大涡模拟预报螺旋桨辐射噪声的三种声学方法

螺旋桨噪声是船舶的三大噪声源之一,研究螺旋桨的噪声现象具有很大的现实意义。首先划分了螺旋桨的结构化网格,进行了网格收敛性分析,然后采用大涡模拟方法计算螺旋桨水动力。在噪声计算中采用了FW-H方程、结合Virtual lab的旋转偶极子辐射、涡声方程三种方法进行预报,并与换算到自由场的试验结果进行对比分析。结果表明,采用大涡模拟可以预报螺旋桨的辐射噪声,三种方法均满足工程应用需求。     

同向旋转涡对发声的数值研究

为深入探索流动发声的机理,将Ribner和Meecham提出的膨胀理论扩展到有背景流场存在的情形下,并对同向旋转涡对的发声问题进行了数值求解。空间离散采用高精度谱元法,时间推进采用隐式Newmark法,并在外边界采用Eliane-Dan-Thomas吸收边界条件。计算结果与用匹配渐进展开法得到的解析解进行比较,数值解与解析解吻合较好。另外计算了同向旋转涡对在不同旋转马赫数、均匀流场、剪切流场影响下的发声情况,并且对其频谱成分进行了分析。研究结果表明:谱元法结合膨胀理论能够高精度求解不可压缩流动引起的气动声学问题;吸收边界条件的使用有效降低了声波在边界处的反射;随着旋转马赫数的增大,声波波长减小,声源强度增大,峰值频率也随之增大;在均匀流场和剪切流场的作用下,声场呈现了典型的多普勒效应。     

发动机低涡轴超声清洗机电气控制系统设计

航空发动机是飞机的核心部件,被喻为飞机的心脏。发动机低涡轴是航空发动机传动系统中的关键部件之一。在发动机修理过程中,需要对低涡轴进行超声清洗,除去其表面附着的物质。本文讲述了发动机低涡轴超声清洗机控制系统研究与设计。     

二维涡斑的运动发声

本文通过对Navier-Stokes方程的直接数值模拟,并应用涡斑模型,研究了腔内二维涡斑运动发声问题,结果表明旋转方向相同的涡斑,不管 始涡斑的涡量分布如何,最后的涡量分布为Burgers型涡斑,呈现“单胞”的稳定结构;旋转方向相反的涡斑,其运动演化转变与初始条件有关,最终呈“双胞”的稳定结构。     

等离子抑制翼尖涡实验研究

该文对利用DBD等离子体注入能量抑制翼尖涡进行了风洞试验研究。通过采用PIV粒子成像测速技术测量了三种不同结构的等离子体激励器影响下的后掠翼下游的尾涡流场,并结合矩形机翼在等离子作用前后的气动力变化,来判断等离子体抑制翼尖涡的效果。试验结果表明:在机翼翼梢的上下表面处布置等离子体激励器,通过等离子体产生的定向诱导气流形成诱导涡对流场注入能量,可以有效地延缓翼梢上翼面流动分离,抑制翼尖涡,增加升力,提高升阻比。在翼尖处流动分离较小时,等离子体抑制翼尖涡强度的效果明显;在大攻角下仍具有一定的作用;其效果与等离子体发生器的结构有关。因此,通过优化等离子体激励器结构,选择合适的等离子体激励器在翼梢表面的安放位置,可以更好地提高抑制翼尖涡的效果。     

分体箱梁中央隔涡板涡激振动抑制机理研究

借助风洞试验和计算流体力学对分体钢箱梁的涡激振动性能及中央隔涡板的抑振效果与机理开展研究,对比不同形式(水平隔涡板、一道竖向隔涡板、两道竖向隔涡板)和尺寸中央隔涡板的抑振效果,并根据流迹与速度云图对不同形式中央隔涡板的抑振机理进行分析。结果表明：开槽顶部水平隔涡板能够阻碍开槽处的气体对流,降低开槽内部及断面周围气流速度,增加旋涡移动和脱落难度,从而在一定程度上抑制涡激振动,抑振效果随隔涡板宽度增大而增加;开槽内部竖向隔涡板能够将旋涡稳定在隔板分隔出的几个小区域内,避免产生稳定的旋涡脱落,进而有效抑制涡激振动,设置两道半高竖向隔涡板比设置一道全高竖向隔涡板效果更好,但需对两板间距进行优化。     

柔性飞机大变形曲面气动力计算及配平分析

柔性飞机在载荷作用下产生较大的弹性变形并呈现出几何非线性特性,机翼升力面呈现出大变形空间曲面的特征,传统平面气动力的工程分析方法无法给出空间变形下的真实载荷状态进而影响柔性飞机气动弹性分析的准确性。该文基于柔性飞机几何非线性气动弹性分析的需求,建立了曲面三维升力线和曲面涡格两种不同的曲面定常气动力方法,并结合曲面样条插值完成了大变形下结构运动信息与气动载荷信息之间的相互作用和交换,实现气动面随结构变形的自适应更新。进行了柔性飞机的全机非线性配平分析,并对两种不同的气动力方法的分析结果进行对比,归纳出柔性飞机几何非线性气动弹性配平分析的特点。升力线方法分析快速简单,涡格法可以考虑机翼弯度影响,便于复杂模型的多轮次反复计算。两种方法的分析结果具有较好的一致性,当飞机变形较小时都与传统的线性分析方法吻合较好;当结构变形较大时,非线性配平结果随风速和结构质量呈非线性变化,与传统线性分析结果产生明显差别需在设计初期引起重视。     

基于CFX圆柱涡激振动的数值模拟研究

基于k-ω SST湍流模型研究了典型亚临界状态下（Re=3900）二维圆柱的涡激振动。通过将圆柱简化为不同刚度的质量弹簧系统,并运用CFX的动网格与CEL功能,研究了刚度系数对圆柱涡激振动的影响。研究观察到了圆柱涡激振动的“锁定”现象、自限定现象以及漩涡脱落模态从2P到2S的转变过程,并得出了圆柱涡激振动特性随刚度变化的规律。     

基于CFX圆柱涡激振动的数值模拟研究

基于k-ω SST湍流模型研究了典型亚临界状态下（Re=3900）二维圆柱的涡激振动。通过将圆柱简化为不同刚度的质量弹簧系统,并运用CFX的动网格与CEL功能,研究了刚度系数对圆柱涡激振动的影响。研究观察到了圆柱涡激振动的“锁定”现象、自限定现象以及漩涡脱落模态从2P到2S的转变过程,并得出了圆柱涡激振动特性随刚度变化的规律。     

螺旋桨梢涡卷起数值计算

通过解析公式计算得到面源和面偶的诱导速度，在面偶附近区域，用涡环代替面偶，计算得到螺旋桨的速度场，根据尾涡面必须和当地流体流速相切的原则，修正原来尾涡面的形状，逐步迭代直至尾涡形状收敛，用这种方法预报了三维水翼和螺旋桨的尾涡面的形状和梢涡卷起，与试验结果有较好的一致性。     

Vortex motion in the early stages of unsteady flow around a circular cylinder

In this paper, processes in the early stages of vortex motion and the development of flow structure behind an impulsively-started circular cylinder at high Reynolds number are investigated by combining the discrete vortex model with boundary layer theory, considering the separation of incoming flow boundary layer and rear shear layer in the recirculating flow region. The development of flow structure and vortex motion, particularly the formation and development of secondary vortex and a pair of secondary vortices and their effect on the flow field are calculated. The results clearly show that the flow structure and vortices motion went through a series of complicated processes before the symmetric main vortices change into asymmetric: development of main vortices induces secondary vortices; growth of the secondary vortices causes the main vortex sheets to break off and causes the symmetric main vortices to become “free” vortices, while a pair of secondary vortices is formed; then the vortex sheets, after breaking off, gradually extend downstream and the structure of a pair of secondary vortices becomes relaxed. These features of vortex motion look very much like the observed features in some available flow field visualizations. The action of the secondary vortices causes the main vortex sheets to break off and converts the main vortices into free vortices. This should be the immediate cause leading to the instability of the motion of the symmetric main vortices. The flow field structure such as the separation position of boundary layer and rear shear layer, the unsteady pressure distributions and the drag coefficient are calculated. Comparison with other results or experiments is also made. This work was presented at the First Asian Congress of Fluid Mechanics, Bangalore in December 1980.     

Transition to Quantum Turbulence and the Propagation of Vortex Loops at Finite Temperatures

We performed numerical simulation of the transition to quantum turbulence and the propagation of vortex loops at finite temperatures in order to understand the experiments using vibrating wires in superfluid ⁴He by Yano et al. We injected vortex rings to a finite volume in order to simulate emission of vortices from the wire. When the injected vortices are dilute, they should decay by mutual friction. When they are dense, however, vortex tangle are generated through vortex reconnections and emit large vortex loops. The large vortex loops can travel a long distance before disappearing, which is much different from the dilute case. The numerical results are consistent with the experimental results.     

Elytra boost lift,but reduce aerodynamic efficiency in flying beetles

Flying insects typically possess two pairs of wings. In beetles, the front pair has evolved into short, hardened structures, the elytra, which protect the second pair of wings and the abdomen. This allows beetles to exploit habitats that would otherwise cause damage to the wings and body. Many beetles fly with the elytra extended, suggesting that they influence aerodynamic performance, but little is known about their role in flight. Using quantitative measurements of the beetle''s wake, we show that the presence of the elytra increases vertical force production by approximately 40 per cent, indicating that they contribute to weight support. The wing-elytra combination creates a complex wake compared with previously studied animal wakes. At mid-downstroke, multiple vortices are visible behind each wing. These include a wingtip and an elytron vortex with the same sense of rotation, a body vortex and an additional vortex of the opposite sense of rotation. This latter vortex reflects a negative interaction between the wing and the elytron, resulting in a single wing span efficiency of approximately 0.77 at mid downstroke. This is lower than that found in birds and bats, suggesting that the extra weight support of the elytra comes at the price of reduced efficiency.     

Dipole influence on Shack-Hartmann vortex detection in scintillated beams

Optical vortices can appear in an optical beam that propagates over a long distance through a turbulent atmosphere. A Shack-Hartmann wavefront sensor can be used to detect such vortices. However, the morphology of these vortices, which changes with beam propagation, and nearby oppositely charged vortices will affect this vortex detection. The influence of the morphology and the separation distance from oppositely charged vortices on the Shack-Hartmann vortex detection is studied. Numerical simulations for vortex detection under these turbulent atmospheric circumstances are also provided.     

Precessing Vortex Motion and Instability in a Rotating Column of Superfluid 3He-B

In a rotating circular cylinder of superfluid ³He-B, an evolving vortex expands longitudinally such that its end point describes a helically spiralling trajectory along the cylinder wall. The spiral motion is found to give rise to a periodically oscillating NMR signal, which is brought about by the modulation in the superfluid counterflow and its influence on the “flare-out” order parameter texture. The new NMR signal becomes observable within a narrow temperature interval close to the onset temperature of turbulence, when new vortices are continuously generated by the single-vortex instability at the cylindrical wall at a slow rate, ～1 vortex/s. We use numerical vortex filament calculations to examine the precessing motion of the evolving vortices, while they expand towards their stable state as rectilinear line vortices.     

马鞍屋盖表面锥形涡流动模型研究

该文基于大跨平屋盖和马鞍屋盖表面锥形涡流动显示试验,对现有的兰金涡模型进行改进,在其涡核区与势流区之间添加过渡区,从而建立了简化的二维锥形涡流动模型,给出了旋涡上部流速、旋涡内部流线曲率以及屋面涡核吸力之间的定量关系。据此流动模型,分析了旋涡强度的影响因素即为旋涡内部流线曲率和旋涡流速:旋涡内部流线曲率越大,旋涡转速越快,旋涡强度越大。基于流动显示试验和测压试验所得平屋盖以及马鞍屋盖表面各旋涡参数之比,以平屋盖表面锥形涡为基准,量化了各风向下马鞍屋盖表面的锥形涡强度。通过加入考虑旋涡效应对传统准定常理论进行修正,给出了锥形涡涡核吸力(均值、峰值)的计算公式,并将计算值与马鞍屋盖刚性模型风洞测压试验数据进行对比,验证了锥形涡流动模型对于预测马鞍表面锥形涡涡核吸力的有效性。     

Vortices Observed and to be Observed

Linear defects are generic in continuous media. In quantum systems they appear as topological line defects which are associated with a circulating persistent current. In relativistic quantum vacuum they are known as cosmic strings, in superconductors as quantized flux lines, and in superfluids and low-density atomic Bose-Einstein condensates as quantized vortex lines. We discuss unconventional vortices in unconventional superfluids and superconductors, which have been observed or have to be observed, such as continuous singly and doubly quantized vortices in ³ He-A and chiral Bose condensates; half-quantum vortices (Alice strings) in ³ He-A and in nonchiral Bose condensates; Abrikosov vortices with fractional magnetic flux in chiral and d-wave superconductors; vortex sheets in ³ He-A and chiral superconductors; the nexus—combined object formed by vortices and monopoles. Some properties of vortices related to the fermionic quasiparticles living in the vortex core are also discussed.     

Modelling with viscous and viscoplastic materials under combining and separating flow configurations

Combining and separating incompressible flow of Newtonian and inelastic Herschel–Bulkley fluids is studied numerically employing a semi-implicit Taylor–Galerkin pressure-correction algorithm, where steady solutions are obtained through a transient finite element procedure. The influence of inertia and fluid rheology is analysed on flow patterns, velocity fields and pressure drops for various flow configurations, with fixed geometric gap width that stimulates the merging and splitting in the flow. For Newtonian fluids and at larger levels of inertia, the appearance of vortices was observed, with an increase in velocity differences and pressure drops across the channel. In this case, the numerical procedure was verified with good agreement against previous numerical and experimental observations. To extend the consideration to non-Newtonian inelastic materials, the material rheological characteristics were approximated with the use of the Herschel–Bulkley fluid model, incorporating the Ostwald–de Waele power-law model and viscoplastic yield stress. Findings for unyielded power-law fluids reveal slight increase in the size of the vortices as power index (m) was decreased. Variation of the consistency index (k) shows strong influence on the streamline patterns with a rapid increase in the vortex formation as k was decreased. For Bingham model solutions, devoid of shear-thinning and increasing yield stress, a higher value of Reynolds number is required for equivalent levels of vortex formation; also one observes the appearance of yielded and unyielded regions. Under Herschel–Bulkley modelling, there was little change noted in the kinematics, but some was apparent in rheological response. Once more, observations reveal the tendency to eliminate vortices at larger yield stress levels, with the appearance of unyielded regions.     

Flow visualization studies of vortices

An actual vortex in the Kármán vortex street downstream of a circular cylinder has a core of finite dimension which increases downstream. The circulation of the vortex is nearly constant. The ratiob/a which is 0.281 according to the theory of Kármán, grows from 0.2 to 0.4 in the near wake. In the flow about a circular cylinder rotating in a uniform flow, a Kármán vortex street, Görtler-type vortices and Taylor vortices are generated at the same time. In the flow about a circular cylinder impulsively started with a constant velocity, the primary twin vortices behind the cylinder induce secondary twin vortices near the separation point. At the beginning of the motion, the separation does not occur even though a reverse flow is observed in the boundary layer. Mutual slip-through of a pair of vortex rings was achieved by increasing the Reynolds number. A vortex ring rebounds from a plane surface due to the separation of the flow on the surface induced by the vortex ring, and the secondary vortex ring is formed from the separated shear layer.     

Numerical solution of Stokes flow generated by vortices: part 1: around an elliptical cylinder

In this paper, the Stokes flow outside a two-dimensional elliptical cylinder is considered for a highly viscous, incompressible fluid flow, driven by a pair of vortices placed at different positions. Relating the coefficients of some of the terms in the asymptotic expansion of the stream function to the force components and the torque on the combined system, together with the imposition of integral constraints, enables the boundary element method to provide a closed system of equations. It is found that excellent agreement is obtained between the numerical results for a single vortex and all the previous analytical expressions. Some new results relating to forces and torques on the elliptical cylinder are presented. The analytical closed-form expression for the single vortex is provided.