scholarly journals Examining the Effects of Fluid Velocity Gradients on 4D Digital Holographic PIV/PTV Measurements

2021 ◽  
Vol 1 (1) ◽  
Author(s):  
Yuan Jing Xia ◽  
Bihai Sun ◽  
Asif Ahmed ◽  
Julio Soria
Keyword(s):  
Velocity Gradient ◽  
Cross Correlation ◽  
Fluid Velocity ◽  
Angular Spectrum ◽  
Bias Error ◽  
Pixel Resolution ◽  
3 Dimensional ◽  
Velocity Gradients ◽  
Piv Measurement ◽  
Gaussian Fit

4D digital holographic PIV/PTV (4D-DHPIV/PTV) methods have demonstrated theoretical viability due to their relative ease of setup and high spatial resolution (Soria (2018)). This study investigates how velocity gradients related to different flow regimes and their magnitudes affect 3-component–3-dimensional (3C-3D) digital holographic PIV measurement uncertainty. The error introduced by velocity gradients within the interrogation volume is studied by simulating particles in a velocity field, with a given constant velocity gradient superimposed on a uniform flow from which a time-series of hologram pairs are generated and the 3C-3D velocity fields and their errors are determined using 4D-DHPIV/PTV Sun et al. (2020). Hologram pairs are simulated by modelling the propagation and particlediffraction of coherent laser light using the angular spectrum method (Goodman (1996)). The hologram reconstruction then involves direct reconstruction, followed by deconvolution, a particle position refinement and a hologram subtraction step (Sun et al. (2020)). The particle positions obtained from 4D-DHPIV/PTV are then used to resolve particle displacement measurements using 3D cross-correlation digital analysis with a 3D Gaussian fit to sub-pixel resolution (Soria (2006)). The effects of velocity gradients on the displacement uncertainty and bias error have been investigated by undertaking Monte Carlo simulations under a range of velocity gradient environments. Specifically, 5 common velocity gradients have been studied, which included pure strain, pure vorticity and x, y and z-directional shear. The results indicate that the novel 4D-DHPIV/PTV has poorer accuracy and precision in the z-propagation axis, resulting in larger minimum uncertainties and bias errors. The errors in the z axis are also significantly less affected by velocity gradients in the z direction when compared to the effects of x and y directional velocity gradients on x and y errors respectively. Furthermore, the rate of cross-correlation maximum and SNR decrease are approximately 1.36 times slower due to velocity gradients in the z axis than other axes.  

scholarly journals Uncertainty Estimation for Ensemble Particle Image Velocimetry

2021 ◽  
Vol 1 (1) ◽  
Author(s):  
Adib Ahmadzadegan ◽  
Sayantan Bhattacharya ◽  
Arezoo M. Ardekani ◽  
Pavlos P. Vlachos
Keyword(s):  
Particle Image Velocimetry ◽  
Uncertainty Quantification ◽  
Velocity Gradient ◽  
Cross Correlation ◽  
Particle Image ◽  
Indirect Methods ◽  
Error Sources ◽  
Piv Measurement ◽  
Image Velocimetry ◽  
Non Gaussian

We present a novel approach to estimate the uncertainty in ensemble particle image velocimetry (PIV) measurements. Ensemble PIV is widely used when the cross-correlation signal-to-noise ratio (SNR) is insufficient to perform a reliable instantaneous velocity measurement. Despite the utility of ensemble PIV, uncertainty quantification for this type of measurement has not been studied. The existing uncertainty quantification algorithms for PIV are developed and used only for instantaneous PIV measurement and do not account for the improved SNR in ensemble PIV. Existing instantaneous uncertainty quantification methods can be divided into direct and indirect categories. Indirect methods require calibration based on the effect of various image parameters (such as noise, particle size, density, velocity gradient, etc.) on the correlation SNR. Indirect methods have not been calibrated for error sources relevant in an Ensemble PIV measurement. Also, they have lower sensitivity to the error sources compared to direct approaches. Direct methods, such as the moment of correlation (MC) and Image Matching (IM), find the uncertainty based on the images and correlation planes without any calibration and are more reliable (Bhattacharya et al., 2018; Sciacchitano et al., 2013). Ensemble PIV is based on ensemble correlations; therefore, MC, which uses the generalized cross-correlation (GCC) plane as a measure of uncertainty, is the most suitable method to be modified to be applicable for the ensemble PIV. The GCC plane is the inverse Fourier transform of the phase correlation and represents the probability density function (PDF) of particles’ displacements (Bhattacharya et al., 2018; Eckstein and Vlachos, 2009). We replaced instantaneous GCC with ensemble GCC and modified MC’s normalization factor to account for the number of ensembles. The MC’s primary limitation is that it assumes a Gaussian shape for the PDF of displacements and estimate the standard deviation of the underlying PDF using a fitted Gaussian. However, the PDF deviates from Gaussian distribution due to velocity gradient or non-Gaussian random displacements. Therefore, MC’s reliability and applicability are reduced for flow fields with non-Gaussian PDFs. Also, our analysis shows that ensemble MC consistently underestimates the uncertainty. So, a generalized and reliable method for uncertainty quantification for ensemble PIV is needed.

scholarly journals Velocity Gradient Separation Reveals a New Extracellular Vesicle Population Enriched in miR-155 and Mitochondrial DNA

Pathogens ◽  
2021 ◽  
Vol 10 (5) ◽  
pp. 526
Author(s):  
Myriam Vaillancourt ◽  
Audrey Hubert ◽  
Caroline Subra ◽  
Julien Boucher ◽  
Wilfried Wenceslas Bazié ◽  
...  
Keyword(s):  
Mitochondrial Dna ◽  
Velocity Gradient ◽  
Messenger Rna ◽  
Sucrose Density Gradient ◽  
Extracellular Vesicle ◽  
Therapeutic Interventions ◽  
Optimal Separation ◽  
Velocity Gradients ◽  
Commercial Kits ◽  
Hiv 1

Extracellular vesicles (EVs) and their contents (proteins, lipids, messenger RNA, microRNA, and DNA) are viewed as intercellular signals, cell-transforming agents, and shelters for viruses that allow both diagnostic and therapeutic interventions. EVs circulating in the blood of individuals infected with human immunodeficiency virus (HIV-1) may provide insights into pathogenesis, inflammation, and disease progression. However, distinguishing plasma membrane EVs from exosomes, exomeres, apoptotic bodies, virions, and contaminating proteins remains challenging. We aimed at comparing sucrose and iodixanol density and velocity gradients along with commercial kits as a means of separating EVs from HIV particles and contaminating protein like calprotectin; and thereby evaluating the suitability of current plasma EVs analysis techniques for identifying new biomarkers of HIV-1 immune activation. Multiple analysis have been performed on HIV-1 infected cell lines, plasma from HIV-1 patients, or plasma from HIV-negative individuals spiked with HIV-1. Commercial kits, the differential centrifugation and density or velocity gradients to precipitate and separate HIV, EVs, and proteins such as calprotectin, have been used. EVs, virions, and contaminating proteins were characterized using Western blot, ELISA, RT-PCR, hydrodynamic size measurement, and enzymatic assay. Conversely to iodixanol density or velocity gradient, protein and virions co-sedimented in the same fractions of the sucrose density gradient than AChE-positive EVs. Iodixanol velocity gradient provided the optimal separation of EVs from viruses and free proteins in culture supernatants and plasma samples from a person living with HIV (PLWH) or a control and revealed a new population of large EVs enriched in microRNA miR-155 and mitochondrial DNA. Although EVs and their contents provide helpful information about several key events in HIV-1 pathogenesis, their purification and extensive characterization by velocity gradient must be investigated thoroughly before further use as biomarkers. By revealing a new population of EVs enriched in miR-155 and mitochondrial DNA, this study paves a way to increase our understanding of HIV-1 pathogenesis.

The influence of agitation on aggregates formed during treatment of water with a content of humic substances

2006 ◽  
Vol 6 (1) ◽  
pp. 211-218 ◽  
Author(s):  
L. Pivokonska ◽  
M. Pivokonsky
Keyword(s):  
Humic Substances ◽  
Velocity Gradient ◽  
Laboratory Tests ◽  
Treatment Plant ◽  
Water Treatment Plant ◽  
Aluminium Chloride ◽  
Velocity Gradients ◽  
Perforated Baffle ◽  
Wide Size Distribution

This study aims to evaluate the influence of agitation conditions on the efficiency of the aggregation process when treating surface water containing humic substances. Laboratory tests were conducted by the jar tests using a variable speed paddle gang stirrer. Optimization of agitation intensity was determined by a Couette flocculator. Suspension was prepared using aluminium chloride as a destabilising reagent. Aggregation efficiency was evaluated by the determination of the degree of aggregation and by the test of aggregation. For all surface waters treated, the optimum treatability was demonstrated by applying higher velocity gradients (G=200–250 s−1). In addition to the laboratory tests, the plant measurements with water containing increased amounts of humic substances were taken to evaluate the aggregation efficiency. The results obtained by the aggregation efficiency measurements show that the intensity of agitation, with the assistance of perforated baffle-type flocculation chamber, attains a low level of velocity gradient (G=22–113 s−1), in contrast to the optimized velocity gradient level (G=200–250 s−1). The aggregates formed in the water treatment plant have an unsuitably wide size-distribution.

Vector Positioning for Cross Correlation PIV

2002 ◽  
Author(s):  
C. N. Young ◽  
R. Gilbert ◽  
D. A. Johnson ◽  
E. J. Weckman
Keyword(s):  
Cross Correlation ◽  
Full Range ◽  
Reduction Process ◽  
Particle Number Density ◽  
Particle Dispersion ◽  
Correlation Peak ◽  
Measured Velocity ◽  
Velocity Gradients ◽  
Positional Bias ◽  
The Cross

Continuing advances in digital imaging technology stimulate greater interest in applying particle image velocimetry (PIV) over increasingly larger fields of view. Unfortunately when larger fields of view are analyzed, velocity gradients in the image become more localized. In addition, non-uniformities in image illumination and particle number density become more prevalent. These factors, coupled with the requirement that large areas of interest (AOIs) must be employed to measure the full range of velocity, cause degradation of correlation results (i.e. broadening and/or splintering of the cross correlation peak) which leads to positional bias errors in the measured velocity field. More advanced super resolution strategies that employ an iterative AOI reduction process inherently reduce positional bias in PIV results but these strategies can break down in complex flows where velocity gradients are steep and particle dispersion does not remain uniformly random. To mitigate these problems a simple but effective technique is presented that enables individual velocity vectors to be placed within an AOI at locations toward which the cross correlation plane is biased. The method involves analysis of the correlation plane to extract the dominant features that are matched in two successive AOIs. To demonstrate the utility of the methodology results obtained from synthetic images are compared against results obtained using the conventional PIV approach.

Experimental Study of Moderator Circulation in CANDU6 Calandria Tank

2014 ◽  
Author(s):  
Hyoung Tae Kim ◽  
Han Seo ◽  
Sunghyuk Im ◽  
Bo Wook Rhee ◽  
Jae Eun Cha
Keyword(s):  
Heat Sink ◽  
Optical Measurement ◽  
Cooling System ◽  
Safety Concern ◽  
Measurement Data ◽  
Validation Data ◽  
3 Dimensional ◽  
Loss Of Coolant Accident ◽  
Piv Measurement ◽  
Accident Conditions

As a CANDU6 reactor has a high pressure primary cooling system and an independently cooled moderator system, the moderator in the calandria would act as a supplementary heat sink during a loss of coolant accident (LOCA) if the primary cooling and emergency coolant injection systems fail to remove the decay heat from the fuel. For the safety concern it is required to predict the 3-dimensional velocity and temperature distribution of moderator fluid to confirm the effectiveness of moderator heat sink. Korea Atomic Energy Research Institute (KAERI) is carrying out a scaled-down moderator test program to simulate the CANDU6 moderator circulation phenomena during steady state operation and accident conditions. This research program includes the construction of the Moderator Circulation Test (MCT) facility, production of the validation data for self-reliant CFD tools, and development of optical measurement system using the Particle Image Velocimetry (PIV). In the present work the PIV technique is used to measure the velocity distributions in the scaled moderator tank of MCT under iso-thermal test conditions. The preliminary PIV measurement data are obtained and compared with CFX code predictions.

scholarly journals Self-gravitating filament formation from shocked flows: velocity gradients across filaments

2020 ◽  
Vol 494 (3) ◽  
pp. 3675-3685 ◽  
Author(s):  
Che-Yu Chen ◽  
Lee G Mundy ◽  
Eve C Ostriker ◽  
Shaye Storm ◽  
Arnab Dhabal
Keyword(s):  
Velocity Gradient ◽  
Unit Length ◽  
Selection Effect ◽  
Major Axis ◽  
Large Area ◽  
North West ◽  
Preferred Direction ◽  
The North ◽  
Velocity Gradients ◽  
Field Lines

ABSTRACT In typical environments of star-forming clouds, converging supersonic turbulence generates shock-compressed regions, and can create strongly magnetized sheet-like layers. Numerical magnetohydrodynamic simulations show that within these post-shock layers, dense filaments and embedded self-gravitating cores form via gathering material along the magnetic field lines. As a result of the preferred-direction mass collection, a velocity gradient perpendicular to the filament major axis is a common feature seen in simulations. We show that this prediction is in good agreement with recent observations from the CARMA Large Area Star Formation Survey (CLASSy), from which we identified several filaments with prominent velocity gradients perpendicular to their major axes. Highlighting a filament from the north-west part of Serpens South, we provide both qualitative and quantitative comparisons between simulation results and observational data. In particular, we show that the dimensionless ratio Cv ≡ Δvh2/(GM/L), where Δvh is half of the observed perpendicular velocity difference across a filament, and M/L is the filament’s mass per unit length, can distinguish between filaments formed purely due to turbulent compression and those formed due to gravity-induced accretion. We conclude that the perpendicular velocity gradient observed in the Serpens South north-west filament can be caused by gravity-induced anisotropic accretion of material from a flattened layer. Using synthetic observations of our simulated filaments, we also propose that a density-selection effect may explain observed subfilaments (one filament breaking into two components in velocity space) as reported in recent observations.

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