PIV measurement of mean and pulsating velocities in a gas-droplet jet with low concentration of dispersed phase

A Particle Image Velocimetry (PIV) system was used to perform an investigation into the effect of low concentration of dispersed phase on time-average and pulsating characteristics of the flow over the self-similar part of a two-phase jet emanating into ambient space filled with the same fluid. A phase discrimination procedure based on reflected intensity was introduced into routine practice. Distributions of mean and pulsating velocities in the carrier and dispersed phases of the gas-droplet jet were obtained. In spite of low concentration of the dispersed phase, large droplets present in the flow were found to reduce the intensity of velocity pulsations in the gas phase.     

射流影响下气固两相流动试验的定量分析

为了研究受限空间内的射流影响下的气固两相流动,本文采用数码颗粒图像测速仪（DpIV）系统测量颗粒分布,并开发了一套用于处理DPIV颗粒图片的算法,可用于准确地定量分析通道内的颗粒分布。使用该算法能够获得颗粒在拍摄平面内的二维相对浓度数据,定量分析该数据发现,在同样的流场条件下,射流对气流场和不同粒径颗粒的影响区域存在明...     

A study of vortex shedding induced by dielectric barrier injection

Previous works have proved that a dielectric barrier injection (DBI) device could be used as an electrohydrodynamic (EHD) actuator. Such a device was used to generate a wall jet with an average velocity of 0.14 m/s.In this study, the liquid flow induced in the surface vicinity by a DBI actuator is measured by Particle Image Velocimetry (PIV). The phase analysis of the flow velocity shows that the DBI actuator mainly acts as a vortex generator. The work presented in this paper underline the relation between the vortex structures and the polarity of the injecting electrode.     

Flow mapping of a jet in crossflow with Stereoscopic PIV

Stereoscopic Particle Image Velocimetry (PIV) has been used to make a three-dimensional flow mapping of a jet in crossflow. The Reynolds number based on the free stream velocity and the jet diameter was nominally 2400. A jet-to-crossflow velocity ratio of 3.3 was used. Details of the formation of the counter rotating vortex pair found behind the jet are shown. The vortex pair results in two regions with strong reversed velocities behind the jet trajectory. Regions of high turbulent kinetic energy are identified. The signature of the unsteady shear layer vortices is found in the mean vorticity field.     

Highlights of the NASA Particle Astrophysics Program

The NASA Particle Astrophysics Program covers Origin of the Elements, Nearest Sources of Cosmic Rays, How Cosmic Particle Accelerators Work, The Nature of Dark Matter, and Neutrino Astrophysics. Progress in each of these topics has come from sophisticated instrumentation flown on long duration balloon (LDB) flights around Antarctica over the past two decades. New opportunities including Super Pressure Balloons (SPB) and International Space Station (ISS) platforms are emerging for the next major step. Stable altitudes and long durations enabled by SPB flights ensure ultra-long duration balloon (ULDB) missions that can open doors to new science opportunities. The Alpha Magnetic Spectrometer (AMS) has been operating on the ISS since May 2011. The CALorimetric Electron Telescope (CALET) and Cosmic Ray Energetics And Mass (CREAM) experiments are being developed for launch to the Japanese Experiment Module Exposed Facility (JEM-EF) in 2014. And, the Extreme Universe Space Observatory (EUSO) is planned for launch to the ISS JEM-EF after 2017. Collectively, these four complementary ISS missions covering a large portion of the cosmic ray energy spectrum serve as a cosmic ray observatory.     

PIV measurements of the flow and turbulent characteristics of a round jet in crossflow

The instantaneous and ensemble averaged flow characteristics of a round jet issuing normally into a crossflow was studied using a flow visualization technique and Particle Image Velocimetry measurements. Experiments were performed at a jet-to-crossflow velocity ratio, 3.3 and two Reynolds numbers, 1,050 and 2,100, based on crossflow velocity and jet diameter. Instantaneous laser tomographic images of the vertical center plane of the crossflow jet show that there exists very different natures in the flow structures of the near field jet due to Reynolds number effect even though the velocity ratio is same. It is found that the shear layer becomes much thicker when the Reynolds number is 2,100 because of the strong entrainment of the inviscid fluid by turbulent interaction between the jet and crossflow. The mean and second order statistics are calculated by ensemble averaging over 1,000 realizations of instantaneous velocity fields. The detail characteristics of mean flow field, streamwise and vertical rms velocity fluctuations, and Reynolds shear stress distributions are presented. The new PIV results are compared with those from previous experimental and LES studies.     

3C PIV and PLIF measurement in turbulent mixing

The present work focuses on the measurements of instantaneous concentration fields of a passive scalar due to an impinging round jet injection into a liquid filled rectangular tank. Simultaneous measurements of velocity and passive scalar concentration fields have been conducted by using Particle Image Velocimetry (planar 2C and 3C PIV) and Planar Laser Induced Fluorescence (PLIF) techniques. The mixing injection behavior is analyzed for several injection values of depth and flow rate. Results showed the classical developing and self-similar regions of the jet, the mixing layer and the coupled concentration and velocity fields due to impingement. Finally, 3C PIV reveals a 3D flow jet structure which seems to be a swirl that does not disturb 2D analysis.     

Experimental estimation of the electric force induced by a blade-plane actuator in dielectric liquids

A blade-plane actuator immersed in a dielectric liquid and connected to a high voltage power supply produces a Coulomb force which sets the liquid in motion from the blade tip to the plane. Jets of more than 1 m/s have already been observed. In this work, a method is proposed to estimate the electric force generated by a blade plane actuator from Particle Image Velocimetry (PIV) velocity field measurements. The originality of this paper comes from the fact that the volume force is not measured directly with an aerodynamic shielded balance but calculated from velocity fields obtained by Particle Image Velocimetry. In this article the global time-averaged electric force is computed in a fixed control volume by the use of the momentum equation in its integral form.     

The flow induced by a jellyfish

The purpose of this study is to understand the propulsion mechanism of a jellyfish during its swimming. We observed the motion of a jellyfish (Aurelia aurita) by a motion-capture camera, and measured the vector field of flow around a jellyfish by using a PIV (Particle Image Velocimetry) measurement. A jellyfish is considered to be principally propelled by a jet at the contracting phase of its motion. If that is true, it is interesting that a jellyfish never stops traveling even at the expanding phase. We found that a vortex ring with the opposite vorticity to shed vortex ring was inside a jellyfish body in the expanding phase. We discussed a cause of an increase in thrust force and keeping constant speed in the expanding phase.     

Time-resolved PIV investigation on the unsteadiness of a low reynolds number confined impinging jet

The flow characteristics in a confined slot jet impinging on a flat plate were investigated in low Reynolds number regime by using time-resolved Particle Image Velocimetry technique. The jet Reynolds number was varied from 404 to 1026, where it is presumed that the transient regime exists. We found that the vortical structures in the shear layer are developed with increase of Reynolds number and that the jet becomes remains steady at the Reynolds number of 404. Vortical structures and their temporal evolution are verified and the results were compared with previous numerical studies.     

Pulsed evaporator of an electrodynamic disperse suspension of particles

We consider the technique for obtaining electrodynamic disperse suspension of metal (Al, W, Cu) and dielectric (semiconductor) particles (SiO₂, Al₂O₃, CuO, Cu₂O) and its evaporation with the help of a diffusive electric discharge. The time dependences of current and integrated luminescence intensity in a pulsed electric discharge in a tube containing a film of a substance (Cu) of the electrodynamic disperse suspension are measured.     

Dynamics of two-phase swirling flow in a vortex chamber with a lower end swirler

The Particle Image Velocimetry (PIV) technique and laser Doppler anemometer (LDA) were used to measure the components of tangential and axial velocities of gas and particles in a vortex chamber with a fluidized bed, particle layer dynamics was estimated qualitatively, and the flow in the vortex chamber with a centrifugal fluidized bed of solid particles was simulated numerically. It is shown that with the growth of gas velocity in the swirler slots, the rotation velocity of bed grows almost linearly, and with an increasing bed mass, the rotation velocity decreases. Data on distributions of the volume fraction of particles and gas flow velocity inside the bed were obtained by numerical calculation.     

A swirling jet with vortex breakdown: three-dimensional coherent structures

The paper reports on shape of a three-dimensional coherent structure in a velocity field of a high-swirl turbulent jet with the bubble-type vortex breakdown. A set of the 3D instantenous velocity fields was measured by using the tomographic particle image velocimetry (tomographic PIV) technique and processed by the proper orthogonal decomposition (POD) method. The detected intensive coherent velocity component corresponded to a helical vortex core of the swirling jet and two secondary spiral vortices. The entire coherent structure was rotating around the jet axis in compliance with the direction of the flow swirl. From the 3D data it is concluded that the dynamics of the strsucture can be described by a traveling wave equation: Re[A(y, r)·e ^{i(mθ + ky - ωt)}] with the number of the spiral mode m = +1 for positively defined k and ω.     

Stereoscopic PIV measurements of a screeching supersonic jet

The effect of acoustic feed back on global flow response is illustrated through an example of a rectangular screeching jet operating at a nominal Mach number of 1.69. Using a stereoscopic Particle Image Velocimetry, the detailed flow characteristics within a screeching cycle are obtained with fidelity. To resolve the “bias” errors inherent with standard PIV image processing technique, a novel mesh-free and high spatial resolution scheme is implemented to yield accurate velocity measurements in a complex three-dimensional supersonic flow. The axis-switching phenomenon that arises due to unusual mixing enhancement in the minor axis plane of a rectangular jet is vividly displayed. Strong streamwise vortex structure in the jet shear layers, enhanced by the inherent instability of the shear layer, is reported.     

Anisotropic elastic properties of microtubules

The present study aims at providing insight into the acceleration mechanism of a bubble chain rising in shear-thinning viscoelastic fluids. The experimental investigation by the Particle Image Velocimetry (PIV), birefringence visualisation and rheological simulation shows that two aspects are central to bubble interactions in such media: the stress creation by the passage of bubbles, and their relaxation due to the fluids memory forming an evanescent corridor of reduced viscosity. Interactions between bubbles were taken into account mainly through a linear superposition of the stress evolution behind each bubble. An analytical approach together with the rheological consideration was developed to compute the rise velocity of a bubble chain in function of the injection period and bubble volume. The model predictions compare satisfactorily with the experimental investigation.     

Experimental investigation of wake evolution behind a couple of flat discs in a hydrochannel

The decay of a far wake and its turbulent fluctuations behind two thin discs of the same diameter D, oriented normal to the incident flow, have been studied using the Particle Image Velocimetry (PIV). The experimental study was carried out in a water flume (Re ≈ 2·10⁵) with varying distances between the discs (L _х = 4–8D) and their axes shift relative to each other (0, 0.5D and 1D). It is found that the velocity deficit behind two discs depends weakly on L _x , and at L _х > 40D, it becomes indistinguishable from the level of turbulent fluctuations of the incident flow. It is found that the decay of the average velocity deficit and its turbulent fluctuations in a wake of a tandem of discs can be described by the same analytical dependence with exponent–2/3 as for the wake decay of a single disc. However, at the same distance downstream, the value of deficit behind two discs is substantially higher than the corresponding value behind a single disc. Velocity fluctuations in a far wake behind a pair of discs depend weakly on longitudinal dimension L _x , but at the same time, in contrast to the velocity deficit, their level does not differ significantly from the level of fluctuations behind a single disc.     

A PIV study of a supersonic impinging jet

The characteristics of supersonic impinging jets are investigated using Particle Image Velocimetry (PIV). The purpose of the experiments is to understand the jet induced forces on STOVL aircraft while hovering close to the ground. For this purpose, a large diameter circular plate was attached at the nozzle exit. The oscillations of the impinging jet generated due to a feedback loop are captured in the PIV images. The instantaneous velocity field measurements are used to describe flow characteristics of the impinging jet. The important flow features such as oscillating shock waves, slipstream shear layers and large scale structures are captured clearly by the PIV. The presence of large scale structures in the impinging jet induced high entrainment velocity in the near hydrodynamic field, which resulted in lift plate suction pressures. A passive control device is used to interfere with the acoustic waves travelling in the ambient medium to suppress the feedback loop. As a consequence, the large scale vortical structures disappeared completely leading to a corresponding reduction in the entrainment.     

Numerical investigation of the accuracy of particle image velocimetry technique in gas-phase detonations

We numerically investigate the accuracy of the Particle Image Velocimetry (PIV) technique for the flow characterization in high-speed, compressible regimes, in particular in gas-phase detonations. We carry out synthetic PIV reconstruction of the flow field in a two-dimensional, planar detonation propagating under atmospheric conditions and modelled using single-step Arrhenius kinetics. The flow is uniformly seeded with monodispersed Al₂O₃ particles with sizes 50 and 200 nm, along with initially co-located massless Lagrangian tracer particles. The effect of massive particles on the detonation speed and thermodynamic state of the flow is investigated and is found to be negligible. We further assess the ability of massive particles to sample the flow field and while it is found that 50 nm particles sample the flow field better than the 200 nm ones, they also exhibit significant clustering. By comparing the trajectories of massive particles with those of massless tracers, it is shown that almost all massive particles rapidly diverge from the actual flow pathlines. Finally, we quantify the accuracy of the PIV reconstruction of the velocity field in comparison with the actual velocity field in the numerical simulations. It is shown that while PIV is generally capable of capturing the bulk flow features in the streamwise direction, its accuracy is not sufficient to characterize the transverse velocity component or velocity fluctuations.     

振荡射流提高风力机叶型升力的PIV实验

本实验在风力机叶片叶型表面头部加入平均质量流量为零的一定频率的振荡射流。PIV(Particle Image Ve- locimetry)实验手段显示出叶片上表面速度场。通过对加入振荡射流前后速度场和涡量场的观察与比较,可以发现在振荡射流的作用下,叶片上表面尾端的流动分离发生了再附,转捩点向后推移。通过比较还可以发现在加入特定的频率和强度的振荡射流之下,该效果更加明显。该实验不仅可以证明振荡射流可以提高叶片升力,而且显示了不同频率的射流对流动的影响。