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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]

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]

• 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]

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]

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]

Experiments are carried out to investigate the effects of mixture stratification, velocity ratio, and swirl on the LPG/air flame structures using a swirl-stabilized axisymmetric burner. Stereo Particle Image Velocimetry (SPIV) technique is used to measure the velocity field of non-reacting and reacting flows. Chemiluminescence imaging technique is used to measure the OH*/CH* signals of premixed and stratified flames. The mixture stratification ratio at the burner exit is varied from 1 to 5 by changing the equivalence ratios of the inner and outer streams between 0.32 and 1.60. The swirl intensity of the inner and outer streams is varied independently by using two different sets of axial swirlers. The ratio of velocities between the inner and outer streams ranged from two to three. Examination of the non-reacting flow field showed that the combination of axial swirlers plays a crucial role in the formation of the streamlines which would help to sustain the flames. Analysis of reacting flow fields of different cases revealed the robustness of the flame structure to the stratification of the mixture. Further investigation on the Abel-inverted chemiluminescence images showed that the transition from premixed to stratified condition makes the flame more compact and closer to the exit of the burner. The flame structure is similar among stratified conditions with the increasing stratification ratios confirming the robustness of the flame structure to the stratification of the mixture. Under the operating conditions studied, the flame structure is more sensitive to changes in the velocity ratio or swirler combination relative to the stratification ratio. • A series of turbulent premixed and stratified LPG/air flames are studied. • The effect of SR, VR, and swirler combinations is investigated. • Stereo particle image velocimetry and chemiluminescence imaging are utilized. • Stratified flames are robust to the variations in mixture stratification. • Stratified flames are much more compact than the premixed flames. [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. [ABSTRACT FROM AUTHOR]