风力机叶尖涡尾迹结构PIV测量研究

空气动力研究与发展中心低速空气动力研究所依托工程型大风洞（实验段直径3.2 m）,采用高分辨率CCD相机（4 008像素×2 672像素）,针对旋转状态下的风力机叶片尾流开展大视场（单个观测区域达到570 mm×380 mm）PIV(particle image velocimetry)测量,以NREL UAE Phase Ⅵ风力机叶片1/8缩比模型为实验对象,获取了叶尖涡产生、发展的流动数据,为研究风力机叶尖涡结构和流动机理研究提供重要的基础数据．观测结果表明,叶尖涡从后缘脱落后首先有一个短时间的向内运动,然后随着尾流的膨胀向外运动,其涡强度则先是短时间内降低,然后随着涡的卷起而增强,从而形成一个强大的叶尖涡．在实验观察范围内叶尖涡在来流方向的迁移规律近似线性．     

Unsteady Flow and its Influence to Aerodynamic and Aeroacoustic of VAWT's

A study is reported of the influence of unsteady flow on the aerodynamics and aeroacoustics of vertical axis wind turbines by numerical simulation. The combination of aerodynamic predictions with a discrete vortex method and aeroacoustic predictions based on Ffowcs Williams-Hawkings equation is used to achieve this goal. The numerical results show that unsteady flow of the turbine has a significant influence on the turbine aerodynamics and can lead to a decrease in generated noise as compared to the conventional horizontal axis wind turbine at the similar aerodynamic performance. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical Solution of Transonic Flow in SE1050 Cascade

The SE1050 cascade is an open test case (QNET network) of plane turbine cascade measured at the IT ASCR wind tunnel. The two regimes with subsonic and supersonic outletMach number were selected for numerical simulation. Several numerical methods have been developed and also several turbulence models have been implemented. Comparison of computed results and experimental data gives us opportunity to discuss main features of transonic flow field in well designed turbine cascade, possibilities of its numerical capturing (grid quality, numerical viscosity, turbulence model, boundary layer transition) and its influence on prediction of energy losses. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

3‐D Inviscid Transonic Condensing Flow around a Swept Wing

Transonic condensing flow is an interesting phenomena because of the large change in temperature over a small area. This drop in temperature allows the moist air to condense. It is the purpose of this paper to examine the effect of sweep on condensing flow. The geometry of the wing model starts with NACA‐0014 at the wall and reduces to a NACA‐0010 at the tip. The span of the wing is 2.5 times the maximum chord length. The effect of sweep is examined by comparing a model wing with a sweep angle of 11.3 with a straight trailing edge that has no thickness and then a straight leading edge with a 11.3 trailing edge sweep. The free stream Mach number is 0.8 and angle of attack is 0. A 2‐D calculation shows that the NACA‐0014 and NACA‐0010 have a region of supersonic flow but due to the effect of sweep the sonic line does not extend to the tip. This change of the supersonic region influences the area of condensation on the wing. The swept wing has a lower total drag coefficient for the adiabatic and all condensation cases compared to the straight leading edge wing and second for the each wing the trend of increasing drag with humidity is shown.     

Large-Eddy Simulations of cavitation in a square surface cavity

We report on the development and application of a multiphase approach to the prediction of cavitation induced by high-speed flow over and within a square surface cavity. The approach entails employing a full cavitation model in conjunction with Large-Eddy Simulations in order to capture the initiation and development of bubble formations in turbulent-flow conditions. The incipient formation of the bubble cloud, and the flow processes of vortex shedding and shear-layer oscillations are tracked using the Volume of Fluid method. The validity of the computational approach was assessed by comparisons with experiments on cavitating flow over a hydrofoil. Application to the case of flow over and within a two-dimensional square cavity with cavitation clearly reveal the presence of traveling cavitation at the corner of the cavity trailing edge, and vortex cavitation within the cavity. It is shown that the collapse of cavitation bubbles results in an impact frequency that is higher than the frequency of the shear-layer oscillations. This implies that structural damage due to cavitation is likely to be most severe at the corner formed at the intersection of the cavity’s trailing edge and the flat surface upstream of it.     

Numerical simulation of vortex induced vibrations and its control by suction and blowing

The vortex formation and shedding behind bluff structures is influenced by fluid flow parameters such as, Reynolds number, surface roughness, turbulence level, etc. and structural parameters such as, mass ratio, frequency ratio, damping ratio, etc. When a structure is flexibly mounted, the Kármán vortex street formed behind the structure gives rise to vortex induced oscillations. The control of these flow induced vibrations is of paramount practical importance for a wide range of designs. An analysis of flow patterns behind these structures would enable better understanding of wake properties and their control. In the present study, flow past a smooth circular cylinder is numerically simulated by coupling the mass, momentum conservation equations along with a dynamical evolution equation for the structure. An active flow control strategy based on zero net mass injection is designed and implemented to assess its efficacy. A three actuator system in the form of suction and blowing slots are positioned on the cylinder surface. A single blowing slot is located on the leeward side of the cylinder, while two suction slots are positioned at an angle α = 100°. This system is found to effectively annihilate the vortex induced oscillations, when the quantum of actuations is about three times the free stream velocity. The dynamic adaptability of the proposed control strategy and its ability to suppress vortex induced oscillations is verified. The exact quantum of actuation involved in wake control is achieved by integrating a control equation to decide the actuator response in the form of a closed loop feed back system. Simulations are extended to high Reynolds number flows by employing eddy viscosity based turbulence models. The three actuator system is found to effectively suppress vortex induced oscillations.     

浅水横流中异重冲击射流的大尺度涡结构

采用RNG湍流模型对浅水横流中异重冲击射流的大尺度涡结构进行了详细的数值研究．分析了冲击区滞止点上游壁面涡结构和近区Scarf涡结构的尺度、形成机理和演化特征．计算得到了上游壁面涡的特征尺度,结果表明上游壁面涡具有高度的三维性,其特征尺度依赖于流速比和环境水深．近区Sarf涡结构对横流冲击射流的横向浓度分布具有重要的影响．当流速比相对较小时,在底层壁射流与环境横流的横向边界附近出现明显的高浓度聚集现象,计算结果表明Scarf涡结构对这一高浓度聚集区的形成起主导作用．     

Coherent structure in flow over a slitted bluff body

The focus of the present investigation is resolution of the coherent structure in the near wake behind a slitted bluff body. The bluff body is two-dimensional with gap ratio from 0.12 to 0.48. The evolution of the structure was numerically investigated using the renormalization group (RNG) k–ε model at Reynolds number of 470,000. Two types of coherent structure are identified: At low gap ratio 0.12, the structure is characterized by a flip–flopping gap flow; at high ratio 0.22–0.48, the gap flow deflects to one side with an asymmetrical wake. The coherent structure is divided by the gap flow into two zones called the primary recirculation zone and the secondary recirculation zone. The coherent structure is intimately related to the gap ratio, and the structure of small gap ratio is different from that of large gap ratio because the interaction between two zones relates to the gap ratio. To explain the vortex shedding, a mechanism that single vortex of large size suddenly immerses between two shear layers was proposed. Experimental results using point-to-point method and particle-image velocimetry (PIV) measurements in a close wind tunnel were also carried out to confirm the observation from the numerical study. The evidence shows that the numerical results are of good agreement with the experiments. The comparison between the RNG k–ε model and the large eddy simulation also indicates that the RNG k–ε model is adequate in computing the bluff body flow.     

带有球凹/球凸的旋转通道内流动结构和强化换热的数值研究

对带有凹坑和凸包的内流通道在不同旋转数下的对流换热特性进行了数值分析,探讨了Coriolis力对通道中流场和换热特征的影响．研究发现,随着旋转数增加,通道前缘呈现出较弱的流动冲击,但存在较大的尾迹和延迟的流动再附着,后缘凹坑内部有一较小旋涡和较强射流使得后缘传热得到强化,最高可达60%．总体Nusselt数随着旋转数的增加先减小而后增大.     

Assessment of an improved turbulence model in simulating the unsteady flows around a D-shaped cylinder and an open cavity

Most engineering flows are still predicted by the conventional Reynolds-averaged Navier-Stokes method because of the low requirements of the computational quantities. However, the resolution capability of Reynolds-averaged Navier-Stokes models is still open to deliberation, especially in the recirculation and wake regions, where the vortical flows dominate. In the present work, an improved turbulence model derived from the original shear stress transport k-ω model is proposed and its superiority is assessed by our modeling the unsteady flows around a D-shaped cylinder and an open cavity, corresponding to two different Reynolds numbers. The results are compared with results from experiments and other turbulence models in terms of the flow morphology and mean velocity profiles. This shows that the predictive accuracy of the modified turbulence model is increased significantly in the bluff body wake flows and in the shear layer and separation flows of the cavity. Some special vortex structures can be captured in the open cavity, in which the secondary vortex emerging from the shear layer and the separation vortex near the trailing edge can induce large flow instability, and this phenomenon should be eliminated in engineering applications. It is believed that this improved turbulence model can be used for the more complex turbomachinery flows with better prediction of the hydrodynamic/aerodynamic performance and the unsteady vortical flows, which can provide some guidelines to design or optimize rotating machines.     

Instability of a Vortex Sheet Leaving a Semi-Infinite Plate

The growth of undulations along an infinite vortex sheet is a classical problem of stability theory. Here we modify that problem by including the effects of a boundary: the vortex sheet is assumed to leave a rigid semi-infinite plate and to undergo spatially growing undulations downstream. The usual solution for a doubly infinite sheet is corrected by the Wiener-Hopf technique to account for the presence of the plate. The correction depends sensitively on whether a Kutta condition is enforced at the trailing edge. Two Kutta conditions, called rectified and full, are suggested to apply depending on conditions in the unperturbed flow. In either case, the correction due to the plate becomes negligible half a wavelength downstream from the trailing edge.     

Mathematical modelling of swirling flow in hydraulic turbines for the full operating range

We introduce and validate a novel mathematical model for computing the radial profiles of both axial and circumferential velocity components, respectively, of the swirling flow exiting the runner of hydraulic turbines within the full operating range. We assume an incompressible, inviscid, axisymmetrical, and steady swirling flow, with vanishing radial velocity at runner outlet. First we find the correlation between the flux of moment of momentum downstream the turbine runner and the operating regime given by turbine’s discharge and head. Second, we express the relationship between the axial and circumferential velocity components, corresponding to the fixed pitch runner blades, using the swirl-free velocity instead of the traditional relative flow angle at runner outlet. It is shown that the swirl-free velocity profile practically does not change with the operating regime. Third, we introduce a constrained variational problem corresponding to the minimization of the flow force while maintaining the prescribed discharge and flux of moment of momentum. This formulation also accounts for a possible central stagnant region to develop when operating the turbine far from the best efficiency point. Fourth, we show that by representing the unknown axial velocity profile with a suitable Fourier–Bessel series, the discharge constraint can be automatically satisfied. The resulting numerical algorithm is robust and produces results in good agreement with available data for both axial and circumferential velocity profiles measured on a model Francis turbine at several operating regimes. Our mathematical model is suitable for the early optimization stages of the runner design, as it provides the swirling flow configuration at runner outlet without actually computing the runner. By optimizing the parameterized swirl-free velocity profile one can achieve through the inverse design approaches the most suitable runner blades configuration at the trailing edge.     

Numerical study on viscoelastic fluid flow past a rigid body

Viscoelastic non-Newtonian fluids can be achieved by adding a small amount of polymer additives to a Newtonian fluid. In this paper, numerical simulations are used to investigate the influence of such polymer additives on the behavior of flow past a circular cylinder. A numerical method is proposed that discretizes the non-linear viscoelastic system on a uniform Cartesian grid, with a penalization method to model the presence of the cylinder. The drag of the cylinder and the flow behavior under the effect of different Reynolds numbers ($\mathrm{Re}$), Weissenberg numbers (Wi) and polymer viscosity ratios (ε) are studied. Numerical results show that different flow characteristics are exhibited in different parameter zones. The polymer viscosity ratio plays an important role at low Weissenberg and Reynolds numbers, but as the Reynolds and Weissenberg numbers increase, the influence of ε weakens. The drag force of the cylinder is mostly affected by the Reynolds and Weissenberg numbers. At low Reynolds numbers, the drag of the cylinder and the flow fields are only affected by a large value of Wi when the elastic forces are strong. Non-trivial drag reduction occurs only when there is vortex shedding in the wake flow, whereas drag enhancement happens when the vortex shedding is inhibited.     

Numerical simulation of unsteady vortex structures in near wake of poorly streamlined bodies on multiprocessor computer system

On the basis of the conservative difference method, spatially unsteady flows near complexly shaped objects are studied. The mathematical model is based on the inviscid gas model. For subsonic, transonic, and supersonic regimes, the nonstationary aerodynamics of various aerospace objects is examined. The three-dimensional structure of the unsteady vortex near wake and its influence on the basic aerodynamic characteristics of aerial vehicles are visualized. The numerical simulation is performed using parallel algorithms on supercomputers of cluster architecture.     

三叶片垂直轴水轮机结构动力响应分析

提出采用改进离散涡和几何精确梁理论混合方法对三叶片垂直轴水轮机进行结构动力响应分析.相比传统的有限元方法,该方法具有求解速度快、建模简单、计算精确等优点.在模态分析中,计算了不同叶片高度下,水轮机叶片和整体的前五阶固有频率,分析了水轮机半径大小和叶片高度对固有频率的影响,结果显示:随着尺寸的增加,叶片和整体固有频率显著减小,整体固有频率更易受到半径大小的影响.在瞬态分析中,考虑了离心载荷和叶片的水动力载荷,得到在工作状况下,旋转一周过程中叶片的最大变形曲线;分析了在不同H/R（叶片高度和半径的比值）的情况下的叶片强度问题,结果显示:当H/R大于3.0时,叶片强度将会失效.     

针对壁面旋转变径管内螺旋流的压力应变项研究

通过壁面旋转变径圆管内螺旋湍流流动特征的分析，确定其切向速度场内涡流区为微团旋转主导的椭圆形流动，外涡流区为微团变形主导且受壁面旋转影响的双曲形流动.进而利用张量的不变量理论，引入旋转率张量与应变率张量的综合不变量作为模型系数，将适用于微团旋转主导的旋转湍流Reynolds应力压力应变项修正模型拓展到了非旋转效应主导的双曲形流动中.将修正压力应变项应用于壁面旋转变径圆管流场的模拟，并将结果与实测结果进行了对比，验证了修正模型的改进效果.     

拓扑方法在叶栅三元流场分析中的应用(Ⅰ)——表面摩擦力线和截面流线的拓扑法则

文中基于Lighthill和Hunt等的工作,建立了适用于分析叶栅表面摩擦力线和截面流线图的拓扑法则.这些法则是:1)对于不带围带的动叶栅,每个节距范围内的表面摩擦力矢量场中的总结点数等于总鞍点数;2)对于叶片端部无间隙的环形叶栅或直列叶栅,每个节距内表面摩擦力矢量场中的总鞍点数比总结点数多两个;3)在叶栅横截面的二次流场中,总鞍点数比总结点数少一个;4)在贯穿流道的子午截面和前后缘截面上的截面流线矢量场中,总的结点数等于总的鞍点数;5)在叶栅跨叶片截面中,截面流线矢量场的总结点数比总鞍点数少一个.     

The effect of modeling of velocity fluctuations on prediction of collection efficiency of cyclone separators

The effect of modeling of velocity fluctuations on the prediction of collection efficiency of cyclone separators has been numerically investigated using the Reynolds stress turbulence model (RSTM) and large eddy simulation (LES). The Eulerian–Lagrangian modeling approach of CFD code Fluent 6.3.26 has been employed to simulate the three dimensional, unsteady turbulent gas–solid flows in a Stairmand high efficiency cyclone. The simulated results have been compared with experimental observations available in the literature. The analysis of results shows that the RSTM and the LES have adequately predicted the mean flow field. Results of the present study demonstrate that the LES has good performance on prediction of fluctuating flow field and collection efficiency for each and every particle size. However, the performance of the RSTM is found poor in terms of prediction of velocity fluctuations and collection efficiency, especially for small particles. This relates to the precessing of the vortex core phenomenon, which is resolved more accurately by LES as compared to the RSTM simulation. The results suggest that the prediction of collection efficiency, especially for small particles is greatly influenced by the simulation of velocity fluctuations in cyclones.     

Influence of pitching on performance of VAWTs

Self- starting is the major obstacle to be overcome for successful design of a vertical axis wind turbine (VAWT). In the past has been suggested that pitching the turbine blades such that pitch angle is not 90 degrees allows for self-starting. To understand the physics surrounding pitching, an analysis is carried out for a common airfoil profile, NACA 0012. The vortex model is used to predict aerodynamic performance of VAWT with pitched blades at various angles. As a result of the analysis carried out for the airfoil at various pitch angles, it was shown that the “dead band” phenomenon could be overcome, but only slightly. At the same time, to overcome the “dead band” with a level on confidence, torques in the tip speed ratio (TSR ) range of 0.75 to 2.75 must be increased to values further above zero. The paper aimed at giving an insight into the small wind turbine starting behavior and its influence parameters. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Variable-Elliptic-Vortex Method for Incompressible Flow Simulation

A variable-elliptic-vortex method, which is a generalization of the elliptic-vortex method proposed by the author in [1], is presented for the numerical simulation of incompressible flows. The most attractive feature of the new method is that the numerical vortex blobs used in this model like actual vortex blobs can be translated, rotated and deformed in elliptic shape. The new method provides a more reasonable and more accurate approach for flow simulation than the fixed-vortex methods. Numerical examples are presented to demonstrate the performance of the new method.