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Particle tracking velocimetry (PTV) and particle image velocimetry (PIV) are popular experimental methods to quantitatively measure flow fields. In many practical applications, hardware limitations result in longer exposure times,...
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Particle tracking velocimetry (PTV) and particle image velocimetry (PIV) are popular experimental methods to quantitatively measure flow fields. In many practical applications, hardware limitations result in longer exposure times, causing particle images to elongate into particle streak images. In this study, the performances of PTV and PIV in relation to particle streak images are evaluated systematically by means of both synthetic and experimental images. For the synthetic images, particle streak images are created via the integration over time of the standard Gaussian approximation of particle images, plus the effects of exposure time, and parameters such as particle image diameter, intensity, and density were investigated in terms of the velocity fields of a 1D uniform flow and the rotational 2D Hill's vortex. The results show that PTV performs well for short exposure times, and its peak-finding criteria can be verified for nondimensional exposure times ET*ET*收起
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In this paper, a novel streak-resolving algorithm is proposed for particle streak velocimetry (PSV) to resolve velocity fields for a single image frame with multiple particle streaks. A model streak function, which was based on th...
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In this paper, a novel streak-resolving algorithm is proposed for particle streak velocimetry (PSV) to resolve velocity fields for a single image frame with multiple particle streaks. A model streak function, which was based on the temporal integral of the particle image intensity moving along its trajectory during the exposure period, was approximated using a multivariable least-square fit procedure to reconstruct the streak information and the corresponding particle trajectory. Lagrangian tracking was achieved, and the velocities were evaluated by differentiating the resolved particle trajectory with respect to time. Two types of flows, accelerating parallel flow and the rotational flow of Hill's vortex, were used to generate synthetic streaks for the performance tests. Three types of relative error were defined and used to evaluate the performance of the algorithm in terms of statistical mean and standard deviation (SD) errors. The accuracy of the fitted streak parameters, such as particle image intensity and diameter, were also evaluated and compared. The results reveal that the error and SD were low if the image noise is below 1.0%; for noise levels of 5.0%, the error was up to 10% with an SD of up to 12%. The error and SD of the particle image intensity and particle image diameter for both flow types were also 0%-7% for clean and up to 12% for noisy images. The processing results for experimental streak images of flow past a cylinder reveal that these images can be resolved using the proposed algorithm with a residual mean error of 4.38 and an SD of 9.48. These results suggest that the proposed novel approach can be used to resolve velocity fields with only a single image frame and without expensive hardware for high-speed imaging and thus is suitable for diverse applications.
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Multiphase flows (MPFs) offer a rich area of fundamental study with many practical applications. Examples of such flows range from the ingestion of foreign particulates in gas turbines to transport of particles within the human bo...
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Multiphase flows (MPFs) offer a rich area of fundamental study with many practical applications. Examples of such flows range from the ingestion of foreign particulates in gas turbines to transport of particles within the human body. Experimental investigation of MPFs, however, is challenging, and requires techniques that simultaneously resolve both the carrier and discrete phases present in the flowfield. This paper presents a new multi-parametric particle-pairing algorithm for particle tracking velocimetry (MP3-PTV) in MPFs. MP3-PTV improves upon previous particle tracking algorithms by employing a novel variable pair-matching algorithm which utilizes displacement preconditioning in combination with estimated particle size and intensity to more effectively and accurately match particle pairs between successive images. To improve the method's efficiency, a new particle identification and segmentation routine was also developed. Validation of the new method was initially performed on two artificial data sets: a traditional single-phase flow published by the Visualization Society of Japan (VSJ) and an in-house generated MPF data set having a bi-modal distribution of particles diameters. Metrics of the measurement yield, reliability and overall tracking efficiency were used for method comparison. On the VSJ data set, the newly presented segmentation routine delivered a twofold improvement in identifying particles when compared to other published methods. For the simulated MPF data set, measurement efficiency of the carrier phases improved from 9percent to 41percent for MP3-PTV as compared to a traditional hybrid PTV. When employed on experimental data of a gas-solid flow, the MP3-PTV effectively identified the two particle populations and reported a vector efficiency and velocity measurement error comparable to measurements for the single-phase flow images. Simultaneous measurement of the dispersed particle and the carrier flowfield velocities allowed for the calculation of instantaneous particle slip velocities, illustrating the algorithm's strength to robustly and accurately resolve polydispersed MPFs.
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We detect the trajectory of particles using the feature matching method to improve the resolution of particle streak velocimetry (PSV), which is used to measure the velocity of particles from a visualized path line. PSV has a more...
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We detect the trajectory of particles using the feature matching method to improve the resolution of particle streak velocimetry (PSV), which is used to measure the velocity of particles from a visualized path line. PSV has a more reliable performance in particle matching as compared to particle tracking velocimetry and is therefore less likely to cause erroneous matching even in high-density images. The center of gravity of the first and last trajectories is obtained to calculate the displacement. The trajectory of the particle is illuminated using a diode laser and imaged using a digital single-lens reflex camera; the trajectory is then divided into three parts and recorded in a single frame using coded illumination. The first and second trajectories are short, and the third trajectory is long. The asymmetry of the trajectories is then used to determine the flow direction. We first evaluate the detection rate by increasing the trajectory density of synthetic images. The image size was fixed at 500 × 500 pixels, and the number of trajectories was increased from 28 to 280, and the detection rate was examined. Then, we evaluated the accuracy of detection of the center of gravity of the first and last trajectories using the root mean square error. Finally, we used the coded illumination method to visualize the swirling flow inside a device to examine its applicability to real flows.
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Particle Image Velocimetry (PIV) measurement in gas-liquid bubbly flows is a challenging task, mainly due to the dispersion of the laser light caused by the gas-liquid interfaces. A common solution adopted is the use of fluorescen...
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Particle Image Velocimetry (PIV) measurement in gas-liquid bubbly flows is a challenging task, mainly due to the dispersion of the laser light caused by the gas-liquid interfaces. A common solution adopted is the use of fluorescent seeding particles associated with a bandpass filter for the laser light. Therefore, the camera captures only the light fluoresced by the seeding particles, filtering the laser light dispersed by the gas-liquid interfaces and measuring a velocity field which corresponds to the liquid phase, seeded with the particles. However, even for relatively low gas fractions, the fluoresced light reflected by the interfaces distorts the measurements, as it illuminates particles out from the laser plane. In addition, the fluoresced light also reflects at the interfaces, distorting the measurements and trending to overshoot the measured liquid velocities In this work, PIV measurements of air-water bubbly flows in a small diameter pipe (D = 26.2 mm) are performed with fluorescent tracer particles (PIV/LIF). A new method for the phase discrimination in PIV was developed to overcome the problems caused by the presence of bubbles in the flow, which uses the pixels intensity information of each interrogation window, to identify if that window corresponds to liquid or bubble region. To validate the PIV procedure, a Particle Tracking Velocimetry (PTV) algorithm was also developed to measure the bubbles velocities, based on similar implementations found in literature, but some corrections were also proposed to overcome the bubble overlap phenomena which arises in bubbly pipe flows, when backward illumination is used. The PIV and the PTV methods were tested and validated by a series of distinct gas void fraction and bubble diameters experimental cases, confirming the accuracy and reliability of both methods. The proposed method was used to analyze a set of upward laminar and turbulent bubbly flows, showing that the averaged axial velocity and the dispersed gas b
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Debris striking the internal structures of aircraft components is capable of causing on-board fires and leading to catastrophic damage to both the aircraft and flight crew. In the present work, two experiments were conducted to ca...
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Debris striking the internal structures of aircraft components is capable of causing on-board fires and leading to catastrophic damage to both the aircraft and flight crew. In the present work, two experiments were conducted to capture the characteristics of high-speed debris strikes and dry-bay fires. Particle tracking velocimetry technique was utilized to investigate the dynamics of debris striking structural components of the aircraft. In conjunction, a flame front detection measurement technique was developed to identify the size and duration of dry-bay fires. Results demonstrated the ability to utilize fundamental image correlation techniques to determine velocity, size, and duration of flash events in support of aircraft survivability and safety research.
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The 3D instantaneous velocity recorded with an acoustic Doppler velocimeter (ADV) in a highly turbulent free surface flow is analysed using several filters in order to eliminate the corrupted data from the sample. The filters used...
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The 3D instantaneous velocity recorded with an acoustic Doppler velocimeter (ADV) in a highly turbulent free surface flow is analysed using several filters in order to eliminate the corrupted data from the sample. The filters used include the minimum/maximum threshold, the acceleration threshold, and the phase-space threshold. Following some ideas of the phase-space filter, a new method based on the 3D velocity cross-correlation is proposed and tested. A way of computing the constants of the acceleration threshold method is proposed, so no parameters need to be fixed by the user, which makes the filtering process simpler, more objective and more efficient. All the samples analysed are highly turbulent. Nevertheless, the turbulence intensity and the air entrainment vary widely in the flow under study, which produces data records of different quality depending on the measurement point. The performance of the filtering methods when applied to samples of different quality, and the effects of the filtering process in the mean velocity, turbulent kinetic energy and frequency spectra are discussed.
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The influence of peak-locking errors on turbulence statistics computed from ensembles of PIV data is considered. PIV measurements are made in the streamwise-wall-normal plane of turbulent channel flow. The PIV images are interroga...
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The influence of peak-locking errors on turbulence statistics computed from ensembles of PIV data is considered. PIV measurements are made in the streamwise-wall-normal plane of turbulent channel flow. The PIV images are interrogated in three distinct ways, generating ensembles of velocity fields with absolute, moderate, and minimal peak locking. Turbulence statistics computed for all three ensembles of data indicate a general sensitivity to peak locking in the single-point statistics, except for the mean velocity profile. Peak-locking errors propagate into the fluctuations of velocity, rendering single-point statistics inaccurate when severe peak locking is present. Multi-point correlations of both streamwise and wall-normal velocity are also found to be influenced by severe levels of peak locking. The displacement range of the measurement, defined by the PIV time delay, appears to affect the influence of peak-locking errors on turbulence statistics. Smaller displacement ranges, particularly those that produce displacement fluctuations that are less than one pixel in magnitude, yield inaccurate turbulence statistics in the presence of peak locking. [References: 8]
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In the present study an experimental analysis of the velocity and pressure fields behind a marine propeller, in non-cavitating regime is reported. Particle image velocimetry measurements were performed in phase with the propeller ...
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In the present study an experimental analysis of the velocity and pressure fields behind a marine propeller, in non-cavitating regime is reported. Particle image velocimetry measurements were performed in phase with the propeller angle, to investigate the evolution of the axial and the radial velocity components, from the blade trailing edge up to two diameters downstream. In phase pressure measurements were performed at four radial and eight longitudinal positions downstream the propeller model at different advance ratios. Pressure data, processed by using slotting techniques, allowed reconstructing the evolution of the pressure field in phase with the reference blade position. In addition, the correlation of the velocity and pressure signals was performed. The analysis demonstrated that, within the near wake, the tip vortices passage is the most important contribution in generating the pressure field in the propeller flow. The incoming vortex breakdown process causes a strong deformation of the hub vortex far downstream of the slipstream contraction. This process contributes to the pressure generation at the shaft rate frequency.
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Tomographic and time resolved PIV measurements were performed to examine the 3D flow topology and the flow dynamic above the upper surface of a low-aspect ratio cylinder at Re approximate to 1 x 10(5). This generic experiment is o...
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Tomographic and time resolved PIV measurements were performed to examine the 3D flow topology and the flow dynamic above the upper surface of a low-aspect ratio cylinder at Re approximate to 1 x 10(5). This generic experiment is of fundamental interest because it represents flow features which are relevant to many applications such as laminar separation bubbles and turbulent reattachment. At Re approximate to 1 x 10(5), laminar separation bubbles arise on the side of the cylinder. Furthermore, on the top of the cylinder a separation with reattachment is of major interest. The tomographic PIV measurement, which allows to determine all three velocity components in a volume instantaneously, was applied to examine the flow topology and interaction between the boundary layer and wake structures on the top of the finite cylinder. In the instantaneous flow fields the tip vortices and the recirculation region becomes visible. However, it is also observed that the flow is quite unsteady due to the large separation occurring on the top of the cylinder. In order to study the temporal behaviour of the separation, time resolved PIV was applied. This technique allows capturing the dynamic processes in detail. The development of vortices in the separated shear layer is observed and in addition regions with different dominant frequencies are identified.
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