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• A phase-locked measurement method for sweeping jets is established based on low-rate PIV devices. • Unsteady features of sweeping jets and secondary flows in a quiescent condition are visualized. • The effects and mechanism of sweeping jet acting on afterbody vortices are well detected. Although low-operation-rate particle image velocimetry (PIV) provides a good spatial accuracy in measurements at relatively affordable costs, it faces some challenges in capturing unsteady features of oscillatory flow. In this paper, a single sweeping jet actuated to control afterbody vortices from a 30◦ slanted-base cylinder is investigated at a Reynolds number of 87,000. Phase-locked stereo PIV measurements combining triggering reference obtaining and real-time processing via field programmable gate array (FPGA) are leveraged to reveal the unsteady characteristics of the sweeping jet. The examined cases show that the phase-locked method can well identify jet's spatiotemporal development process in each oscillation cycle. A sinusoidal-like interaction along phases between the jet and the afterbody vortex can be reasonably detected. At each moment, coherent small vortical structures form at the upper and bottom jet/ambient interfaces, which are caused by Kelvin-Helmholtz instability. Since the induced vortex has the same rotation direction as the afterbody vortex on each side, they merge with each other as the jet approaches the vortex, causing an increase in vorticity. Meanwhile, the sweeping jet's intrusion into the vortex region induces a rise in turbulent kinetic energy in that area, causing turbulence ingestion of the vortex which weakens the velocity gradient. The sweeping behavior of the jet dominates the afterbody vortex to meander as the jet pushes its way underneath the vortex. The findings of this study provide encouraging evidence for future applications of sweeping jets in control of afterbody vortices. [ABSTRACT FROM AUTHOR]

This paper describes an experimental study aimed at the characterization of the steady-state tumble motion in the cylinder of an engine using stereoscopic particle image velocimetry (Stereo-PIV). More specifically, a pent-roof four valves gasoline direct injection (GDI) engine head was mounted on a modified FEV steady-state flow rig for applying Stereo-PIV at different measurement vertical tumble planes at mid cylinder, mid injector and mid valve. The flow field was described by the distribution of the ensemble average flow patterns for 1000 pairs of images for every case, vorticity contours, turbulent kinetic energy and tumble ratio. The results revealed that the higher velocities acquired at the mid valve plane improved the turbulent kinetic energy and tumble ratio compared to the other planes. There was a good level of agreement between direct and indirect methods used for calculating the tumble ratio.

Fuel and air mixture in the combustion chamber is not the homogeneous distribution. Although various models have been developed to understand the characterization of the mixture movement inside the cylinder, it is not well understood. Air movements in the cylinder have a great effect on engine combustion performance and engine emissions. During the suction of the engine, the air can discharge with different movements through to the cylinder as swirl and tumble. In this study, swirl performance is investigated with laser and camera technologies. Within the scope of this work, a prototype part that represents the air intake port of a 9l diesel engine was manufactured with additive manufacturing method. Swirl coefficients were measured using the honeycomb torque method. The error level of the test was determined by the uncertainty analysis. Swirl measurement was performed with the particle imaging velocimetry (PIV) technique in two dimensions and three dimensions. The degree of error was determined by performing uncertainty analysis for the PIV technique as well. A comparison between the methods and evaluation of suitability was completed. [ABSTRACT FROM AUTHOR]

We report on a study of the impact of coherent helical vortex structures on the shape of the reaction zone and heat release in swirling methane/air flames in regimes with a vortex breakdown. Three kinds of atmospheric flames are considered, viz., fuel-lean and fuel-rich premixed flames and a partially premixed fuel-rich lifted flame. Based on the measurements of the velocity fields by a stereo PIV in combination with the OH PLIF and HCHO PLIF, the impact of the coherent flow structures on large-scale corrugations of the reaction zone is evaluated. Helical vortex structures, detected in both the non-reacting and reacting high-swirl flows by using proper orthogonal decomposition, are found to promote combustion both in the lean premixed and fuel-rich partially premixed flames. In the first case, based on the phase-averaged intensity of the HCHO×OH signal and the location of the helical vortex structure in the inner mixing layer, it is concluded that the vortex locally increases the heat release rate by enlarging the flame front and enhancing the mass exchange between the combustion products inside the recirculation zone and the fresh gases. The events of the local flame extinctions are detected in the instantaneous PLIF snapshots for the lean mixture, but they do not cause extinction of the entire flame or a blow-off. In case of the lifted flame, the outer helical vortex structure promotes combustion by locally intensifying the mass exchange between the fuel-rich jet with the surrounding air. [ABSTRACT FROM AUTHOR]

• Surface pressure distribution was measured when a circular jet impinges on a cylinder. • Effects of the impingement angle, Reynolds number, and diameter ratio were examined. • Pressure gradient in flow direction decreases when the impingement angle increases. • The three-dimensional nature of the surface pressure changes with the flow parameters. When the circular jet impinges onto a cylinder, the three-dimensional (3D) curved wall jet was attached to the wall due to the Coanda effect and the flow separation was delayed with increasing Reynolds number and impinging angle. These interesting phenomena can be studied in depth through a variety of parameters and measurement methods. Time-resolved stereo PIV measurement was conducted to investigate the 3D flow characteristics with instantaneous velocity and vorticity fields. Pressure sensitive paint (PSP) method was used to examine the pressure distribution on the cylinder surface when a circular jet impinges on it. The effects of the impingement angle, Reynolds number, and diameter ratio between the jet nozzle and the cylinder (d/D) were examined at a fixed nozzle to wall distance ratio of two (H/d = 2). Experimental results show that the pressure gradient in the flow direction decreases and the separation is delayed when the Reynolds number and impingement angle are increased at the same distance ratio. As the diameter ratio decreases with the same impingement angle and Reynolds number, the maximum suction pressure increases, and the pressure gradient in the wall jet flow direction increases. The three-dimensional nature of the surface pressure distribution changes with the flow parameters, which are well explained with three-dimensional flow field measurements in the curved 3D wall jet due to the Coanda effect. [ABSTRACT FROM AUTHOR]

The stereo PIV, commonly known as the SPIV, is capable of measuring all the three velocity components in a two-dimensional measurement plane and thus classified as 2-D 3-C PIV. Even though the SPIV utilizes a set of two cameras, the positions of the measurement points are assumed to be located in the central plane of the illuminated volume, which is usually the zone of laser light sheet. This assumption makes the SPIV versatile and its calibration procedure very handy. But, if the three-dimensional orientation of the cameras can be calibrated precisely, the three-dimensional position of the particles that may be distributed in the depth of the illuminated zone can be calculated. The authors of this study have proposed a new algorithm that enables the fully three-dimensional measurement by SPIV and it can be classified as 3-D 3-C SPIV. Presently, the three-dimensional turbulent measurement of a plane jet has been carried out with the use of presently proposed 3-D 3-C SPIV and the transverse distributions of the turbulence statistics have been obtained in the depth direction of 3-D 3-C SPIV. The results are compared with those by standard PIV, i.e., 2-D 2-C PIV, and thus, the accuracy and the applicability of 3-D 3-C SPIV have been evaluated. [ABSTRACT FROM AUTHOR]

This paper describes an experimental study aimed at the characterization of the steady-state tumble motion in the cylinder of an engine using stereoscopic particle image velocimetry (Stereo-PIV). More specifically, a pentroof four valves gasoline direct injection (GDI) engine head was mounted on a modified FEV steady-state flow rig for applying Stereo-PIV at different measurement vertical tumble planes at mid cylinder, mid injector and mid valve. The flow field was described by the distribution of the ensemble average flow patterns for 1000 pairs of images for every case, vorticity contours, turbulent kinetic energy and tumble ratio. The results revealed that the higher velocities acquired at the mid valve plane improved the turbulent kinetic energy and tumble ratio compared to the other planes. There was a good level of agreement between direct and indirect methods used for calculating the tumble ratio.This paper describes an experimental study aimed at the characterization of the steady-state tumble motion in the cylinder of an engine using stereoscopic particle image velocimetry (Stereo-PIV). More specifically, a pentroof four valves gasoline direct injection (GDI) engine head was mounted on a modified FEV steady-state flow rig for applying Stereo-PIV at different measurement vertical tumble planes at mid cylinder, mid injector and mid valve. The flow field was described by the distribution of the ensemble average flow patterns for 1000 pairs of images for every case, vorticity contours, turbulent kinetic energy and tumble ratio. The results revealed that the higher velocities acquired at the mid valve plane improved the turbulent kinetic energy and tumble ratio compared to the other planes. There was a good level of agreement between direct and indirect methods used for calculating the tumble ratio.This paper describes an experimental study aimed at the characterization of the steady-state tumble motion in the cylinder of an engine using stereoscopic particle image velocimetry (Stereo-PIV). More specifically, a pentroof four valves gasoline direct injection (GDI) engine head was mounted on a modified FEV steady-state flow rig for applying Stereo-PIV at different measurement vertical tumble planes at mid cylinder, mid injector and mid valve. The flow field was described by the distribution of the ensemble average flow patterns for 1000 pairs of images for every case, vorticity contours, turbulent kinetic energy and tumble ratio. The results revealed that the higher velocities acquired at the mid valve plane improved the turbulent kinetic energy and tumble ratio compared to the other planes. There was a good level of agreement between direct and indirect methods used for calculating the tumble ratio.This paper describes an experimental study aimed at the characterization of the steady-state tumble motion in the cylinder of an engine using stereoscopic particle image velocimetry (Stereo-PIV). More specifically, a pentroof four valves gasoline direct injection (GDI) engine head was mounted on a modified FEV steady-state flow rig for applying Stereo-PIV at different measurement vertical tumble planes at mid cylinder, mid injector and mid valve. The flow field was described by the distribution of the ensemble average flow patterns for 1000 pairs of images for every case, vorticity contours, turbulent kinetic energy and tumble ratio. The results revealed that the higher velocities acquired at the mid valve plane improved the turbulent kinetic energy and tumble ratio compared to the other planes. There was a good level of agreement between direct and indirect methods used for calculating the tumble ratio. [ABSTRACT FROM AUTHOR]

The aerodynamic and thermal behaviour of multiperforated zones in combustors is essential to the development of future combustion chambers. Detailed databases are therefore crucial for the validation of RANS/LES solvers, but also regarding the derivation of heat transfer correlations used in 0D/1D in-house codes developed by engine manufacturers. In the framework of FP7 EU SOPRANO Program, the test-rig used in a previous study is modified to be compatible with anisothermal conditions. The plate studied is a 12:1 model with a 90∘ compound angle injection. A heating system is used to generate a moderate temperature gradient of about 20 K between the secondary hot flow and the main cold flow. The aerodynamic field is acquired by a PIV 2D-3C (Stereo Particle Image Velocimetry) system. The surface heat transfer coefficient is derived based on surface temperature distribution acquisitions. Several heating power levels are tested, which allows evaluating the convective heat transfer coefficient and reference temperature through a linear regression. Measurements are conducted on both sides of the plate, which also gives access to those quantities on the injection/suction sides. From a numerical point of view, the configuration is studied using the unstructured ONERA in-house CEDRE solver with an advanced Reynolds Stress Model. A systematic comparison is presented between the experimental and numerical database. Due to the high blowing ratio, the film protection is low in the first rows, with a convective heat transfer coefficient enhancement around three, and freestream cold air brought close to the wall by vortices created at injection. After four rows, the film is building up, leading gradually to a better insulation of the wall. The comparison with the numerical simulation exhibits a qualitative agreement on the main flow structures. However, the mixing between the jets, the film and the freestream is underestimated by the calculation.

Several studies exist modeling the Fontan connection to understand its hemodynamic ties to patient outcomes (Chopski in: Experimental and Computational Assessment of Mechanical Circulatory Assistance of a Patient-Specific Fontan Vessel Configuration. Dissertation, 2013; Khiabani et al. in J Biomech 45:2376-2381, 2012; Taylor and Figueroa in Annu Rev Biomed 11:109-134, 2009; Vukicevic et al. in ASAIO J 59:253-260, 2013). The most patient-accurate of these studies include flexible, patient-specific total cavopulmonary connections. This study improves Fontan hemodynamic modeling by validating Fontan model flexibility against a patient-specific bulk compliance value, and employing real-time phase contrast magnetic resonance flow data. The improved model was employed to acquire velocity field information under breath-held, free-breathing, and exercise conditions to investigate the effect of these conditions on clinically important Fontan hemodynamic metrics including power loss and viscous dissipation rate. The velocity data, obtained by stereoscopic particle image velocimetry, was visualized for qualitative three-dimensional flow field comparisons between the conditions. Key hemodynamic metrics were calculated from the velocity data and used to quantitatively compare the flow conditions. The data shows a multi-factorial and extremely patient-specific nature to Fontan hemodynamics. [ABSTRACT FROM AUTHOR]

This paper describes experimental investigation on the three-dimensional internal flow structures of the acoustic levitated volatile droplets by stereo PIV. The acoustic levitation is one of the effective techniques to levitate and manipulate droplets in air. In order to apply the acoustic levitation to various fields such as the material science, analytical chemistry, drug discovery and so on, it is essential to understand the flow structures and transport phenomena on the acoustically levitated droplet. Our observation shows the internal flow structure is changed with the increase of the saturated vapor pressure of the levitated droplets. In the case of decane, nonane, octane, heptane, hexane and pentane droplets, the rotational motion is generated in each droplet. In the case of ethanol and methanol droplets, there is the flow from the equator toward the center of the droplet in the horizontal plane. In addition, the two or four vortices are generated in the horizontal plane with the increase of the droplet diameter. [ABSTRACT FROM AUTHOR]

Experimental flow field investigations are presented on a 53° leading-edge sweep diamond wing configuration with rounded leading-edge contour. The analyses pertain to recent work that was conducted within the NATO Science and Technology Organization (STO) task group AVT-183 (Applied Vehicle Technology panel). The results obtained in a low-speed wind tunnel facility depict the overall aerodynamic characteristics as function of the angle of attack, and mean and turbulent flow field characteristics at one specific angle of attack, namely α = 12 ° . Both Stereo Particle Image Velocimetry and Hot-Wire Anemometry are applied to detect the emerging flow phenomena in several chordwise sections including near-wall data. Details of the leading-edge vortex formation and progression are analyzed and discussed. Over almost the entire length of its existence, the leading-edge vortex is characterized by retarded axial flow, thus showing vortex bursting tendencies. The corresponding turbulent fluctuations show maxima close to the evolution of the leading-edge vortex and decrease towards the trailing edge. The observed flow separation onset and progression are significant for the AVT-183 diamond wing configuration, and more generally for a moderately-swept low-aspect-ratio wing with rounded leading-edge contour. Following, the derived flow field characteristics provide a high-quality data base, which is suitable for general CFD validation. [ABSTRACT FROM AUTHOR]

This paper presents a computational investigation of the wake of the MEXICO rotor. The compressible multi-block solver of Liverpool University was employed, using a low-Mach scheme to account for the low-speed flow near the blade and in the wake. In this study, computations at wind speeds of 10, 15 and 24 m s − 1 were performed, and the three components of the velocity were compared against experimental data around the rotor blade up to one and a half rotor diameters downstream. Overall, fair agreement was obtained with the computational fluid dynamics showing good vortex conservation near the blade. Vorticity values revealed discontinuities in the wake at approximately 70%R, where two different aerofoils with different zero-lift angles are blended. The results suggest that all-Mach schemes for compressible computational fluid dynamics methods can deliver good performance and accuracy over all wind speeds for flows around wind turbines, without the need to switch between incompressible and compressible flow methods. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of flame propagation through a partially-premixed flow following ignition. A combination of simultaneous high-speed acetone planar laser induced fluorescence (PLIF) and stereoscopic particle image velocimetry (SPIV) was utilized for time-resolved measurements of mixture fraction, flow field and flame position. This provides access to the major quantities needed to characterize non-premixed flames. High quality mixture fraction measurements with signal-to-noise ratios up to 120 for unity mixture fraction were made feasible using a combination of a conventional high-speed laser at 10 kHz for LIF excitation and a wavelet based de-noising algorithm to reject camera noise. It was observed that flame propagation in the far-field of a partially-premixed jet takes place in a premixed mode, with the flame propagating through highly stratified mixtures until it approaches locations containing mixtures outside the flammability limits. In these areas the flame recedes and further propagation is controlled by mixing processes of air and fuel. Even though flame propagation is then mixing-controlled, the flame is not observed to switch into a non-premixed mode. Instead, mixing ahead of the flame takes place until locally premixed flammable mixtures are recovered for subsequent flame propagation. [ABSTRACT FROM AUTHOR]

A modern method of velocity measurement in a flow volume on the basis of few-projections optical tomography and correlation of moving particles is considered. The method is used to measure instantaneous three-dimensional velocity distributions in non-swirling and swirling turbulent jet flows. In addition to measurement of three-dimensional velocity distributions, specific features of this experiment are a larger depth of the measurement region (up to 40 mm) and a higher resolution of cameras (up to 4 MP as compared to previous authors' tomographic measurements). To verify the correctness of tomographic measurements, the velocity in the plane passing along the jet axis is measured by the Stereo PIV method. The difference between the stereoscopic and tomographic measurements of three components of the mean velocity for the flows considered in this study is smaller than 10%. A three-dimensional vortex structure of the core of a swirling jet is visualized with a spatial resolution of 3 mm. [ABSTRACT FROM AUTHOR]

Die Ausbildung der Strömung fließenden Wassers einer gravimetrischen Kalt- und Warmwasser-Normalmessanlage für große Volumenströme wird anhand von Stereoscopic Particle Image Velocimetry (SPIV)-Messungen an verschiedenen Positionen gezeigt. Dabei wird nachgewiesen, dass an dieser Prüfanlage nahezu optimale Strömungszustände zur Kalibrierung von Durchflusssensoren gewährleistet werden. SPIV eignet sich als schnelles und präzises Messverfahren zur zeitlich und räumlich hochaufgelösten Ermittlung von 3D-Geschwindigkeitsverteilungen in Rohrleitungen. The conditions of a cold and hot water standard test section have been investigated with Stereo PIV in different positions. Optimal requirements for the calibration of flow meters could be proved. Stereoscopic Particle Image Velocimetry (SPIV) is a fast and precise measurement method for the investigation of 3D velocity distributions in pipes with a high time resolution and spatial resolution. [ABSTRACT FROM AUTHOR]

Abstract: This paper carried out LES of the fluctuating wind pressures on a three-dimensional square cylinder for PIV-based inflow turbulence. Numerical simulation has generated inflow turbulence by directly using the stereo PIV measurement data at inflow condition. Since this PIV experiment was executed at low Reynolds number, the Reynolds number of inflow turbulence numerically obtained was also low. So we try to raise the Reynolds number for inflow turbulence by employing the new numerical technique on the basis of the scale similarity concept. Also, using the obtained inflow data at the higher Reynolds number, we performed LES of a square-cylinder flow. By comparison with wind tunnel experimental results, it can be confirmed that high Reynolds number inflow turbulence tends to modify the pressure characteristics on a 3D square cylinder. We can validate the proposed method which can appropriately transform the original flows to the turbulent boundary layer flows at higher-Reynolds number. [Copyright &y& Elsevier]

Three-component, turbulent flow measurements are performed using stereo PIV technique, in a cylindrical, stirred, water-flow mixer, with commercially available HR-100 as the impeller, which has three identical blades, separated by 120° with each other. The flow is analyzed at twelve axisymmetric, vertical planes, with the regular angular interval of 10°, so that the 12 planes cover the whole 120° separation between any two of the impeller blades. The results show that the turbulence mixing below the impeller is much higher than that above it, as the bulk fluid motion convects down the turbulence produced at the blade-water interfaces. Moreover, vortices with high level of turbulence, overwhelmingly dominated by the tangential velocity fluctuations, are created near the tips of the blades and convected downward, with their turbulence gradually dissipating with time. When these vortices are connected throughout the domain, they constitute three disconnected, helical arcs, each of which is connected to the tip of a blade and extends downward in the opposite direction to the rotation of the impeller. The arcs rotate with the same angular velocity of the impeller. The mean velocity and turbulence statistics of the flow are discussed at each of the alternate measurement planes. Graphical Abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Abstract: CFD has come to be used in the prediction of wind loads by the development of computer. Wind loads acting on buildings located in turbulent boundary layer are affected by the characteristic of approach flow strongly. Because inflow wind fluctuations affect the results of calculation with CFD, various kinds of methods to generate inflow wind fluctuations are suggested. On the other hand in wind tunnel experiment, PIV (Particle image velocimetry) technology has improved and been able to catch the high frequency turbulent fluctuations. We can acquire time–dependent–data of three-dimensional wind velocity components directly by stereo PIV. This report presents the result of LES calculation of turbulent boundary layer developed on flat floor for inflow generation using stereo PIV measurement results, and we discuss the applicability and the utility of this method. The simulated results show that the turbulent boundary layer is formed and reaches the equilibrium state at a relatively short distance from the starting point of computation. It can be thought that this method is useful for reproducing a turbulent boundary layer by LES. [Copyright &y& Elsevier]

Abstract: With the aid of the Stereo PIV technique, the local structure of swirling free turbulent jets is investigated under the following flow conditions: the Reynolds number was equal to 8900, and the swirl number varied from 0 to 1.0. The effect on the jet flow structure of external periodical forcing, applied with an axisymmetric mode to the inlet velocity, was studied. Also investigated were the flow response to the forcing at two Strouhal numbers, St =0.52 and 1.2, and with various forcing amplitudes. Additional measurements with application of the conditional sampling approach were performed in order to analyze spatio-temporal dynamics of the large-scale ring-like vortices, generated in the forced jet at a low swirl rate. The greatest effect of the forcing on the swirling jet structure was observed in the case of the high swirl rate (S =1.0), which was previously considered to be largely insensitive to external forcing. The forcing at St =1.2 with relatively high amplitude resulted in an abrupt change in the turbulent structure of the flow: an increase of total turbulent kinetic energy and strong anisotropy of its components took place. [Copyright &y& Elsevier]

Abstract: The control of an axisymmetric air jet equipped with a small-angle diffuser exhaust using a single non-thermal plasma actuator is studied with a stereoscopic PIV system. A single Dielectric Barrier Discharge (DBD) actuator is used to separate a naturally attached flow (12° angle diffuser) or to reattach a naturally separated one (22° angle diffuser). The experimental set-up allows us to measure the flow modifications occurring at the bevel of the diffuser. The actuator can generate a local flow (electric wind) of 6ms−1 used to modify an axisymmetric air jet with centreline velocities of 10, 20 and 30ms−1. The flow modifications under actuation are investigated in terms of time-averaged, phased-averaged fields as well as eddy localization. The analysis is mainly focused on the lower primary jet velocity at which the actuator presents a high efficiency. The results highlight that a plasma actuator can force the flow separation or attachment in a few dozen of milliseconds and that this device can be effective to manipulate the coherent structures surrounding the potential core. The global conclusion of the present study is that a single DBD actuator could be an effective device to promote the vortex shedding at the diffuser lip of an axisymmetric air jet. This may results in an improvement of the mixing characteristics, at least in the near-field region. [Copyright &y& Elsevier]

Abstract: A stereo PIV technique using advanced pre- and post-processing algorithms is implemented for the experimental study of the local structure of turbulent swirling impinging jets. The main emphasis of the present work is the analysis of the influence of swirl rate on the flow structure. During measurements, the Reynolds number was 8900, the nozzle-to-plate distance was equal to three nozzle diameters and the swirl rate was varied from 0 to 1.0. For the studied flows, spatial distributions of the mean velocity and statistical moments (including triple moments) of turbulent pulsations were measured. The influence of the PIV finite spatial resolution on the measured dissipation rate and velocity moments was analyzed and compared with theoretical predictions. For this purpose, a special series of 2D PIV measurements was carried out with vector spacing up to several Kolmogorov lengthscales. All terms of the axial mean momentum and the turbulent kinetic energy budget equations were obtained for the cross-section located one nozzle diameter from the impinging plate. For the TKE budget, the dissipation term was directly calculated from the instantaneous velocity fields, thereby allowing the pressure diffusion term to be found as a residual one. It was found that the magnitude of pressure diffusion decreased with the growth of the swirl rate. In general, the studied swirling impinging jets had a greater spread rate and a more rapid decay in absolute velocity when compared to the non-swirling jet. [Copyright &y& Elsevier]