Visualization of the inflow ahead of a rotating propeller attached to a container ship model

The inflow ahead of a rotating propeller attached to a container ship model was visualized using a two-frame particle image velocimetry (PIV) technique. For illuminating the inflow region, a transparent window was installed at the stern of the ship model. Ensemble-averaged mean velocity fields were measured at four different blade phases under the design loading condition. The characteristics of the inflow in the upper plane above the propeller axis are quite different from those below the propeller axis. In the far upstream region above the propeller axis, most of the inflow comes from the hull wake and the axial velocity is very small. As the inflow moves toward the propeller plane, its axial velocity component increases rapidly. In addition, the variation of the inflow characteristics with respect to phase angle becomes apparent. The thick hull boundary layer and out-of-plane motion resulting from the propeller rotation produce a large turbulent kinetic energy around the tip of the propeller blade in the upper inflow region. The axial velocity distribution of the propeller inflow is asymmetric with respect to the vertical center axis, exhibiting different axial velocities on the port and starboard sides.     

Vortex identification in the wake of a model wind turbine array

Vortex identification techniques are used to analyse the wake of a 4 × 3 array of model wind turbines. The Q-criterion, Δ-criterion, and λ₂-criterion are applied to particle image velocimetry data gathered fore and aft of the last row centerline turbine. The Q-criterion and λ₂-criterion provide a clear indication of regions where vortical activity exists, while the Δ-criterion is not successful. Galilean decomposition, Reynolds decomposition, vorticity, and swirling strength are used to further understand the location and behaviour of the vortices. The techniques identify and display the high-magnitude vortices in high-shear zones resulting from the blade tips. Using Galilean and Reynolds decomposition, swirling motions are shown encapsuling vortex regions in agreement with the identification criteria. The Galilean decompositions selected are 20% and 50% of a convective velocity of 7 m/s. As the vortices convect downstream, the strength of the vortices decreases in magnitude, particularly in the far wake of the array, to approximately 25% of those present in the near wake. A high level of vortex activity is visualised as a result of the top tip of the wind turbine blade -- the location where the highest vertical entrainment is present. Analysing the full frame set, the Q-criterion, λ₂-criterion, and swirling strength prove comparable, while the Δ-criterion under-performs in regions of high turbulence activity, namely in the back of the turbine. Entraining flow into the turbine canopy interacting with the turbine generates high-magnitude vortices concentrated at the blade tips. The count of vortices decreases when moving from the top tip down to the wall, as well as their strength for each Galilean technique when a non-zero threshold is applied. Vortex sizes in the near wake are found comparable to turbine blade, hub, and mast dimensions. In the far wake, the resulting size of the vortices is approximately 30% of those in the near wake. These vortices increase in velocity as they convect downstream, following the mean velocity behaviour. The lowest magnitude vortices reside at the hub height in the near-wake region, where they convect at nearly half the speed of those at the blade tips.     

PIV measurements on the change of the three-dimensional wake structures by an air spoiler of a road vehicle

The three-dimensional vortical structures formed in the wake behind a road vehicle were measured using a particle image velocimetry (PIV) technique and the change of the structures by the existence of an air spoiler was investigated. The measurements were carried out in severalx-y, y-z andz-x planes to obtain full three-dimensional flow fields, including an out-of-plane velocity component, obtained by interpolating the velocities in the other plane. Then, the velocity gradient tensor is evaluated to obtain the vortex core by theλ ₂-definition. The relationship between streamwise, longitudinal and spanwise vortices is systematically analyzed by overlapping the vortex lines and vortex cores and the whole flow topology is compared in both cases with and without an air spoiler. As a result, an air spoiler was found to weaken the C-pillar vortices producing strong wing tip vortices, which reduce downwash flow and longitudinal vortices increase in the vertical direction. The recirculation zone formed when an air spoiler is installed is higher and narrower than without a spoiler.     

Flow diagnostics downstream of a tribladed rotor model

This paper presents results of a study of vortex wake structures and measurements of instantaneous 3D velocity fields downstream of a triblade turbine model. Two operation modes of flow around the rotor with different tip speed ratios were tested. Initially the wake structures were visualized and subsequently quantitative data were recorded through velocity field restoration from particle tracks using a stereo PIV system. The study supplied flow diagnostics and recovered the instantaneous 3D velocity fields in the longitudinal cross section behind a tribladed rotor at different values of tip speed ratio. This set of data provided a basis for testing and validating assumptions and hypothesis regarding classical theories of rotors.     

Measurements of aerodynamic noise and wake flow field in a cooling fan with winglets

The aerodynamic noise and the wake flow field in a cooling fan under actual operating conditions are studied with and without winglets on the fan blades. In order to understand the influence of the winglet, the aerodynamic noise and the wake velocity distribution are measured. The results indicated that overall noise level decreased and the noise spectrum was changed in a low frequency range when the winglet was installed. It was found from the flow visualization and PIV measurement that the influence of the winglet appeared in the traces of the tip vortices and the magnitude of vorticity was reduced in the near wake region, which suggest the observed reduction in aerodynamic noise     

水平轴风力机叶尖涡流动的PIV测试

在风洞开口实验段,针对不同尖速比应用PIV技术对风轮附近1/3圆周内的瞬时速度场进行测试。随着尖速比的增加,叶尖涡运动的径向位移增大,涡流诱导效应区域半径扩大,使整体尾迹半径增大;同时,尖速比增加使尾迹轴向速度亏损增大,叶尖涡运动的轴向位移减小。叶尖涡诱导效应与风轮旋转的相互作用使风轮附近形成强速度梯度区域,诱导效应延伸到风轮上游,激励叶片产生不同的振动形态,使叶片上脱落的叶尖涡结构在尾迹中出现波动,越向下游运动波动越强烈。通过叶尖涡核位置平均发现,在10°～120°方位角尾迹区域内,涡核运动轨迹与方位面交点的连线近似为直线,且该线的斜率随着尖速比的增加而增大。     

旋转离心叶轮内部非定常流动实验研究

利用激光成像速度仪(PIV)测量了旋转离心叶轮内部的非定常流场,获得了旋转离心叶轮内部相对速度的非定常流场分布。详细分析了叶轮内部非定常流动现象和流动规律。通过实验研究发现旋转离心叶轮内部的流动是非定常,在叶轮出口处,叶片的吸力面与轮盖的夹角区存在一个低速区,并观察到了明显的射流/尾迹结构。射流区和尾流区的大小和范围在沿盘盖方向和跨叶片方向上是不同的。射流区和尾流区之间不存在明显的分界线。     

Flow visualization and PIV measurement of flow field around a darrieus rotor in dynamic stall

The flow field around a Darrieus rotor in dynamic stall is studied by flow visualization and PIV measurements. The visualization is carried out by dye injection technique while the phase averaged velocity distributions around the blade are measured by PIV combined with a conditional imaging technique. The results indicate the appearance of dynamic stall phenomenon due to the shedding of two pairs of vortices from the blade during one rotation of the rotor. These stall vortices are produced by the separation of flow over the inner surface of the blade and the formation of roll-up vortices from the outer surface. The second stall vortices develop along the blade and strongly interact with the flow field near the blade, affecting the aerodynamic performance of the rotor.     

Visualization of aerodynamic noise source around a rotating fan blade

The purpose of this study is to understand the aerodynamic noise source distribution around a rotating fan blade by measuring the noise signal and velocity field around the blade. The local noise-level distribution over the fan blade is measured by microphone arrays, and the flow field is visualized by smoke and phase-averaged PIV measurement. The noise source distribution is examined by cross-correlation analysis between noise signal and velocity fluctuation. It is found that the noise source is located near the rotating fan blade, especially around leading and trailing edges. The separation and reattachment of flow are observed near the leading edge, and the tip vortices and vortex shedding are found near the trailing edge. The cross-correlation distribution of the noise signal and the radial velocity fluctuation shows large magnitude in the correlated regions, which indicates the noise generation by the formation of vortex structure around the blade.     

Vortex generation in the cross-flow around a cylinder attached to an end-wall

A cylinder attached to an end-wall normal to its axis is a common feature of many practical flow systems, e.g. in turbo-machinery or when a bridge is supported by a pillar from the bed of a river. In this situation, the nominally two-dimensional boundary layer flow incident upon the cylinder develops strong three-dimensional features and a very pronounced vortex structure may arise in the upstream flow close to the wall. For the appropriate Reynolds number range, the upstream vortical structure is nominally steady and is commonly referred to as the “horseshoe vortex system”. In contrast, the flow downstream is unsteady and periodic over a wide range of Reynolds numbers and vortices aligned with the cylinder axis are shed at a regular frequency into the wake. The generation of both these vortex systems requires energy to be extracted from the incident flow with the result that the drag force on the cylinder is increased.This paper concentrates on the upstream region of the cylinder and discusses an investigation in which two-component Particle Image Velocimetry (PIV) has been used to visualise the flow behaviour for a circular cylinder on a plane end-wall. The use of PIV has enabled two orthogonal velocity components to be measured in planes defined by the upstream flow direction and the axis of the cylinder. The third (out-of-plane) velocity component was then calculated by integrating the continuity equation. Subsequently, the velocity field information has been manipulated and converted into time-averaged information.Discussion of the measured results confirms that colour displays are an invaluable aid to understanding this complex fluid flow situation since they reveal substantially more information than grey-scale plots of the same data. In particular, the source of the horseshoe vortex system can be identified when colour plots of the time-averaged velocity and vorticity distributions are obtained. A limited amount of information on the unsteady vortex structures appearing in the end-wall region upstream of the cylinder is also presented. Finally, the experimental findings are discussed in relation to the results of previous workers.     

旋翼尾流时空发展二维速度场的实验研究

旋翼作为直升机的主要升力和操纵部件, 具有复杂的流场结构, 如非定常性, 桨-涡干扰和桨尖涡等, 导致旋翼流场研究十分困难. 针对这一问题, 结合锁相技术和粒子图像测速(particle image velocimetry, PIV)技术开展了悬停状态下旋翼流场的实验研究, 并通过本征正交分解(proper orthogonal decomposition, POD)提取主要含能模态, 刻画流场时空演化. 结果显示, 旋翼尾流发展过程中向旋转轴靠近, 二维流场结构呈现倒三角结构, 即扩展到三维流动中会呈现倒锥型结构的特性; 通过POD进行含能模态分析, 旋翼尾流中对湍动能贡献最大的为桨叶涡结构, 其次是桨尖涡结构.      

On vortex–airfoil interaction noise including span-end effects,with application to open-rotor aeroacoustics

A linear analytical model is developed for the chopping of a cylindrical vortex by a flat-plate airfoil, with or without a span-end effect. The major interest is the contribution of the tip–vortex produced by an upstream rotating blade in the rotor–rotor interaction noise mechanism of counter-rotating open rotors. Therefore the interaction is primarily addressed in an annular strip of limited spanwise extent bounding the impinged blade segment, and the unwrapped strip is described in Cartesian coordinates. The study also addresses the interaction of a propeller wake with a downstream wing or empennage. Cylindrical vortices are considered, for which the velocity field is expanded in two-dimensional gusts in the reference frame of the airfoil. For each gust the response of the airfoil is derived, first ignoring the effect of the span end, assimilating the airfoil to a rigid flat plate, with or without sweep. The corresponding unsteady lift acts as a distribution of acoustic dipoles, and the radiated sound is obtained from a radiation integral over the actual extent of the airfoil. In the case of tip–vortex interaction noise in CRORs the acoustic signature is determined for vortex trajectories passing beyond, exactly at and below the tip radius of the impinged blade segment, in a reference frame attached to the segment. In a second step the same problem is readdressed accounting for the effect of span end on the aerodynamic response of a blade tip. This is achieved through a composite two-directional Schwarzschild's technique. The modifications of the distributed unsteady lift and of the radiated sound are discussed. The chained source and radiation models provide physical insight into the mechanism of vortex chopping by a blade tip in free field. They allow assessing the acoustic benefit of clipping the rear rotor in a counter-rotating open-rotor architecture.     

Characteristic correlation between propellers cavitating wake and cavitation noise

The characteristic correlation between the propellers cavitating wake and their cavitation noise has been studied. Based on the viscous multiphase flow theory, the Navier–Stokes equations, a turbulence model and a cavitation model were applied to predict numerically the vapor volume fraction and the pressure in the wake. Compared with experimental results, the numerical predictions of the sheet cavitation and the tip vortex cavitation are in agreement with the corresponding experimental data, which indicates the numerical method in present paper is feasible. The characteristic of the low line spectra of the numerical pressure fluctuation signals and the measured noise signals for uniform inflow and wake flow is analyzed, and the characteristic correlation of the two line spectra is also validated. Therefore, it can be concluded that due to the wake and the radiated noise both modulated by the revolving propeller blades, the features of the low line spectra for noise signals and the pressure signals in the wake are close to each other.     

Unsteady wake vortices in jets in cross-flow

Abstract  

The current flow visualization study investigates unsteady wake vortices of jets in cross-flow in order to (1) advance the understanding of their origin and characteristics and (2) explore various excitation techniques for organizing and accentuating them. An isolated circular jet passed through a nozzle and entered the cross-flow normal to the wall. Free stream velocities up to 5 m/s and jet-to-cross-flow velocity ratio range between 1 and 10 were covered. While mechanical perturbation did not result in any significant periodic organization of the wake vortices, the database obtained for the unperturbed flow provided further insight into their behavior. The key finding was that the wake vortices always originated from the lee-side of the jet where the jet efflux boundary layer and the wall boundary layer intersected. In no case these vortices were seen to form either from the wall boundary layer or the jet shear layer at downstream locations. After formation the wake vortex twists and stretches as it convects downstream with the base still attached to the near-wall region on the jet’s lee side. The top remains connected to the underside of the jet where the tracer particles dissipate due to high turbulence in the shear layer.     

开式轴流风扇叶尖流动的PIV实验研究

粒子成像测速仪(PIV)技术是一种测量叶轮机械内部流场的有效手段。本文利用对开式前缘弯掠轴流风扇进行PIV实验,捕捉到了开式轴流风扇的叶尖涡。实验结果表明,叶尖涡产生于叶顶前缘的吸力面,其涡核沿一条与旋转方向相反的斜线延伸,一直到转子下游出口。PIV实验数据和CFD内流计算结果吻合良好。本实验为改善风扇性能和认识其内流机制提供了可靠的实验依据。     

Combined stereoscopic particle image velocimetry and line integral convolution methods

The flow fields for a sphere sedimenting through a Newtonian and two non-Newtonian liquids near a wall in a square tank are investigated using 3-D stereoscopic particle image velocimetry (PIV) and line integral convolution (LIC) methods. The PIV data were taken using an angular stereoscopic configuration with tilt and shift arrangements for the Scheimpflug condition and a pair of liquid correction prisms. Data were recorded from planes perpendicular and parallel to the wall for each fluid case over a range of distances from the wall. The PIV and LIC results highlight significant differences in the wake structure for all three cases. Out of plane flow was also found to persist up to two sphere diameters downstream in the wake for all cases.     

前缘弯掠子午加速叶轮尾缘脱落涡的研究

本文采用DPIV实验研究方法,在不同转速工况下,对开式前缘弯掠子午加速风扇转子叶尖涡在叶轮尾缘的脱落流动现象进行了测量.结果表明:在不同转速下,转子尾缘处均存在明显的叶尖涡流动现象.该叶尖脱落涡流动现象具有相似性,随着转速的增大,叶尖涡区和吸力面之间的径向速度差增大,叶尖涡强度随转速增加而增强.随着转速的增大,叶尖涡和主流相互作用加剧,涡量负值增大,叶尖涡影响区域也逐渐扩大.径向速度梯度同样随转速增大而增大的结果,增强了叶尖涡对尾迹区作用的影响.最后,本文通过CFD对该开式风扇叶尖涡流动现象进行模拟,其结果给出了相似的叶尖涡在转子尾缘脱落流动现象,为前缘弯掠开式子午加速风扇在户室中央空调室外机上的应用和风机系统的优化设计及其降噪研究提供了重要的内流数据.     

Effects of an air spoiler on the wake of a road vehicle by PIV measurements

A particle image velocimetry (PIV) technique has been used to analyze the flow characteristics behind a road vehicle with/without an air spoiler attached on its trunk and also to estimate its effect on the wake. A vehicle model scaled in the ratio of 1/43 is set up in the mid-section of a closed-loop water tunnel. The Reynolds number based on the vehicle length is 10⁵. To investigate the three-dimensional structure of the recirculation zone and vortices, measurements are carried out on the planes both parallel and perpendicular to the free stream, respectively. The results show significantly different vorticity distributions in the recirculation region according to the existence of the air spoiler. The focus and the saddle point, appearing in the wake, are disposed differently along the spanwise direction. Regarding the streamwise vortices, the air spoiler produces large wing tip vortices. They have opposite rotational directions to C-pillar vortices which are commonly observed in the case that an air spoiler is absent. The wing tip vortices generate the down force and as a result, they might make the vehicle more stable in driving.     

Application of DPIV to study both steady state and transient turbomachinery flows

Digital particle imaging velocimetry (DPIV) is a powerful measurement technique, which can be used as an alternative or complementary approach to laser doppler velocimetry (LDV) in a wide range of research applications. The instantaneous planar velocity measurements obtained with PIV make it an attractive technique for use in the study of the complex flow fields encountered in turbomachinery. The planar nature of the technique also significantly reduces the facility run time over point-based techniques. Techniques for optical access, light sheet delivery, CCD camera technology and particulate seeding are discussed. Results from the successful application of the PIV technique to both the blade passage region of a transonic axial compressor and the diffuser region of a high speed centrifugal compressor are presented. Both instantaneous and time-averaged flow fields were obtained. The averaged flow field measurements are used to estimate the flow turbulence intensity. The instantaneous velocity vector maps obtained during compressor surge provide previously unobtainable insight into the complex flow field characteristics occurring during short lived surge events. These flow field maps illustrate the true power of the DPIV technique.     

Visualization and characterization of small-scale spanwise vortices in turbulent channel flow

High-resolution particle-image velocimetry (PIV) measurements are made in the streamwise-wall-normal plane of turbulent channel flow at Re_τ=566, 1184 and 1759, facilitating documentation of the population trends and core diameters of small-scale spanwise vortices. Swirling strength, an unambiguous vortex-identification criterion and hence a local marker of rotation, is used to extract small-scale spanwise vortex cores from the instantaneous velocity fields. Once the small-scale vortices are properly extracted from the PIV realizations, their characteristics are studied in detail. The present results indicate that the very-near-wall region (y < 0.1h) is densely populated by spanwise vortices with clockwise (negative) rotation. This behavior supports the notion that hairpin-like vortices are generated very close to the wall and grow into the outer layer as they advect downstream. In contrast, counterclockwise (positive) spanwise vortices are scarce in the very-near-wall region, but their presence steadily increases within the logarithmic layer presumably due to a localized generation mechanism. The average core diameter of negative spanwise vortices is found to be larger than the average diameter of positive vortices, with few positive vortices having core diameters exceeding 80y.