PTV System Research Articles

Wall Shear Stress Characteristics Of A Turbulent Boundary Layer Obtained With Multi-Aperture Defocusing Micro-PTV And High-Speed Stereo Profile-PIV

A multi-aperture 3d 3c micro PTV system is presented which relies on single window optical access for both high-speed tracer illumination and image recording. Similar to the “defocusing” concept described by C. Willert & Gharib (1992), the wall distance of individual particles is obtained from the size of projected particle image triplets formed by a triplet of apertures on the entrance pupil of the microscope lens. The present article extends upon previously published material Klinner & Willert (2022) by applying multi-aperture micro particle tracking velocimetry (MA-micro PTV) on a canonical turbulent boundary layer (TBL) to track the near-wall motion of tracer particles with the aim of estimating the unsteady wall-shear stress from particle tracks. The technique is validated with measurements of a TBL inside the closed test section of the 1 m wind tunnel of DLR in Göttingen (1mWK) at free- stream velocities of 5.2 to 20 m/s with corresponding shear Reynolds numbers between 560 and 1630. In the viscous sublayer, the probability density distributions of stream- and span-wise wall shear stress (WSS) components could be reliably captured down to probability densities of 10E−3. As far as we know, this has not been achieved in the past, particularly not for the span-wise component. For the stream-wise component the measured Reynolds-number dependency of the root mean squared fluctuations agrees to correlations by Örlü & Schlatter (2011). Furthermore, the skewness of the stream-wise WSS agrees well to values from DNS. On the other hand, the span-wise WSS fluctuations consistently underestimate the predicted DNS values, for which it is assumed that the deviation can be explained by the rapid decrease of the span-wise velocity fluctuations with increasing wall distance. Wall-normal profiles of 3c velocity statistics were obtained by bin averaging of particle velocities. Up into the buffer layer, these profiles agree well with profiles from stereo PIV as well as from DNS and LES, indicating an accurate determination of both near-wall velocities and inner scaling.

Volumetric particle tracking velocimetry with improved algorithms using a two-view shadowgraph system

Determining the time-resolved three-dimensional (3D) three-component (3C) velocity is essential for complex turbulent flow measurements. The current study is an extension of a recently developed temporal-spatial three-dimensional particle tracking velocimetry (TS 3D-PTV) technique established for two-view imaging systems. Two improvements have been embedded in TS 3D-PTV algorithm to improve the accuracy at high particle image densities (up to 0.03 ppp). One is using the neighboring particle information to correct the predicted positions and select the temporal particles with higher probability; the other is to iteratively optimize the 2D particle positions during the tracking process using the temporal and image information. The synthetic particle tests indicate that the correctness can be increased by 4.7%–5.8%, to reach a value about 92% with the improved algorithm around 0.03 ppp. The comparative results also indicate that using an advanced particle identification algorithm can improve the correctness over 20%. Two experiments, including a buoyancy jet in water and a transient droplet splashing process, have been conducted with a two-view shadowgraph imaging system. Different tracking algorithms have been conducted to determine the 3D trajectories of seeding particles or secondary droplets comparatively. The new algorithm has shown the best performance with much longer and more reliable trajectories, which indicates the tracking interruption caused by particle overlapping is reduced. The newly developed algorithms have further improved the performance under high seeding density conditions, which makes the two-view shadowgraph 3D PTV system adaptable to more experimental conditions.

Simultaneous two-phase flow measurements in a high-speed particle-laden under-expanded jet

The objective of this study is to understand the dynamics of a high-speed particle-laden under-expanded jet, motivated by landings on extraterrestrial bodies. In this setup, inertial particles are entrained and accelerated by an under-expanded jet. But, due to their inertia, the particle velocity is significantly lower than that of the surrounding gas close to the nozzle, so the two phases are coupled through aerodynamic drag. Sub-micron oil droplets are dispensed upstream to serve as tracers, whose velocity is determined through a PIV system; inertial particles, after image segmenting is performed to separate them from PIV data, will be tracked over time with a PTV system. This was accomplished with a single laser pulse and the camera straddle time to produce image pairs and shorten the pulse width. The results will help to understand particle-laden flow in a new regime where the background flow is compressible and the Mach number based on the slip velocity is not negligible, which may help to pave a foundation for future studies in compressible multiphase flows.

Application of simultaneous time-resolved 3-D PTV and Two Colour LIF in Studying Rayleigh-Benard Convection

To study the flow topology and temperature distribution of Rayleigh-Benard convection in a highly slender cell, measurement of the simultaneous velocity and temperature in the 3-D domain is required. For this aim, implementing a simultaneous time-resolved 3-D PTV and two-colour PLIF is planned. As a part of this development, for both PTV and two-colour PLIF techniques, the experimental setup has been implemented separately to measure time-resolved 2-D velocity and temperature and is presented in this paper. For PTV, a scanning system is also utilized to scan the flow field to capture the planar velocity in different depths of the flow domain. Progress on calculation of the out-of-plane velocity component including the theory is discussed. Finally, results of the time-resolved 2-D PTV and PLIF systems are presented.

Stereoscopic Correspondence of Particles for 3-Dimensional Particle Tracking Velocimetry by using Genetic Algorithm

The genetic algorithm (GA) based stereo particle-pairing algorithm has been developed and applied to the spatial particle-pairing problem of the stereoscopic three-dimensional (3-D) PTV system. In this 3 D PTV system, particles viewed by two (or more than two) stereoscopic cameras with a parallax have to be correctly paired at every synchronized time step. This is important because the 3-D coordinates of individual particles cannot be computed without the knowledge of the correct stereo correspondence of the particles. In the present study, the GA algorithm is applied to the epipolar line proximity analysis for establishing correspondence of particles pairs between two co-instantaneous stereoscopic particles images, in order to compute the 3-D coordinates of every individual particle. The results are tested with various standard images and it’s found that the new strategy using GA works better than conventional particle pairing methods of 3-D particle tracking velocimetry for steoroscopic PTV. Journal of the Institute of Engineering, 2016, 12(1): 10-26

Development of a tomographic PTV

A new tomographic PTV (Tomo-PTV) was proposed, and then its performance was compared with that of tomographic PIV (Tomo-PIV). To construct the new tomographic PTV for calculation of vector fields, an affine transformation was introduced. The vectors obtained by match probability method were used as the initial data for the final vectors. Four camera-based tomographic PTV and PIV systems were constructed. By introducing a new factor called degree of reality, the real particles were easily separated from the ghost particle groups. Two flow fields, a ring vortex and an impinging jet, were measured by the constructed tomographic PTV and the conventional tomographic PIV. Eight image frames (two consecutive image frames for each camera) were used for the construction of threedimensional voxel images. The construction method used for the voxel images was MLOS-MART.

E113 投影法ステレオストリークPTVを用いた3D3C時系列速度分布計測法の開発(OS-18: 熱・物質移動の計測および現象解析(1))

The internal blood flow of an aneurysm changes temporally and spatially according to the heartbeat and has a complex form; accordingly, time series volumetric velocity measurements are required. In the present study, we have developed a shadow imaged streak PTV system which enables time series volumetric velocity measurement with only two cameras. Applying this system to two types of aneurysm models in a cube measurement area, we have obtained three dimensional velocity streak results. The measuring volume size was 7.0 mm × 6.9 mm × 6.8 mm and velocity fluctuation was from 0.5 mm/s to 20.0 mm/s. Moreover, this system could accurately measure the small and large eddy changing spatially and temporally. From these results, it is concluded that the shadow imaged streak stereo PTV is applicable to milli-scale flows like an aneurysm.

9J-07 共焦点PTVシステムによるがん細胞表面タンパクの挙動計測(OS-11(3) 細胞・生体分子のバイオメカニクス(2))

9J-07 共焦点PTVシステムによるがん細胞表面タンパクの挙動計測(OS-11(3) 細胞・生体分子のバイオメカニクス(2))

Investigation of large-scale circulations in room air flows using three-dimensional particle tracking velocimetry

Large-scale circulations in room air flows have been investigated with three-dimensional particle tracking velocimetry (3D PTV). This technique has been refined for large scales, enabling us to determine flow patterns, trajectories, and velocity vectors within a large cylindrical enclosure used as a model room for thermal convection. This test enclosure is called Rayleigh-Bénard (RB) cell and addresses the limiting case of room air with very high internal loads. The flow patterns are only created by buoyancy forces. The 3D PTV system consists of four CMOS cameras, two flash lamps, and an image recording and data processing system. Helium-filled latex balloons were used as tracer particles. The spatial resolution and accuracy were investigated in a test cell with the dimensions of 4 × 3 × 4 m 3 by means of a given trajectory of a small glass sphere. First applications of the developed 3D PTV system in the large RB cell with a diameter of 7.15 m and a height of 3.58 m showed different characteristic flow patterns of large-scale circulation outside the boundary layers. From the recorded and analyzed long-term particle trajectories further important data of room air flows like velocity time series, probability density functions (PDF) of the velocity, and acceleration fluctuations were calculated. The proposed technique has a wide range of potential applications for the three-dimensional and time-dependent analysis of indoor air flows.

An improved PTV system for large-scale physical river model

To measure the surface flow in a physical river model, an improved system of Large-Scale Particle Tracking Velocimetry (LSPTV) was proposed and the elements of the PTV system were described. Usually the tracer particles of a PTV system seeded on water surface tend to form conglomerates due to surface tension of water. In addition, they can not float on water surface when water flow is shallow. Ellipsoid particles were used to avoid the above problems. Another important issue is particle recognition. In order to eliminate the influence of noise, particles were recognized by the processing of multi-frame images. The kernel of the improved PTV system is the algorithm for particle tracking. A new 3-frame PTV algorithm was developed. The performance of this algorithm was compared with the conventional 4-frame PTV algorithm and 2-frame PTV algorithm by means of computer simulation using synthetically generated images. The results show that the new 3-frame PTV algorithm can recover more velocity vectors and have lower relative error. In addition, in order to attain the whole flow field from individual flow fields, the method of stitching individual flow fields by obvious marks was worked out. Then the improved PTV system was applied to the measurement of surface flow field in Model Yellow River and shows good performance.

Turbulence structure and fluid–particle interaction in sediment-laden flows over developing sand dunes

In this study, we conducted the fluid–particle simultaneous measurements in order to reveal the fluid–particle interaction mechanism in the developing stages of sand dunes. We measured the instantaneous velocities over the growing sand dunes by the use of both the discriminator particle-image velocimetory (D-PIV) and the discriminator particle-tracking velocimetory (D-PTV). In this D-PIV&PTV system, fluid tracers and sediment particles are discriminated accurately by their occupied particle sizes, and thus, the particle velocity Up and fluid velocity Uf can be measured simultaneously. It was found from the present measurements that turbulence intensities in the trough area became larger after the generation of the reverse flow, i.e., Uf < 0. There are two kinds of coherent eddies behind dunes, i.e., one is convected along the shear layer behind dune and the other is lifted up from the reattachment point due to the kolk-boil vortex.

Neural network approach to stereoscopic correspondence of three‐dimensional particle tracking velocimetry

AbstractParticle tracking velocimetry (PTV) is a reliable measurement technique for the quantitative study of fluid flows by observing the motion of the particles seeded in them and is widely used in several industrial applications. The nature of the flow can be precisely observed only if all the three components of the velocity are computed. In 3‐D PTV system, particles viewed by two (or more than two) stereoscopic cameras with a parallax have to be correctly paired at every synchronized time step. This is important because the 3‐D coordinates of individual particles cannot be computed without the knowledge of the correct stereo correspondence of the particles. In the present work, a neural network–based algorithm has been proposed for the stereoscopic particle pairing process. The correspondence between the particle pairs is modeled as a constrained optimization problem. The constraints are provided on the basis of the epipolar geometry of the particle images and on the basis of the uniqueness of the matched pairs. The results are tested with various standard images. Copyright © 2008 Institute of Electrical Engineers of Japan. Published by John Wiley &amp; Sons, Inc.

208 PET樹脂微粒子の燃焼形態に及ぼす粒子速度と周囲燃焼ガス流速との相対速度の影響(再資源化・リサイクル・熱利用技術,資源循環・廃棄物処理技術)

The burning process of micro PET particles is observed, when they are supplied into a sinusoidally oscillating lean propane-air premixed flame. Since PET particles follow the gas flow out of phase and produce relative velocity due to the difference in inertia force, effects of the relative velocity on the flame patterns in PET particle combustion can be systematically examined. The velocity vector fields of hot combustion gas are conditionally measured with a PIV system, whereas the velocities of objective PET particles are measured with a PTV system. It can be certified that, when a PET particle flies faster than the ambient gas flow, a long thin envelop flame tailed to the upstream direction is formed behind it, and that the boundary of the thin envelop flame transforms into a regular cluster of brilliant spots having diameters of several microns in the final stage of PET particle combustion.

A102 河床波上の浮遊砂流れにおける乱れと流砂濃度のPIV・PTV計測

In the present study, we developed the discriminator PIV and PTV systems (D-PIV&PTV), in which fluid tracers and sediment particles were discriminated reasonably by their occupied particle sizes, and thus, the velocities of particles and fluid could be measured simultaneously. As the results, we have investigated experimentally the sediment effects on coherent motion of kolk-boil vortices in suspended sediment-laden flows over sand dunes by using these D-PIV&PTV.In particular, it was found that the kolk-boil vortex has a significant relation with the local distribution of suspended sediments near the reattachment point behind the dune crest.

Development of fiberscope PIV system by controlling diode laser illumination

Because of the inherent small size of optical fiberscopes, they provide access and relative handling ease in given closed vessels, which are hardly equipped with extra windows for conventional flow visualization. The use of an optical fiberscope in conjunction with a conventional particle image velocimetry/particle tracking velocimetry (PIV/PTV) system without optimization can lead to degraded transmission of images. The present study proposes a processing technique to filter background noise contained within the coarse bundle image by subtracting the original image of the bundle as reference image. Additionally, efforts were made to increase the reliability of vector processing using particle streak images via judicious pulse interval and duration adjustments. As an applications test we measured classic jet flow using the developed system and using established conventional measurement techniques. Our tests confirmed that our fiberscope PTV system provides vector fields with sufficient accuracy.

A Quantitative Visualization Method for Wind Tunnel Experiments Based on 3D Particle Tracking Velocimetry (3D-PTV)

The development of a measurement system to visualize, classify (based on topological features) and quantify complex flows in wind tunnel experiments is described. The basic idea is to extract topological features directly instead of postprocessing large sets of raw data (as generated by conventional PIV or PTV systems). As a particular aspect, the seeding methods in industrial wind tunnels are discussed. While helium filled soap bubbles are well suited because of their neutral buoyancy, there is a problem with their visibility, especially when reflecting model surfaces are being imaged at the same time. Various methods were evaluated in order to enhance the visual contrast of the bubbles.

A Quantitative Visualization Method for Wind Tunnel Experiments Based on 3D Particle Tracking Velocimetry (3D-PTV)

The development of a measurement system to visualize, classify (based on topological features) and quantify complex flows in wind tunnel experiments is described. The basic idea is to extract topological features directly instead of postprocessing large sets of raw data (as generated by conventional PIV or PTV systems). As a particular aspect, the seeding methods in industrial wind tunnels are discussed. While helium filled soap bubbles are well suited because of their neutral buoyancy, there is a problem with their visibility, especially when reflecting model surfaces are being imaged at the same time. Various methods were evaluated in order to enhance the visual contrast of the bubbles.

Hole diameter effect on flow characteristics of wake behind porous fences having the same porosity

The hole diameter effect on the flow characteristics of wake behind porous fences has been investigated experimentally in a circulating water channel having a test section of 300w × 200h × 1200l (mm). Three porous fences having different hole diameters of d = 1.4,2.1,2.8 mm were tested in this study, but they have the same ϵ = 38.5% geometric porosity. One thousand instantaneous velocity fields for each fence were measured consecutively by the hybrid PTV system employing a high-speed CCD camera. Free stream velocity was fixed at 10 cm/sec and the corresponding Reynolds number based on the fence height was Re = 2,985. Consequently, the fence with the smallest hole diameter d = 1.4 mm (d1.4) decreases the streamwise velocity component and increases the vertical velocity component. Among the three hole diameters tested in this study, the d1.4 fence has the largest turbulence intensity in the shear layer developed from the fence top. Regardless of the hole diameter, however, all three fences having the same porosity reduce the reduction of turbulent intensity in the lower region below the fence height (y/H < 1).

Concentration dependence of the short-range order in the Ni-V and Pt-V systems

The short-range order (SRO) in Pt-V and Ni-V systems has been studied by diffuse scattering of neutrons over a large concentration range. In both systems, a c-dependent SRO has been observed. We show that this concentration sensitivity has a different origin in the two cases. In Ni-V, the change of the SRO pattern is mainly due to c-dependent effective pair interactions (EPI), while in Pt-V, the EPI are c independent. We find explicit discrepancies between our results, deduced from measurements in the disordered state, and the electronic structure calculations, if they are performed in the ordered state. These discrepancies are due to the sensitivity of the electronic structure to the state of order.

Velocity Field Measurement of Flow Around an Axial Fan Using a Phase Averaged 2-Frame PTV Technique

The flow structure around a rotating axial-fan was experimentally investigated using a phase averaging velocity field measurement technique. The fan blades were divided into 4 different phases, for which 500 velocity fields were acquired for each phase angle with a 2-frame PTV system. Velocity field measurements were also carried out at two planes parallel to the axis of rotation, with offsets toward the radial direction of the fan. For accurate synchronization of the PTV system with the phase of the axial fan, two synchronization circuits were employed with a photo-detector attached to the rotating shaft. The phase averaged velocity fields show periodic variations with respect to the blade phase. The periodic formation of vortices at the blade tip is also observed in vorticity contour plots. Locations of local maximum turbulence intensities in the axial and radial directions are found to be located in an alternating pattern. These experimental results can be used to validate numerical calculations and to understand the flow characteristics of an axial fan.