振荡射流改善翼型气动性能的实验研究

本文针对采用振荡射流控制流动分离改善大攻角下翼型气动特性的问题,在NACA0015翼型上进行了多种工况的风洞实验。结果表明:在失速攻角附近,振荡射流抑制流动分离提高翼型升力系数的作用十分明显,可将翼型失速攻角推迟2°左右。存在最佳的振荡射流频率段、射流动量范围和射流位置,使得翼型性能的改善最大。实验还得到了振荡射流的频率、动量和施加位置等参数对翼型气动性能的影响规律。     

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.     

Fourier time spectral method for subsonic and transonic flows

The time accuracy of the exponentially accu-rate Fourier time spectral method (TSM) is examined and compared with a conventional 2nd-order backward differ-ence formula (BDF) method for periodic unsteady flows. In particular, detailed error analysis based on numerical com-putations is performed on the accuracy of resolving the local pressure coefficient and global integrated force coefficients for smooth subsonic and non-smooth transonic flows with moving shock waves on a pitching airfoil. For smooth sub-sonic flows, the Fourier TSM method offers a significant accuracy advantage over the BDF method for the predic-tion of both the local pressure coefficient and integrated force coefficients. For transonic flows where the motion of the discontinuous shock wave contributes significant higher-order harmonic contents to the local pressure fluctuations, a sufficient number of modes must be included before the Fourier TSM provides an advantage over the BDF method. The Fourier TSM, however, still offers better accuracy than the BDF method for integrated force coefficients even for transonic flows. A problem of non-symmetric solutions for symmetric periodic flows due to the use of odd numbers of intervals is uncovered and analyzed. A frequency-searching method is proposed for problems where the frequency is not known a priori. The method is tested on the vortex shedding problem of the flow over a circular cylinder.     

合成射流对失速状态下翼型大分离流动控制的试验研究

为研究低速状态合成射流在抑制翼型气流分离和推迟失速方面的控制机理, 开展了NACA0021 翼型失速特性射流控制的风洞试验研究. 通过系统性的模型测力、翼型瞬态流场粒子图像测速和边界层速度测定的对比试验, 深入探索了合成射流各参数对翼型失速特性控制效果的影响规律. 试验结果表明射流偏角在翼型升力和失速迎角控制方面的效果对射流动量系数较为敏感: 当动量系数较大时, 近切向射流的控制效果更好. 射流动量系数为0.033 时, 偏角30°的射流使得翼型升力系数峰值提高23.56%, 失速迎角增大5°; 而动量系数较小时, 偏角较大的射流能够获得最佳控制效果. 射流动量系数为0.0026 时, 法向射流对翼型最大升力系数控制效果最好(提高9.2%).      

Inflow broadband noise from an isolated symmetric airfoil interacting with incident turbulence

Inflow noise from a symmetric airfoil interacting with homogeneous and isotropic turbulence is investigated focusing on the effects of airfoil geometry. The numerical method employed is based on computational aeroacoustic techniques using the high-order dispersion-relation-preserving finite-difference schemes for solving two-dimensional linearized Euler equations. Effects on inflow noise of the airfoil thickness, leading-edge radius, and freestream Mach number are examined by comparing the acoustic power spectrum of the airfoils and their flow field characteristics. Acoustic power levels of airfoils are found to exponentially decrease in the high-frequency range as airfoil thickness increases because incident turbulent velocities are more distorted in the larger stagnation region near the leading edge. This distortion is shown to be related to the slope angle of the streamline of steady mean flow near the leading edge. However, this high-frequency reduction weakens as the Mach number increases due to the decreasing slope angle. In addition, the chordwise velocity component in the incident turbulence contributes more to the radiating acoustic pressure level as the freestream Mach number increases, which also results in less high-frequency reduction at higher freestream Mach number. At fixed airfoil thickness, increasing the leading-edge radius leads to decreases in the acoustic power level, which may also be explained by size variation of the stagnation region around the leading edge. An approximate algebraic formula for acoustic power spectra is derived on the basis of these observations. Acoustic power spectra predicted using this formula are shown to closely follow the numerical results. Finally, the applicability of the algebraic formula and the current numerical methods to more realistic problems are confirmed by comparing their predictions with the measured data.     

横摆振荡翼型动态气动掠效应试验研究

大型风力机设计对获取翼型更加全面、准确的动态载荷提出更高要求, 研究翼型横摆振荡动态气动特性具有重要意义. 借助"电子凸轮"技术和动态数据同步采集手段, 针对翼型动态“掠效应”首次开展了横摆振荡风洞试验研究, 研究表明: 横摆振荡翼型的气动曲线存在明显迟滞效应, 吸力面压力周期性波动是主要诱因, 且随着振荡频率、初始迎角和振幅的增大, 气动迟滞特性均增强; 升力和压差阻力随横摆角变化的迟滞回线呈"W"形, 俯仰力矩迟滞回线呈"M"形, 升力差量迟滞回线呈"$\infty$"形; 负行程下翼型气动力相对于正行程下的更高, 且负行程下翼型气动力随振荡频率的增大而略有增大, 正行程下则明显减小; 升力系数功率谱密度分布在振荡频率倍频处的能量集中的幅值随着振荡频率增大有增大趋势; 吸力面1.2%和40%弦长处压力的滞回特性较强, 是由于翼面剪切层涡和动态分离涡周期性发展、运动、破裂和重建; 振幅为$10^{\circ}$时, 升力迟滞曲线呈"$^{\wedge}$"形, 振幅为$30^{\circ}$ 时, 升力迟滞曲线呈"$^{\wedge\wedge\wedge}$"形.      

满足几何守恒律的WENO格式及其应用

对几何守恒律的来源进行了分析,发展了一种满足几何守恒律的WENO格式,并应用于翼型层流分离现象的数值模拟中。为消除网格质量影响,采用守恒型方法计算网格导数,并将标准的WENO格式分解为中心差分部分和数值耗散部分。算例计算结果表明,几何守恒律对高精度有限差分WENO格式计算结果具有重要影响,本文方法能够消除网格导数计算误差,保证来流保持性。将本文方法应用于SD7003翼型层流分离现象的数值模拟中,计算结果与文献中计算及试验数据吻合较好,同时能够精细捕捉小尺度流场结构,准确模拟翼型层流分离现象中的复杂流动过程。     

基于几何不确定性的翼型多目标稳健优化设计

提出在优化设计进程中引进基于各种不确定性波动的稳健优化设计思想, 进行多目标进化优化算法与代理模型技术在稳健优化设计中的应用研究. 提供翼型确定性优化和稳健性优化实例, 并对结果进行对比, 结果表明该稳健优化设计方法可以得到更有实际应用价值的翼型气动外形.      

Numerical discussions on complete empirical correlation in Langtry's transition model

Recently, introducing a transition predicting model into the Reynolds averaged Navier-Stokes (RANS) environment has been paid more and more attention. Langtry proposed a correlation-based transition model in 2006, which was built strictly on local variables. However, two core correlations in the model had not been published until 2009. In this paper, after considerable analyses and discussions of the mechanism of this transition model and a series of numerical validations in the skin friction coefficient of flat plate boundary layers, a new correlation based on free-stream turbulence intensity is developed, and the empirical correlation of the transition onset momentum thickness Reynold number aiming at the hypersonic transition is improved. Low-speed/transonic airfoils and a hypersonic double wedge flat are tested to prove the reliability and practicability of this correlation.     

Numerical discussions on complete empirical correlation in Langtry＇s transition model

Recently, introducing a transition predicting model into the Reynolds averaged Navier-Stokes （RANS） environment has been paid more and more attention. Langtry proposed a correlation-based transition model in 2006, which was built strictly on local variables. However, two core correlations in the model had not been published until 2009. In this paper, after considerable analyses and discussions of the mechanism of this transition model and a series of numerical validations in the skin friction coefficient of flat plate boundary layers, a new correlation based on free-stream turbulence intensity is developed, and the empirical correlation of the transition onset momentum thickness Reynold number aiming at the hypersonic transition is improved. Low-speed/transonic airfoils and a hypersonic double wedge fiat are tested to prove the reliability and practicability of this correlation.