic transonic flow problem around the profile exists.

     

Saturation-free numerical scheme for computing the flow past a lattice of airfoils and the determination of separation points in a viscous fluid

A numerical method for computing the potential flow past a lattice of airfoils is described. The problem is reduced to a linear integrodifferential equation on the lattice contour, which is then approximated by a linear system of equations with the help of specially derived quadrature formulas. The quadrature formulas exhibit exponential convergence in the number of points on an airfoil and have a simple analytical form. Due to its fast convergence and high accuracy, the method can be used to directly optimize the airfoils as based on any given integral characteristics. The shear stress distribution and the separation points are determined from the velocity distribution at the airfoil boundary calculated by solving the boundary layer equations. The method proposed is free of laborious grid generation procedures and does not involve difficulties associated with numerical viscosity at high Reynolds numbers.     

Design of a subsonic airfoil with upstream blowing

The problem is solved of designing a symmetric airfoil with upstream blowing opposite to subsonic irrotational steady flow of an inviscid incompressible fluid. The solution relies on Sedov’s idea of a stagnation region developing in the neighborhood of the stagnation point. An iterative solution process is developed, and examples of airfoils are constructed. The numerical results are analyzed, and conclusions are drawn about the effect of blowing parameters on the airfoil geometry and the resultant force acting on the airfoil.     

A two-algebraic turbulence model for compressible flow in turbomachinery cascade

The objective of this study is to examine the Baldwin–Lomax turbulence model in a finite volume solver to introduce a computer code for complex two–dimensional flows in turbomachinery. The turbulent model was tested with investigating the turbulent flow over a flat plate and other test cases. Three test cases are presented and the computed results are compared with experimental data. The calculated velocity profile agreed well with the experimental data in plate test case and the solver is validated in test case of flow over a semi NACA–0012 airfoil. The solver is used for flow through a multi–blade cascade of an axial compressor in design condition to show its capability of multi–block solution. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

风力机叶型增升及叶尖流动控制研究

研究了两种改善风力机叶型气动性能的流动控制技术,分别对风力机专用S809翼型和较大升阻比的FX 60-100翼型进行应用研究．首先,通过在叶型前缘加装流动偏转器,研究流动偏转器对叶型流动分离的控制效果．并采用多岛基因算法,对流动偏转器进行多参数优化．结果表明:流动偏转器可以有效控制叶型的失速特性,推迟失速攻角和增加升力;基因优化算法能更大地提升流动偏转器的控制效果．其次,基于对风力机叶尖旋涡和尾涡特征以及叶片表面压力分布的分析,在叶片尖部加装不同倾斜角的旋涡扩散器控制叶尖涡．结果表明:涡扩散器能够提高叶尖涡涡核的总压,削弱其旋涡强度,使风力机尾流旋涡耗散更快,从而可以减小噪声,提高叶片效率．     

翼涡干扰前缘开孔被动控制数值研究

开孔方法是一种简单的流动被动控制方法.为找到一种有效降低桨涡干扰效应的被动控制方法,以NACA 0012翼型作为研究对象,建立了4种前缘开孔的模型.在不同来流速度、涡的强度和干扰距离条件下,对4种前缘开孔模型和无孔的基准翼型进行了二维平行桨涡干扰（翼涡干扰）数值模拟,对比了升力系数的变化.结果表明:前缘开孔可以降低翼涡干扰效应,但对翼型升力系数有一定的影响;宽度为2.5%弦长的直孔能在翼型升力系数损失较小的情况下有效地降低翼涡干扰效应,且适用范围较广.     

基于线性涡模型的风洞孔壁干扰特性分析

发展了一种适用于二元翼型试验洞壁干扰特性的评估和修正方法.基于Prandtl-Glauert速度势方程和布置在模型及洞壁表面的线性涡,采用迭代方法计算了风洞孔壁对翼型表面压力分布特性的影响,分析了不同孔壁透气特性参数的影响规律和量值,利用与国外参考结果及风洞试验结果的对比确定了该方法的准确性.结果表明,孔壁对翼型绕流的影响主要反映在上翼面吸力峰和最大厚度位置之间,使压力系数减小,积分后的升力系数降低,且随着孔壁透气特性参数的增大,洞壁干扰由实壁特性向开口特性发展,洞壁干扰、影响量急剧增大.与传统方法相比,该方法计算快速,结果可靠,同时具备试验前评估的能力,可用于亚临界范围内翼型表面压力的快速估算,以及翼型试验的洞壁干扰修正.     

二元拉伐尔喷管不可压缩流动精确解

在物理平面上,仔细分析沿拉伐尔喷管中心线和喷管型线的流动,可以发现拉伐尔喷管流动的上下两半部分在速度平面中是两个相同的具有尾缘点前后错开的双尾的裂缝厚翼型。该两个翼型处在不同的黎曼面内。翼型的内部在复位势平面中可转绘成无限长的条带。利用这些结果得到了二元拉伐尔喷管内不可压缩位势流动的精确解。精确解对任意给定的收缩比n₁、扩张比n₂和喉部壁面曲率半径R*都适用。作为应用的举例,给出了一些典型的喷管型线,喷管内的流速分布以及不同瞬间流体质点的所在位置。     

Computational fluid dynamics simulation of aerodynamic performance and flow separation by single element and slatted airfoils under rainfall conditions

This study investigated the effects of rainfall on flow separation and the aerodynamic performance of single element and slatted NACA 0012 airfoils by using a mathematical model developed with the commercial computational fluid dynamics solver ANSYS FLUENT 18.2. A two-way momentum coupled Eulerian–Lagrangian multiphase approach was used to simulate the formation of the water film layer on the airfoil's surface. According to the results, very low values of the lift-to-drag ratio at low angles of attack reflected severe degradation of the aerodynamic performance of the airfoil in the presence of water accumulated on its surface. The impact of rain droplets on the leading-edge slat surface led to less water accumulating on the main section of the airfoil. In particular, the maximum water film mass concentrated on the airfoil surface decreased from 15 g to 1 g compared with the single element airfoil. Hence, the thickness of the water film layer was not sufficiently large to significantly affect the aerodynamic coefficients of the slatted airfoil, especially the maximum lift coefficient, compared with the thicker water film layer on the single element airfoil. In addition, the use of slats clearly enhanced the aerodynamic coefficients and increased the stall angle from 13° to 22° in dry conditions, and from 16° to 24° in rainy conditions. Slats also significantly decreased the boundary layer thickness and delayed the separation at higher angles of attack.     

Design and veri?cation of airfoil families for large-size wind turbine blades?

For the design of wind turbine blades, the use of a family of specially tailored airfoils is particularly important. The dedicated airfoils can dramatically improve the capability of capturing wind power, reduce the structural weight to save the cost of manufacturing and transportation, and lower the inertial loads as well as the loads due to gust. An overview of the world-wide wind turbine airfoil families developed since 1990’s is presented, such as the S series, the DU series, the Ris? series, and the FFA series. The design and wind-tunnel tests of the Northwestern Polytechnical University (NPU) airfoil family for megawatt-size wind turbines, called the NPU-WA series, are summarized. All tests for the NPU-WA series are carried out in the NF-3 low-speed wind-tunnel with a two-dimensional (2D) test section of 1.6 m?0.8 m and at the Reynolds number ranging from 1.6?106 to 5?106 . The research activities for further improving the NPU-WA airfoils towards lower roughness sensitivity are also reviewed. The development of the new NPU-WA series dedicated for multi-megawatt wind turbines is discussed.     

Turbulent Flow with Embedded Vortical Structures Induced by Vortex Generators in a Cascade

Presented work is the next step after several experimental examinations of vortex generator influence on a flow separation occurring on a model of the NACA 63A421 airfoil with deflected simple flap. In this stage of research the vortices produced by vortex generators (VGs) were studied using Particle Image Velocimetry technique (PIV) and numerical simulations. Vane type VGs with two spacings among VGs pairs in straight channel with turbulent flow were tested. The average velocity flow field, peak of vorticity and circulation decay downstream of VGs were evaluated. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

翼型绕流干涉噪声的实验与数值研究

来流湍流干扰噪声在风力机叶片气动总噪声级中占有重要地位．选取圆柱/翼型干涉模型从实验和数值两方面研究此类干涉发声现象．实验中通过对翼型表面非定常载荷的测量,重点研究了圆柱位置和翼型攻角的影响,选取的翼型包括两个NACA系列翼型（NACA0012和NACA0018）和两个风力机翼型（s809和s825）,同时利用PIV（particle image velocimetry）技术对低攻角状态下翼型的前缘流场进行了研究．实验结果表明翼型表面非定常压力与圆柱涡脱落存在一定相关性．与此同时采用非定常Reynolds平均(URANS)方法对圆柱/NACA0012翼型的干涉流场进行了非定常数值模拟,并将得到的翼型表面压力频谱与实验结果进行了对比．     

An assessment of turbulence models applied to the simulation of a two-dimensional submerged jet

This paper is concerned with the investigation of the performance of different turbulence models in the numerical prediction of transient flow caused by a confined submerged jet. Several widely used models, i.e., the standard k–ε, RNG k–ε, low Reynolds number k–ε models and the differential Reynolds stress model, as included in CFD codes, were compared with each other for a two-dimensional, incompressible, turbulent jet flow and with reported experimental data. A flapping oscillation was predicted regardless of the model used. A chosen Strouhal (St) number definition brought the fundamental frequencies from both the experiments and computations into close proximity. However, different turbulence models have exhibited quite different behaviours in terms of the frequency and regularity of the oscillation and in terms of the scale and duration of the vortices generated. All versions of the k–ε model yielded regular oscillations, which agree with experimental observations. On the other hand, the Reynolds stress (RS) model produced a complex pattern but a slower dissipation of vortices. In addition, some aspects of gridding and inflow representation are also discussed.     

Asymptotic behaviour of commutation errors and the divergence of the Reynolds stress tensor near the wall in the turbulent channel flow

The derivation of the space averaged Navier–Stokes equations for the large eddy simulation (LES) of turbulent incompressible flows introduces two groups of terms which do not depend only on the space averaged flow field variables: the divergence of the Reynolds stress tensor and commutation errors. Whereas the former is studied intensively in the literature, the latter terms are usually neglected. This note studies the asymptotic behaviour of these terms for the turbulent channel flow at a wall in the case that the commutation errors arise from the application of a non‐uniform box filter. To perform analytical calculations, the unknown flow field is modelled by a wall law (Reichardt law and 1/αth power law) for the mean velocity profile and highly oscillating functions model the turbulent fluctuations. The asymptotics show that near the wall, the commutation errors are at least as important as the divergence of the Reynolds stress tensor. Copyright © 2006 John Wiley & Sons, Ltd.     

Investigations of shear free turbulent diffusion in a rotating frame

We reconsider the problem of shear free turbulent diffusion in a rotating frame, rotating about x₁. Shear free turbulence is generated at a vibrating grid in the x₂–x₃ plane and diffuses away from the grid in x₁ direction. An important property of this flow case is that there is no mean flow‐velocity. With the help of Lie‐group methods Reynolds‐stress transport models can be analyzed for this kind of flow in a rotating frame. From the analysis it can be found, that the turbulent diffusion only influences a finite domain. Implicating this solution in the model equations shows that even fully nonlinear Reynolds‐stress transport models (non‐linear in the Reynolds‐stresses for the pressure‐strain model) are insensitive to rotation for this type of flow. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)