Natural convective flows: A comparison with electrovortex flows

Fluid flow distribution among parallel channels usually plays an important role on the global performance improvement of tubular process equipment, but remains difficult to be properly measured by experimental methods. This paper presents a systematic study on the measurement of flow distribution in a multi‐channel fluidic network using Particle Image Velocimetry (PIV) technique. For the precise measurement of flow‐rate in each individual channel, standard 2 dimensions, 2 components PIV technique is used to obtain velocity vectors on multiple sampling planes parallel to the flow direction, so as to reconstruct accurate velocity profiles on the cross‐sectional surface. Such procedure is repeated for every micro‐ (mini‐) channel to obtain the actual flow distribution properties in the fluidic network.Pre‐test results on a double‐channel device indicate that the maximum possible deviation of PIV measurements is 13.1% with respect to those of a flange flowmeter, implying that the PIV technique is relatively accurate and reliable. PIV results on the flow distribution in a 15‐channel fluidic network are then compared with Computational Fluid Dynamics simulation results under the same working conditions. A quite good agreement could be observed. Moreover, the locations and sizes of vortices in the distributor have a significant influence on the flow distribution. © 2016 Curtin University of Technology and John Wiley & Sons, Ltd.

Throughout their useful life structures demand monitoring to verify their conditions of use. Thus, the need arises to seek methodologies capable to evaluate displacements and deformations in structural components of buildings. The objective of this research was to evaluate the use of the particle image velocimetry (PIV) technique to analyze test specimen behavior during a static bending test, additionally a displacement map is presented. The PIV technique measures variations of position of a region in an object from consecutive images from a loading session. The tests were performed on wood of Pinus oocarpa and Eucalyptus grandis, plywood panels, laminated veneer lumber (LVL) and oriented strand board (OSB). The results obtained by the PIV technique were compared to values obtained from dial indicators. It was verified that in all the materials tested, the PIV technique presented results similar to those found in the dial indicators. By means of the Student t test, with a significance level of 1%, some regions of analysis the values found by the PIV technique and the dial indicators were statistically similar. For the analyzed regions that did not achieve statistical equality, a correction equation was calculated (R² higher than 0.99). It was concluded that the PIV technique, with properly adjusted setup, presented results similar to those obtained in the conventional tests, proving its accuracy and reliability in displacement measurements, with the advantage of being a non-destructive and contactless structure monitoring technique.

Thermocapillarity is a physical phenomenon used in many industrial processes, mainly in the field of miniaturization. Thermocapillary forces are the base of thermocapillary pumping (TCP), in which a drop of liquid moves through a microchannel or flat surface after temperature gradient occurs. The objective of this work was to study the thermocapillary convection during pumping without inserting external particles, monitored by the Particle Image Velocimetry (PIV) technique. The experiment consisted of a Pasteur tube containing a yellow fluorescein solution (0.02, 0.04, 0.06, 0.08, and 0.1 Molar), illuminated by a laser beam of 545 nm, 40 mW. After 2 minutes of illumination, the thermocapillary movement occurred at a distance of 0.05 mm below the meniscus when using a laser beam of 545 nm, 3 mW. The images were captured by a charge-coupled device (CCD) camera and processed using the PIV technique. The results showed an internal conversion capacity between the intersystem crossing, vibrational, and relaxation phenomena, also demonstrating the potential for applying the proposed approach. The images presented velocity distribution caused by thermocapillarity. The PIV was a useful tool for convective flow analysis if connected to appropriate image processing and enhancement techniques. In conclusion, the research showed the images with velocity distribution caused by thermocapillarity.

The effect of drag reducing additive on the structure of turbulence in concentric annular pipe flow was investigated using Particle Image Velocimetry (PIV) technique. Experiments were conducted using a 9m long horizontal flow loop with concentric annular geometry (inner to outer pipe radius ratio = 0.4). The drag reducing additive was a commercially available partially hydrolyzed polyacrylamide (PHPA). The experiments were conducted using 0.1% V/V polymer concentration, giving a drag reduction of 26% at a solvent Reynolds number equal to 56400. Near wall local fluctuating velocity values were determined by analysing the PIV data. The root mean square (RMS) values of radial velocity fluctuations showed a significant decrease with the use of drag reducing additive. The RMS values of axial velocity fluctuations near the wall (Y+<10) were similar for both water and polymer fluid flow; though, higher peaks were obtained during the polymer fluid flow. As compared to water flow, a strong reduction in vorticity was observed during polymer fluid flow. The degree of vorticity reduction on the inner wall was higher than that of the outer wall. Results of the viscous dissipation and the shear production terms in the kinetic energy budget showed that less energy was produced and dissipated by the route of turbulence when using polymer fluid.

For this study, a scaled model test is conducted to investigate the deflection of a laterally loaded pile and soil deformation in loose sand. An optical experimental setup is developed to capture images of soil movement and the readings of strain gauges along the pile. The deflection of the pile, derived from readings of the strain gauges, increases with the increasing lateral loads. Additionally, the displacement fields of ground surface and profile from a series of images are calculated using the particle image velocimetry (PIV) technique. This technique provides a more accurate estimate of the pile deflection compared with the double integration (DI) method. The images illustrate that the sand rotated in a position approximately 160 mm deep, which is identical to the results of the DI method. Finite-element analysis is used to simulate the model test, which verifies the accuracy of the PIV technique. The results demonstrate that the PIV technique and this optical setup are suitable for solving ...

Ensemble correlation PIV applied to bubble plumes rising in a bubble column

Vacuum preloading combined with prefabricated vertical drains is a widely used system for improving soft clay-dredged slurry deposits. However, the influence of the initial water content on clogging is not yet fully understood. In this study, laboratory model tests were conducted on slurry with initial water contents of 106%, 133%, and 159% during vacuum preloading. The process of vacuum preloading was monitored from the perspectives of discharged water volume, pore water pressure, as well as the displacement and strain profile of soil patches by using the novel particle image velocimetry (PIV) technique. The results showed that the clogging formation process could be divided into three stages based on the water discharge rate: rapid formation, transition, and slow formation. An increase in initial water content resulted in a larger horizontal displacement, leading to a thinner and denser clogging zone with lower permeability coefficients and a reduced rate of pore pressure dissipation. Empirical equations for the thickness of the clogging zone versus time were proposed using the PIV technique, providing a reference for theoretical consolidation calculations and making the calculations more practical by considering the variation of the clogging zone during vacuum preloading.

This paper mainly introduces the study that use particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) techniques to measure the natural convection flow field in a cabinet. In order to simulate the natural convection flow, a square cabinet is built, and different boundary temperatures are set to generate natural convection. The research shows that the experimental results measured by the PIV and PLIF technologies are close to the results of computational fluid dynamics (CFD) numerical simulation. This verifies the accuracy of the test technique and numerical simulation. Furthermore, the measurement technologies of PIV and PLIF will contribute to in-depth study on natural convective flow phenomena.

The fluid particle velocities in the overturning jet of a breaking wave have been measured by the Particle Image Velocimetry (PIV) technique. Monochromatic waves with wave height of 14.5 cm and wavelength of 121 cm were generated in the water depth of 20 cm. The measured fluid particle velocity at the tip of the overturning jet reached 1.68 times of the phase velocity calculated from the linear wave theory. Fluid particle accelerations were estimated from the velocity data with the following results: The overturning jet enters the horizontal water surface with an acceleration of 1.1 g at an angle of 88° downward. The PIV technique was also used to measure the instantaneous vertical vorticities generated by breaking waves. The number and locations of the vortices on the horizontal plane appear to be random. The maximum instantaneous vorticity was in the order of magnitude of 20–30 s−1, whereas the ensemble-averaged vorticity was quite small.

The 3D structure of the tide-induced Lagrangian residual current was studied using the particle image velocimetry (PIV) technique in a long shallow narrow tank in the laboratory. At the mouth of the tank, a wave generator was used to make periodic wave which represents the tide movement, and at the head of the tank, a laterally sloping topography with the length of one fifth of the water tank was installed, above which the tide-induced Lagrangian residual current was studied. Under the weakly nonlinear condition in the present experiment setup, the results show that the Lagrangian residual velocity (LRV) field has a three-layer structure. The residual current flows inwards (towards the head) in the bottom layer and flows outwards in the middle layer, while in the surface layer, it flows inwards along the shallow side of the sloping topography and outwards along the deep side. The depth-averaged and breadth-averaged LRV are also analyzed based on the 3D LRV observations. Our results are in good agreement with the previous experiment studies, the analytical solutions with similar conditions and the observational results in real bays. Moreover, the volume flux comparison between the Lagrangian and Eulerian residual currents shows that the Eulerian residual velocity violates the mass conservation law while the LRV truly represents the inter-tidal water transport. This work enriches the laboratory studies of the LRV and offers valuable references for the LRV studies in real bays.

Hydrodynamic study of bubble characteristics and bubble rise velocities in batch electrocoagulation with vibration-induced electrode plates using the PIV technique

Experimental study on multiphase flow in fracture-vug medium using 3D printing technology and visualization techniques

Particle image velocimetry (PIV) is a well-established and powerful technique for performing whole-field velocity measurements under a broad range of flow conditions, and is widely used in fluid mechanics laboratories.The aim of this feature issue of Measurement Science and Technology is to provide the reader with an overview of some recent progress in PIV applications developed in the EUROMECH 411 colloquium, which was held at CORIA, Rouen University, France from 29 to 31 May 2000.The topics of the meeting organized by M Trinité and B Lecordier were focused on three main aspects: Advanced PIV techniques (2D2C, 2D3C or 3D) used to study turbulent flows. Accuracy and limitations of PIV treatments for measurement of turbulence properties. Relevant post-processing tools for PIV data to extract information on turbulent flows.In total, 24 papers were presented on these topics and four keynote lectures were given by recognized specialists. Seven papers have been selected by the EUROMECH Scientific Committee for publication in this feature issue.If 3C, 3D (4D with time) is the ultimate goal of PIV developments, then holography is the only practical way to extend sheet-oriented PIV to measuring a volume of considerable depth in 3D. However, one major problem is the pollution from particles located at different depths: image quality is spoiled by noise from out-of-focus particles. Digital holography (Coëtmellec et al) using a 2D wavelet algorithm is a very simple promising new method in this area despite poor resolution.Stereoscopic PIV, using two cameras viewing the flow field from different angles, is one technique for measuring all three components in a fluid plane. To reconstruct the three-dimensional velocity field, a calibration stage is necessary since the imaging geometry causes significant aberrations. A 3D vector is calculated at a physical point in the object plane. The first step consists in computing two 2D vectors from both cameras at the physical point. Two methods are discussed (warping and mapping) in the paper by Coudert and Schon.Cross-correlation based PIV algorithms have a limited spatial resolution, which is determined by the size of the measurement window. The study and development of an integrated PIV-PTV method is described by Stitou and Riethmuller. This combines the robustness of Particle Image Velocimetry with the spatial resolution of Particle Tracking Velocimetry for super resolution analysis.Different experimental studies using known flow fields such as grid turbulence or pipe flows have been used to try to show the capability of the PIV technique to extract the information from turbulent vector maps. Nevertheless, from these studies, it always seems difficult to evaluate which treatment is the most accurate to measure a given flow. This is mainly due to the difficulty in knowing the exact characteristics of the flows. A solution can be to numerically impose the properties of the flow and to generate synthetic images (Lecordier et al). This approach is now possible by using Direct Numerical Simulation (DNS).If PIV has now evolved from its embryonic state to reach a certain level of confidence, we are now facing a problem directlylinked to the high spatial resolution and frequency ofacquisition that can nowadays be achieved by manufactured CCDcameras: the overabundance of information, which has to beappropriately reduced if one desires to draw conclusions fromthe data set. Two recent tools can now be used to resolve theproblem of unsteady turbulent flows and to try to separate thefluctuation intensity due to the unsteadiness and the turbulenceitself: Proper Orthogonal Decomposition (POD) and wavelettransform analysis. Promising results are obtained using POD inrotating flows (Graftieaux et al, Patte-Rouland et al). With regard to spatial correlation and patternrecognition this tool is also promising in evaluating thecontribution of coherent structures to the generation andself-sustaining of wall turbulence in boundary layers. Wavelettransform analysis can be applied to analyse temporal evolutionof coherent structures and to characterize the leading vortexgenerated in a starting flow (Schram and Riethmuller).We believe that this special feature provides a valuable overview of the proceedings at the colloquium and we would like to express our thanks to all the participants and to the contributing authors and reviewers who have made this special feature possible.

Impinging streams technology has been widely used in many applications in recent years because of its enhancement to the heat and mass transfer between phases. In this paper, in order to investigate the influences of the impinging distance and flow rate on the characters of the flow field, gas-gas impinging streams flow fields are tested experimentally and analyze qualitatively with particle image velocimetry (PIV). The experimental equipment consists of two opposite nozzles which are the same axis. A PIV system is used to measure the characters of the 2-D flow field between two opposite nozzles. The gas is delivered by a compressor through two opposite jets which could be seeded with oil droplets as tracer particles. The effects of the flow rate and impinging distance on the velocity fields of impinging zone are investigated in detail. As the flow rate increases from 0.2 m3/h to 0.8 m3/h, the width of impinging zone increases from 0.25 to 0.5. However, the range of impinging zone does not change significantly as the impinging distance increases from 61mm to 94mm. The results indicate that the PIV technique is an effective method to measure and analyze the characters of impinging streams.

The study of microdischarge dynamics (MD) in dielectric barrier discharge (DBD) is gaining much attention due to high interest in the collective interaction of MD filaments, memory effects, and the formation of self-organized patterns. In this work, the MD dynamics of atmospheric pressure air DBD were studied under the impact of forced external gas flow and natural convective flow. For this, rail type and parallel-plate discharge electrode arrangements were used. The discharge between rail electrodes in external airflow along the electrodes clarified the role of volume plasma and surface charges in the memory effect of microdischarges [1] . The change in the gas flow regime from laminar to turbulent shows the complex dynamics of MD filaments with trajectories downstream and upstream to the gas flow direction. The impact of convective flow due to the temperature gradient between discharge cell electrodes and ambient air was studied for parallel-plate electrode arrangement. The convective gas flow velocity increased at a higher thermal gradient, and the MD channels follow the gas flow direction. The velocity of convective flow was estimated by the CFD simulation and compared with the mean velocity of MD channels obtained by the particle image velocimetry method.