An efficient CFD-based method for aircraft icing simulation using a reduced order model

In-flight icing is a critical technical issue for aircraft safety and, in particular, the droplet impingement areas on aircraft surfaces must be investigated for anti-/de-icing devices. As a step toward the prediction of droplet impingement on aircraft, an Eulerian-based droplet impingement code that provides collection efficiency for air flows around an airfoil containing water droplets is developed. A computational fluid dynamics (CFD) solver was also developed to solve the clean airflow. Then, a proper orthogonal decomposition (POD) method, a reduced order model (ROM), that optimally captures the energy content from a large multi-dimensional data set is utilized to efficiently predict the collection efficiency and ice accretion shapes on an airfoil following the mean volume diameter, liquid water contents and angle of attacks. As a result it is shown that the collection efficiency and ice shapes were in good agreement with the simulated and predicted results.     

Computations of droplet impingement on airfoils in two-phase flow

The aerodynamic effects of leading-edge accretion can raise important safety concerns since the formulation of ice causes severe degradation in aerodynamic performance as compared with the clean airfoil. The objective of this study is to develop a numerical simulation strategy for predicting the particle trajectory around an MS-0317 airfoil in the test section of the NASA Glenn Icing Research Tunnel and to investigate the impingement characteristics of droplets on the airfoil surface. In particular, predictions of the mean velocity and turbulence diffusion using turbulent flow solver and Continuous Random Walk method were desired throughout this flow domain in order to investigate droplet dispersion. The collection efficiency distributions over the airfoil surface in simulations with different numbers of droplets, various integration time-steps and particle sizes were compared with experimental data. The large droplet impingement data indicated the trends in impingement characteristics with respect to particle size; the maximum collection efficiency located at the upper surface near the leading edge, and the maximum value and total collection efficiency were increased as the particle size was increased. The extent of the area impinged on by particles also increased with the increment of the particle size, which is similar as compared with experimental data.     

Sliding friction of polyethylene on ice: tribometer measurements

It is well known that the low kinetic friction experienced when sliding on snow and ice is due to water films generated through frictional heating. There is, however, uncertainty concerning the thickness and the distribution of these water films. Since direct observation of the water films is difficult, tribometer studies coupled with temperature measurements have been carried out on a large-scale, pin-on-disc tribometer (diameter 1.80 m). IR sensors were used to measure the temperature of the ice track in front of and behind the contact region. In addition, thermocouples integrated into the polyethylene slider measured the temperature close to the interface. The kinetic friction between polyethylene and ice has been measured as a function of temperature, velocity, load, and apparent contact area. The friction coefficient, as well as the temperature increase of the slider and the ice track, depends on all of these parameters. Interpretation of the results is given on the basis of hydrodynamic lubrication, taking into account the generation and shearing of thin water films in the contact regions.     

Aerodynamic design and analysis of a small-scale vertical axis wind turbine

Small-scale vertical axis wind turbines are regarded today as an attractive source of green energy, still insufficiently implemented and tested. This paper presents a fast design methodology of such a VAWT, in terms of choosing its main parameters: airfoil, rotor diameter and solidity. For obtaining generated power of each considered geometrical model at different undisturbed wind velocities and rotor angular velocities two models were used — momentum and vortex-wake model, combined with experimentally measured airfoil data (airfoil lift and drag coefficients). Even though the former model is simpler, it is the most utilized model, known to provide good results in stationary working regimes. Both models still present fairly accurate and fast tools for computation and optimization, particularly useful in the phase of conceptual design. In this research, the use of the momentum model resulted in determination of the maximal power coefficient, optimal- and minimal freestream velocity for every considered VAWT model. From these output parameters, a selection of the optimal geometric model was done, and a more detailed transient analysis and flow representation around the selected solution was obtained by the vortex-wake model. The results obtained by the two used computational models coincide satisfactorily.     

Novel fiber optic sensor for ice type detection

This paper reports on a novel fiber-optic ice sensor featuring oblique fiber end-faces, not perpendicular to the fiber axis. The inclination angles of the end-faces were respectively 50° and 30° for the source and signal fiber bundles. Ice-growth curves obtained in an icing wind tunnel showed that with increasing ice thickness, the output voltage of this sensor would reduce rapidly for glazed ice, increase rapidly for rime ice, and make no remarkable change for mixed ice. This significant difference is caused by the different optical intensity distributions for different ice types and can be used to accurately identify the ice type for aerospace applications.     

Physical characteristics of frost formation in semi-closed cycle turbine engines

The power, water extraction, and refrigeration (PoWER) system generates electric power, potable water and refrigeration effects simultaneously, and is composed of a micro-turbine and vapor absorption refrigeration system, which typically uses ammonia, heat exchangers, and a turbocharger. In order to improve the efficiency and the electric output power of the micro-turbine portion of the PoWER system, attempts have previously been made to reduce the inlet temperature of the compressor. However, it was problematic to drive it below the freezing point since frost or ice forms from the humidity in re-circulated air. As a result, the ice accretion that attaches on the bell mouth or guide vane might increase the pressure drop, leading to performance loss. Furthermore, large sections that break from the ice accretion may cause damage to the compressor blades. In this paper experiments have been conducted under the same environmental conditions as the PoWER system in order to observe the physical characteristic of the frost formation on cylindrical tubes. The results show the thickness of the frost formation for different air velocities (3, 5, 7 m/s) and surface temperatures (?9.8, ?16.6, ?24.4°C) with respect to time.     

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.     

基于气动性能与刚度特性的风力机翼型优化设计

结合翼型泛函集成理论与叶片截面刚度矩阵数学计算模型,提出了风力机中等厚度翼型气动性能与结构刚度特性的一体化设计方法,实现了翼型气动性能与叶片截面刚度特性的同时提高。对考虑叶片截面铺层参数变化设计的WQ-B300翼型与DU97-W-300翼型进行了气动性能与结构刚度特性对比分析,结果表明:相比于DU97-W-300翼型,WQ-B300翼型的气动性能与叶片截面刚度性能均有显著提高,其挥舞刚度和摆振刚度分别提高了6.2%和8.4%,验证了该设计方法的可行性,给风力机中等厚度及大厚度翼型设计提供了一种思路。     

影响风力发电机翼型气动性能的因素研究

翼型气动性能的优劣影响着风力发电机的发电效率,研究影响叶片翼型气动性能的因素具有重要意义。本文采用数值方法计算了文献中NACA0012翼型在Re=10^6时的气动性能参数并与试验值比较,验证了数值方法的正确性。通过对相对厚度、相对弯度、雷诺数等影响翼型气动特性的参数进行研究,结果表明:相对厚度小的翼型在小攻角范围可以获得更好的气动性能;当攻角大于失速角12°后,相对厚度大的翼型的气动性能更佳。在0°~20°攻角范围内,相对弯度和雷诺数越大,翼型的气动性能越好。     

一种测量液膜厚度的超声相控阵实验装置

气液两相流存在于核反应堆蒸发、飞行器冷却、化工生产降膜蒸发等过程,界面波的动态测量对工业过程监控和生产优 化具有重要意义。 界面波的准确识别与特性参数测量是开展科学研究与工程实践的重要前提。 基于超声相控阵测量系统,设 计了扇扫的测量方式,可以用于气液界面清晰的流型中液膜厚度和界面波形态三维测量。 通过静态标定和圆管验证,确定了像 素点和液膜厚度之间的关系,在气相表观流速为 0. 071 9~ 0. 431 6 m/ s,液相表观流速为 0. 056 7~ 1. 416 1 m/ s 的工况下进行实 时动态实验,获得了实时流动过程中较高精度的截面气液相界面信息,并构建了管道内部界面波三维分布形态,为界面波特性 研究提供了一种实验参考方法。     

可识别冰型的光纤结冰传感器

现有结冰传感器没有识别冰型的能力或识别冰型的可靠性偏低。针对这个问题,提出了一种能以较高可靠性识别冰型的光纤结冰传感器。在介绍传感器光学结冰探测原理和分析光路的基础上,设计出了此传感器,并在实验室环境下模拟传感器结冰,通过大量的试验,验证了其结果的有效可靠。最后得出结论:该传感器可用于对物体表面冰型(明冰和霜冰)及结冰厚度进行较高可靠性的识别和定量探测,具有广阔的应用前景。     

低雷诺数旋翼翼型设计及气动仿真

研究了低雷诺数下薄圆弧旋翼的翼型,考虑其对高气动性能、高结构强度和便于制造和轻量化的要求,提出一种具有上凸结构的薄圆弧翼型。通过在翼型上表面增加凸起结构,增加部分弦长的翼型厚度并安装加强筋来提高翼型延展向的结构强度;设计出了最大厚度为4.3%、圆弧均匀厚度为2.5%、最大弯度为5.5%和均匀弯度为4.5%的薄圆弧翼型。采用基于二维定常、不可压缩Navier-Stoke方程的数值仿真方法计算了该翼型在雷诺数为40,000~100,000,迎角为-4°~12°下的气动性能,并获得了该翼型上下表面的压力系数分布线和速度矢量图。采用该翼型制作了直径为40cm,质量为15g,桨距为15.7cm的碳纤维旋翼;在悬停状态下完成了它的升力和结构强度试验。实验结果显示其性能满足使用要求。目前,研制的旋翼已成功地应用于某型多旋翼飞行器。     

冰面上橡胶滑动摩擦特性的数值分析

基于摩擦熔化理论,考虑流体动力润滑和热传导的耦合作用,采用打靶法和线性插值法获得了冰面上滑动橡胶块摩擦界面上的压力、流体膜厚及温度的无量纲分布规律,研究了胶块的摩擦特性。结果表明,摩擦界面上的压力峰值随载荷的减小向界面中部移动,摩擦因数与滑动速度的平方根成正比,而与载荷和试样长度的平方根成反比。所用方法可应用于其它材料在冰面上的摩擦及其它情况下的摩擦熔化问题。     

Experimental study of frost growth on a horizontal cold surface under forced convection

Frost formation on a horizontal copper surface under low air temperature and forced convection conditions is investigated experimentally. Both the frost crystals pattern and the frost layer thickness formed on the cold plate are compared under different experimental conditions. The environmental variables considered in this study include the ambient temperature (T _∞ ), air relative humidity (φ), and velocity (v), as well as the cold surface temperature (Tw). The tested ranges are −5≤T _∞ ≤5 °C, 50%≤ φ≤80%, 2.2≤v≤8.0 m/s, −16.8≤T _w ≤−25.5 °C. The experimental results show the cold surface temperature and the air relative humidity have obvious effects on the frost growth: the frost layer thickness increases strongly with the decreasing cold surface temperature and increasing air relative humidity. The air temperature and air velocity or Reynolds number are also important factors affecting the frost crystals’ growth and thickness. With the increase of the air temperature and velocity, the frost crystals become denser, and the frost layer thickness become thicker, but this trend becomes weaker under higher air temperature and velocity.     

A technique and device for ice thickness and drift velocity measurements with hydrostatic-pressure sensors sliding along the ice bottom

The results of laboratory experiments performed with a prototype of a measuring device intended for monitoring the thickness and drift velocity of ice in Arctic seas are presented. This device is a system of hydrostatic-pressure sensors located inside a hollow flexible tube. Owing to the positive buoyancy, the system of sensors is pressed to the bottom of drifting ice. The load (anchor) on the sea bottom ensures a constant geographical position of the measuring device. During the ice drift, the tube slides along the ice bottom and sensors record the depths of separate segments of the tube. A data logger collects information and determines the ice thickness and drift velocity.     

明渠海水取水泥沙起动试验研究

针对明渠海水取水所处岸边海域环境与地质条件,进行物理模型试验,以获得工程项目所在处海床泥沙起动条件和岸边波浪场的底流速,预测明渠所在海域出现泥沙冲淤现象的可能性,为工程设计提供科学依据。     

翼型厚度对风力机叶片翼型气动特性的影响

建立了预测翼型气动特性的理论模型并进行了数值计算,研究了翼型厚度对风力机叶片翼型的气动特性影响,给出了翼型厚度对翼型的升力系数、阻力系数、升阻比和流场、压力系数的影响。研究结果表明,对于同一弯度不同厚度的NACA系列翼型,在较小攻角时,较小厚度翼型可获得较大的升阻比,在大攻角时,增加厚度翼型可以提高翼型的升阻比,扩宽大升阻比范围,而且较大厚度翼型的分离点前移速度较缓慢,涡分布范围较小。     

Investigation of load prediction on the Mexico rotor using the technique of determination of the angle of attack

Blade element moment(BEM) is a widely used technique for prediction of wind turbine aerodynamics performance,the reliability of airfoil data is an important factor to improve the prediction accuracy of aerodynamic loads and power using a BEM code.The method of determination of angle of attack on rotor blades developed by SHEN,et al is successfully used to extract airfoil data from experimental characteristics on the MEXICO(Model experiments in controlled conditions) rotor.Detailed surface pressure and particle image velocimetry(PIV) flow fields at different rotor azimuth positions are examined to determine the sectional airfoil data.The present technique uses simultaneously both PIV data and blade pressure data that include the actual flow conditions(for example,tunnel effects),therefore it is more advantageous than other techniques which only use the blade loading(pressure data).The extracted airfoil data are put into a BEM code,and the calculated axial and tangential forces are compared to both computations using BEM with Glauert’s and SHEN’s tip loss correction models and experimental data.The comparisons show that the present method of determination of angle of attack is correct,and the re-calculated forces have good agreements with the experiment.     

Image comparisons of snow and ice crystals photographed by light (video) microscopy and low-temperature scanning electron microscopy

Light (video) microscopy and low-temperature scanning electron microscopy (SEM) were used to examine and record images of identical precipitated and metamorphosed snow crystals as well as glacial ice grains. Collection procedures enabled numerous samples from distant locations to be shipped to a laboratory for storage and/or observation. The frozen samples could be imaged with a video microscope in the laboratory at ambient temperatures or with the low-temperature SEM. Stereo images obtained by video microscopy or low-temperature SEM greatly increased the ease of structural interpretations. The preparation procedures that were used for low-temperature SEM did not result in sublimation or melting. However, this technique did provide far greater resolution and depth of focus over that of the video microscope. The advantage of resolution was especially evident when examining the small particles associated with rime and graupel (snow crystals encumbered with frozen water droplets), whereas the greater depth of focus provided clearer photographs of large crystals such as depth hoar, and ice. Because the SEM images contained only surface information while the video images were frequently confounded by surface and internal information, the SEM images also clarified the structural features of depth hoar crystals and ice grains. Low-temperature SEM appears to have considerable promise for future investigations of snow and ice.     

Profiles of jet polished electron microscope specimens

The cross-sectional profiles of jet polished electron microscope disc specimens have been measured as a function of (1) jet diameter, (2) electrolyte velocity, (3) jet to specimen distance, (4) lip thickness of clamping disc and (5) material used for the clamping disc. It is found that the optimum values of these parameters varies according to the material/electrolyte combination. However, a fault-finding table has been devised which enables the optimum conditions to be found rapidly for any material.