Particle Image Velocimetry Study Research Articles

PIV studies around a bus model

The three-dimensional flow structures around a bus model have been investigated experimentally. The time-averaged and instantaneous velocity vector maps, vorticity contours, streamline topology and other turbulent quantities were obtained using the particle image velocimetry (PIV) technique. The measurements were carried out in vertical, horizontal and cross-sectional planes. The three-dimensional wake region was asymmetric in the vertical plane while the wake flow was almost symmetric in the horizontal plane. In cross-planes the flow field downstream of the model is dominated by a pair of counter rotating vortices originating from the bottom and upper corners of the model.

On the flow characteristics of minor-plane inclined elliptic jets

Flow characteristics derived from a particle-image velocimetry study on 30° and 60° minor-plane inclined elliptic jets are reported and compared to their major-plane inclined counterparts. Flow dynamics are clarified and found to be more sensitive towards the presence of inclined nozzle exits than the basic nozzle geometry and exact incline-plane used. Results show that the inclination of the inclined vortex roll-ups increases and decreases for 30° and 60° inclined nozzles respectively, similar to major-plane inclined elliptic jets. Based on available data, a critical incline-angle where the resultant inclined vortex roll-ups will neither increase nor decrease their inclinations has been estimated to be about 41° for minor-plane inclined jets. Flow quantities associated with cross-stream mixing characteristics are found to be dependent upon the exact incline-plane used. 60° minor-plane inclined jet attains the highest centerline turbulence levels and results demonstrate that flow events affecting radial velocity characteristics have significant influences upon the jet developments. Lastly, developments of mean streamwise and radial velocity components distributions indicated that minor-plane inclined elliptic jets incurred less flow asymmetry within the present measurement range than their major-plane inclined counterparts.

Stereoscopic and tomographic PIV of a pitching plate

This paper applies particle image velocimetry (PIV) to a simplified, canonical, pitch-hold-return problem of a pitching plate in order to gain some understanding of how three dimensionality develops in such flows. Data from a progression of PIV studies, from stereoscopic PIV yielding three-component, two-dimensional (3C-2D) data to tomographic PIV yielding three-component, three-dimensional (3C-3D) data are presented thus providing progressively more detailed information. A comparison of results is made between the two techniques. The PIV study is performed in a water tunnel facility with cross-sectional area 500 × 500 mm, and involves a full-span (nominally two-dimensional) plate, suspended between a wall end boundary condition and a free surface, pitching at a dimensionless pitch rate of K c = 0.93 in flow at Re = 7,500. Results demonstrate the existence of spanwise flows in both the leading edge and trailing edge vortices, but with strong directionality in the leading edge vortex towards the wall end boundary condition. Observations of instantaneous flow patterns suggest also the existence of three-dimensional coherent vortex filament structures in the outer regions of the leading edge vortex.

Particle image velocimetry study of the impinging height effect on an overexpanded supersonic impinging free jet at Ma = 1.754. Part 1: global coherent structure

Results of an experimental study of an overexpanded axisymmetric supersonic impinging jet at Mach 1.754 with particle image velocimetry (PIV) are presented in this article. The focus of Part I is on the influence of the impinging height on the global coherent flow structure, while in Part II the detailed velocity magnitude, both axial and radial, as well as the position variation of the plate shock will be presented. The PIV results clearly demonstrate the formation of the stagnation bubble as well as the presence of the recirculating flow in it. When the impinging height reaches five times the nozzle exit diameter, the stagnation bubble disappears, which is thought to be the result of the increase in the thickness of the shear layer of the main jet and consequently the decrease in the difference of the stagnation pressure between the centre of the stagnation zone and the impinging point of the main jet shear layer. The comparison between instantaneous and mean PIV measurements demonstrates that there exists highly unsteady behaviour of the impinging jet. The shock wave structure and the unsteady entraining vortex of the nozzle jet and the wall jet are all clearly recorded by PIV. All these results are crucial to understand the effects of the impinging height on the global flowfield coherent structure of the overexpanded axisymmetric supersonic jet impingement.

Effects of Simple Wall-Mounted Cylinder Arrangements on a Turbulent Boundary Layer

Effects of Simple Wall-Mounted Cylinder Arrangements on a Turbulent Boundary Layer

PIV study of the flow field generated by a sawtooth impeller

Stereoscopic and high-speed particle image velocimetry (PIV) techniques have been employed to study the flow field induced by a sawtooth (EkatoMizer) impeller, operated in the fully turbulent flow regime at an impeller speed of 1500 rpm. Ensemble-averaged mean flow fields and turbulence quantities were calculated for a region close to the impeller blades. The flow was found to be anisotropic near the impeller and exhibited return-to-isotropy behaviour further away from it. Macro-instabilities were found to have a high probability of occurrence in the discharge stream. All three velocity components from the stereo-PIV measurements were used to estimate the dissipation rate, by adopting a large eddy simulation (LES) analogy. Spurious vectors distorting the dissipation rate calculation were identified, and various standard deviation filters were applied for vector validation. By evaluating the filtered dissipation rate profiles against the multifractal intermittency model of Meneveau and Sreenivasan (1991), the global standard deviation filter was found to be the most suitable type. The ratio of the maximum to the mean ( ε max / ε ¯ ) dissipation rate for the EkatoMizer discharge stream was found to be similar to that reported for Rushton disk turbine and pitched-blade turbine impellers in the literature, raising questions about the reported high-shear advantage of sawtooth impellers. However it should be noted that these PIV experiments were conducted outside the sawtooth blades and it is possible that the maximum dissipation rate occurs within the impeller swept volume, where ε max / ε ¯ could be significantly higher.

Visualization and PIV study of wing-tip vortices for three different tip configurations

The planform, profile, and cross-sectional views of the wing-tip region of a half-wing model with an aspect ratio of 3.2 and three different wing configurations, namely, square-cut, simple fairing, and Whitcombʼs full winglet wing-tip, were visualized at various angles of attack using smoke-wire visualization technique. Visualization pictures clearly show that the wing-tip vortices at different angles of attack and wing-tip configurations had distinct formation and structure characteristics. A comparison of simple fairing and Whitcombʼs winglet configurations shows that the wing-tip vortices of the Whitcombʼs winglet configuration were reduced in strength and displaced outboard and upward, at least in the near-wake region. This resulted in an increased lift-to-drag ratio for the Whitcombʼs winglet configuration. The changes in the wing-tip vortex characteristics and the improved aerodynamic performance of the winglet were confirmed by Particle Image Velocimetry (PIV) measurements of the cross-flow velocity of the wing-tip trailing regions and the force measurement of the model.

Multilaboratory Particle Image Velocimetry Analysis of the FDA Benchmark Nozzle Model to Support Validation of Computational Fluid Dynamics Simulations

This study is part of a FDA-sponsored project to evaluate the use and limitations of computational fluid dynamics (CFD) in assessing blood flow parameters related to medical device safety. In an interlaboratory study, fluid velocities and pressures were measured in a nozzle model to provide experimental validation for a companion round-robin CFD study. The simple benchmark nozzle model, which mimicked the flow fields in several medical devices, consisted of a gradual flow constriction, a narrow throat region, and a sudden expansion region where a fluid jet exited the center of the nozzle with recirculation zones near the model walls. Measurements of mean velocity and turbulent flow quantities were made in the benchmark device at three independent laboratories using particle image velocimetry (PIV). Flow measurements were performed over a range of nozzle throat Reynolds numbers (Re(throat)) from 500 to 6500, covering the laminar, transitional, and turbulent flow regimes. A standard operating procedure was developed for performing experiments under controlled temperature and flow conditions and for minimizing systematic errors during PIV image acquisition and processing. For laminar (Re(throat)=500) and turbulent flow conditions (Re(throat)≥3500), the velocities measured by the three laboratories were similar with an interlaboratory uncertainty of ∼10% at most of the locations. However, for the transitional flow case (Re(throat)=2000), the uncertainty in the size and the velocity of the jet at the nozzle exit increased to ∼60% and was very sensitive to the flow conditions. An error analysis showed that by minimizing the variability in the experimental parameters such as flow rate and fluid viscosity to less than 5% and by matching the inlet turbulence level between the laboratories, the uncertainties in the velocities of the transitional flow case could be reduced to ∼15%. The experimental procedure and flow results from this interlaboratory study (available at http://fdacfd.nci.nih.gov) will be useful in validating CFD simulations of the benchmark nozzle model and in performing PIV studies on other medical device models.

軸対称放物体の剥離渦の可視化とPIV計測

Visualization and PIV measurements were conducted in a towing air-chamber for wake vortices behind a paraboloidal-nose cylinder at high angles of attack. The evolution of the wake was then measured by a particle image velocimetry (PIV) using cross-sectional and longitudinal cuts which were produced by the sheet illumination of the Nd:YAG laser. To visualize the three-dimensional evolution of flow in the vicinity of wall, we employed a titanium tetrachloride (TiCl4). It was predicted by the PIV study that serial vortex loops with almost constant spacing appear around the main vortex.

Particle Image Velocimetry Study of Flow Patterns and Mixing Characteristics in Multiple Impeller Stirred Tank

The flow generated in a stirred tank of 0.48 m diameter, agitated by three impellers of 0.19 m diameter, including one half-elliptical blade disc turbine (HEDT) below and two up-pumping wide-blade hydrofoils (WHU), were investigated by using 360° ensemble-averaged particle image velocimetry (PIV) technique. The flow characteristics for seven impeller combinations with different impeller spacings were investigated. Results show that global circulation patterns in the stirred tank are strongly dependent on the impeller spacing. Three types of flow patterns were observed, including five, four, and three circulation loops in the flow field, by changing the spacing between the bottom and the middle impellers (C1), the spacing between the middle and the top impellers (C2), and the submersion depth of the top impeller (C3). While the clearance (ΔC) of the bottom impeller is equal to 0.4T, the five circulation loops flow pattern occurs when C1 = 0.48T, C2 = 0.48T, and C3 = 0.4T, and the four circulation loops flow pattern occurs when C1 = 0.48T, C2 = 0.4T, and C3 = 0.4T, and the three circulation loops flow pattern occurs when C1 = 0.48T, C2 = 0.4T, C3 = 0.32T. The mixing efficiency defined based on the mixing time and the power consumption were also discussed and the results show that less circulation loops could lead to higher mixing efficiency, but the mixing efficiency does not solely increase with the decreasing of the circulation loops.

Fluid Dynamic Investigation of the ATS 3F Enable Sutureless Heart Valve

: Currently, sutureless heart valves (SHV) reveal good clinical results during aortic valve replacement. The aim of this study was to evaluate the fluid dynamics of the ATS 3F Enable SHV in the ascending aorta and their influence on the aortic wall in an in vitro setup. : A two-dimensional particle image velocimetry study with an image rate of 15 Hz was conducted to evaluate the fluid dynamics of the SHV in the aortic flow field. The prosthesis (diameter, 23 mm) was placed inside a silicone mock aorta under pulsatile flow conditions. Velocities, vorticity, and strain rate were obtained and calculated with a fixed frequency (70 Hz) at constant stroke volume (70 mL). : 3F Enable showed a jet flow type profile with a maximum velocity of 1.01 ± 0.13 m/s during peak flow phase (PFP). The jet flow was surrounded by ambilateral vortices with a slightly higher percentage of clockwise than counterclockwise vorticity (377 ± 57/s vs 389 ± 76/s), strain rate (370 ± 79/s for elongation vs -370 ± 102/s for contraction) was nearly similar. The point-of-interest analysis revealed a higher velocity for bottom compared with upper aortic wall (0.28 ± 0.07 m/s vs 0.31 ± 0.06 m/s, P = 0.024). All values were lower during acceleration and deceleration phase compared with PFP. : The peak flow of the 3F Enable SHV seems to be little higher compared with native aortic valves, thus simulating nearly physiologic conditions. Vorticity and strain rate are high during PFP and low during other phases and might have an influence on the aortic wall as well.

PIV study of near-field tip vortex behind perforated Gurney flaps

The impact of Gurney flaps, of different heights and perforations, on the growth and development of a tip vortex, both along the tip and in the near field of a finite NACA 0012 wing, at Re = 1.05 × 105 was investigated by using particle image velocimetry (PIV). Wind-tunnel force balance measurements were also made to supplement the PIV results. This study is a continuation of the work of Lee and Ko (Exp Fluids 46(6):1005–1019, 2009) on the near-wake measurements behind perforated Gurney flaps. The present results show that along the tip, the overall behavior of the secondary vortices and their interaction with the primary, or tip, vortex remained basically unchanged, regardless of flap height and perforation. The peak vorticity of the tip vortex, however, increased with flap height and always exhibited a local maximum at x/c = 0.8 (from the leading edge). In the near field, the strength and structure of the near-field tip vortex were found to vary greatly with the flap height and perforation. The small flaps produced a more concentrated tip vortex with an increased circulation, while the large Gurney flaps caused a disruption of the tip vortex. The disrupted vortex can, however, be re-established by the addition of flap perforation. The larger the flap perforation the more organized the tip vortex. The Gurney flaps have the potential to serve as an alternative off-design wake vortex control device.

Particle image velocimetry studies around a rectangular body close to a plane wall

Flow structures around a rectangular body located close to a ground board in a free-surface water channel were investigated using the particle image velocimetry (PIV) technique. The rectangular body is set with α = 0 ° and α = 10 ° yaw angles referenced to the flow direction, and measurements were performed on the vertical and horizontal planes. The PIV technique provided instantaneous and time-averaged flow fields for Reynolds number Re H =1.2×10 4 based on the model height. For α = 0 ° , the results indicate that the flow structure in the wake region vary significantly with the elevation level from the ground surface. An asymmetric large circulating flow region is identified in the wake region for α = 10 ° . The instantaneous flow fields reveal the presence of small-scale vortices in the main flow over the separation line. These small-scale vortices coalesce to form larger-scale vortices.

Assessment of Seeder Performance for Particle Velocimetry in a Scramjet Combustor

The design and performance of two unique fluidized bed particle seeders were assessed experimentally. The seeders were designed to seed the fuel stream and freestream of a scramjet combustor for particle image velocimetry measurements. The fuel stream seeder used alumina powder to generate particles and has been used in the past to obtain particle image velocimetry measurements. The freestream seeder used silica powder and is a new design. Using a probe and filter paper, samples were collected from the output of each seeder at operating conditions near those encountered in scramjet particle image velocimetry studies. The samples were imaged using a scanning electron microscope to obtain qualitative and quantitative data about the particles generated. Both seeders produced particles near the target size of 0.3 μm, although the silica particles were generally smaller in size and more spherical than the alumina particles. Based on these results, it was predicted that the freestream particles would scatter light more uniformly and track the flow more faithfully than the fuel stream particles. The results of this study confirm assumptions on particle size and shape made in previous particle image velocimetry experiments and also indicate that the fuel stream seeder will generate more effective particles using silica powder.

Particle Image Velocimetry Study of Flow near Trashrack Models

This paper provides results of an experimental study of turbulent flow near trashrack models that are comprised of an array of three rectangular bars. The bar thickness, bar depth, and center-to-center spacing were maintained constant. The flow characteristics were studied by aligning the bars with the approach flow and conducting measurements at three different approach freestream velocities. Subsequently, the freestream velocity was kept constant and detailed measurements were conducted for four different bar inclinations relative to the approach flow. For each test condition, a high-resolution particle image velocimetry (PIV) technique was used to conduct detailed velocity measurements in streamwise-spanwise planes at middepth of flow. From these measurements, isocontours and profiles of the mean velocities, turbulence intensities, Reynolds shear stress, and production term in the transport equation for the turbulent kinetic energy were obtained to study the flow characteristics around and downstream of the aligned and inclined bars. Flow characteristics near hydroelectric station trashracks are important for efficient turbine operation and reduction of fish entrainment.

A hybridized snapshot proper orthogonal decomposition-discrete wavelet transform technique for the analysis of flow structures and their time evolution

A novel hybrid technique has been proposed in order to reveal in a greater detail the turbulent flow structures and their time evolution, and to address the issues and limitations related to the application of snapshot proper orthogonal decomposition (POD) and wavelet transform technique. The proposed hybrid technique combines the inherent abilities of the snapshot proper orthogonal decomposition and the two-dimensional discrete wavelet transform technique. The POD gives us the overall view of the most energetic flow pattern in an ensemble by decomposing the flow field into spatial and temporal modes, while two-dimensional wavelet transform gives us the localized spatial information through scale wise decomposition of the flow field. In this work, we apply the wavelet transform on the POD spatial modes. This enables us to understand the space scale structure of the flow events captured by the spatial POD modes, and the scale wise selectivity of these spatial POD modes. Thus, we are able to relate the most energetic flow events over a period of time (as obtained in spatial modes of snapshot POD) with the localized dominant scales that are contributing to it. Further, this information is utilized in the selection of those pod spatial modes that can effectively reconstruct a flow structure and its time evolution. The proposed technique has also been able to address the issues in the literature concerning the application of POD when the flow is less deterministic, as then a single POD mode may not reveal the flow structure and combination of modes is required to reconstruct it. In the present work, this hybrid methodology has been used to reveal the near wall intermittent events in channel flow: the ascending streaks and the bursts and their time evolution, the vortex tube and leading edge vortices in jet and the Taylor–Couette and irregular small chaotic vortices in Taylor–Couette flow. The planar dataset used for such an analysis has been obtained from particle image velocimetry and large eddy simulation studies.

Flow Visualization Study of a Novel Respiratory Assist Catheter

Respiratory assist using intravenous catheters may be a potential therapy for patients with acute and acute-on-chronic lung failure. An important design constraint is respiratory catheter size, and new strategies are needed that enable size reduction while maintaining adequate gas exchange. Our group is currently developing a percutaneous respiratory assist catheter (PRAC) that uses a rotating bundle of hollow fiber membranes to enhance CO(2) removal and O(2) supply with increasing bundle rotation rate. In this study, particle image velocimetry (PIV) was used to analyze the fluid flow patterns and velocity fields surrounding the rotating fiber bundle of the PRAC. The goal of the study was to assess the rotational flow patterns within the context of the gas exchange enhancement that occurs with increasing fiber bundle rotation. A PRAC prototype was placed in a 1-in. internal diameter test section of an in vitro flow loop designed specifically for PIV studies. The rotation rate of the PRAC was varied between 500 and 7000 rpm, and PIV was used to determine the velocity fields in the primary (r-theta) and secondary (r-z) flow planes. The secondary flow exhibited time-varying and incoherent vortices that were consistent with the classical Taylor vortices expected for Taylor numbers (Ta) corresponding to the rotation speeds studied (2200 < Ta < 31 000). In the primary flow, the tangential velocity exhibited boundary layers of less than (1/2) mm adjacent to the fiber bundle and vessel wall. The estimated shear stress associated with the Taylor vortices was approximately 11 dyne/cm(2) at 7000 rpm and was over 10 times smaller than the shear stress in the primary flow boundary layers.

A PIV study of co-rotating disks flow in a fixed cylindrical enclosure

This study investigates the coherent flow between enclosed co-rotating disks using flow visualization and particle image velocimetry (PIV). To simulate the 3.5-inch hard disks the tested disks have a diameter of 275 mm and a gap-to-disk radius ratio of 0.09. In the flow measurements the Reynolds number is fixed at Re = 5.25 × 10 5 corresponding to a disk rotating speed of 12,000 rpm. The rotation speed and disk space selected above is for the next generation hard disk consideration, and the related parameters are determined through the Reynolds dynamic similarity Re ( model , water 30 ° C ) = Re ( prototype , air 30 ° C ) . The results show that the flow between the disks can be characterized by eight sub-flow regions: namely 1st solid body rotation region, 2nd large-scale vortex structure region, 3rd detached shear layer region, 4th boundary layer region, 5th core region, 6th Ekman layer region, 7th Stewartson layer region, and 8th shroud shear layer region. Among these sub-flow regions the 5th to 7th flow regimes are not identified previously. In addition, the additional measurements of the circumferential velocity component in the 5th core region reveal that a pair of circular flow structure acts like an annular chain to surround the 2nd sub-region large-scale vortex. This finding is also not reported in the literature. Moreover, the present PIV phase velocity measurements provide the physical explanation of the cyclic variation in the 1st solid body rotation region reported previously by our Laser Doppler Velocimetry spectra measurements.

Particle image velocimetry study of acoustic field in miniature traveling wave device.

Particle imaging velocimetry (PIV) techniques were employed to investigate the flow patterns inside a 2 kHz annular thermoacoustic device. This thermoacoustic device uses air at one atmosphere as its working fluid and smoke for seeding particles. For these measurements, the sound level output of the device was kept to 140 dB to help maintain the suspension of seeding particles. Several types of flow patterns were observed, including Gedeon streaming, start-up noise characterized by instabilities such as vortices, and a traveling wave throughout accompanied by radial standing waves localized near corners. Such measurements are challenging in small high frequency devices. In order to satisfy the Shannon-Nyquist theorem, a 2 kHz engine would need to be sampled at a minimum of 4 kHz; however, a new technique was developed that allows for the measurements to be made using a camera with a maximum frame rate per sec of 300. This new method makes it possible to perform measurements on a flow without the need for more expensive and delicate high speed equipment, thereby, making observation of high speed flows more readily available. [Research supported by the U.S. Army Space and Missile Defense Command/U.S. Army Forces Strategic Command.]

PIV study on the development of double-diffusive convection during the solidification effected by lateral cooling for a super-eutectic binary solution

The solidification of a binary solution occurs in a variety of industrial applications. The fundamental mechanism of the development of “double-diffusive convection” during solidification was one of the main subjects of study in the past. This study focused on the effect of the initial concentration of a super-eutectic aqueous ammonium chloride solution on the development of double-diffusive convection during the solidification process. Particle image velocimetry (PIV) technique was employed to measure the flow velocity and observe the morphological conditions associated with the solidification effected by cooling from the side wall. The transient temperature distribution within a test cell was also measured by using the designed experimental system. PIV measurement revealed that the flow structure was composed of a major circulation flow within the test cell for the binary solution with low-concentration and multiple layers of circulation flows developed in sequence in the melt of the test cell for the high-concentration binary solution. Test-cell images captured by CCD camera indicated the presence of more A-segregates within the mushy zone as the initial concentration of the binary solution increased. For the low-concentration binary solution, under circumstances of without or with very weak double-diffusive convection, transient temperature distribution presented thermal stratification within the test cell. However, strong double-diffusive convection resulted in intersection on temperature curves for the high-concentration binary solution.