Measurement of Velocity Field and Turbulent Parameters in a Downward Conical Channel (original) (raw)
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Engineering Reports, 2020
Particle image velocimetry (PIV) measurement technique provides an excellent opportunity for investigating instantaneous spatial structures which are not always possible with point measurements techniques like laser Doppler velocimetry. In this review, it was shown that PIV technique provides an effective means of visualizing important structures of Newtonian and drag-reducing fluid flows. Such structures include large-scale-events that constitute an important portion of the Reynolds stress tensor; shear layers of drag-reducing flows, which have been suggested to constitute the mechanism of drag reduction (DR); and near wall vortices/low speed streaks which constitute the mechanism of turbulence production. PIV investigations of turbulence statistics in Newtonian and drag-reducing fluid flows were reviewed with the view of providing explanation to DR by additives. Results of turbulence statistics, for Newtonian fluid flow, showed that streamwise velocity fluctuations and turbulence intensity had peak values close to the wall, in a region of high mean velocity gradient, while radial fluctuating velocity and Reynolds stress tensor had peaks further from the wall and at approximately the same detachment from the wall. In single-and two-phase flows in horizontal channels, the velocity profile of polymer solution, in the turbulent regime, show asymmetric behavior. This review highlighted important interfacial characteristics in gas-liquid flows such as S-shaped velocity profile as well as the turbulences statistics in each phase and across the interface region. Drag-reducing agents (DRAs)-imposed changes on turbulence statistics and flow structures were also examined. PIV studies of drag-reducing flows showed that DRAs act to dampen wall-normal-, streamwise fluctuating velocity, and Reynolds stress tensor. The reduction in Reynolds stress tensor is higher than the reduction of both wall-normal and streamwise velocity fluctuations and this discrepancy has been associated with the decorrelation of the component of fluctuating velocity. Furthermore, the addition of DRA produces a shift of the peak of wall-normal velocity fluctuations further from the wall due to increased buffer layer thickness. DRAs do not only act to reduce drag but also to modify the flow structure. The major influence of DRAs on flow structures is seen in the This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Engineering reports, 2020
Particle image velocimetry (PIV) measurement technique provides an excellent opportunity for investigating instantaneous spatial structures which are not always possible with point measurements techniques like laser Doppler velocimetry. In this review, it was shown that PIV technique provides an effective means of visualizing important structures of Newtonian and drag-reducing fluid flows. Such structures include large-scale-events that constitute an important portion of the Reynolds stress tensor; shear layers of drag-reducing flows, which have been suggested to constitute the mechanism of drag reduction (DR); and near wall vortices/low speed streaks which constitute the mechanism of turbulence production. PIV investigations of turbulence statistics in Newtonian and drag-reducing fluid flows were reviewed with the view of providing explanation to DR by additives. Results of turbulence statistics, for Newtonian fluid flow, showed that streamwise velocity fluctuations and turbulence intensity had peak values close to the wall, in a region of high mean velocity gradient, while radial fluctuating velocity and Reynolds stress tensor had peaks further from the wall and at approximately the same detachment from the wall. In single-and two-phase flows in horizontal channels, the velocity profile of polymer solution, in the turbulent regime, show asymmetric behavior. This review highlighted important interfacial characteristics in gas-liquid flows such as S-shaped velocity profile as well as the turbulences statistics in each phase and across the interface region. Drag-reducing agents (DRAs)-imposed changes on turbulence statistics and flow structures were also examined. PIV studies of drag-reducing flows showed that DRAs act to dampen wall-normal-, streamwise fluctuating velocity, and Reynolds stress tensor. The reduction in Reynolds stress tensor is higher than the reduction of both wall-normal and streamwise velocity fluctuations and this discrepancy has been associated with the decorrelation of the component of fluctuating velocity. Furthermore, the addition of DRA produces a shift of the peak of wall-normal velocity fluctuations further from the wall due to increased buffer layer thickness. DRAs do not only act to reduce drag but also to modify the flow structure. The major influence of DRAs on flow structures is seen in the This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Experiments in Fluids, 2005
The turbulence structure of a horizontal channel flow with microbubbles is experimentally investigated using combined particle image velocimetry (PIV) in order to clarify the mechanism of drag reduction caused by microbubbles. A new system which simultaneously measures the liquid phase and the dispersed bubbles is proposed, based on a combination of particle tracking velocimetry (PTV), laser-induced fluorescence (LIF) and the shadow image technique (SIT). To accurately obtain the velocity of the liquid phase, tracer particles which overlap with the bubble shadow images are almost entirely eliminated in the post-processing. Finally, the turbulence characteristics of the flow field are presented, including measurements for both phases, and the bubble effect on the turbulence is quantified.
Journal of Membrane Science, 2010
Liquid and liquid/gas flows through spacer filled channels were studied using Particle Imaging Velocimetry (PIV) to provide experimental support for velocity distributions obtained from Computational Fluid Dynamics studies available in the literature. It is shown that PIV is a suitable technique for measuring velocity profiles in spacer filled channels, although care has to be taken when interpreting the results. PIV measurements were carried out for an entire flow cell (5 cm × 15 cm), a smaller area of roughly 7 mm × 8 mm and for a single spacer cell (2.5 mm × 3 mm). The experimental results show that fluid flow is well distributed across the entire flow cell in the case of single-phase flow. The recordings of the single spacer cell showed that liquid flow is mainly parallel to the spacer filaments and therefore the direction of flow changes 90 • over the height of the channel. Introduction of air bubbles introduced strong local velocity gradients. The liquid velocity in two-phase flows is shown to be more unsteady than in the case of single-phase flow, which is advantageous when trying to prevent fouling or concentration polarization.
Characterizing drift velocity for two-phase flow using particle image velocimetry
Advances in Fluid Mechanics XI, 2016
Two-phase flow analysis is important in the design of gas and oil pipelines, the design of fuel channels in power generation industries, the prediction of heat transfer characteristics in process equipment, the operation of the gas-oil separation plants in oil production companies, the safe operation of nuclear reactors, the design of thermal desalination processes, and in many other industrial systems. The reliability or failure of these systems depends on the ability to analyze and model these types of flows. Drift velocity is an important parameter for the models used to analyse two-phase flow in vertical pipes and in the oilwater separation processes. Several studies are available for large pipe diameters. However, very few studies have been conducted in small pipe diameters and the effect of liquid phase properties has not been thoroughly investigated. In small pipe diameters, the liquid surface tension can play a significant role in determining the flow behaviour, and consequently the accuracy of the available models and correlations. In this study the drift velocity and the liquid flow characteristics for two-phase slug flow in a vertical pipe are experimentally investigated. Particle image velocimetry (PIV) along with both image pre-and post-processing techniques are adapted to determine the drift velocity, void fraction, and vorticity fields in the liquid phase.
17th Brazilian Congress of Thermal Sciences and Engineering, 2018
The present work aims the development of a Particle Tracking Velocimetry (PTV) technique capable to identify and then track the motion of the different gas phase spatial structures present on gas-liquid vertical flows. Due to its transient and strong spatial variations, the vertical slug flow pattern is studied, since the gas phase are spatially distributed as dispersed small bubbles or large elongated bubbles, which are commonly referred as Taylor bubbles. The different morphologies can be classified on two groups: i) small scale interfacial scale phase, the dispersed bubbles and ii) large scale interfacial scale phase, the Taylor bubble. This morphological classification arises from the different length of the gas phase structures presented on the flow pattern. In order control the interfacial length scales and test out the proposed method, the real slug flow is simplified to a "quasi-slug" flow pattern, where a fabricated flow pattern is created by the injection of Taylor bubble into gas-liquid bubbly vertical flow, where it is possible to vary the Taylor bubble length and the of the bubbly flow liquid and gas superficial velocities, simulating a real slug flow pattern. The Particle Tracking Velocimetry implementations found on literature can only be used for flows with a single interfacial length scales, such as the rising of a Taylor bubble or the individual tracking of dispersed bubbles found on gas-liquid bubbly flows. The proposed method consists on two different tracking algorithms that are coupled to be used on a gas-liquid flows with different interfacial length scales, being able to track the motion of the different interfacial length scales and also extract information about its shape and size.
Application of Large Scale Particle Image Velocimetry (LSPIV) to Identify Flow Pattern in a Channel
Procedia Engineering, 2015
This study has three main goals, first, to map the flow structure in a channel using a unique technique named Large Scale Particle Image Velocimetry (LSPIV). Second, to test the sensitivity of LSPIV results to LSPIV parameters (e.g., Interrogation Area, and Searching Area). Third, to test the capability of LSPIV method in predicting the magnitude and direction of flow velocity in a complex flow structure. An LSPIV system was set to observe 14 (fourteen) runs of laboratory re-circulating trapezoidal open channel with a sudden expansion-constriction shape in its middle reach. LSPIV technique was successfully mapping the core flow and the swirling motions near the wall in the expansion-constriction reach. In the core flow region, longitudinal velocity, U, estimate was sensitive to the interrogation area and searching area parameters, while lateral velocity, V, estimate was insensitive to those parameters. The opposite condition occurred in the wall region. The maximum forward flow velocity U was 0.56 m/s occurred in the core flow region. This value was closed to channel bulk velocity Ubulk=0.4 m/s measured using flow meters. The largest backward velocity was 0.068 m/s, obtained in left wall region where the swirling motion occurred.
Particle Image Velocimetry: Recent Improvements, 2004
The presence of a large number of different software codes for image analysis, including several variants for the image interrogation, suggests the need for testing the suitability and accuracy of the developed algorithms. One of the possible approaches is testing these systems with experiments of well-known flow properties. Otherwise, tests can be performed by analyzing synthetically generated images. The advantage of the latter approach is that there is no need to set-up an experiment and the flow field is known in detail. On the other hand, there are obvious doubts on how close a synthetic image can describe the reality. This paper provides some insight on the relation between results on real and synthetic images both in a turbulent channel flow. A classical Particle Tracking Velocimetry (PTV) algorithm will be presented as well as the advanced PTV using Feature Tracking.
PIV Methods for Turbulent Bubbly Flow Measurements
Analysis of particle properties in dispersed multiphase flow and simultaneous determination of velocity fields for both phases is main concern in the study of many industrial problems. Particle Image Velocimetry (PIV) is a powerful tool to study the structure of multiphase, three-dimensional, transient fluid flows . The aim of this study is to find the most suitable PIV method to study turbulent bubbly flows and to measure the properties of bubbles in a mixing vessel. The method should be accurate, inexpensive, fast and easy to use in varying measurement conditions. Back lighting is used to detect bubble outlines and the laser light sheet is produced to illuminate the tracer particles. Images of bubble shadows and particles are recorded with the same camera. Two experiments with different measurement methods are performed. The first set uses silver-coated hollow glass spheres as tracer particles and the second fluorescent particles. The optical filter blocks the scattered light from bubbles in the second experiment. In the first experiment the scattered light from bubbles is enhanced by digital image processing methods and by geometrical alignment of the camera. Also a stereo-PIV setup using two cameras at Brewster angles is tested for turbulent bubbly flow. In order to study the performance of the proposed methods, velocity fields and relative velocities of the continuous phase and bubbles are measured. The sizes and shapes of the bubbles are also measured and the quality of the results is analyzed. The accuracy of three dispersed particle velocity measurement methods is analyzed with simulated particle images.
Journal of Applied Fluid Mechanics, 2016
In this work, we investigated experimentally the hydrodynamics of flows crossing conical diffusers. On our previous work (Aloui et al., 2011), CFD turbulent models were validated for flows crossing the critical angle (2=16°). Indeed, the PIV data base constructed was exploited to validate a variant of SST-RLC model. Taking into account the conical diffuser angle effect, the apparition and the development of vortices were observed and studied. The dynamics of the recirculation zones which may be observed at the lower and higher parts of the singularity, has not formed the subject of numerous studies. There were no studies that characterize the vortices at the conical diffusers in terms of size, centre positions, and vortex intensity. Consequently, two conical diffusers were studied using the Particle Image Velocimetry technique (PIV). The results illustrate effects of "opening angle" (2=16°) and (2=30°) on the flow structures developed in such type of diffusers. From such opening angle of conical diffusers, the progressive angle increasing generates a detachment of the boundary layer of the conical diffuser depending on the turbulence level. This detachment may lead to a coherent flow structures. We applied the coherent structures criterion 2 to the recorded velocity fields to detect and characterize the vortices at the conical diffusers. We used the Proper Orthogonal Decomposition (POD) to filter the PIV data base constructed and to extract the most energetic modes. The results illustrate that the turbulent flow structures can be constituted using a limited number of energetic modes.