Flow-field dynamics of the non-reacting and reacting jet in a vitiated cross-flow

The dynamics of a premixed ethylene-air jet injected transverse to a vitiated cross-flow were investigated using high-repetition rate particle image velocimetry (PIV). Both non-reacting and reacting jets were found to be characterized by a dominant frequency associated with the jet wake vortex system. For the isothermal jet, increasing the momentum flux ratio (J) has only a slight effect on the frequency of the oscillation but significantly increases its magnitude. The reacting jet was found to exhibit different behavior, with a monotonic increase in the dominant frequency with J. The jet equivalence ratio (?_j) was found to have little effect on the rate of wake vortex shedding but affects the overall magnitude of the oscillation. Comparison with data reported in the literature suggests the relationship between the wake Strouhal number (St_w) and J is fuel dependent. Application of a vortex detection algorithm shows a stark difference in the location of the wake vortices under non-reacting and reacting conditions. Under isothermal conditions, the vortices are found close to the jet centerline and dissipate relatively quickly. Reaction confines the vortices to a narrow shear layer until a farther distance downstream and the vortices convect through the entire area of interest. Additionally, the vortex circulation strength was found to increase with J. Proper orthogonal decomposition (POD) analysis of the non-reacting and reacting jets demonstrates the dominance of the wake vortex structures in the oscillating flow fields. In both cases, the temporal information extracted from the most energetic modes is identical to the dominant frequencies measured in the flow fields. The primary effect of heat release is to reduce the overall amount of coherence and to delay the appearance of elevated coherence levels until a larger streamwise distance from the jet exit.     

基于速度梯度张量的四元分解对若干涡判据的评价

本文基于速度梯度张量分析,对其中四种ω判据、Q判据、w判据、λci判据的物理意义和局限性进行分析,揭示各判据常用等值面展示的涡形态或强度的实际物理意义.首次采用基于速度梯度张量正规性的四元分解,将流体微元的运动分解为胀缩、沿正规标架的轴向变形、做平面运动和简单剪切,使得各涡判据的运动学意义更加清晰.涡量ω反映的流体微元的平均转动中总是包含简单剪切运动;Q判据可揭示流体微元在复特征向量平面上净转动相对于轴向变形的强弱,是净转动存在的充分但非必要条件;fi判据能准确辨别净转动是否存在,却无法表示出净转动的强度;在净转动存在的前提下,λci可反映其绝对强度大小,净转动是复特征向量平面内正规转动和简单剪切的总和效果,正规转动是最基本的转动.新引入的四元分解方法有利于深入了解流体的涡及其运动.     

Visualisation and analysis of large-scale vortex structures in three-dimensional turbulent lid-driven cavity flow

In this paper, large eddy simulation (LES) of a three-dimensional turbulent lid-driven cavity (LDC) flow at Re = 10,000 has been performed using the multiple relaxation time lattice Boltzmann method. A Smagorinsky eddy viscosity model was used to represent the sub-grid scale stresses with appropriate wall damping. The prediction for the flow field was first validated by comparing the velocity profiles with previous experimental and LES studies, and then subsequently used to investigate the large-scale three-dimensional vortical structures in the LDC flow. The instantaneous three-dimensional coherent structures inside the cavity were visualised using the second invariant (Q), Δ criterion, λ₂ criterion, swirling strength (λ_ci) and streamwise vorticity. The vortex structures obtained using the different criteria in general agree well with each other. However, a cleaner visualisation of the large vortex structures was achieved with the λ_ci criterion and also when the visualisation is based on the vortex identification criteria expressed in terms of the swirling strength parameters. A major objective of the study was to perform a three-dimensional proper orthogonal decomposition (POD) on the fluctuating velocity fields. The higher energy POD modes efficiently extracted the large-scale vortical structures within the flow which were then visualised with the swirling strength criterion. Reconstruction of the instantaneous fluctuating velocity field using a finite number of POD modes indicated that the large-scale vortex structures did effectively approximate the large-scale motion. However, such a reduced order reconstruction of the flow based on the large-scale vortical structures was clearly not as effective in predicting the small-scale details of the fluctuating velocity field which relate to the turbulent transport.     

Stereoscopic PIV measurements of flow around a marine propeller

A stereoscopic PIV (Particle Image Velocimetry) technique has been employed to measure the 3 dimensional flow structure of turbulent wake behind a marine propeller with 5 blades. The out-of-plane velocity component was measured using particle images captured simultaneously by two CCD cameras installed in the angular displacement configuration. 400 instantaneous velocity fields were acquired for each of four different blade phases of 0°, 18°, 36° and 54°. They were ensemble averaged to investigate the spatial evolution of propeller wake in the near wake region up to one propeller diameter (D) downstream. The phase-averaged velocity fields show clearly the viscous wake formed by the boundary layers developed along both surfaces of the blade. Tip vortices were generated periodically and the slipstream contraction occurs in the near-wake region. The out-of-plane velocity component has large values at the locations of tip and trailing vortices. With going downstream, the axial turbulence intensity and the strength of tip vortices were decreased due to the viscous dissipation, turbulence diffusion and blade-to-blade interaction. The difference in the mean velocity fields measured by SPIV and 2-D PIV methods was about 5% ≈ 10%. However, the 2-D PIV results also give sufficient information on propeller wake beyond the region of X/D=0.2.     

Visualization of the tip vortices in a wind turbine wake

Abstract  

In the present study, an experimental study was conducted to characterize the formation and the evolution of the helical tip vortices and turbulent flow structures in the wake of a horizontal axis wind turbine model placed in an atmospheric boundary layer wind. A high-resolution particle image velocimetry system was used to make detailed flow field measurements to quantify the time evolution of the helical tip vortices in relation to the position of the rotating turbine blades in order to elucidate the underlying physics associated with turbine power generation and fatigue loads acting on the wind turbines.     

涡轮叶栅尾迹对泄漏流影响的试验研究

本文利用运动圆柱排模拟上游叶片尾迹,在平面涡轮叶栅上研究了上游尾迹与叶栅通道内泄漏流的相互作用,初步讨论了尾迹与泄漏流相互作用的机理,研究发现利用这种相互作用可以控制泄漏涡的强度、减小泄漏损失,进而可实现提高涡轮效率的目的。     

New omega vortex identification method

A new vortex identification criterion called W-method is proposed based on the ideas that vorticity overtakes deformation in vortex.The comparison with other vortex identification methods like Q-criterion and λ_2-method is conducted and the advantages of the new method can be summarized as follows:(1) the method is able to capture vortex well and very easy to perform;(2) the physical meaning of W is clear while the interpretations of iso-surface values of Q and λ_2 chosen to visualize vortices are obscure;(3)being different from Q and λ_2 iso-surface visualization which requires wildly various thresholds to capture the vortex structure properly, W is pretty universal and does not need much adjustment in different cases and the iso-surfaces of W=0.52 can always capture the vortices properly in all the cases at different time steps, which we investigated;(4) both strong and weak vortices can be captured well simultaneously while improper Q and λ_2 threshold may lead to strong vortex capture while weak vortices are lost or weak vortices are captured but strong vortices are smeared;(5) W=0.52 is a quantity to approximately define the vortex boundary. Note that, to calculate W, the length and velocity must be used in the non-dimensional form. From our direct numerical simulation, it is found that the vorticity direction is very different from the vortex rotation direction in general 3-D vortical flow,the Helmholtz velocity decomposition is reviewed and vorticity is proposed to be further decomposed to vortical vorticity and non-vortical vorticity.     

Multiple andreev reflections spectroscopy of superconducting LiFeAs single crystals: Anisotropy and temperature behavior of the order parameters

The superconducting state of LiFeAs single crystals with the maximum critical temperature T _c ≈ 17 K in the 111 family has been studied in detail by multiple Andreev reflections (MAR) spectroscopy implemented by the break-junction technique. The three superconducting gaps, Δ_Γ = 5.1–6.5 meV, Δ_L = 3.8–4.8 meV, and Δ_S = 0.9–1.9 meV (at T ? T _c), as well as their temperature dependences, have been directly determined in a tunneling experiment with these samples. The anisotropy degrees of the order parameters in the k space have been estimated as <8, ～12, and ～20%, respectively. Andreev spectra have been fitted within the extended Kümmel-Gunsenheimer-Nikolsky model with allowance for anisotropy. The relative electron-boson coupling constants in LiFeAs have been determined by approximating the Δ(T) dependences by the system of the two-band Moskalenko and Suhl equations. It has been shown that the densities of states in bands forming Δ_Γ and Δ_L are approximately the same, intraband pairing dominates in this case, and the interband coupling constants are related as λ_ΓL ≈ λ_LΓ ? λ_SΓ, λ_SL.     

Covariance measurements in the initial region of an annular jet

This paper describes the results of correlation measurements in the initial region of a basic annular jet, which is under the effect of the jet vortices and of the wake induced vortices. Evidence has been found that the wake induced vortices are excited by the disturbances associated with the wake vortices. For these wake induced vortices the azimuthal constituent (m = 1) is dominant. The similarity observed between the behaviour of the jet vortices and that of the wake induced vortices suggests that the jet vortices are also excited by these disturbances of the wake vortices, having the same Strouhal number. Thus the jet vortices are also dominated by the azimuthal constituent (m = 1). However, evidence has been found that within the first two outer diameters an axisymmetrical constituent (m = 0) is also present within the jet vortices. Comparison of the wake induced vortices with the jet vortices shows that these two helical waves are out of phase and are axially separated.     

Low-coherence interferometric tip-clearance probe

We propose an all-fiber, self-calibrating, economical probe that is capable of near-real-time, single-port, simultaneous blade-to-blade tip-clearance measurements with submillimeter accuracy (typically < 100 microm, absolute) in the first stages of a gas turbine. Our probe relies on the interference between backreflected light from the blade tips during the 1-micros blade passage time and a frequency-shifted reference with variable time delay, making use of a low-coherence light source. A single optical fiber of arbitrary length connects the self-contained optics and electronics to the turbine.     

风力机翼型在失速工况下非定常流场的本征正交分解分析

风力机气动性能受静态失速与动态失速影响很大,对风力机翼型的失速问题研究具有重要意义。本文通过计算流体力学方法得到的风力机翼型在固定大攻角工况,以及大攻角震荡工况下的非定常流场,来研究翼型静态失速与动态失速。采用本征正交分解方法(POD),对非定常流场降阶,得到流场的POD模态以及对应的系数。POD模态结果表明在静态失速下,主要非定常流动结构是尾迹区域交替脱落的涡结构;在动态失速下,除了尾迹区域,前缘和整个吸力面都存在流动分离结构。     

Streak instability in near-wall turbulence revisited

The regeneration cycle of streaks and streamwise vortices plays a central role in the sustainment of near-wall turbulence. In particular, the streak breakdown phase in the regeneration cycle is the core process in the formation of the streamwise vortices, but its current understanding is limited particularly in a real turbulent environment. This study is aimed at gaining fundamental insight into the underlying physical mechanism of the streak breakdown in the presence of background turbulent fluctuation. We perform a numerical experiment based on direct numerical simulation, in which streaks are artificially generated by a body forcing computed from previous linear theory. Upon increasing the forcing amplitude, the artificially driven streaks are found to generate an intense fluctuation of the wall-normal and spanwise velocities in a fairly large range of amplitudes. This cross-streamwise velocity fluctuation shows its maximum at λ⁺_x ≈ 200 ? 300 (λ⁺_x is the inner-scaled streamwise wavelength), but it only appears for λ⁺_x ? 3000 ? 4000. Further examination with dynamic mode decomposition reveals that the related flow field is composed of sinuous meandering motion of the driven streaks and alternating cross-streamwise velocity structures, clearly reminiscent of sinuous-mode streak instability found in previous studies. Finally, it is shown that these structures are reasonably well aligned along the critical layer of the secondary instability, indicating that the surrounding turbulence does not significantly modify the inviscid inflectional mechanism of the streak breakdown via streak instability and/or streak transient growth.     

透平叶片中的二次流旋涡结构的研究

当今在透平机械行业中的研究人员常常会被二次流旋涡结构的研究所吸引。本文作者主要用数值计算,部分地也用实验手段,试图研究、分析与讨论叶片通道中马蹄涡、通道涡、角隅涡和间隙涡等二次流旋涡的生成、演绎与发展。对于壁涡由于尺寸很小而不容易发现。现在应用弯扭叶片作为一种手段来降低叶片损失的人多了,从而需要对弯扭叶片对上述旋涡的影响进行研究。因此,本文对周向弯扭叶片的影响以及其对损失影响的机理也进行了讨论。     

Experimental investigation on laminar separation control for flow over a two-dimensional bump

The laminar boundary layer separation flow over a two-dimensional bump controlled by synthetic jets is experimentally investigated in a water channel with hydrogen-bubble visualisation and particle image velocimetry (PIV) techniques. The two-dimensional synthetic jet is applied near the separation point. Two Reynolds numbers (Re = 700 and 1120) based on the bump height and free-stream velocity are adopted in this experiment, and seven different excitation frequencies at each Reynolds number are considered, focusing on the separation control as well as the vortex dynamics. The experimental results show that the optimal control can only be achieved within some excitation frequencies at both Reynolds numbers. However, beyond this range, further increasing the excitation frequency leads to an increase in the separation region. The proper orthogonal decomposition (POD) technique and vortex identification by swirling strength (Λ_ci) are applied for the deeper analysis of the separated flow. The reconstructed Λ_ci field by the first four POD modes is used and vortex lock-on phenomenon is observed. It is found that the negative synthetic jet vortex with clockwise rotation draws the separated wake shear layer as it is convected downstream, and then they syncretise together. Thus, the new vortex is induced and shedding downstream periodically.     

POD analysis on the sphere wake at a subcritical Reynolds number

Abstract  

Flow characteristics of turbulent wake behind a sphere at a subcritical flow regime were experimentally investigated. The particle image velocimetry measurements and proper orthogonal decomposition (POD) modal analysis were employed to get detailed flow information such as the wavy structure, swirling motion and coherent structures of the sphere wake. The variation of turbulent intensities of the radial and circumferential velocity components showed the swirling motion of sphere wake in the cross-sectional planes. The relative contribution of the POD mode 1, 2 and 3 in eigenvalues was 26, 11, and 8%, respectively. The general pattern of velocity fields for the POD mode 1 in the near-wake region of x/d = 0.7–1.4 is similar with that of time-averaged mean velocity fields. In addition, the sweeping flow in the region from x/d = 1.5 to x/d = 2.0 possesses wavy structure of the sphere wake. The experimental results of the present study would contribute to the fundamental understanding of the turbulent near-wake behind a sphere.     

压气机级间尾迹非定常流动的分析与利用途径探索

在真实轴流压气机中,上游静子叶排尾迹经由静止/旋转两套坐标系转换,对下游转子叶排流场进口气流产生准周期性扰动.对尾迹干扰下的转子非定常流场进行数值模拟,与传统设计定常流动假设下均匀进气的流场对比分析了尾迹干扰的影响.结果表明,在适当条件下,上游静子叶排尾迹运动对下游转子分离流旋涡运动能够产生有益的影响,改善叶背分离旋涡运动与转子叶片静压分布,提高转子流动效率与压比.     

动静干涉效应对轴流透平级气动性能的影响

本文对Aachen一级半轴流式透平进行了准定常及非定常流动的数值模拟,讨论了引发动静干涉效应的主要因素,分析了非定常效应对叶栅通道中脉动速度、气流角、型面压力分布以及损失结构的影响。结果表明,对于Aachen透平而言,上游叶片排的尾迹区及二次涡是引发下游叶栅通道流场强非定常特性的最主要因素,引起了脉动速度、型面压力、攻角以及损失的较大变化,从而对透平级性能产生较大的影响。     

Onset of chaos in finite interacting Boson systems with F-spin

For m bosons, carrying spin (f = 1 2) degree of freedom, in Ω single particle levels which are doubly degenerate, one-plus two-body Embedded Gaussian Orthogonal random matrix Ensemble which conserves spin (F) [BEGOE(1 + 2)-F] is defined and spectral properties are examined. Using mean field defined by random single particle energies, for fixed-(m,F) BEGOE(1 + 2)-F, density of states being close to Gaussian, we analyze spectral fluctuation properties, like Nearest Neighbour Spacing Distribution (NNSD) and Dyson-Mehta (Δ₃) statistic, as a function of λ- the two-body interaction strength. Numerical calculations are used to demonstrate that for very small value of λ, NNSD and Δ₃ are found close to Poisson form and they move steadily to GOE form as λ increases. Moreover, spin dependence of the transition point λ _C is obtained. It is also verified using periodogram analysis that BEGOE(1 + 2)-F spectra shows 1/f–noise behaviour.     

Taylor-Görtler vortices in the flow driven by a rotating magnetic field in a cylindrical container

Taylor-Görtler vortices play an important role in the flow driven by rotating magnetic field in a closed cylindrical container. The visualization of these structures is essential to understand their physics and their impact on the dynamics of the flow. We present a study of various methods for identifying vortex cores, including the so-called λ₂ andQriteria. While λ₂ is, probably, the better indicator it is more sensitive to numerical noise and thus less effective in devising 3D vortex structures. Here, the Q criterion proved particularly useful when applied to the fluctuation velocity field.     

Vortex stretching and reconnection in a compressible fluid

Vortex stretching in a compressible fluid is considered. Two-dimensional (2D) and axisymmetric cases are considered separately. The flows associated with the vortices are perpendicular to the plane of the uniform straining flows. Externally-imposed density build-up near the axis leads to enhanced compactness of the vortices — “dressed" vortices (in analogy to “dressed" charged particles in a dielectric system). The compressible vortex flow solutions in the 2D as well as axisymmetric cases identify a length scale relevant for the compressible case which leads to the Kadomtsev-Petviashvili spectrum for compressible turbulence. Vortex reconnection process in a compressible fluid is shown to be possible even in the inviscid case — compressibility leads to defreezing of vortex lines in the fluid.