Aerodynamic drag of bodies in transonic flow. Theory and applications to computational aerodynamics

Formulas for all the components of the aerodynamic drag (total, friction, inductive, wave, pressure, and heat-transfer) are uniformly derived as applied to flows governed by the Navier-Stokes and Reynolds equations. For flows of this type the definition of the aerodynamic drag components is refined and the physical basis of the chosen method of breaking up the total drag into components is discussed. Ways of calculating the aerodynamic drag components using the methods of computational aerodynamics are considered. On the basis of the refined formulas the drag components are calculated for flows around airfoils and a high-aspect-ratio wing in transonic flow.     

AERODYNAMIC OPTIMIZATION OF HIGH-SPEED TRAIN BASED ON RBF MESH DEFORMATION

基于局部型函数三维参数化方法、改进的蚁群算法和改进的克里金（Kriging）代理模型,开展了列车头型的三维气动减阻优化设计研究。为了避免复杂几何外形大变形情况下千万量级网格的重复生成,提高高速列车头型优化设计的效率,引入了缩减控制点的径向基函数网格变形技术。优化结果表明：径向基函数网格变形技术在不降低网格质量的情况下可以有效缩短网格变形的时间消耗,能够用于复杂几何外形的气动优化设计;在给定的设计空间内,控制鼻锥外形的6个关键设计参数对列车气动阻力的影响呈单调递增关系;优化后,在满足约束条件的情况下,简化外形列车的整车气动阻力减小5.41%,头尾车减阻效果明显,中间车气动阻力基本不变。     

Experimental investigation on the flow around a simplified geometry of automotive engine compartment

In the current sustainable development context, car manufacturers have to keep doing efforts to reduce the aerodynamic drag of automotive vehicle in order to decrease their CO₂ and greenhouse gas emissions. The cooling airflow, through the engine compartment of vehicles, contributes from 5 to 10% to the total aerodynamic drag. By means of simplified car geometry, equipped with an engine compartment, the configurations that favor a low contribution to total drag are identified. PIV (particle image velocimetry) velocity measurements in the wake of the geometry allow explaining these drag reductions. Besides, the cooling flow rate is also assessed and gives indications on the configurations that favor the engine cooling.     

Aerodynamic mechanism of forces generated by twisting model-wing in bat flapping flight

The aerodynamic mechanism of the bat wing membrane Mong the lateral border of its body is studied. The twist-morphing that alters the angle of attack （AOA） along the span-wise direction is observed widely during bat flapping flight. An assumption is made that the linearly distributed AOA is along the span-wise direction. The plate with the aspect ratio of 3 is used to model a bat wing. A three-dimensional （3D） unsteady panel method is used to predict the aerodynamic forces generated by the flapping plate with leading edge separation. It is found that, relative to the rigid wing flapping, twisting motion can increase the averaged lift by as much as 25% and produce thrust instead of drag. Furthermore, the aerodynamic forces （lift/drag） generated by a twisting plate-wing are similar to those of a pitching rigid-wing, meaning that the twisting in bat flight has the same function as the supination/pronation motion in insect flight.     

等离子体减阻技术的研究进展

利用等离子体的特性减小飞行器的气动阻力是一种新概念的减阻思路.目前国外的一些风洞实验和计算结果已证明了等离子体减阻效果的显著性.本文介绍了国外等离子体减阻技术的研究进展,包括:逆向等离子体喷流减阻、基于等离子体边界层控制减阻和局部施加能量点源减阻.分析了等离子体减阻的基本原理及所涉及的一些关键技术点.最后针对我国的减阻研究情况,提出了等离子体减阻技术的研究方向.      

NUMERICAL STUDY OF BASE BLEED EFFECTS ON AERODYNAMIC DRAG FOR A TRANSONIC PROJECTILE

ABSTRACT

A numerical study is made to analyze the performance of a secant-ogive-cylinder projectile in the transonic regime in terms of aerodynamic drag. At transonic speeds, the base drag contributes a major portion of the total aerodynamic drag, and hence affects projectile's performances significantly. The base bleed method is applied to reduce the base drag by varying the value of parameters, the bleed quantity (I) and the bleed area ratio (?). The implicit, diagonalized, symmetric Total Variation Diminishing (TVD) scheme, accompanied by a suitable grid, is employed to solve the thin-layer axisymmetric Navier-Stokes equations coupled with the Baldwin-Lomax turbulence model. The computed results show that, in comparison with the case without base bleed, an increase in bleed quantity or a higher injection speed due to a smaller bleed area ratio at fixed bleed quantity can result in a base (and total) drag reduction. At Mach number 0.96, the reductions in base drag and total drag can be as high as 64% and 44%, respectively, for I = 0.1 and ? = 0.3.     

汽车外流场数值仿真中的湍流模型分析与应用

采用较为合理的计算流体力学分析技术对汽车高速行驶时其外部气动特 性进行研究. 以带有轮胎、侧后视镜以及天线等突出部件的某轿车外流场模型为研究对象, 在稳态下采用RNS法中Realizable $k$-$\varepsilon$两方程湍流模型结合增强壁 面函数法对不同车速工况下整车外流场进行数值仿真计算. 研究表明：沿车辆X纵 向上所形成压力差是产生气动阻力的主要原因; 随车速的提高, 气动阻力 的增长速度快于气动升力的增长.     

Effect of vortex shedding in unsteady aerodynamic forces for a low Reynolds number stationary wing at low angle of attack

This study elucidates the relation between wake vortex shedding and aerodynamic force fluctuations for a low Reynolds number wing from time resolved particle image velocimetry (TR-PIV) experimental measurements. The results reveal a periodic lift and drag variation within the shedding cycle and resolve the frequencies of those fluctuations from a proper orthogonal decomposition (POD) and power spectral density (PSD) analysis. To show the effect of vortex shedding on the body force fluctuations, the evolution of instantaneous aerodynamic forces is compared to the pressure field of the fluid flow and to the vortical structures in the wake of the airfoil. A six step model describing the vortex-force relation is proposed. It shows that changes in lift such as maximum lift and minimum lift are associated with the detachment of a vortex. It also shows that the minimum or local minimum drag value is obtained at the onset formation of a vortex on the airfoil wake. Similarly, the maximum or local maximum drag is obtained at the onset formation of the saddle on the airfoil wake. The model further explains the asymmetry observed in the unsteady drag force evolution. The model can be used to optimize flow control and fluid-structure interaction applications.     

Optimization of 3D wings based on Navier-Stokes solutions and genetic algorithms

The problem of the aerodynamic shape optimization to minimum drag, subject to geometrical and aerodynamic constraints, is considered. An accurate and computationally efficient approach to the multiobjective constrained design of 3D aerodynamic wings is proposed. The optimization is driven by full Navier-Stokes computations and Genetic Algorithms (GAs). The verification results include a variety of optimization cases for a classical test-case of ONERA M6 wing in transonic flight conditions. The method allows to significantly reduce the total drag of optimized wings, while exhibiting high robustness and keeping CFD computational volume to an acceptable level.     

Numerical modelling of the entrainment of particles in inviscid supersonic flow

The interaction between particles situated in close proximity and moving at supersonic speeds is investigated computationally. The simplest case of the motion of a single particle travelling behind a lead particle is used to elucidate the role of aerodynamic forces in the motion of a group of particles. The effect of the following parameters on the drag and lift forces acting on each of two particles of equal diameter in proximity is investigated: the free-stream Mach number, and the axial and lateral displacements of the trailing particle. The two-dimensional flow field is numerically simulated using an unsteady Euler CFD code to find the steady-state drag and lift coefficients for both particles. Three static zones of aerodynamic influence in the wake of the lead particle are identified, which are denoted as the entrainment, lateral attraction, and ejection zones. A non-dimensional representation of the zones of influence is given. It is shown that the dynamic entrainment of particles can occur even when the path of the trailing particle originates outside the entrainment and lateral attraction zones.     

褶皱幅度对仿生翼型滑翔气动特性的影响

蜻蜓翅膀具有独特的褶皱状形貌．研究者们致力于利用仿生学原理,设计在低雷诺数条件下具有更优气动性能的褶皱翼型．本文采用计算流体力学方法,求解二维不可压Navier-Stokes方程组,探讨了四种翼型（平板翼型、流线翼型、小幅度褶皱翼型和大幅度褶皱翼型）的气动表现．在低雷诺数条件下得到以下结果：(1) 较小幅度的褶皱结构有利于增加升力和减小阻力．(2) 雷诺数变化时褶皱翼型的升力系数呈非线性变化;在特定雷诺数区间,幅度相近的褶皱翼型会发生相对气动优势的转变．(3) 褶皱结构内的回流区通过减小粘性阻力,使得翼型总阻力下降．(4) 翼型前缘的极小区域会产生脉冲高升力,对升力表现产生较大影响．这些结果表明,调整褶皱幅度是实现褶皱翼型气动优化的有效方案．     

Numerical evaluation of passive control of shock wave/boundary layer interaction on NACA0012 airfoil using jagged wall

Shock formation due to flow compressibility and its interaction with boundary layers has adverse effects on aerodynamic characteristics, such as drag increase and flow separation. The objective of this paper is to appraise the prac-ticability of weakening shock waves and, hence, reducing the wave drag in transonic flight regime using a two-dimensional jagged wall and thereby to gain an appropriate jagged wall shape for future empirical study. Different shapes of the jagged wall, including rectangular, circular, and triangular shapes, were employed. The numerical method was validated by experimental and numerical studies involving transonic flow over the NACA0012 airfoil, and the results presented here closely match previous experimental and numerical results. The impact of parameters, including shape and the length-to-spacing ratio of a jagged wall, was studied on aerodynamic forces and flow field. The results revealed that applying a jagged wall method on the upper surface of an airfoil changes the shock structure significantly and disinte-grates it, which in turn leads to a decrease in wave drag. It was also found that the maximum drag coefficient decrease of around 17%occurs with a triangular shape, while the max-imum increase in aerodynamic efficiency (lift-to-drag ratio) of around 10%happens with a rectangular shape at an angle of attack of 2.26?.     

A novel MAV with treadmill motion of wing

The Magnus effect is well known phenomena for producing high lift values from spinning symmetrical geometries such as cylinders, spheres, or disks. But, the Magnus force may also be produced by treadmill motion of aerodynamic bodies. To accomplish this, the skin of aerodynamic bodies may circulate with a constant circumferential speed. Here, a novel wing with treadmill motion of skin is introduced which may generate lift at zero air speeds. The new wing may lead to micro aerial vehicle configurations for vertical take-off or landing. To prove the concept, the NACA0015 aerofoil section with circulating skin is computationally investigated. Two cases of stationary air and moving air are studied. It is observed that lift can be generated in stationary air although drag force is also high. For moving air, the lift and drag forces may be adopted between the incidence angles 20° to 25° where lift can posses high values and drag can remain moderate.     

高速列车头型长细比对气动噪声的影响

高速列车的头尾车外形对气动噪声具有重要的影响.工程实践中随着车速的增加,车辆头部越来越细长,日本高速磁悬浮列车实践中甚至出现了具有极端长细比的头部形状.本文以讨论头型长细比对列车气动噪声的影响规律为出发点,应用非线性声学求解器(NLAS)和FW-H声学比拟法的混合算法,在3种运行速度下对基于CRH380A高速列车头型概化的4种不同头型长细比的模型车的气动噪声进行了数值模拟.给出了不同头型长细比列车的流场特征、气动阻力和气动噪声.结果表明,列车的气动总阻力随头型长细比的增大而减小,且头型长细比对列车总气动阻力的影响随运行速度的增加而增强.而头型长细比对气动噪声的影响呈现出较为复杂的影响,并不存在单调的影响关系;综合考虑气动阻力和气动噪声,长细比最大的头型综合性能较优,但差异并不显著,因此在不考虑微气压波等因素的条件下,简单增加车头长细比并不一定能带来明显的气动噪声性能提升.     

Simulation of the aerodynamic drag of three-dimensional star-shaped bodies at hypersonic velocities

The domain of the parameters in which the aerodynamic drag of hypersonic pyramidal bodies, whose wave component is calculated within the framework of conical flows with the boundary layer displacement thickness taken into account, agrees satisfactorily with the experimental data is found. The calculation model is also applicable in the region of minimum aerodynamic drag of star-shaped bodies in the class of conical bodies equivalent in length and mid-sectional area.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 5, pp. 69–79, September–October, 1996.     

Auto-aerodynamic drag-force analysis

This paper describes an inexpensive experimental method of evaluating the aerodynamic drag forces encountered by automobiles under actual roadtraveling conditions. The data are directly applicable to stress, power and efficiency analyses. The procedure consists of applying Newton's second law,F=Ma, to a freely decelerating vehicle to obtain the opposing force-velocity relationship. By greatly increasing tire-inflation pressure, the rolling resistance is made constant. The remaining aerodynamic drag force is found to vary directly with the square of velocity, allowing the theoretical drag coefficients to be calculated. An illustrated auto-aerodynamic drag study of a recent production vehicle indicates that significant improvements in efficiency can be achieved by simple streamlining of the leading body surfaces.     

Drag measurement and dynamic simulation of Martian wind-driven sensor platform concepts

The purpose of this work is to (a) determine the drag coefficient of three wind-driven systems (referred to as tumbleweeds) in a simulated Martian atmospheric boundary layer; two concepts from NASA Langley (LaRC) and one from Texas Tech University (TTU), and (b) perform a dynamic analysis of the TTU tumbleweed to establish the feasibility of operation in a simulated Martian environment. The TTU Wind Tunnel is used in order to determine the drag coefficient for the tumbleweeds in both the aerodynamic and atmospheric boundary layer (ABL) test sections. A comparison of the two mean drag coefficients for each tumbleweed model reveals the extent to which an ABL affects drag on the models. It appears that no transformation exists that can be used to transform aerodynamic-based drag coefficients into boundary-layer-based drag coefficients; therefore, reliance upon ABL tests is important. It is generally accepted that a complete ABL test under conditions of neutral atmospheric stability requires knowledge of the incoming (approach) mean velocity and turbulent intensity profile, spectral distribution, roughness height, and Reynolds number. Given the fact that limited data exists for the Martian flows, physical simulations of an atmospheric surface layer with knowledge of the mean velocity and general turbulence characteristics was developed in order to obtain drag coefficients for several tumbleweed wind platform designs. The tumbleweeds drag coefficients were effectively constant with the boundary layer affected coefficient less than the respective aerodynamically obtained coefficient. Of particular interest in this study is the TTU tumbleweed, which underwent extensive testing in order to obtain a force function to describe its aerodynamic characteristics in any orientation relative to the wind.     

顺桨风力机塔叶干涉翼型非稳气动力时频特性

由于风力机叶片与塔筒流场相互干涉,实际气动力与理想情况存在较大差异,这种干涉作用造成的气动力差异给叶片与塔筒结构可靠性带来不可忽视的影响.以翼型DU91-W2-250为研究对象,采用瞬态数值分析与本征正交分解方法,考虑叶片和塔筒流场相互干涉作用,分析顺桨工况翼型非稳气动力时频特性及其影响规律,量化不同雷诺数下塔叶相对位置及几何参数对气动力均值、波动幅度和频率的影响程度,通过流场模态能量分布形态分析,揭示流场干涉对气动力的影响机制.结果表明,翼型气动中心至塔筒几何中心的垂直距离、水平距离以及塔筒直径相对于翼型弦长的无量纲参数y*,x*和D*对气动力均有不同程度影响,其中y*对升阻力系数均值影响最大,对频率无明显影响,y*绝对值越大,C_l均值越接近单翼型C_l值,y*绝对值越小升阻力系数波动幅度越大,y*从-12增大到12,升力系数均值最小值为-0.48,最大值为1.16;x*减小和D*增大,反向阻力均值增大,波动幅度增大,波动频率略有下降,当x*小于临界值5时,带塔翼型阻力均值反向;在计算范围内,带塔翼型升力系数均值相对于单翼型升力系数最大偏差为...     

Particle and drop dynamics in the flow behind a shock wave

The results of an investigation of the dynamics of hard particles and liquid drops in the flow behind a transmitted shock wave are presented. From the equation of motion of a particle in the shock wave, relations for the displacement, velocity and acceleration as functions of time and certain velocity-relaxation parameters taking into account the properties of the gas and the aerodynamic drag of the particles are obtained for unsteady flow around the particles at an acceleration of 10³–10⁴ m/s². It is shown that the velocity-relaxation parameters are universal. Approaches to finding the aerodynamic drag of freely-accelerating bodies from the dynamics of their acceleration after being suddenly exposed to the flow are considered. It is established that under these conditions the drop dynamics observed can be well described in terms of the same velocity-relaxation parameters with account for linear growth of the transverse drop size. All the kinematic functions obtained are confirmed experimentally.     

高亚声速运输机后体减阻措施研究

为了降低高亚声速运输机的阻力,改善飞机的气动性能,本文在某一典型运输机机身外形的基础上,研究了后体参数和涡流发生器对机身后体流场及阻力的影响。通过数值模拟计算,得出了不同外形的后体流态分布及阻力特性数据。结果表明,小的上翘角和收缩比、大的扁平度和长细比能够显著地减小压差阻力,而涡流发生器能够有效地减小摩擦阻力。该结果可为后续的型号研制提供参考。