基于iSIGHT平台翼型气动优化CAD/CFD集成技术研究

传统的翼型设计中,CFD大多用来检验设计结果,而不是驱动翼型优化设计。两者基本上处于孤岛状态,没有实现有机的集成。为使CFD更好的应用到翼型优化设计,实现翼型设计/仿真的协同优化设计,需要针对现有的仿真分析软件和设计模型,利用集成优化平台进行流程集成和数据集成。采用集成优化平台iSIGHT实现了翼型参数化程序、UG建模软件、CFD计算分析软件Fluent之间的数据集成与过程集成,从而为基于翼型的多学科协同优化研究打下了基础。     

Unsteady thin airfoil theory revisited for a general deforming airfoil

This paper examines the influence of negative work on energy required for the flapping wing flight of birds, or other flapping wing animals or vehicles. Because negative energy is typically required by muscles or actuators to produce negative work, it must be accounted for when determining the most efficient flapping configuration. The present work provides a simple theoretical analysis for determining the influence of negative work by introducing a simple actuator/muscle model, which specifies the amount of input energy required to produce negative or positive work. The influence of aerodynamic, structural, and inertial forces are treated in the study. The aerodynamic forces are modeled using unsteady thin airfoil theory, which is appropriate for the relatively high aspect ratio wings of most birds. The influence of springs on a flapping system are discussed, and theoretical approach to determine the spring stiffness required to minimize negative work is presented. The developed analysis is applied to a Pigeon and a Pied Flycatcher. It is found that the flight speed requiring minimum muscle energy is dependent upon the energy cost of negative work.     

On the development of airfoil section inspection and analysis technique

In blade inspection, the measurement and analysis of airfoil section parameters and geometric tolerance are very important but complicated. The aim of this study was to apply coordinate measurement techniques to blade sections measurement and to develop methods for the evaluation of dimensions and geometric tolerance. Due to significant variation in the blade shape, especially near the upper sections, the leading edge and the trailing edge, the measurement paths based on the CAD model may lead to incorrect results. Thus, a two-step measurement procedure, including a rough measurement and a fine measurement, along with the calculation of the back-off directions was proposed to improve the measurement accuracy. In blade profiles analysis, several algorithms, including a best-fit algorithm, blade parameters computation and profile and tolerance zone analysis, were used to evaluate section parameters and geometric tolerance. Successful experimental results have demonstrated the feasibility of the proposed method.     

Effect of the Gurney flap on a NACA 23012 airfoil

A numerical investigation was performed to determine the effect of the Gurney flap on a NACA 23012 airfoil. A Navier-Stokes code, RAMPANT, was used to calculate the flow field about the airfoil. Fully-turbulent results were obtained using the standardk-ε two-equation turbulence model. The numerical solutions showed that the Gurney flap increased both lift and drag. These results suggested that the Gurney flap served to increase the effective camber of the airfoil. The Gurney flap provided a significant increase in the lift-to-drag ratio relatively at low angle of attack and for high lift coefficient. It turned out that 0.6% chord size of flap was the best. The numerical results exhibited detailed flow structures at the trailing edge and provided a possible explanation for the increased aerodynamic performance.     

Separation blockage-correction method for the airfoil of a wind turbine blade

A new blockage-correction method for the separated flows around the airfoil of a wind turbine blade was developed for the wall interference correction of the closed test-section wind tunnel. A wind tunnel test was performed for the airfoil at an angle-of-attack range of 0–180°. The freestream velocity was 15 m/s, which corresponds to a Reynolds number of 2.3 × 10⁵ based on the chord. Then a blockage correction for the separated flows was obtained with respect to the multiplication of the blocking area and the separation drag coefficient based on the test. The present method was validated by comparing the corrected results with those of the existing classical and measured-boundary-condition methods. The results of the classical method are similar to those of the measured-boundary-condition method at the attached flow region; however, at high angles of attack, the difference in the corrected results between the classical and MBC methods becomes significantly large. The present method is in good agreement with the measured-boundary-condition method, enabling better wall corrections than the classical method in post-stall region.     

赛车空气动力学套件设计与气动特性研究

FSAE赛车的空气动力学特性对其动力性、燃油经济性、操纵稳定性及制动性有重要影响.文中依据大赛规则设计了FSAE赛车空气动力学套件,基于CFD方法对比分析了3种不同参数的尾翼组合方案的气动特性,确定了最优的尾翼组合方案,根据最优方案的尾翼参数对前翼的参数进行了匹配;研究了有、无空气动力学套件的赛车的空气动力学特性,安装空气动力学套件后,赛车的气动升力系数由0.28下降到-1.19,气动阻力系数由0.44变为0.68,下压力显著增加;基于MATLAB对有、无空气动力学套件的赛车在特征赛道上的性能进行了计算,结果表明:赛车加装空气动力学套件后有效提高了过弯速度,推迟了刹车点,提高了圈速,有效提高了赛车的操纵稳定性和动力性.     

Aeroacoustic simulations of a blunt trailing-edge wind turbine airfoil

We demonstrate the effectiveness of semi-empirical Brooks, Pope and Marcolini model and hybrid large eddy simulations in calculating the blunt trailing edge wind turbine noise at higher Reynolds number conditions. The 4 million element meshes of sharp and blunt trailing edge airfoils were tested at a Reynolds number of 3.2 million and an angle of attack of 4 degrees. The predicted airfoil self-noise by the modified semi-empirical formula with a low frequency directivity function and an additional term for large thickness ratio was compared to the experiments. The sound pressure level spectra from the hybrid large eddy simulation show that the predictions agree well with experimental measurements at the same observer location in the peak frequencies of the blunt trailing edge noise and sound pressure level rates of change at lower frequencies are also similar to experiments. The modified semi-empirical formula and the hybrid large eddy simulation can be considered as promising tools for high vorticity flow problems, especially for flatback airfoils for use on large wind turbines.     

夹具中接触力的计算

以无摩擦的点接触假设为前提,建立夹具中工件与定位元件和夹紧元件之间接触力计算模型,并针对欠定位、完全定位和过定位等不同的定位模式,讨论接触力的计算方法问题。利用提出的接触力计算模型可以检验工件夹紧的封闭性,并为定位元件和夹紧元件的结构设计提供理论指导。最后,给出一个事例验证导出的计算模型。     

导流罩对厢式货车空气阻力特性的影响

为了明确厢式货车导流罩的减阻机理,利用数值模拟的方法对厢式货车原形以及加装空心和实心导流罩后货车外部流场进行计算.分析了导流罩高度、导流罩与厢体间距离对货车阻力系数的影响.证实了在同等条件下,空心薄壁导流罩与实心导流罩相比减阻效果更好.通过计算分析,合理匹配导流罩高度和导流罩与厢体间距离,可以使厢式货车阻力系数进一步减小.     

On the calculation of autoresonance vibroimpact systems perturbed by random forces

The investigation into autoresonance vibroimpact systems is continued. As an example, the system containing the sensor for impact pulses is selected. The main purpose of the study is to the calculation procedure for systems in the presence of small random perturbations. The corresponding calculation procedure based on the determination of fluctuation corrections obtained by the averaging method is given. Examples of calculations for two types of perturbations, namely, small random forces of the white noise type and the forces with a short correlation time are presented. The solutions have a clear mechanical interpretation.     

风轮叶片翼型气动特性分析水

应用CFD流体力学软件对风力机叶片常用翼型FFA-W3-211进行数值分析,得出了其升力系数、阻力系数、升阻比以及翼型表面压力随来流攻角变化关系,并依据计算结果对FFA-W3-211翼型的气动性能进行分析.     

On modeling of cutting forces in micro-end milling operation

Micro-end milling is used for manufacturing of complex miniaturized components precisely in wide range of materials. It is important to predict cutting forces accurately as it plays vital role in controlling tool and workpiece deflections as well as tool wear and breakage. The present study attempts to incorporate process characteristics such as edge radius of cutting tool, effective rake and clearance angles, minimum chip thickness, and elastic recovery of work material collectively while predicting cutting forces using mechanistic model. To incorporate these process characteristics effectively, it is proposed to divide cutting zone into two regions: shearing- and ploughing-dominant regions. The methodology estimates cutting forces in each partitioned zone separately and then combines the same to obtain total cutting force at a given cutter rotation angle. The results of proposed model are validated by performing machining experiments over a wide range of cutting conditions. The paper also highlights the importance of incorporating elastic recovery of work material and effective rake and clearance angle while predicting cutting forces. It has been observed that the proposed methodology predicts the magnitude and profile of cutting forces accurately for micro-end milling operation.     

Axial forces of a V-belt CVT

A simple analytical formula is derived for the axial forces of a V-belt CVT drive. For the driver pulley axial force, belt tension is assumed to be constant by self-locking. For the driven pulley axial force, the whole arc of contact between the belt and the pulley is divided into two regions; (1) inactive and (2) active area. The sliding angle ø for the belt friction is approximated as ø=0 in the active area. It is found that the results of the new formula are in close agreement with the exact solution. The new formula is suggested for the V-belt CVT design due to its simplicity and sufficient accuracy.     

Measurement of unsteady transition on a pitching airfoil using dynamic pressure sensors

Unsteady boundary layer transition on a pitching OA209 airfoil in a wind tunnel was detected by using pressure fluctuation measurement at different oscillation frequency. Thirty Kulite dynamic pressure transducers flush-mounted on the airfoil surface recorded pressure signatures, and root mean square of pressure signatures were calculated. Results indicated that the criterion of transition for static airfoil defined as the peak of root mean square of pressure fluctuation was still suitable for detection of transition on a pitching airfoil. Fixed transition experiment for pitching airfoil was performed to validate the conclusion. Effect of oscillation frequency on transition was investigated. For small reduced frequency, the hysteresis loop is larger near leading edge. With increasing in the oscillation frequencies, the transition was promoted and relaminarization was enhanced.

     

Flow past airfoil moving reciprocally in a channel by vortex method

The velocity and pressure fields of a ship’s propulsion mechanism of the Weis-Fogh type, in which a airfoil moves reciprocally in a channel, are studied in this paper using the advanced vortex method. The airfoil and the channel are approximated by a finite number of source and vortex panels, and the free vortices are introduced from the body surfaces. The viscous diffusion of fluid is represented using the core-spreading model to the discrete vortices. The velocity is calculated on the basis of the generalized Biot-Savart law and the pressure field is calculated from integrating the equation given by the instantaneous velocity and vorticity fields. Two-dimensional unsteady viscose flows of this propulsion mechanism are numerically clarified, and the calculated results agree well with the experimental ones.     

Unsteady boundary layer for a pitching airfoil at low Reynolds numbers

An experimental study was conducted in order to investigate unsteady boundary layers for a pitching airfoil. An NACA0012 airfoil sinusoid-pitched at quarter chord was employed, and its mean angle-of-attack and oscillation amplitude were 0° and 6°, respectively. To explore the unsteady boundary layers, smoke-wire visualization and surface-mounted probe measurements were pursued for three different cases, varying with Reynolds numbers (Re_c=2.3×10⁴, 3.3×10⁴, and 4.8×10⁴). A reduced frequency of 0.1 was identically set in all cases. Results show that in the presented Reynolds number range, the separation bubble dependent on both angle-of-attack and Reynolds number was observed, accompanied with unsteady laminar separation after reattachment. The unsteady laminar separation occurred at the saddle point, which was formed by the two vortices, the wall, and the external flow, and it was independent of reverse flow. This result indicates that the unsteady laminar separation occurs during the process of transition after the reattachment of separated boundary layer for an unsteady flow. The reverse flow observed over the trailing edge significantly interacted with the trailing edge vortex that rotates in the streamwise direction. This trailing edge vortex prevents the uppermost of the reverse flow from reaching to the unsteady laminar separation point during the upstroke, and this induces that the boundary layer breakdown does not occur in spite of the occurrence of laminar separation. The discrete vortices are formed by unsteady laminar separation, and its formation is ultimately affected by the Reynolds number. Consequently, it is obvious that the unsteady boundary layers are ultimately sensitive to Reynolds number in a low Reynolds number regime.     

Ultra-rapid freezing by spraying/plunging: pre-cooling in the cold gaseous layer

A thermophysical model is established to analyse the influence of pre-cooling of a biological specimen in the cold gas layer associated with spray-freezing techniques. The basic principles governing the process of pre-cooling are provided. It is concluded that pre-cooling is one of the major limiting steps in attaining an overall ultra-rapid cooling rate. Pre-cooling has a substantial influence on the nature of the final frozen specimens. In order completely to avoid crystallization before entry into the liquid cryogen and maximize the overall cooling rate of the specimen, precautions should be taken to control the height of the gaseous layer and the size of the specimen. The probability of the specimen being frozen in the cold gaseous layer is reduced by increasing the entry speed. The effectiveness, however, becomes less marked at speeds greater than 10 m/s. In order to minimize the risk of misinterpreting the measured cooling rate, it is necessary to specify the pre-cooling conditions. The pre-cooling effect is much more evident in liquid helium than in cryogens such as propane, ethane, Freon 12, 13 and 22.     

Numerical investigation of the aerodynamic benefits of wing-wing interactions in a dragonfly-like flapping wing

Journal of Mechanical Science and Technology - Numerical simulations are performed to investigate the aerodynamic benefits of wing-wing interactions on a dragonfly-like flapping wing while...     

Aerodynamic performance improvement by divergent trailing edge modification to a supercritical airfoil

A computational study has been performed to determine the effects of divergent trailing edge (DTE) modification to a supercritical airfoil in transonic flow field. For this, the computational result with the original DLBA 186 supercritical airfoil was compared to that of the modified DLBA 283. A Navier-Stokes code, Fluent 5. 1, was used with Spalart-Allmaras’s one-equation turbulence model. Results in this study showed that the reduction in drag due to the DTE modification is associated with weakened shock and delayed shock appearance. The decrease in drag due to the DTE modification is greater than the increase in base drag. The effect of the recirculating flow region on lift increase was also observed. An airfoil with DTE modification achieved the same lift coefficient at a lower angle of attack while giving a lower drag coefficient. Thus, the lift-to-drag ratio increases in transonic flow conditions compared to the original airfoil. The lift coefficient increases considerably whereas the lift slope increases just a little due to DTE modification.