Parametric study of turbine NGV blade lean and vortex design

The effects of blade lean and vortex design on the aerodynamics of a turbine entry nozzle guide vane (NGV) are considered using computational fluid dynamics. The aim of the work is to address some of the uncertainties which have arisen from previous studies where conflicting results have been reported for the effect on the NGV. The configuration was initially based on the energy efficient engine turbine which also served as the validation case for the computational method. A total of 17 NGV configurations were evaluated to study the effects of lean and vortex design on row efficiency and secondary kinetic energy. The distribution of mass flow ratio is introduced as an additional factor in the assessment of blade lean effects. The results show that in the turbine entry NGV, the secondary flow strength is not a dominant factor that determines NGV losses and therefore the changes of loading distribution due to blade lean and the associated loss mecha-nisms should be regarded as a key factor. Radial mass flow redistribution under different NGV lean and twist is demonstrated as an addition key factor influencing row efficiency.     

Influence of artificial tip perturbation on asymmetric vortices flow over a chined fuselage

An experimental study was conducted with the aim of understanding behavior of asymmetric vortices flow over a chined fuselage.The tests were carried out in a wind tunnel at Reynolds number of 1.87 · 105 under the conditions of high angles of attack and zero angle of sideslip.The results show that leeward vortices flow becomes asymmetric vortices flow when angle of attack increases over 20.The asymmetric vortices flow is asymmetry of two forebody vortices owing to the increase of angle of attack but not asymmetry of vortex breakdown which appears when angle of attack is above 35.Asymmetric vortices flow is sensitive to tip perturbation and is nondeterministic due to randomly distributed natural minute geometrical irregularities on the nose tip within machining tolerance.Deterministic asymmetric vortices flow can be obtained by attaching artificial tip perturbation which can trigger asymmetric vortices flow and decide asymmetric vortices flow pattern.Triggered by artificial tip perturbation, the vortex on the same side with perturbation is in a higher position, and the other vortex on the opposite side is in a lower position.Vortex suction on the lower vortex side is larger, which corresponds to a side force pointing to the lower vortex side.     

NUMERICAL SIMULATION OF UNSTEADY FLOW AROUND A CIRCULAR CYLINDER AT HIGH REYNOLDS NUMBERS

A Lagrangian-Eulerian hybrid scheme to solve unsteady N-S equation in two-dimensional incompressible fluid at high Reynolds numbers is presented in this paper. A random walk is imposed to simulate the viscous diffusion, the vortex-in-cell method is used to obtain the convection velocity, and nascent vortices are created on a cylinder to satisfy the zero-slip condition. The impulsively started flow around a circular cylinder and the separation induced by a pair of incident vortices symmetrically approaching a circular cylinder have been successfully simulated by the hybrid scheme. The impulsively started flow from rest has been computed at Reynolds numbers 3000 and 9500. Comparisons are made with those results of finite-difference method, vortex method and flow visualization. Agreement is good. The particular attention has been paid to the evolutions of flow pattern. A topological analysis has been proposed in the region of the near wake. The bulge, isolated secondary vortex, a pair of secondary vortices     

Numerical Investigation of Flow Separation Control on a Highly Loaded Compressor Cascade by Plasma Aerodynamic Actuation

To discover the characteristic of separated flows and mechanism of plasma flow control on a highly loaded compressor cascade, numerical investigation is conducted. The simulation method is validated by oil flow visualization and pressure distribution. The loss coefficients, streamline patterns, and topology structure as well as vortex structure are analyzed. Results show that the numbers of singular points increase and three pairs of additional singular points of topology structure on solid surface generate with the increase of angle of attack, and the total pressure loss increases greatly. There are several principal vortices inside the cascade passage. The pressure side leg of horse-shoe vortex coexists within a specific region together with passage vortex, but finally merges into the latter. Corner vortex exists independently and does not evolve from the suction side leg of horse-shoe vortex. One pair of radial coupling-vortex exists near blade trailing edge and becomes the main part of backflow on the suction surface. Passage vortex interacts with the concentrated shedding vortex and they evolve into a large-scale vortex rotating in the direction opposite to passage vortex. The singular points and separation lines represent the basic separation feature of cascade passage. Plasma actuation has better effect at low freestream velocity, and the relative reductions of pitch-averaged total pressure loss coefficient with different actuation layouts of five and two pairs of electrodes are up to 30.8% and 26.7% while the angle of attack is 2°. Plasma actuation changes the local topology structure, but does not change the number relation of singular points. One pair of additional singular point of topology structure generates with plasma actuation and one more reattachment line appears, both of which break the separation line on the suction surface.     

Generalized Kutta–Joukowski theorem for multi-vortex and multi-airfoil flow with vortex production——A general model

By using a special momentum approach and with the help of interchange between singularity velocity and induced flow velocity, we derive in a physical way explicit force formulas for twodimensional inviscid flow involving multiple bound and free vortices, multiple airfoils, and vortex production. These force formulas hold individually for each airfoil thus allowing for force decomposition, and the contributions to forces from singularities(such as bound and image vortices,sources, and doublets) and bodies out of an airfoil are related to their induced velocities at the locations of singularities inside this airfoil. The force contribution due to vortex production is related to the vortex production rate and the distance between each pair of vortices in production, thus frameindependent. The formulas are validated against a number of standard problems. These force formulas, which generalize the classic Kutta–Joukowski theorem(for a single bound vortex) and the recent generalized Lagally theorem(for problems without a bound vortex and vortex production) to more general cases, can be used to identify or understand the roles of outside vortices and bodies on the forces of the actual body, optimize arrangement of outside vortices and bodies for force enhancement or reduction, and derive analytical force formulas once the flow field is given or known.     

Research of Step-down Stress Accelerated Degradation Data Assessment Method of a Certain Type of Missile Tank

The performance degradation rates of the missile tank are generally time-varying functions uneasily evaluated by general classical evaluation methods. This paper develops a segmented nonlinear accelerated degradation model (SNADM) based on the equivalent method of accumulative damage theory, which tackles the problem that product life is difficult to be determined with degradation rate being a function of the variable of time. A segmented expression of the function of population accumulative degradation is derived. And combined with nonlinear function, an accelerated degradation function, i.e., SNADM is obtained. The parameters of the SNADM are identified by numerical iteration, and the statistical function of degradation track is extrapolated. The reliability function is determined through the type of random process of the degradation distribution. Then an evaluation of product storage life is undertaken by combining the statistical function of degradation track, reliability function and threshold. An example of a missile tank undergoes a step-down stress accelerated degradation test (SDSADT), in which the results with the SNADM and the classical method are evaluated and compared. The technology introduced is validated with the resultant coincidence of both evaluated and field storage lives.     

Generalized Kutta–Joukowski theorem for multi-vortex and multi-airfoil flow(a lumped vortex model)

For purpose of easy identification of the role of free vortices on the lift and drag and for purpose of fast or engineering evaluation of forces for each individual body,we will extend in this paper the Kutta–Joukowski(KJ) theorem to the case of inviscid flow with multiple free vortices and multiple airfoils. The major simplification used in this paper is that each airfoil is represented by a lumped vortex,which may hold true when the distances between vortices and bodies are large enough. It is found that the Kutta–Joukowski theorem still holds provided that the local freestream velocity and the circulation of the bound vortex are modified by the induced velocity due to the outside vortices and airfoils. We will demonstrate how to use the present result to identify the role of vortices on the forces according to their position,strength and rotation direction. Moreover,we will apply the present results to a two-cylinder example of Crowdy and the Wagner example to demonstrate how to perform fast force approximation for multi-body and multi-vortex problems. The lumped vortex assumption has the advantage of giving such kinds of approximate results which are very easy to use. The lack of accuracy for such a fast evaluation will be compensated by a rigorous extension,with the lumped vortex assumption removed and with vortex production included,in a forthcoming paper.     

Effect of boundary conditions on downstream vorticity from counter-rotating swirlers

Particle image velocimetry(PIV) is utilized to measure the non-reacting flow field in a reflow combustor with multiple and single swirlers. The velocity field, vortex structure and total vorticity levels are experimentally obtained using two different boundary conditions, representing a single confined swirler and multiple swirlers in an annular combustor. The influence of the boundary conditions on the flow field at several locations downstream of the swirlers is experimentally investigated, showing that the central vortex in the multi-swirler case is more concentrated than in the single-swirler case. The vorticity of the central vortex and average cross-sectional vorticity are relatively low at the swirler outlet in both cases. Both of these statistics gradually increase to the maximum values near 20 mm downstream of the swirler outlet, and subsequently decrease. It is also found that the central vortex in the multi-swirler case is consistently greater than the single-swirler case. These results demonstrate the critical influence of boundary conditions on flow characteristic of swirling flow, providing insight into the difference of the experiments on testbed combustor and the full-scale annular combustors.     

A ROBUST FLOW CALCULATION TECHNIQUE WITH MULTIPLE FINITE CONTROL VOLUMES

The control volume method gives the forces which act on the system, but not necessarily the wall pressure of the system. The author has made an attempt to develop a control volume method which makes it possible to obtain the wall pressure of the control volume. The 2-D inviscid incompressible steady duct flow is considered. The conservation equations in integral form are discretized for a control volume. The circulation along the control surface is expressed as a nonlinear function of the vertical velocity component at the inlet and is set equal to zero for the inviscid flow. The equation is solved by the Newton method, and the other aerodynamic properties can be obtained. The calculated results have been compared to the experiment and the agreement has been found fairly satisfactory.     

Real-time total system error estimation:Modeling and application in required navigation performance

In required navigation performance（RNP）, total system error（TSE） is estimated to provide a timely warning in the presence of an excessive error. In this paper, by analyzing the underlying formation mechanism, the TSE estimation is modeled as the estimation fusion of a fixed bias and a Gaussian random variable. To address the challenge of high computational load induced by the accurate numerical method, two efficient methods are proposed for real-time application, which are called the circle tangent ellipse method（CTEM） and the line tangent ellipse method（LTEM）,respectively. Compared with the accurate numerical method and the traditional scalar quantity summation method（SQSM）, the computational load and accuracy of these four methods are extensively analyzed. The theoretical and experimental results both show that the computing time of the LTEM is approximately equal to that of the SQSM, while it is only about 1/30 and 1/6 of that of the numerical method and the CTEM. Moreover, the estimation result of the LTEM is parallel with that of the numerical method, but is more accurate than those of the SQSM and the CTEM. It is illustrated that the LTEM is quite appropriate for real-time TSE estimation in RNP application.     

基于N-S方程的地面效应模拟

模拟了低速情况下二维翼型的地面效应问题，其中翼面流动利用一种改进的、基于不可压N－S方程的离散涡方法来模拟，地面模拟则利用一系列连续分布的涡板块来实现。利用上述模型研究了地面对翼型表面流动和压力分布的影响，并对有地面存在条件下的尾涡发展规律进行了讨论。计算结果表明，该方法能够有效地模拟低速情况下翼型的近地面流动，具有一定的工程应用价值。     

侧风条件下短舱进气道地面涡数值模拟

为了研究起飞状态下进气道地面涡的形成与发展规律,针对缩比的短舱进气道模型进行了3维数值仿真,分析了来流速度和短舱进气道距地面高度对地面涡的影响,得到了地面涡的特点以及对进气道流场品质的影响。研究结果表明:侧风来流速度越低,越容易形成地面涡,随着侧风来流速度的增高,地面涡会消失;短舱进气道距地面高度越低,越容易形成地面涡,地面涡的环量越大;地面涡的形成会增大进气道出口截面的流场畸变程度,短舱进气道距地面高度对畸变的影响很小,畸变主要受侧风来流速度的影响。最后给出了侧风条件下地面涡的分界线方程,可作为是否存在地面涡的判断依据。     

Equilibrium state of trailing vortices: two-dimensional model of a far-field vortex wake

The equilibrium statistical mechanics of a system composed of a large number of two-dimensional point vortices is employed to describe the vortex system shed from aircraft wings. According to this theory, these higher energy states of the vortex system can only be achieved by segregating the point vortices of like kind into two clusters that descend with a constant velocity. The solution is given in terms of the integral constraints for each cluster: total circulation, center of inertia, and kinetic energy. The negative non-dimensional inverse temperature of the system and the length scale related to angular momentum of a single trailing vortex are obtained versus initial interaction energy of the vortex system. Comparison of the theoretical results with available experimental data shows good agreement between the calculated tangential velocity distribution in the trailing vortex and the data. The flow characteristics for three different wing loads are also compared to emphasize the effect of initial circulation distribution along a lifting wing on the vorticity distribution in the equilibrium trailing vortices.     

滑跑条件下短舱进气道地面涡数值仿真

针对滑跑条件下短舱进气道地面涡进行了三维数值仿真研究,分析了来流风速、滑跑速度和短舱进气道距地面高度对地面涡的影响,得到了地面涡的变化规律以及对进气道流场品质的影响。研究结果表明:随着滑跑速度的增加,地面涡会向下游移动,当滑跑速度达到一定值时,地面涡消失;短舱进气道距地面高度越低,越容易形成地面涡,随着短舱进气道距地面高度的增加,地面涡向下游移动;在风速一定的情况下,随着滑跑速度的增加,地面涡强度先增加后减弱。文中给出了滑跑条件下地面涡的分界线方程,可作为是否存在地面涡的判断依据。     

火焰稳定器稳焰机理的非定常观点探讨

本文在低速风洞中,利用在线式互相关PIV(ParticleImageVelocimetry)系统,对尾缘吹气式火焰稳定器及V型火焰稳定器的近尾迹流动进行了测量,在瞬态场尾流结构分析的基础上,提出了稳焰机理的非定常观点,探讨了近尾迹流动结构中旋涡的交替生成和脱落在火焰稳定过程中起了重要的作用。      

高压比离心压气机二次流旋涡结构研究

为揭示高压比离心压气机的流动特性,采用数值方法对高压比离心压气机的旋涡结构和流动损失的产生及演变规律进行了研究。根据不同类型旋涡的具体特征,给出了分别适用于受迫涡和自由涡的二次流识别方法,包括截面旋线法和拟定主流的截面流线法。应用给出的二次流识别方法并结合耗散函数,探讨了压气机内旋涡的形成机理以及旋涡与损失的关联性。研究表明:当涡量与截面法矢量夹角的余弦值大于零时,旋线方向与实际气流方向定性一致,否则相反;旋线显示的涡轴方向与截面法矢量夹角大于90°时,识别出的旋涡不存在;刮削涡和泄漏涡既是低能流体的聚集区也是能量的耗散区,是影响离心压气机损失产生及分布的关键因素;诱导轮尾迹会抑制导风轮流道内叶表通道涡的形成。     

旋翼地面效应的气动建模与特性

地面对旋翼气动特性影响明显,且导致旋翼流场更加复杂。为分析地面效应下的旋翼桨尖涡和流场变化特性,基于涡面和无滑移边界条件,求解第2类Fredholm方程获得地面涡面矢量分布,且将涡面矢量按涡扩散方程扩散到流体中,建立考虑黏性效应的地面气动模型,并耦合非定常面元/黏性涡粒子混合法以体现旋翼桨叶气动特性和旋翼尾迹的非定常效应,构建旋翼地面效应气动分析方法。通过计算Lynx尾桨地面效应下的性能和桨尖涡轨迹,并计算Maryland大学模型旋翼和NASA缩比旋翼地面效应下的垂向、径向速度分布,且与试验和CFD计算结果对比,验证了本文方法能较好捕捉地面效应下的旋翼尾迹变化特性和复杂速度场特性,且结果表明本文方法能较好模拟地面效应下旋翼桨尖涡的收缩、扩散、井喷、地面射流等物理现象。     

旋涡运动研究的某些进展

 本文简要介绍研究旋涡运动在以下问题上的某些结果:低速不同后掠角三角翼在各个迎角下的九种分离流类型及其边界;应用微分方程定性论与拓扑学对三维分离流与旋涡流的分析;旋涡破裂形态,对三角翼前缘涡破裂的实验研究与理论分析;受控分离与旋涡的干扰,二旋涡的位移、绕转与合并等。     

电磁控制涡激振荡的实验研究

利用电介质溶液中圆柱体侧表面附近分布的电磁场产生电磁力可有效改变流体边界层及尾涡结构。本文以减振为目标,对电磁力控制涡激振荡进行了实验研究。实验在转动水槽中进行,通过吊杆将装有电磁激活板的圆柱插在槽内液体中。吊杆上的应变片用于测试圆柱的升力,注入适当的染料用来显示流场。结果表明：对称电磁力作用下,脱体涡被抑制,从而使升力的振荡受到有效抑制,进而抑制圆柱的振动,双排方向相反的涡变为单排正负交错的涡。当电磁力足够大时,圆柱的振荡被完全消除,流场达到定常。     

有翼飞行器大攻角分离涡模拟

基于势流方程,采用涡格法和三维离散涡相结合,模拟具有升力面分离涡的大攻角弹冀、弹体、尾翼组合体绕流。该方法适用于高亚音速、小攻角或大攻角有翼飞行器的压力分布、法向力和压力中心等气动特性的数值计算,并具有显示三维涡流场的能力。