小尺度涡流发生器强化传热机理的研究

在矩形槽道内布置了不同高度的小尺度涡流发生器,采用湍流模型对流体在其中的流动与传热特性进行了数值模拟,并对流体在槽道中的传热和流动特性进行了对比研究.分析了小尺度涡流发生器诱导涡的特性及其对湍流相干结构的影响与作用,讨论了湍流相干结构对温度场的作用机理,解释了涡流发生器强化换热的机理.     

纵向涡强化换热特性及机理分析

本文在尼Re=190～1125范围内对两种不同形状纵向涡发生器(矩形、三角形)以两种不同方式(渐缩式、渐扩式)布置于平行通道内的流动换热特性进行了三维数值模拟研究,并利用场协同原理对其换热机理进行了分析.结果表明:纵向涡使通道换热得到很大提高,通道平均Num数最大可提高46%.比较了通道性能评价指标(Num/Num0)/(f/f0),综合性能三角翼优于矩形翼,对于三角翼布置方式不同对综合性能影响不大,对于矩形翼渐扩方式布置优于渐缩方式.纵向涡使速度与温度梯度的平均夹角减小,通道中流场和温度场协同程度得到改善.     

纵向涡强度衰减及其干涉的数值分析

本文以空气为介质,在Re=600-2100的范围内对布置有三角翅片的平行通道内的初始段层流状态下涡干涉的流动及换热进行了数值模拟。展示了旋向相反的涡在流道中的产生及发展情况,说明了涡产生器在横向位置变化时对横截面上平均Nu数和涡旋强度(?)的影响,并对单个涡产生器的有效换热区域长度进行了探讨。     

Energy constraints in vortex-excited vibrations

Excitation or damping of flow-induced vibrations depends upon the magnitude and the phase relationship between the fluid dynamic force and the oscillatory motion of the body. These have been measured for two aerofoil shapes in mechanically controlled heaving motion in a water channel. At the same time, measurements have been made of the strength and spacing of the associated vortex trails. To help interpret the significance of these measurements, an analysis based upon energy conservation has been performed. When taken together with the classical momentum equation, this yields a relationship giving the energy available for vibration work in terms of three dimensionless vortex trail parameters. Comparing this analysis with the experimental data leads to a qualitative discussion of the significance of the vortex formation mechanism in controlling the excitation or damping of that class of vibrations which is affected by vortex shedding.     

三分螺旋折流板换热器壳侧流场温度场模拟

建立了三分螺旋折流板换热器壳侧流动与传热的数学模型,对其流场、温度场和压力场的数值模拟结果在多个纵剖面和横切片上进行了展示。可见在螺旋通道的轴心和靠近壳体的折流板外缘区域局部速度较高,在折流板背流面呈现出有利于强化掺混作用的回流区,流道内几乎没有流动死区;壳侧流体温度为稳步均匀下降趋势,并呈现从圆周的外缘向轴心方向逐渐递减;而压力场则呈现明显的周期性和阶梯性。     

A coaxial vortex ring model for vortex breakdown

A simple-yet plausible-model for B-type vortex breakdown flows is postulated; one that is based on the immersion of a pair of slender coaxial vortex rings in a swirling flow of an ideal fluid rotating around the axis of symmetry of the rings. It is shown that this model exhibits in the advection of passive fluid particles (kinematics) just about all of the characteristics that have been observed in what is now a substantial body of published research on the phenomenon of vortex breakdown. Moreover, it is demonstrated how the very nature of the fluid dynamics in axisymmetric breakdown flows can be predicted and controlled by the choice of the initial ring configurations and their vortex strengths. The dynamic intricacies produced by the two ring + swirl model are illustrated with several numerical experiments.     

Experimental Investigation of Opposing Mixed Convection in a Channel with an open Cavity Below

Abstract

In this article, mixed convection in an open cavity with a heated wall bounded by a horizontal unheated plate is investigated experimentally. The heated wall is on the opposite side of the forced inflow. The results are reported in terms of wall temperature profiles of the heated wall and flow visualization. The range of pertinent parameters used in this experiment are Reynolds numbers (Re) from 100 to 2,000 and Richardson numbers (Ri) from 4.3 to 6,400. Also, the ratio between the length and the height of cavity (L/D) ranges from 0.5–2.0, and the ratio between the channel and cavity height (H/D) is equal to 1.0. The lack of experimental results on mixed convection in a channel with an open cavity below was an impetus for investigating this configuration when one cavity vertical wall is heated at uniform heat flux. The present results show that at the lowest investigated Reynolds number, the surface temperatures are lower than the corresponding surface temperatures for Re = 2,000 at the same ohmic heat flux. The flow visualization shows that for Re = 1,000, there are two nearly distinct fluid motions: a parallel forced flow in the channel and a recirculation flow inside the cavity. For Re = 100, the effect of a stronger buoyancy determines a penetration of thermal plumes from the heated plate wall into the upper channel. Moreover, the flow visualization shows that for lower Reynolds numbers, the forced motion penetrates inside the cavity, and a vortex structure is adjacent to the unheated vertical plate. At higher Reynolds numbers, the vortex structure has a larger extension while L/D is held constant.     

Quenching Properties of an Arc in a Double Flow with a Vortex

The effect of a vortex in a gas flow on an air-blast arc is investigated. The radial density of a vortex in the compressible flow is evaluated with a simple model. The experiments show that the width of a low pressure channel on the axis of the nozzle is comparable to the theoretical values. The measured electric field strength profile is strongly influenced by the presence of such a vortex. In addition, the thermal interrupting capability is drastically lowered by vortex superimposed on the axial gas flow.     

Viscous near-wall flow in a wake of circular cylinder at moderate Reynolds numbers

Here we present the results of experimental investigation of a cross flow around a circular cylinder mounted near the wall of a channel with rectangular cross section. The experiments were carried out in the range of Reynolds numbers corresponding to the transition to turbulence in a wake of the cylinder. Flow visualization and SIV-measurements of instantaneous velocity fields were carried out. Evolution of the flow pattern behind the cylinder and formation of the regular vortex structures were analyzed. It is shown that in the case of flow around the cylinder, there is no spiral motion of fluid from the side walls of the channel towards its symmetry plane, typical of the flow around a spanwise rib located on the channel wall. The laminar-turbulent transition in the wake of the cylinder is caused by the shear layer instability.     

Viscoelastic effects on asymmetric two‐dimensional vortex patterns in a strongly coupled dusty plasma

Strongly coupled dusty plasma medium is often described as a viscoelastic fluid that retains its memory. In a flowing dusty plasma medium, vortices of different sizes appear when the flow does not remain laminar. The vortices also merge to transfer energy between different scales. In the present work, we study the effect of viscoelasticity and compressibility over a localized vortex structure and multiple rotational vortices in a strongly coupled viscoelastic dusty plasma medium. In case of single rotating vortex flow, a transverse wave is generated from the localized vortex source and the evolution time of generated waves is found to be reduced due to finite viscoelasticity and compressibility of the medium. It is found that the viscoelasticity suppresses the dispersion of vorticity. In the presence of multiple vortices, we find, the vortex mergers get highly affected in the presence of memory effect of the fluid, and thus the dynamics of the medium gets completely altered compared to a non‐viscoelastic fluid. For a compressible fluid, viscoelasticity dampens the energy in the sonic waves generated in the medium. Thus a highly viscoelastic and compressible fluid, in some cases, behaves similarly to an incompressible viscoelastic fluid. The wave‐front like rings propagate in elliptical orbits keeping the footprint of the earlier position of the point‐vortex. The rings collide with each other even within the patch vortex region forming regions of high vorticity at the point of intersection and pass through each other.     

Asymmetric opening of a simple bileaflet valve

The opening motion of a bileaflet valve driven by a pulsed stream is studied in the limiting condition of rigid massless leaflets closing a two-dimensional channel. This simple arrangement allows us to write simple dynamical equations for the solid that are solved numerically with the Navier-Stokes equation for the fluid. The analysis is focused on the influence of asymmetry on the coupled fluid-valve dynamics when parameters are taken with references to cardiac valves. Results show that the wake generation from the leaflet's trailing edge can be partly inhibited; the primary vortex downstream does not occur in an intermediate range of asymmetry. The potential emergence of such a phenomenon in realistic cases would present implications in development of diagnostic schemes.     

Observation of vortex packets in direct numerical simulation of fully turbulent channel flow

A number of experimental studies have inferred the existence of packets of inclined, hairpinlike vortices in wall turbulence on the basis of observations made in two-dimensional x−y planes using visualization and particle image velocimetry (PIV). However, there are very few observations of hairpins in existing three-dimensional studies made using direct numerical simulation (DNS), and no such study claims to have revealed packets. We demonstrate, for the first time, the existence of hairpin vortex packets in DNS of turbulent flow. The vortex packet structure found in the present study at low Reynolds number,Re _t=300, is consistent with and substantiates the observations and the results from twodimensional PIV measurements at higher Reynolds numbers in channel, pipe and boundary layer flows. Thus, the evidence supports the view that vortex packets are a universal feature of wall turbulence, independent of effects due to boundary layer trips or critical conditions in the aforementioned numerical studies. Visualization of the DNS velocity field and vortices also shows the close association of hairpin packets with long low-momentum streaks and the regions of high Reynolds shear stress.     

Long-range nonlocal flow of vortices in narrow superconducting channels

We report a new nonlocal effect in vortex matter, where an electric current confined to a small region of a long and sufficiently narrow superconducting wire causes vortex flow at distances hundreds of intervortex separations away. The observed remote traffic of vortices is attributed to a very efficient transfer of a local strain through the one-dimensional vortex lattice (VL), even in the presence of disorder. We also observe mesoscopic fluctuations in the nonlocal vortex flow, which arise due to "traffic jams" when vortex arrangements do not match a local geometry of a superconducting channel.     

4Pi focusing of spatially modulated radially polarized vortex beams

We propose a method for generating focal beams with special intensity distributions using radially polarized vortex beams in a 4Pi configuration. A spherical dark-hollow beam and hollow beam array can be obtained by vortex beams with topological charge of m=1. A dark channel can be generated using vortex beams with topological charge of m=2. The length of the well-defined hollow beam array and the dark channel is about 30λ. These interesting beams are useful in optical trapping and manipulation.     

Stability of solitons on vortex filaments

A vortex filament is a filament on which fluid vorticity is concentrated. This concept is particularly important in superfluidity and turbulence. This Letter focuses on the vortex filament equation (VFE), which is a model for the motion of a vortex filament in an incompressible and inviscid fluid. The VFE soliton solutions are considered and their spectral stability is proven by developing a straightforward method to solve the corresponding eigenvalue problem. A similar analysis is performed on the planar vortex filament equation. We discuss the applicability of the methods introduced in this Letter to other physically relevant curve equations and other types of solutions.     

Majorana modes at the ends of superconductor vortices in doped topological insulators

Recent experiments have observed bulk superconductivity in doped topological insulators. Here we ask whether vortex Majorana zero modes, previously predicted to occur when s-wave superconductivity is induced on the surface of topological insulators, survive in these doped systems with metallic normal states. Assuming inversion symmetry, we find that they do but only below a critical doping. The critical doping is tied to a topological phase transition of the vortex line, at which it supports gapless excitations along its length. The critical point depends only on the vortex orientation and a suitably defined SU(2) Berry phase of the normal state Fermi surface. By calculating this phase for available band structures we determine that superconducting p-doped Bi(2)Te(3), among others, supports vortex end Majorana modes. Surprisingly, superconductors derived from topologically trivial band structures can support Majorana modes too.     

掠过双板流动引起的旋涡脱落特性数值模拟

板状燃料组件在先进核反应堆中得到重要应用.流体以一定的流速轴掠板状组件时会导致板后产生旋涡脱落现象.旋涡脱落有可能引发板状燃料组件的流致振动.使用BELIEF程序,通过改变方柱间距模拟了Re=200情况下刚性矩形通道内不同节距条件下双平行方柱的旋涡脱落现象,得出了双柱间节距对双平行方柱旋涡脱落特性的影响,并进一步对双平...     

Absolute measurement of vortex line density in counterflowing hell

The vortex line density in a thermally induced counterflow in helium II is measured by means of attenuation of second sound in a long channel 1 cm × 1 cm in cross section. The line density is calibrated by rotating the channel about its vertical axis. The results are compared with current theories of vortex line turbulence.     

Topological excitations in rotating spin–orbit-coupled spin-1 Bose–Einstein condensates with in-plane gradient magnetic field

We investigate the topological excitations of rotating spin-1 ferromagnetic Bose–Einstein condensates with spin–orbit coupling (SOC) in an in-plane quadrupole field. Such a system sustains a rich variety of exotic vortex structures due to the spinor order parameter and the interplay among in-plane quadrupole field, SOC, rotation, and interatomic interaction. For the nonrotating case, with the increase of the quadrupole field strength, the system experiences a transition from a coreless polar-core vortex with a bright soliton to a singular polar-core vortex with a density hole. Without rotation but with a fixed quadrupole field, when the SOC strength increases, the system transforms from a central Mermin–Ho vortex into a criss-crossed vortex–antivortex string lattice. For the rotating case, we give a phase diagram with respect to the quadrupole field strength and the SOC strength. It is shown that the rotating system supports four typical quantum phases: vortex necklace, diagonal vortex chain cluster, single diagonal vortex chain, and few vortex states. Furthermore, the system favors novel spin textures and skyrmion excitations including an antiskyrmion, a criss-crossed half-skyrmion–half-antiskyrmion lattice, a skyrmion-meron necklace, a symmetric half-skyrmion lattice, and an asymmetric skyrmion-meron lattice.