Quality Assuring a Ring Vortex Flow Phantom in Real-Time

Introduction: The ring vortex phantom is a novel, cost-effective prototype which generates complex and well-characterised reference flows in the form of the ring vortex. Although its reproducibility has been demonstrated, with ring speeds routinely behaving within 10% tolerances at speeds of approximately 10 - 70 cm/s, a form of real-time QA of the device at the time of imaging is needed to confirm correct function on demand in any environment. Methods: The technology described here achieves real-time QA, comprising a linear encoder, laser-photodiode array, and Doppler probe, measuring piston motion, ring speed and intra-ring velocity respectively. This instrumentation does not interfere with imaging system QA, but allows QA to be performed on both the ring vortex and the device in real-time. Results: The encoder reports the reliability of the piston velocity profile, whilst ring speed is measured by laser behaviour. Incorporation of a calibrated Doppler probe offers a consistency check that confirms behaviour of the central axial flow. For purposes of gold-standard measurement, all elements can be related to previous Laser PIV acquisitions with the same device settings. Conclusion: Consequently, ring vortex production within tolerances is confirmed by this instrumentation, delivering accurate QA in real-time. This implementation offers a phantom QA procedure that exceeds anything seen in the literature, providing the technology to enhance quantitative assessment of flow imaging modalities.

Spark-generated bubble collapse near or inside a circular aperture and the ensuing vortex ring and droplet formation

This paper aims to study the oscillation of a sparkgenerated submerged bubble located near or inside a circular aperture made in a flat plate using high-speed visualization technique. In the case of a bubble oscillating near an aperture the initial free surface of the water was set at the bottom surface of the plate. The effects of aperture size and bubblefree surface distance on the bubble behavior as well as on the ensuing droplet dynamics are investigated. It was found that the direction of the bubble reentrant jet was towards the aperture or away from it respectively when the normalized aperture size was smaller or greater than a certain critical value. In addition, a toroidal vortex ring was observed to form, which rotated inwards as it moved away from the aperture. It was also found that if the bubble was incepted at a distance sufficiently away from a supercritical size aperture a single droplet could be produced. In the case of a bub- ble initiated in the middle of a circular aperture submerged just beneath the water free surface, the bubble was found to take the shape of an ellipsoid during its expansion. Then a reentrant jet was initiated and pierced the bubble from its top side.

Lagrangian analysis of the formation and mass transport of compressible vortex rings generated by a shock tube

In order to understand the mass transport and the dynamic genesis associated with a compressible vortex formation,a dynamic analysis of compressible vortex rings (CVRs) generated by shock tubes by using the framework of Lagrangiancoherent structures (LCSs) and finite-time Lyapunov exponents field (FTLE) is performed. Numerical calculation is performed to simulate the evolution of CVRs generated by shock tubes with 70 mm, 100 mm, and 165 mm of the driver sectionat the circumstances of pressure ratio = 3. The formation of CVRs is studied according to FTLE fields. The mass transportduring the formation is obviously seen by the material manifold reveled by FTLE fields. A non-universal formation numberfor the three CVRs is obtained. Then the elliptic LCSs is implemented on three CVRs. Fluid particles separated by ellipticLCSs and ridges of FTLE are traced back to t = 0 to identify the fluid that eventually forms the CVRs. The elliptic LCSsencompass around 60% fluid material of the advected bulk but contain the majority of the circulation of the ring. The otherparts of the ring carrying almost zero circulation advect along with the ring. Combining the ridges of FTLE and the ellipticLCS, the whole CVR can be divided into three distinct dynamic parts: vortex part, entrainment part, and advected part. Inaddition, a criterion based on the vortex part formation is suggested to identify the formation number of CVRs.

A PREDICTION OF THE VORTEX-RING STATE BOUNDARY BASED ON MODEL TESTS

To understand the vortex-ring state and to develop an approach for predicting its boundary, a series of model rotor tests of vertical descent and oblique descent have been conducted on a newly-built test apparatus - the Whirling Beam. The test results showed some unsteady aerodynamic behavior of the model rotor operating in the vortex-ring state. A very irregular variation of the rotortorque at low rate-of-descent was observed here for the first time. We considered it to be the start of the 'power settling' and determined the critical descent velocity according to this observation. A previous criterion for the vortex-ring state was modified to give a semi-empirical method for predicting the entire vortex-ring state boundary. The computed boundary shows a good correlation with the model test results and the flight experiences.

Large eddy simulation of a vortex ring impinging on a three-dimensional circular cylinder

Abstract A vortex ring impacting a three-dimensional circular cylinder is studied using large eddy simulation （LES） for a Reynolds number Re = 4 × 10^4 based on the initial translation speed and diameter of the vortex ring. We have investigated the evolution of vortical structures and identified three typical evolution phases. When the primary vortex closely approaches to the cylinder, a secondary vortex is generated and its segment parts move inward to the primary vortex ring. Then two large-scale loop-like vortices are formed to evolve in opposite directions. Thirdly, the two loop-like vortices collide with each other to form complicated small-scale vortical structures. Moreover, a series of hair-pin vortices are generated due to the stretching and deformation of the tertiary vortex. The trajectories of vortical structures and the relevant evolution speeds are analyzed. The total kinetic energy and enstrophy are investigated to reveal their properties relevant to the three evolution phases.

THE NONLINEAR INTERACTION OF VORTEX RINGS WITH A FREE SURFACE

Nonlinear interactions of vortex rings with a free surface are considered in an incompressible, ideal fluid using the vortex contour dynamics technique and the boundary integral equation method. The flow is axisymmetric and the vorticity is linearly distributed in the vortex. Effects of the gravity and the surface tension as well as the initial geometric parameter of the vortex on the interaction process are investigated in considerable detail. The interaction process may be divided into three major stages: the vortex free-traveling stage, the collision stage, and the vortex stretching and rebounding stage. Time evolutions of both the vortex and free surface under various conditions are provided and analyzed. Two kinds of waves exist on the free surface during interaction. In a special case where the gravity and surface tension are very weak or the vortex is very strong, an electric-bulb-like 'cavity' is formed an the free surface and the vortex is trapped in the 'cavity' for quite a. long time, resulting in a large amount, of fluid above the mean fluid surface.

EVOLUTION OF SINGLE ELLIPTIC VORTEX RINGS

The evolution of single elliptic vortex rings for initial aspect ratio (AR) = 2,4,6 has been studied. The incompressible Navier-Stokes equations are solved by a dealiased pseudo-spectral method with 64(3) grid points in a periodic cube. We find that there are three kinds of vortex motion as AR increases and bifurcation occurs at certain AR. The processes of advection, interaction and decay of vortex ring are discussed. Numerical results coincide with experiments and other authors' numerical simulation.

Three-dimensional numerical simulation of a vortex ring impacting a bump

A vortex ring impinging on a three-dimensional bump is studied using large eddy simulation for a Reynolds number Re = 4 × 104 based on the initial translation speed and diameter of the vortex ring. The effects of bump height on the vortical flow phenomena and the underlying physical mechanisms are inves- tigated. Based on the analysis of the evolution of vortical structures, two typical kinds of vortical structures, i.e., the wrapping vortices and the hair-pin vortices, are identified and play an important role in the flow state evolution. The circu- lation of the primary vortex ring reasonably elucidates some typical phases of flow evolution. Furthermore, the mechanism of flow transition from laminar to turbulent state has been revealed based on analysis of turbulent kinetic energy.

Effect of pressure evolution on the formation enhancement in dual interacting vortex rings

In the biological locomotion,the ambit pressure is of particular importance to use as a means of propulsion.The multiple vortex rings have been proved to generate additional thrust by interaction,but the mechanism of this thrust enhancement is still unknown.This study examines the effect of ambit pressure on formation enhancement in interacting dual vortex rings.The vortex rings,which have the same formation time,are successively generated in a piston-cylinder apparatus.The finite-time Lyapunov exponent(FTLE)visualizes the flow fields as an indication of Lagrangian coherent structures(LCSs),and the pressure field is calculated based on the digital particle image velocity(DPIV).We extract the back pressure of the rear vortex in dual vortices and the back pressure circulationΓ_(b),which is defined as a form of overpressure circulationΓ_(p).TheΓ_(b)has a positive linear relationship withΓ_(p).A critical interval distance d*_(cr)in a range of0.32-0.42 is found whereΓbandΓp reach the maximum synchronously,leading to a full-interaction mode.Moreover,an over-interaction mode and an under-interaction mode are proposed when the dimensionless interval distance d*_(is)smaller or larger than d*_(cr).To conclude,the high back pressure caused by vortex interaction can enhance the formation of vortex rings and lead to high thrust.

A modeling method of microburst based on multiple vortex ring

Microburst is a special kind of low-level wind shear, which may do great damage to aircrafts. Modelling of a microburst is significant for flight simulations. In this paper we adopt multiple vortex ring principle to model microburst and propose a new parameter selection method of multiple vortex ring model. We treat the parameters selection as an optimization problem, and introduce the differential evolution algorithm into it. A nested differential evolution algorithm is proposed to complete the two optimization process, objective optimization and intermediate optimization. The simulation results show that this method can flexibly generate microburst with any maximum wind velocity.

Compression and stretching of ring-vortex solitons in a bulk nonlinear medium

Compression and stretching of ring-vortex solitons, which is a novel self-similar solution of（2＋1）-dimensional diffraction decreasing waveguide, is investigated analytically and numerically. We obtain the ring-vortex solitons via the similarity transformation method. The distance modulation for the width, the diffraction, and the nonlinear response, strongly affects the form and the behavior of the self-similar vortex, and facilitates the efficient compression of optical waves. This approximate ring-vortex solitons can reflect the real properties of self-similar optical vortex beams during propagation under certain parameter window selection. Specific examples and figures are given to illustrate discussed features. The results obtained in this paper may have potential values for all-optical data-processing schemes and the design of beam compressors and amplifiers.

Interaction of Bubbles with Vortex Ring Launched into Bubble Plume

This study is concerned with an experimental exploration for the interactions of bubbles with a vortex ring launched vertically upward into a bubble plume. A vortex ring launcher, composed of a cylinder and a piston, is mounted at the bottom of a water tank. Small hydrogen bubbles are released into still water from a cathode, which is wound around the cylinder outlet, by the electrolysis of water. The bubbles rise by the buoyant force and induce a bubble plume. The water in the cylinder is discharged into the bubble plume by the piston, resulting in a laminar vortex ring convecting along the central axis of the plume. Just after the launch of the vortex ring, the bubbles are spirally entrained into the vortex ring with the roll up of the shear layer. The void fraction within the vortex ring increases with the convection of the vortex ring until a certain displacement of the vortex ring, where the reduction occurs. The vortex ring convects with a constant velocity higher than that in still water. The entrained bubbles reduce the strength of the vortex ring.

ON THE FORMATION OF VORTEX RINGS AND PAIRS

The axisymmetric vortex sheet model developed by Nitsche and Krasny had been extended to study the formation of vortex rings (pairs) at the edge of circular (2D) tube and opening. Computations based on this model were in good agreement with the experiments (Didden (1979) for circular tube and Auerbach (1987) for 2D tube and opening). Using this new model, evidences are provided to show that the main failure of the similarity theory (the false prediction of axial trajectory of vortex ring) is due to its ignorance of the self-induced ring velocity (mutual induction for vortex pair). The Glezer (1988)'s summary on the influence of piston speed upon the shedding circulation was also discussed, and finally the variation of core distribution of vortex ring with turning angle and piston speed was given. (Edited author abstract) 22 Refs.

红外照相机的热浮法湍射流流态转捩机理探究

为了探究湍射流3种流动状态的转捩机理,设计并搭建了可控式湍射流实验仪,利用热浮法红外拍摄了3种流态,实现了湍射流的可视化及精确测量。通过实验观察和分析了包络态中流体转捩速度与雷诺数的关系,确定了转捩临界雷诺数大小范围,并推导和验证了涡环态中推进力与流体流速的半经验公式;同时通过颜色仿真展示了喷口截面处的压强分布图像;利用网格剖分的方式分析了涡环转捩机理,完善了流体运动过程的精细测量,为实验室测量雷诺数提供了一种可行方法。

基于半解析涡环模型的农用单旋翼直升机流场快速计算

农业航空领域中,定量获取风场(流场)速度分布对单旋翼直升机辅助授粉作业、农药施用效率提升具有重要意义。基于此提出一种半解析理论方法能够快速完整计算单旋翼流场速度。以涡环模型为基础,将农用直升机旋转桨盘下的尾涡系等效为涡环连续叠加所形成的圆柱涡面,再由涡环速度诱导公式半解析计算直升机旋翼空间各点速度。理论模型与仿真和文献试验数据比较,结果表明:涡环模型简单快速,普通计算机(CPU 2 GHz,内存2 GB)上2.8 s完成计算;悬停时与计算流体力学仿真结果下洗速度径向平均误差小于1.9 m/s,平均相对误差小于39.1%,轴向平均误差小于2.26 m/s,平均相对误差小于54.6%;悬停时,诱速从中央往桨尖递增;前飞时,平飞速度越大诱速越小;流场主要影响因素依次为前飞速度、旋翼半径、飞机总重、空气密度。为定量计算农用直升机旋翼风场、辅助授粉作业和田间施药喷头布置提供一种快速方法和参考。

喷气涡流纺新型纱线开发探析

为提升开发喷气涡流纺纱线的技术水平,介绍喷气涡流纺纱在我国的发展历程及存在问题,对比环锭纺、转杯纺、喷气涡流纺的优势、局限及技术特性;详细分析喷气涡流纺开发多种纤维混纺纱、包芯包覆纱、有色纱等特色纱线开发途径;总结发展喷气涡流纺需注意的问题。指出:喷气涡流纺纱尚无法替代环锭纺与转杯纺;开发喷气涡流纺纱线产品时,应避免同质化竞争,合理配置纺纱速度,不断优化纺纱工艺配置,努力拓宽喷气涡流纺纱线的应用领域。

往复式涡环生成装置性能的多目标优化

为了将涡环更好地应用到送风等流体输送领域,需要提高涡环生成频率以获得更高的流体输送效率。本文设计一种能够提高涡环生成频率的往复式涡环生成装置,该装置在一个活塞运动周期中可以生成两个涡环,有效地利用了活塞往复运动过程中的推力。针对涡环远距离输送的特点,设计并进行实验;基于响应曲面法与NSGA-II算法对往复式涡环生成装置性能进行多目标优化,使其能够将涡环输送更远距离。结果表明:涡环1与涡环2的输送距离难以同时达到最大值,涡环平均输送距离最大为295 cm,相比优化前增加了2.25%;单个涡环输送距离最大为377 cm,相比优化前增加了6.2%。

涡环输送特性的实验与仿真分析

为探究结构变化对涡环输送的作用,设计一种基于空气驱动活塞运动模式的空气涡环发生器,采用实验方法研究了涡环发生器的长径比、环径比及活塞停顿时间对涡环输送特性的影响,通过数值仿真对涡环输送过程的速度场和压力场进行分析。结果表明:长径比与环径比对涡环输送特性影响较大,活塞停顿时间的影响较小,其中涡环输送的最远距离为277 cm;长径比过大会造成较大沿程损失,环径比过大会造成较大局部损失,阻碍涡环的生成和输送,而涡环具有在较小能量耗散的同时远距离输送的特性。

多轨道角动量模式谐振环形行波天线设计

为解决目前轨道角动量(OAM)天线难以在同一频率下多模工作的问题,推动OAM在无线通信领域的实际应用,设计、加工并测量了一种基于单层板的谐振环形行波天线,引入4个馈电探针将不同相位差的信号馈入行波天线。经仿真分析和实际测量,该天线可以在3 GHz上谐振并分别产生了携带有OAM模式l=0,l=±1,l=-2的涡旋电磁波束。该天线结构简单并且可以在同一频率下产生多种模式OAM,能够有效增加无线通信的信道容量,具有一定的理论意义和实用价值。

基于直接数值模拟的涡环碰撞过程涡结构分析

涡环间的碰撞涉及复杂的多层级涡结构拓扑关系和涡量转移机理.深入研究该模型,有助于揭示涡结构之间相互作用过程所包含的机理.区别于现有研究一般专注于涡环小尺度次级结构的瞬态演化,本研究使用基于一种改进的SIMPLE算法的直接数值模拟,结合整体动态演变与其涡结构细节对涡环碰撞模型进行分析.基于高分辨率网格,分析了在不同雷诺数下涡环碰撞后由涡重联分裂出的次级涡结构.研究表明,涡环碰撞过程伴随着复杂的涡结构拓扑变化,如高涡量区域的偏转及涡丝间的此消彼长.在碰撞时,环本身的不稳定性导致了局部接触后分裂出沿方位角规律分布的次级涡环.而次级结构本身随流场雷诺数的增加呈现出从无到有、从近似圆环状到湍流态的转变.最后,对多雷诺数下的涡拟能曲线进行横向比较以反映碰撞对流场的整体涡流强度影响.结果表明,雷诺数的提高对涡拟能的调控整体表现为初始时刻耗散速率降低;因碰撞而抬升的峰值提高;峰值对应的时间点先延后再前移.本研究基于直接数值模拟展示了涡结构演化的偏转、重联、湍流化等特征,揭示了雷诺数对涡环碰撞的部分调控机制,对复杂流动的涡系演化机理研究有启示作用.