Nonlinear temporal filtering of time-resolved digital particle image velocimetry data

Nonlinear filtering methods have been developed to identify and replace outlying data points in velocity time series obtained with time-resolved digital particle image velocimetry (PIV) of the flow around a surface-mounted cube at a Reynolds number of 20,000. Nuances associated with the spectral computation of the cross-correlation are highlighted, including the requirement of zero-padding an image interrogation area to eliminate the circular components of the cross-correlation. Three nonlinear filtering methods for the replacement of outliers are applied to the velocity time series sampled at 1,000 Hz: a median filter, a decision-based Hampel filter, and a PIV-specific Hampel filter. The particular benefit of the PIV-specific Hampel filter is that it allows the retention of actual measured data, sometimes derived from alternate peaks in the cross-correlation function, while still providing for the removal of outliers when a consistent, nonoutlying measurement is not available.     

Tracer particle generation in superfluid helium through cryogenic liquid injection for particle image velocimetry (PIV) applications

This paper reports the first experimental study of liquid neon injection into superfluid helium (He II) through a plain orifice atomizer to explore different means of introducing micron-size tracer particles into a He II bath for particle image velocimetry (PIV) applications. The obtained results verify that the direct injection of liquid neon into He II introduces seed particles into the He II bath. It is also demonstrated that the particle sizes can be controlled by changing the pressure above the injected liquid. Additionally, the size distribution of the particles is calculated from the PIV results through the use of the correlations to the standard drag curve.     

Two-color particle image velocimetry technique for an internal combustion engine

A two-color particle image velocimetry (PIV) technique has been applied to a single-cylinder motored research engine. Two-color PIV is a quantitative planar velocity measurement technique that can unambiguously determine the velocity magnitude and direction.

The work includes the development of an interrogation system, a series of computer simulations to determine the performance of the technique under various conditions, the comparison of these results to similar ones obtained for an autocorrelation PIV system, and a test of the technique by reconstructing the velocity field of a uniform jet flow.

The technique was then applied to the in-cylinder flow field of a motored single-cylinder, cup-in-head, research engine. A total of 27 instantaneous velocity fields were obtained at a single measurement plane for a single operating condition of the engine. The data were analyzed to yield ensemble-averaged velocity and velocity fluctuation.     

Analysis of soil flow and traction mechanics for lunar rovers over different types of soils using particle image velocimetry

Grouser wheels have been used in planetary rovers to improve mobility performance on sandy terrains. The biggest difference between a wheel with and without grousers is the soil behavior beneath the wheel as the grousers shovel the soil. By analyzing the soil flow, we gain insight into the mechanics dominating the interaction between the wheel and the soil, directly impacting performance. As the soil flow varies depending on the soil properties, the effects of soil type on soil behavior and wheel-traveling performance should be studied. This paper reveals the difference in soil flow and wheel performance on cohesive and non-cohesive soils. We conducted a series of single wheel tests over different types of soils under several wheel-traveling conditions. Soil flow was visualized by using particle image velocimetry (PIV). The experimental results indicate that soil flow characteristics highly depend on the shear strength of the soil. The cohesive soil exhibited lower fluidity due to its higher shear strength. At the same time, the wheel displayed a higher traveling performance over the cohesive soil, that is, a lower slip ratio.     

Assessment of PIV-based analysis of water entry problems through synthetic numerical datasets

The phenomenon of hull-slamming, that is, the sudden impact of a solid body on the water surface, is critical in the design of naval structures. Thus, the development and validation of schemes to predict the slamming load and elucidate energy exchange during water entry are of fundamental importance in a wide range of engineering applications. Recent studies have demonstrated the possibility of using direct flow measurements from particle image velocimetry (PIV) to investigate the kinetics of water entry. Specifically, these efforts have contributed a first characterization of the hydrodynamic loading on impacting wedges and of the energy imparted to the water pile-up and the spray jets. Here, we seek to provide a thorough assessment of such a PIV-based approach through synthetic datasets, in which PIV parameters, such as the camera acquisition rate and the size of the interrogation area, are systematically varied, without experimental confounds. We implement a direct computational framework to study the two-dimensional flow physics generated during the water entry of a rigid wedge. Water and air are treated as immiscible phases and their relative motion is utilized to track the free surface dynamics. Our results show that the PIV-based methodology allows for an accurate reconstruction of the pressure field from the measured velocity field, except for early stages of the impact and for a small region close to the free surface. We also demonstrate that the reconstruction is only marginally affected by the spatial resolution, while a sufficiently high acquisition frequency is required to correctly predict the pressure field in the pile-up region. The proposed computational framework can also find application in the analysis of less studied aspects of water entry problems, such as cycling loading, flow transitions and separation, and formation of spray jets.     

Estimation of particle dynamics in 2-D fluidized beds using particle tracking velocimetry

The experimental characterization of particle dynamics in fluidized beds is of great importance in fostering an understanding of solid phase motion and its effect on particle properties in granulation processes. Commonly used techniques such as particle image velocimetry rely on the cross-correlation of illumination intensity and averaging procedures. It is not possible to obtain single particle velocities with such techniques. Moreover, the estimated velocities may not accurately represent the local particle velocities in regions with high velocity gradients. Consequently, there is a need for devices and methods that are capable of acquiring individual particle velocities. This paper describes how particle tracking velocimetry can be adapted to dense particulate flows. The approach presented in this paper couples high-speed imaging with an innovative segmentation algorithm for particle detection, and employs the Voronoi method to solve the assignment problem usually encountered in densely seeded flows. Lagrangian particle tracks are obtained as primary information, and these serve as the basis for calculating sophisticated quantities such as the solid-phase flow field, granular temperature, and solid volume fraction. We show that the consistency of individual trajectories is sufficient to recognize collision events.     

Experimental investigation of hydrodynamic loads and pressure distribution during a pyramid water entry

In the maritime environment slamming is a phenomenon known as short duration impact of water on a floating or sailing structure. Slamming loads are local and could induce very high local stresses. This paper reports a series of impact test results and investigate the slamming loads and pressures acting on a square based pyramid. In this study the slamming tests have been conducted at constant velocity impact with a hydraulic high speed shock machine. This specific experimental equipment avoids the deceleration of the structure observed usually during water entry with drop tests. Three velocities of the rigid pyramid have been used (10, 13 and 15 m s⁻¹). Time-histories of local pressures, accelerations and slamming loads were successfully measured. The relationship between the pressure magnitude and the impact velocity is obtained and the spatial distribution of pressures on pyramid sides is characterized. The impact velocity was found to have a negligible influence in predicting the maximum pressure coefficient.     

Non-linear dynamics of semi-dilute polydisperse polymer solutions in microfluidics: A study of a benchmark flow problem

The complex flow behaviour of semi-dilute (15 < c/c^* < 22.5) polydisperse polyethylene oxide (PEO) aqueous solutions flowing through a planar microfluidic geometry with an 8:1:8 contraction-expansion is systematically studied. The molecular weight and distribution of the PEO samples are analysed by Gel Permeation Chromatography (GPC). Full rheometric characterizations using various techniques including piezoelectric axial vibrator (PAV) measurements at frequencies as high as 6700 Hz are carried out for one semi-dilute PEO solution. Complex flows over a wide range of elasticity numbers (20 ? El ? 120), Weissenberg numbers (7 ? Wi ? 121) and Reynolds numbers (0.08 ? Re ? 4.5) are characterized using micro-particle image velocimetry (μ-PIV) and pressure drop measurements. The evolution of vortex formation and dynamics has been visualized through a step-flow-rate experiment. The effect of El on vortex stability has been studied. Various flow dynamics regimes have been quantified and are presented in a Wi-Re diagram. The experimental results reveal that the elastic behaviour of polymer solutions is very sensitive to high molecular weight polymer in the polydisperse polymer samples, and the contraction ratio and the aspect ratio of flow geometry are the important design parameters in controlling the non-linear dynamics of semi-dilute polymer solutions in microfluidics.     

A study of the flow characteristics in micro-abrasive jets

An experimental study of particle velocities in micro-abrasive jets by using the particle image velocimetry (PIV) technique is presented. It has been found that the particle jet flow has a nearly linear expansion downstream. The particle velocities increase with air pressure, and the increasing rate increases with nozzle diameter within the range considered. The instantaneous velocity profile of the particle flow field in terms of the particle velocity distribution along the axial and radial directions of the jets is discussed. For the axial profile in the jet centerline downstream, there exists an extended acceleration stage, a transition stage, and a deceleration stage. For the radial velocity profiles, a relatively flat shape is observed at a jet cross-section near the nozzle exit. Mathematical models for the particle velocities in the air jet are then developed. It is shown that the results from the models agree well with experimental data in both the variation trend and magnitude.     

Effect of solid particle loading on nucleophilic addition of epoxy-resin to isophorone diisocyanate

In the preparation of surface coatings made of conductive composites consisting of conductive particulate fillers in a soft matrix, cracks will develop with increase of the particulate loading, which is believed to be related to the nucleophilic addition reaction between glycidyl end-capped poly （bisphenol A-co-epichlorohydrin） and isophorone diisocyanate molecules. This curing reaction is responsible for the generation of crosslinking network throughout the coatings. The influence of solid particle loading on the chemical reaction may be described as a volume-excluded effect, that is, the high solid particle loading will occupy the space between the functional groups thus preventing the chemical reaction to take place. As a direct consequence, the cross-linking network cannot develop properly due to the insufficiency of curing reaction. This will lead to the generation of cracks, as was supported by FT-IR analysis in this work.     

Particle image velocimetry （PIV） measurements of tip vortex wake structure of wind turbine

Large-view flow field measurements using the particle image velocimetry (PIV) technique with high resolution CCD cameras on a rotating 1/8 scale blade model of the NREL UAE phase VI wind turbine are conducted in the engineering-oriented Φ3.2 m wind tunnel.The motivation is to establish the database of the initiation and development of the tip vortex to study the flow structure and mechanism of the wind turbine.The results show that the tip vortex first moves inward for a very short period and then moves out...     

Study on local topology model of low/high streak structures in wall-bounded turbulence by tomographic time-resolved particle image velocimetry

The relationship between the bursting event and the low/high-speed streak in the logarithmic law (log-law) region of a turbulent boundary layer is investigated. A tomographic time-resolved particle image velocimetry (TRPIV) system is used to measure the instantaneous three-dimensional-three-component (3D-3C) velocity field. The momentum thickness based Reynolds number is about 2 460. The topological information in the log-law region is obtained experimentally. It is found that the existence of the quadrupole topological structure implies a three-pair hairpin-like vortex packet, which is in connection with the low/high-speed streak. An idealized 3D topological model is then proposed to characterize the observed hairpin vortex packet and low/high-speed streak.     

A study of the flow field characteristics around star-shaped artificial reefs

In this paper, a three-dimensional numerical model is devised to calculate the unsteady flow field around star-shaped artificial reefs. The model is based on Reynolds-averaged Navier–Stokes (RANS) equations embedded within a renormalization group (RNG) k–ε turbulence model. The RANS equations are solved using the finite volume method (FVM) with an unstructured tetrahedral mesh. The pressure and velocity coupling is solved at each time step with the SIMPLEC algorithm. Non-invasive particle image velocimetry (PIV) laboratory measurements are employed to verify the simulation results. Good agreement is found between the simulation and experimental results with respect to the major flow fields. Based on the flow-field verification, the influence of arrangement and spacing on the flow field of one and two artificial reefs are discussed in light of the numerical method. A large-scale slow flow region is obtained when the reef is arranged in the second form. In the parallel combination, a slight mutual effect exists between the two reefs when the spacing is larger than 3.0L. In the streamwise combination, the interaction of two reefs is at its strongest at spacings of 3.0L to 4.0L.     

Application of particle image velocimetry to a transonic centrifugal compressor

As part of an ongoing research project the performance and internal flow field of a high-pressure ratio centrifugal compressor is being investigated. Based on previous, primarily, point-wise laser-optical measurements the compressor was redesigned and resulted in an improved impeller and diffuser with a single-stage pressure ratio of 6:1 at 50,000 rpm. Current research activities involve the use of particle image velocimetry (PIV) to analyze and further improve the understanding of the complex flow phenomena inside the vaned diffuser given the capability of PIV of capturing spatial structures. The study includes phase-resolved measurements of the flow inside a diffuser vane passage with respect to the impeller blade position. Both, instantaneous and phase-averaged velocity fields are presented. The flow field results obtained by PIV are to be used for future validation of the related CFD calculations, which in turn are expected to lead to further improvements in compressor performance. In addition, the potential of stereo PIV for this type of turbomachinery application could be successfully demonstrated.     

Three-dimensional flow structure measurements behind a queue of studied model vehicles

The three-dimensional flow structures of a queue of studied model vehicles (i.e., one-, two- and three-vehicle cases) were investigated comprehensively in a closed-circuit wind tunnel using particle image velocimetry (PIV) for the typical urban vehicle speeds (i.e., 10, 30 and 50 km/h). In this three-dimensional vehicle wake, a pair of longitudinal vortices is characterized by counter-rotating and moving downstream at relatively low velocity than their surrounding flow. The flow structures of multiple studied model vehicles are dominated by the wake generated from the last studied model vehicle but the preceding studied model vehicle(s) also has/have some minor effects. Cross-sectional turbulence distribution is non-uniform in the far-wake region for all studied cases. The lowest turbulence occurs at the center part of the vehicle wake while high turbulence occurs at its two sides. As such, it may lead to considerable underestimation in turbulence magnitude if the measurement is only taken along the centerline of the vehicle wake.     

Measurement of surface velocities and shears at a wavy air–water interface using particle image velocimetry

 Measurements have been accomplished within the aqueous surface viscous sublayer of wind-forced microscale breaking waves using particle image velocimetry. These measurements have been used to derive surface velocities and tangential stresses. Assessment is made of the performance of the experimental equipment and techniques developed to accomplish these measurements. Comparison is made with previous studies of viscous sublayer behaviour at the air–water interface and at smooth solid surfaces. Received: 7 June 1996/Accepted: 30 April 1997     

An experimental investigation of jet flows through a row of forward expanded holes into a mainstream over a concave surface

This study performed detailed measurements of jet flows through a row of forward expanded holes into a mainstream over a concave surface using digital particle image velocimetry. Each of ejected holes had a streamwise inclined angle of 35° bounded on a concave surface with constant radius of 382 mm. The spacing of adjacent holes is 1.5D. The density and the momentum flux ratio of the mainstream to the jet flow were 1.0. Results show detailed 2D mean velocity maps on several horizontal and vertical planes and a 3D streamline pattern of jet mean velocity. The streamlines of 3D mean velocity clearly display different flow characteristics of the ejected jet flow along the transverse direction. In addition, the particle trajectory of a ring enclosing an ejected jet above the injection hole was also presented to show movement of jet.     

Pressure measurements on the impingement surface of sonic and sub-sonic jets impinging onto a flat plate at inclined angles

The flow field associated with a jet impinging onto a surface at an inclined angle is investigated using pressure-sensitive paint (PSP) and particle image velocimetry. The PSP yields continuous measurements of pressure on the jet impingement surface. The jet footprint on the impingement surface is visualized using the half-maximum pressure contour. The results indicate that the impingement angle of the jet is the dominant parameter in determining the footprint of the jet on the impingement surface. This contour is similar in shape to an ellipse that is created by projecting the nozzle through the impingement surface. The ellipse is centered at the location of maximum pressure and the width of the minor axis is just over one jet diameter. The location of maximum pressure is found upstream of the geometric impingement point and this location is a strong function of the impingement angle. A curve fit for the location of maximum pressure can be constructed using an exact solution of the Navier–Stokes equations for a non-orthogonal stagnation flow. The maximum value of pressure is a function of impingement angle and varies as the sine of the impingement angle squared; the maximum pressure is also a function of jet impingement distance. Using these results, a simple procedure for predicting the overall structure of the jet on the impingement surface is presented.