Stereoscopic Particle Image Velocimetry System Research Articles

Turbulence structure under the free-surface wave around an advancing surface-piercing cylindrical body

Free-surface wave effects on the turbulent boundary layer and wake of a surface-piercing slender body are investigated by measuring the flow and free-surface elevation around the model in three different Froude number (Fr) conditions—0.126, 0.282, and 0.400. The flow is measured by a towed underwater stereoscopic particle image velocimetry system. At the lowest Fr of 0.126, there appears no significant wave pattern, but the flow retards under the free-surface. The free-surface hinders the flow underneath, as the solid surface does. The steady and smooth free-surface wave generated at the intermediate Fr of 0.282 affects the pressure gradient on the turbulent boundary layer. The momentum thickness increases due to the adverse pressure gradient from the wave trough to the wave crest. At the highest Fr of 0.400, a strong adverse pressure gradient results in flow separation and violent wave breaking. The wave-induced separation at the high Froude number condition stimulates momentum transfer and turbulence dissipation, resulting in isotropic turbulence.

Large-scale energetic coherent structures and their effects on wall mass transfer rate behind orifice in round pipe

This paper first uses a low-speed stereoscopic particle image velocimetry (SPIV) system to measure the convergent statistical quantities of the flow field and then simultaneously measure the time-resolved flow field and the wall mass transfer rate by a high-speed SPIV system and an electrochemical system, respectively. We measure the flow field and wall mass transfer rate under upstream pipe Reynolds numbers between 25 000 and 55 000 at three specific locations behind the orifice plate. Moreover, we apply proper orthogonal decomposition (POD), stochastic estimation and spectral analysis to study the properties of the flow field and the wall mass transfer rate. More importantly, we investigate the large-scale coherent structures’ effects on the wall mass transfer rate. The collapse of the wall mass transfer rates’ spectra by the corresponding time scales at the three specific positions of orifice flow suggest that the physics of low-frequency wall mass transfer rates are probably the same, although the flow fields away from the wall are quite different. Furthermore, the spectra of the velocity reconstructed by the most energetic eigenmodes agree well with the wall mass transfer rate in the low-frequency region, suggesting that the first several energetic eigenmodes capture the flow dynamics relevant to the low-frequency variation of the wall mass transfer. Stochastic estimation results of the velocity field associated with large wall mass transfer rate at all three specific locations further reveal that the most energetic coherent structures are correlated with the wall mass transfer rate.

Novel method to accurately align the laser sheet for planar and stereoscopic PIV

Several techniques including two-dimensional (2D) and three-dimensional (3D) calibration are used for the calibration of two-component two-dimensional (2C-2D) particle image velocimetry (PIV) and three-component two-dimensional (3C-2D) stereoscopic PIV (SPIV) systems. A major requirement of these techniques is to keep the calibration target exactly at the position of the laser sheet within the field of view (FOV), which is very difficult to achieve (Raffel et al., 2018). In 3C-2D SPIV, several methods offer different correction schemes based on the disparity between the FOV of two stereo cameras produced due to misalignment, to account for the misalignment error. These techniques adjust the calibration or the measured displacement field in different ways to reduce the error which may introduce an unintended error in the measurement position and/or velocity such as a bias in the measured three-component 3C displacements. This paper introduces a novel method to align the laser sheet with the calibration target so that the uncertainty in displacement measurements is minimal. Ideally, it should be of the order of the uncertainty associated with PIV measurement so that no ad hoc post-correction scheme is required.

Working mechanism of pre-swirl stator based on stereoscopic particle image velocimetry

The pre-swirl stator (PSS), as a type of energy-saving device, has garnered increasing attention due to efforts aimed at global energy-saving and emission reduction. Owing to the influence of the stern profile, the wake field at the stern is complex, and the installation of such energy-saving appendages further increases its complexity. To explore the flow mechanism of a ship's wake field and the working principles of energy-saving appendages, this study used a scale model of a bulk carrier as its research object and conducted a wake field experiment with the help of the large underwater stereoscopic particle image velocimetry system at Harbin Engineering University. The wake field data of several sections were obtained, including the axial velocity, tangential velocity, average kinetic energy, vorticity, swirl strength, streamline, pulsating velocity, turbulent kinetic energy, and axial velocity at different radii. The wake field data with or without a PSS in the bare hull condition and with or without a PSS in the self-propelled condition were compared. The time-averaged flow field clearly demonstrated a “hook-like” velocity profile, bilge vortex, hub cap vortex, and other flow structures, as well as the flow details around the PSS. The results showed that the PSS could destroy the contour structure of the axial velocity, which mainly affects the range of 0.5–0.8R, making the flow field more uniform. The PSS has a certain influence on the flow field on the side without the stator blade and a great influence on the tangential velocity by producing a pre-swirl flow in a direction opposite to the propeller rotation in front of the propeller plane. This pre-swirl flow can increase the inflow of the propeller disk and improve the working conditions of the propeller. The PSS also has an influence on radial velocity; however, the influence degree and range are smaller than that on the tangential velocity. The PSS changed the water inflow direction, increased the interaction angle between the water flow and propeller blade, improved the wake conditions and propulsion performance, and was conducive to improving the working efficiency of the propeller. Moreover, the existence of a PSS reduced the local pulsating velocity and turbulent kinetic energy, benefiting the structural strength of the propeller.

Interaction Between a Crossflow and a Spatially Oscillating Jet at Various Angles

The quasi-time-resolved flowfields of spatially oscillating jets emitted by fluidic oscillators at various installation angles, velocity ratios, and oscillation frequencies are examined experimentally. The velocity fields are acquired plane by plane using a stereoscopic particle image velocimetry system. Dominant streamwise vortices are identified and found to prevail downstream. The flowfield at a given velocity ratio is independent of the oscillation frequency because velocity ratio and Strouhal number are proportional. For small Strouhal numbers, the flowfield behaves similar to that of a vortex generating jet. For higher Strouhal numbers, the crossflow experiences a steady, wide-spread jet. When the skew angle decreases, one of the former two dominating vortices diminishes until at a skew angle of 45 deg one vortex dominates the flowfield. At a skew angle of 0 deg (i.e., the plane spanned by the oscillating jet is parallel to the crossflow), the jet penetration is comparable to that of a steady jet, and a pair of co-existing, counter-rotating vortices is identified. When changing the inclination angle, the flow features in the far field reduce to two dominant streamwise vortices whose spanwise effect increases when the inclination angle decreases. Additionally, the Strouhal number effects are shifted toward higher Strouhal numbers.

Wake Structures and Surface Patterns of the DrivAer Notchback Car Model under Side Wind Conditions

The flow field topology of passenger cars considerably changes under side wind conditions. This changes the surface pressure, aerodynamic force, and drag and performance of a vehicle. In this study, the flow field of a generic passenger vehicle is investigated based on three different side wind angles. The study aimed to identify vortical structures causing changes in the rear pressure distribution. The notchback section of the DrivAer model is evaluated on a scale of 1:4. The wind tunnel tests are conducted in a closed section with a splitter plate at a Reynolds number of 3 million. The side wind angles are 0 ∘ , 5 ∘ , and 10 ∘ . The three-dimensional and time-averaged flow field downstream direction of the model is captured by a stereoscopic particle image velocimetry system performed at several measurement planes. These flow field data are complemented by surface flow visualizations performed on the entire model. The combined approaches provide a comprehensive insight into the flow field at the frontal and side wind inflows. The flow without side wind is almost symmetrical. Longitudinal vortices are evident along the downstream direction of the A-pillar, the C-pillars, the middle part of the rear window, and the base surface. In addition, there is a ring vortex downstream of the vehicle base. The side wind completely changes the flow field. The asymmetric topology is dominated by the windward C-pillar vortex, the leeward A-pillar vortex, and other base vortices. Based on the location of the vortices and the pressure distributions measured in earlier studies, it can be concluded that the vortices identified in the wake are responsible for the local minima of pressure, increasing the vehicle drag.

Flow dynamics in the vicinity of a gravel embedded vertical retaining wall: conditions corresponding to the initial stages of local erosion

The present study examines the turbulent flow characteristics in the vicinity of the leading edge of a vertical retaining wall structure embedded in a gravel bed. Conditions before and during the initial stages of local scour development were considered. To that end, three laboratory experiments were carried out: one with fixed boundaries as a reference case, a second one with an erodible bed, but immobile channel bank, and a third one characterized by an erodible channel bed and bank. Instantaneous and time-averaged flow features, and turbulence statistics associated with the three-component velocity fields, obtained with a stereoscopic particle image velocimetry system, were analyzed. Water surface examination determined that channel bank erosion alters flow behavior upstream of the retaining wall. This change is manifested through more frequent formation of eddies near the top corner of the junction between the upstream face of the protrusion and the channel bank. Time-averaged and instantaneous velocity vector fields demonstrate the presence of two counter-rotating vortices in the experiments with the fixed channel bank, one located near the junction of retaining wall and the channel bed and one away from it generated as a result of a strong downflow. The latter motion represents a section of a horseshoe vortex. During the channel bank erosion, however, these vortices are not as significant and persistent. The results also indicate that the coherency and magnitude of the streamwise vorticity field diminish as a result of scour. However, it was confirmed that local turbulent kinetic energy and shear stresses increase in the presence of scour due to enhanced local flow separation.

Analysis of the influence of an interceptor on the transom flow of a fast ship by pressure reconstruction from stereoscopic scanning PIV

The present study describes the application of the particle image velocimetry (PIV) technique for the reconstruction of hydrodynamic pressures and loads on a ship model from measured velocity fields during towing tank tests. As an alternative to conventional pressure and force measurement techniques the method simultaneously pictures the velocity field and captures the dynamic aspect of the flow. The presented measurements are conducted in the transom region of a generic hull of a planing vessel which is equipped with an interceptor to create a stagnating flow, associated with a high pressure peak. The flow close to the hull is captured with an underwater stereoscopic PIV system and the pressure peak in front of the interceptor is reconstructed from time-averaged velocity fields. Results show the effect of different interceptor heights on the pressure distribution in the center-plane of the model. Further, a 3D flow field is reconstructed from scanning PIV measurements to analyze the lift reduction due to the finite span of the interceptor. The spatial variation of the measurement uncertainty is analyzed and propagated to the pressure field uncertainty and the potential of the method is further evaluated by comparison with numerical results from steady Reynolds Averaged Navier-Stokes (RANS) simulations.

A technique for measuring ensemble-averaged, three-component liquid velocity fields in two-phase, gas–liquid, intermittent pipe flows

Gas–liquid intermittent flows can be found in many engineering applications, nevertheless a detailed knowledge of this flow pattern is still not fully available. In the present work, an experimental study was conducted with the objective of developing a measurement procedure capable of providing ensemble-averaged three-component velocity fields in the liquid phase of a gas–liquid, intermittent, horizontal flow in a pipe. To this end, a high-frequency stereoscopic particle image velocimetry system (SPIV) was employed, combined with the laser induced fluorescence (LIF) technique to separate the light scattered by the liquid–gas interfaces from that emitted by the fluorescent tracer particles. A set of photogates was used to trigger the SPIV system, allowing for the measurement of velocity fields in the liquid plug, downstream of the elongated bubble, and in the liquid film, upstream of the elongated bubble nose position. The triggered measurements allowed the determination of ensemble-averaged three-component velocity fields at different positions in relation to the bubble nose, obtained from the replication of a sufficiently large number of bubble passage events. Contours of the liquid flow streamwise vorticity component in cross-stream planes upstream and downstream of the bubble nose tip were also obtained from the SPIV measurements. The photogate system was also employed to measure the bubble velocity. This information was used to transform time-based into space-based velocity field data. This allowed the construction of a three-dimensional representation of the ensemble-averaged structure of the gas bubble nose and the associated vortical structures induced in the liquid flow. The three-component velocity information obtained revealed the influence of the gas bubble motion on the liquid flow in the plug and liquid film regions.

Uncertainty assessment for a towed underwater stereo PIV system by uniform flow measurement

The present study aims to assess test uncertainty assessment method of nominal wake field measurement by a Stereoscopic Particle Image Velocimetry (SPIV) system in a towing tank. The systematic uncertainty of the SPIV system was estimated from repeated uniform flow measurements. In the uniform flow measurement case, time interval between image frames and uniform flow speed were varied to examine the effects of particle displacement and flow around the SPIV system on the systematic standard uncertainty. The random standard uncertainty was assessed by repeating nominal wake field measurements and the estimated random standard uncertainty was compared with that of laser Doppler velocimetry. The test uncertainty assessment method was applied to nominal wake measurement tests of a very large crude oil carrier model ship. The nominal wake measurement results were compared with existing experimental database by other measurement methods, with its assessed uncertainty.

Influence of Design Parameter Variations for Propeller-Boss-Cap-Fins on Hub Vortex Reduction

The present study aimed to identify significant design parameters on the propeller open water efficiency and characteristics of the wake field of a propeller with propeller-boss-cap-fins (PBCFs), with special attention to hub vortex dynamics. A towed underwater stereoscopic particle image velocimetry system was used to measure the near wake fields of a five-bladed propeller and a PBCF model. Through model tests, it was confirmed that the disappearance of a low-pressure area behind a boss cap, i.e., a reduction of the hub vortex, was the dominant contribution to open water efficiency gains from PBCF rather than a decrease in net torque. Global force and wake fields were also measured in response to design parameter variations of PBCF, i.e., fin chord length, fin span height, pitch angle, and phase lag. In the cases of pitch angle and fin chord length variations, PBCF with a light loading configuration had better open water efficiency than the baseline PBCF. However, in the case of a short fin span height for light loading configuration, the hub vortex was not entirely prevented, and the total open water efficiency decreased. Under heavy loading configurations, excessive negative and torque degraded the open water efficiency. Phase lag variations were less effective with respect to the open water efficiency than other design parameter variations.

Experimental and theoretical study of wind turbine wakes in yawed conditions

This work is dedicated to systematically studying and predicting the wake characteristics of a yawed wind turbine immersed in a turbulent boundary layer. To achieve this goal, wind tunnel experiments were performed to characterize the wake of a horizontal-axis wind turbine model. A high-resolution stereoscopic particle image velocimetry system was used to measure the three velocity components in the turbine wake under different yaw angles and tip-speed ratios. Moreover, power and thrust measurements were carried out to analyse the performance of the wind turbine. These detailed wind tunnel measurements were then used to perform a budget study of the continuity and Reynolds-averaged Navier–Stokes equations for the wake of a yawed turbine. This theoretical analysis revealed some notable features of the wakes of yawed turbines, such as the asymmetric distribution of the wake skew angle with respect to the wake centre. Under highly yawed conditions, the formation of a counter-rotating vortex pair in the wake cross-section as well as the vertical displacement of the wake centre were shown and analysed. Finally, this study enabled us to develop general governing equations upon which a simple and computationally inexpensive analytical model was built. The proposed model aims at predicting the wake deflection and the far-wake velocity distribution for yawed turbines. Comparisons of model predictions with the wind tunnel measurements show that this simple model can acceptably predict the velocity distribution in the far wake of a yawed turbine. Apart from the ability of the model to predict wake flows in yawed conditions, it can provide valuable physical insight on the behaviour of turbine wakes in this complex situation.

Note: Device for obtaining volumetric, three-component velocity fields inside cylindrical cavities.

We describe a device designed and built to obtain the three-component, steady state velocity field in the whole volume occupied by a fluid in motion contained in a cavity with cylindrical walls. The prototype comprises a two-camera stereoscopic particle image velocimetry system mounted on a platform that rotates around the volume under analysis and a slip ring arrangement that transmits data from the rotating sensors to the data storage elements. Sample observations are presented for natural convection in a cylindrical container but other flows can be analyzed.

Dynamic stall process on a finite span model and its control via synthetic jet actuators

An experimental study of the process by which dynamic stall occurs on a finite span S809 airfoil was conducted at the Center for Flow Physics and Control at Rensselaer Polytechnic Institute. Understanding the flow field around a dynamically pitching airfoil helped in controlling the dynamic stall process through active flow control via synthetic jet actuators. The three component, two dimensional flow fields were measured with a stereoscopic particle image velocimetry system. This study demonstrated that, through the introduction of periodic momentum near the leading edge of this model, the evolution of the dynamic stall vortex, which forms and convects downstream under dynamic conditions, could be delayed or suppressed in favor of the preservation of a trailing edge vortex that arises due to trailing edge separation and recirculation in the time averaged sense. This process seems to be the result of changing how the flow field transitions from trailing edge separation to a fully separated flow. In a phase-averaged sense, absent of flow control, this process is defined by the creation of a phase averaged leading edge recirculation region, which interacts with the trailing edge separation. Through the introduction of momentum near the leading edge, this process can be altered, such that the phase averaged trailing edge separation region is the dominant structure present in the flow. Additionally, a cursory investigation into the instantaneous flow fields was conducted, and a comparison between the phase averaged flow field and instantaneous fields demonstrated that while similar effects can be observed, there is a significant difference in the flow field observed in the instantaneous fields versus the phase averaged sense. This would imply that a different method of analyzing dynamic stall from PIV measurements may be necessary.

PIV study of large-scale flow organisation in slot jets

The paper reports on particle image velocimetry (PIV) measurements in turbulent slot jets bounded by two solid walls with the separation distance smaller than the jet width (5–40%). In the far-field such jets are known to manifest features of quasi-two dimensional, two component turbulence. Stereoscopic and tomographic PIV systems were used to analyse local flows. Proper orthogonal decomposition (POD) was applied to extract coherent modes of the velocity fluctuations. The measurements were performed both in the initial region close to the nozzle exit and in the far fields of the developed turbulent slot jets for Re⩾10,000. A POD analysis in the initial region indicates a correlation between quasi-2D vortices rolled-up in the shear layer and local flows in cross-stream planes. While the near-field turbulence shows full 3D features, the wall-normal velocity fluctuations day out gradually due to strong wall-damping resulting in an almost two-component turbulence. On the other hand, the longitudinal vortex rolls take over to act as the main agents in wall-normal and spanwise mixing and momentum transfer. The quantitative analysis indicates that the jet meandering amplitude was aperiodically modulated when arrangement of the large-scale quasi-2D vortices changed between asymmetric and symmetric pattern relatively to the jet axis. The paper shows that the dynamics of turbulent slot jets are more complex than those of 2D, plane and rectangular 3D jets. In particular, the detected secondary longitudinal vortex filaments and meandering modulation is expected to be important for turbulent transport and mixing in slot jets. This issue requires further investigations.

Influence of light sheet separation on SPIV measurement in a large field spanwise plane

Stereoscopic particle image velocimetry (SPIV) is nowadays a well-established measurement technique for turbulent flows. However, the accuracy and the spatial resolution are still highly questionable in the presence of complex flow with both strong gradients and out-of-plane motions. To give guidelines for both setup and measurements of such flow configurations, a large region of overlap between two SPIV systems on the same laser light sheet is acquired in a plane normal to the streamwise direction of a high Reynolds turbulent boundary layer flow. A simple separation of the two light sheets is used to improve the accuracy of the measurements by increasing the velocity dynamic range especially. It also presents the enhancement of accuracy due to the light sheet separation for characterizing streamwise vortices (i.e. perpendicular to the sheet). The proposed technique has been demonstrated in the Laboratoire de Mécanique de Lille wind tunnel facility which has been specially designed to study fully developed turbulent boundary layers at high Reynolds numbers. The outlook is to study in detail the physics of the streamwise vortices generated from vortex generators taking advantage of the large scales of this turbulent boundary layer.

Interactions between aquatic plants and turbulent flow: a field study using stereoscopic PIV

AbstractA stereoscopic particle image velocimetry (PIV) system for use in shallow (${\sim }$0.5 m deep) rivers was developed and deployed in the Urie River, Scotland, to study the interactions between turbulent flow and a Ranunculus penicillatus plant patch in its native environment. Statistical moments of the velocity field were calculated utilizing a new method of reducing the contribution of measurement noise, based on the measurement redundancy inherent in the stereoscopic PIV method. Reynolds normal and shear stresses, their budget terms, and higher-order moments of the velocity probability distribution in the wake of the plant patch were found to be dominated by the presence of a free shear layer induced by the plant drag. Plant motion, estimated from the PIV images, was characterized by travelling waves that propagate along the plant with a velocity similar to the eddy convection velocity, suggesting a direct coupling between turbulence and the plant motion. The characteristic frequency of the plant velocity fluctuations (${\sim }$1 Hz) may suggest that the plant motion is dominated by large eddies with scale similar to the flow depth or plant length. Plant and fluid velocity fluctuations were, in contrast, found to be strongly correlated only over a narrow (${\sim }$30 mm) elevation range above the top of the plant, supporting a contribution of the shear layer turbulence to the plant motion. Many aspects of flow–aquatic plant interactions remain to be clarified, and the newly developed stereoscopic field PIV system should prove valuable in future studies.

Stereo‐PIV experiments and large eddy simulations of flow fields in stirred tanks with Rushton and curved‐Blade turbines

The flow characteristics in pilot‐scale stirred tanks with Rushton and curved‐blade turbines were investigated by using stereoscopic particle image velocimetry (SPIV) experiments and large eddy simulation (LES) methods. The velocity and turbulent kinetic energy (TKE) in the impeller discharge regions were carefully resolved with a high resolution SPIV system, and the detailed phase‐resolved velocity and TKE profiles were used to validate the LES results. The effects of Reynolds number and blade shape on the flow characteristics were discussed. The LES results of velocity, TKE, and the evolution of trailing vortices were compared with the SPIV experimental data, and good agreement was obtained at various phase angles. The effects of subgrid scale model and hybrid grid with different mesh resolutions on the LES results were investigated. LES is a computationally affordable method for the accurate predictions of the complex flow fields in pilot‐scale stirred tanks is presented. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3986–4003, 2013

Experimental investigation of multilayer particle deposition and resuspension between periodic steps in turbulent flows

Multilayer particle deposition and resuspension between periodic steps in a turbulent channel flow was experimentally studied. A particle laden turbulent flow in a horizontal channel was generated by means of a small-scale wind tunnel. Periodic steps were placed on the channel floor to induce a complex flow field characterised by flow separations, recirculation and reattachment zones.The turbulent flow field was recorded by means of a stereoscopic Particle Image Velocimetry system. The dimensions of the flow field structures agree well with the specifications from comparable studies. Polydisperse graphite particles were injected into the flow field during a pure deposition regime. The multilayer build up between the periodic steps was measured using a laser distance sensor coupled with a linear stage. The particle layer build up shows a linear tendency against time and the deposition velocities are in good agreement with similar investigations. After the deposition scenario the particle dispersion was switched off and particle resuspension was induced by the stepwise increase of the fluid velocity. The particle resuspension was found to begin in clusters close to the recirculation zones and was accomplished at lower friction velocities than necessary for single particle resuspension. Surface roughness analysis of the multilayer interface characterises the growth of the loose particle agglomerates during particle deposition and the instant removal of these structures during particle resuspension. Furthermore, particle bed porosity measurements were performed by gravimetrical and x-ray methods to characterise the bed porosity. Spatial correlations between the turbulent flow field and the multilayer thickness distribution indicate the influence of inertia impaction, turbulent dispersion and gravitational settling on the particle deposition process. The multilayer particle resuspension seems to be influenced by the mean and fluctuating flow field structures. The results indicate similarities between single particle and multilayer particle formation during particle deposition and resuspension processes.

Modification of Swirling Jet Flow by Premixed Combustion

Effect of combustion of propane-air mixture in a weakly and strongly swirling jet was studied. The rate of swirl was classified in terms of the absence / presence of a stable recirculation zone in the non-reacting jet flow. Sequences of the instantaneous velocity fields were measured in lean and rich flames at atmospheric pressure, and also in the non-reacting flow, by using a high-repetition stereoscopic particle image velocimetry system. Spatial distributions of the time-averaged velocity and Reynolds stresses, calculated from the sequences, revealed that combustion of rich mixture dramatically affected structure of flow and type of vortex breakdown