Modification of the ultrasound induced activity by the presence of an electrode in a sonoreactor working at two low frequencies (20 and 40 kHz). Part I: Active zone visualization by laser tomography

Sonoelectrochemical experiments differ from sonochemical ones by the introduction of electrodes in the sonicated reaction vessel. The aim of the study is to characterize the changes of the ultrasonic activity induced by the presence of an electrode located in front of the transducer. The scope of our investigations concerns two low frequencies vibration modes: 20 and 40 kHz. For this purpose, two laser visualization techniques have been used. The first part of the study, described in the present paper (part I), deals with the laser tomography technique which provides an accurate picture of the reactor actives zones, related to numerous cavitation events. For each frequency, two parameters were studied: the electrical power supplied to the transducer and the electrode transducer distance. At both frequencies, without electrode, we can observe distinct zones corresponding to cavitation production and stationary waves establishment. When increasing the input power, bubble clouds tend to form a unique cloud near the transducer. In presence of the electrode, bubble cavitation clouds are largely influenced by the obstacle. The second part of the paper (part II) will describe the Particle Image Velocimetry (P.I.V.) technique which allows to measure the velocity vector field in the fluid portion between the horn and the electrode.     

Direct measurement of fluid velocity gradients at a wall by PIV image processing with stereo reconstruction

Velocity gradient is typically estimated in Particle Image Velocimetry (PIV) by differentiating a measured velocity field, which amplifies noise in the measured velocities. If gradients near a boundary are sought, such noise is usually greater than in bulk fluid, because of small tracer displacement, uncertainty in the effective positions of velocity vectors, intense deformation of tracer patterns, and laser reflection. We consider here a modified form of the Particle Image Distortion (PID) method todirectly calculate velocity gradients at a fixed wall, and refer it as “PIV/IG” (“Interface Gradiometry”). Results from synthetic 2D PIV images suggest our method achieves higher SNR and accuracy than velocity differentiation. Also, we have developed a procedure to reconstruct three-dimensional velocity gradient at a fixed wall the two non-zero components from PIV/IG data obtained in stereo views; these equations simplify considerably thanks to the no-slip condition. Experimental data from the bottom wall of turbulent open channel flow appear to suffer from a form of pixel locking. As with standard PIV, this underlines the importance of adequate tracer diameter in the images, sufficient seeding density, and of dynamic range of the camera sensor.     

Investigation of the structure of a polydisperse gas-droplet jet in the initial region. Experiment and numerical simulation

The structure of gas-droplet polydisperse jet was studied experimentally and numerically. Experiments were carried out using the Particle Image Velocimetry/Particle Tracking Velocimetry/Interferometric Particle Imaging methods, which allowed simultaneous measurements of parameters for the average and pulsation carrying and disperse phases. Gas phase turbulence was calculated using the model of Reynolds stresses transfer. Significantly non-isotropic character of velocity pulsation of particles along and across the section of the gas-droplet jet was demonstrated in experiments and calculations. Strong dependence of axial pulsations of disperse phase velocity on the size of its particles is observed.     

PIV and LDA velocity measurements near walls and in the wake of a delta wing

Velocity measurements in flow regions with high turbulent intensities can be performed with high accuracy by means of optical methods. Despite the validity ranges of these methods, great care must be used in taking measurements near solid walls, where high noise levels are present due to the scattering of the light on the wall, and in flows with high vorticity regions like the ones in the wake of a finite wing. In this paper, Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV) velocity measurement techniques are used in these two experimental situations and their results compared. This comparison shows that good measurements can be obtained from both techniques and that the resulting data sets do not provide alternative but rather complementary information.     

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.     

Visualization of molten pools and invisible laser beam profiles using an ultraviolet CCD camera

Experiments were conducted on the effects of a wall distance and velocity ratio of suction flow to injection flow on the flow and heat transfer characteristics of a circular impinging jet accompanying an annular suction flow. As a result, it is found that in the case of accompanying suction flow, a higher Nusselt number can be obtained compared with in the case without suction flow, under a condition of the wall distance within eight times of injection pipe diameter from the near pipe exit edge. In addition, when the effect of velocity ratio is examined at a fixed arbitrary wall distance, it is found that there exists an optimum velocity ratio where the Nusselt number becomes the maximum. It is shown that these heat transfer characteristics are closely associated with the fluctuating velocity and the mean velocity in the two-dimensional velocity field observed by Particle Image Velocimetry (PIV).     

弯管内气液固三相流中液膜区流场的PIV测量

为了深入认识螺旋管内多相流相分离现象,应用粒子图象测速仪(PIV),对组合弯管内气水砂三相流底部水平段和上升段中液膜区的流场进行了测量。研究表明:底部水平段流动主流速度分布比较均匀,颗粒跟随流体运动;二次流十分微弱,颗粒在离心力作用下由内弯侧向外弯侧运动;速度脉动强烈,不利于相分离。上升段中流动受到重力的阻碍,同底部平段相比主流速度降低,离心力作用减弱,颗粒径向时均速度较低;流动的间歇性更强,两相速度脉动更剧烈。     

A visual description of the convective flow field around the head of a human

Mean velocity data obtained by PIV (Particle Image Velocimetry) around the head of a real-life size breathing thermal manikin are presented for two cases of ‘no breathing’ and ‘continuous exhalation through nose’. Experiments were conducted in a special chamber which provided stationary convective flows around the seated manikin. Results are limited to the plane of symmetry. The paper aims to describe the physical structure of the turbulent flow field by presenting velocity and vorticity data in color graphics.     

Particle pairing using genetic algorithms for PIV

This paper presents a new particle pairing algorithm using “Genetic Algorithms” for DPIV (Digital Particle Image Velocimetry), which are searching algorithms for obtaining an optimal solution based on the mechanism of evolution. The particle pairing between two tracer images with a constant time interval is needed to obtain a velocity vector field. Since the algorithm adopts a fitness function which totally evaluates the similarity between respective small particle patterns in the two tracer images over the field, it promises to give a more correct velocity vector distribution than the conventional PTV (Particle Tracking Velocimetry) which identifies each particle based on its local information. In addition, a particle pattern matching for the similarity is performed after correcting fluid rotation. It therefore is robust against a high particle density and an increase in the time interval. The algorithm is applied to the PIV standard images distributed through the Internet (http://www.vsj.or.jp/piv). It gives a correct velocity vector distribution as a result even if a pair of the successive images has a large time interval.     

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.     

Modelling the pressure field in the vicinity of a microphone membrane using PIV

A new approach for estimating the acoustic pressure in the near field of a microphone based on non-intrusive direct measurement of acoustic particle velocity is proposed.This method enables the estimation of the acoustic pressure inside a domain located in front of the microphone membrane. The acoustic pressure is calculated using the acoustic particle velocity on the frontiers of this domain and a physical model based on the Green function of the system.Results are obtained using the acoustic velocity measured with Particle Image Velocimetry (PIV) in front of a microphone excited with a plane wave inside a rectangular waveguide. They show that the diffraction of the plane wave by the microphone leads to an increase of the acoustic pressure on the microphone edge in the order of magnitude of 0.1 dB.     

Time resolved particle image velocimetry

The temporal evolution of the velocity field of an unsteady fluid flow can be tracked by combining Particle Image Velocimetry (PIV) and high speed photography. Two alternative techniques for PIV are discussed: the classical light sheet technique and a method which makes use of the light scattered in forward direction. We applied time resolved PIV to investigate the flow around cavitation bubbles during their collapse near a solid boundary. The light source was an argon laser with an external acousto-optic deflector which produces series of short pulses. Using a drum camera for high speed photography, we achieved a temporal resolution of10 kHz and a spatial resolution of better than2 points/mm². Velocities could be determined without directional ambiguity in a range from2 m/s to30 m/s.     

Planar velocity visualization in high-speed wedge flow using Doppler Picture Velocimetry (DPV) compared with Particle Image Velocimetry (PIV)

A technique for visualizing a velocity field in an entire plane has been developed by taking ‘Doppler Pictures’ using Michelson interferometry. With the Doppler Picture Velocimetry (DPV), information about the instantaneous and local velocities of tracers passing through a light sheet is available. The technique for taking and processing the Doppler pictures has been improved recently and the state-of-the-art of the DPV method will be described with an application in high-speed fluid flows showing the velocity distribution in a light sheet plane crossing a supersonic wedge flow generated in the high-energy shock tunnel STB of ISL. A comparison with Particle Image Velocimetry (PIV) velocity visualizations is also presented.     

Planar Droplet Sizing for the Characterization of Droplet Clusters in an Industrial Gun‐Type Burner

An important problem in spray combustion deals with the existence of dense regions of droplets, called clusters. To understand their formation mechanism, the droplet dynamics and fuel concentration profile are investigated by means of planar laser techniques in an industrial gun‐type burner. The simultaneous measurement of elastic Mie scattering and Laser Induced Fluorescence (LIF) allows the instantaneous measurement of the Sauter Mean Diameter (SMD), after proper calibration. Using two different CCDs to get the two signals requires a detailed calibration of the CCD response before getting absolute diameters. Pixels are binned 6 by 6 to obtain the final SMD map, this is a compromise between spatial accuracy and noise. Velocity field is measured on both sets of images using standard Particle Image Velocimetry (PIV) algorithms. The comparison of cross‐correlation technique with PDA results shows that the velocity measured on the LIF images are close to the velocity based on D₃₀, whereas the Mie scattering results are similar to D₂₀. On Mie scattering images, regions of high interfacial area forming clusters can be detected. A special tracking scheme is used to characterize their dynamics in terms of velocity and diameters by ensuring that the same volume of fluid is tracked. It is shown that the clusters have a velocity similar to the velocity of droplets with the same diameter as the mean SMD of the cluster. It is also shown that an increase of pressure tends to trigger the appearance of such a group of droplets, due to a smaller diameter of the droplets caused by the increase of pressure discharge. Uncertainties for the different techniques used are discussed.     

The application of particle image velocimetry to the study of water waves

The application of Particle Image Velocimetry (PIV) to the measurement of velocity distributions under water waves is described. Two-dimensional water waves are generated in a laboratory wave flume with the measurement area illuminated by a powerful CW Argon ion laser. Photographic recording and subsequent electro-optical analysis of the film provides the displacement and hence velocity field for passing waves. The accuracy and reliability of the technique is illustrated by typical experimental results and a comparison with Linear wave theory.     

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.     

Large-scale PIV surface flow measurements in shallow basins with different geometries

Shallow depth flow fields and low velocity magnitudes are often challenges for traditional velocity measuring instruments. As such, new techniques have been developed that provide more reliable velocity measurements under these circumstances. In the present study, the two-dimensional (2D) surface velocity field of shallow basins is assessed by means of Large-Scale Particle Image Velocimetry (LSPIV). The measurements are carried out at the water surface, which means that a laser light sheet is not needed. Depending on the time scales of the flow and the camera characteristics, it is even possible to work with a constant light source. An experimental application of this method is presented to analyze the effects of shallow basin geometry on flow characteristics in reservoirs where large coherent two-dimensional flow structures in the mixing layer dominate the flow characteristics. The flow and boundary conditions that give rise to asymmetric flow are presented. Asymmetric flow structures were observed starting from basin shape ratios that are less than or equal to 0.96. By decreasing the basin length and increasing the shape ratio to greater than 0.96, the flow structure generally tends towards a symmetric pattern.     

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。