Correcting for sub-grid filtering effects in particle image velocimetry data

Particle Image Velocimetry methodology results in a spatial averaging of the real velocity field into a set of discrete measured velocities: one for each interrogation cell. In the absence of measurement noise this filtering process results in a reduction of the measured turbulent kinetic energy and other second order statistics of the velocity field. The reduction in this energy will naturally be dependent upon the amount of turbulent energy at lengthscales smaller than can be resolved by the interrogation cells that make up the measurement grid. This paper investigates the effects of sub-grid scale filtering on the second order statistics of velocity. Several experiments are reported for which interrogation cell size to turbulent integral length scale ratios were varied. In addition, synthetic turbulent velocity fields with known spatial correlation functions are used to support experimental results and provide calibration for the estimation of the level of sub-grid filtering. It is suggested that to accurately capture all turbulent kinetic energy using PIV the interrogation cell should be at least of order 10 times smaller than the integral lengthscale of the flow. A method is then provided to estimate the level of sub-grid filtering should the interrogation cell be larger than this limit up to around the size of the integral lengthscale. With interrogation cells larger than this lengthscale then sub-grid filtering is such that second order statistics are reduced by over 50% and it should be considered unwise to rely on any second order statistics from such a scenario, corrected or otherwise

Current methods for characterising mixing and flow in microchannels

This article reviews existing methods for the characterisation of mixing and flow in microchannels, micromixers and microreactors. In particular, it analyses the current experimental techniques and methods available for characterising mixing and the associated phenomena in single and multiphase flow. The review shows that the majority of the experimental techniques used for characterising mixing and two-phase flow in microchannels employ optical methods, which require optical access to the flow, or off-line measurements. Indeed visual measurements are very important for the fundamental understanding of the physics of these flows and the rapid advances in optical measurement techniques, like confocal scanning laser microscopy and high resolution stereo micro particle image velocimetry, are now making full field data retrieval possible. However, integration of microchannel devices in industrial processes will require on-line measurements for process control that do not necessarily rely on optical techniques. Developments are being made in the areas of non-intrusive sensors, magnetic resonance techniques, ultrasonic spectroscopy and on-line flow through measurement cells. The advances made in these areas will certainly be of increasing interest in the future as microchannels are more frequently employed in continuous flow equipment for industrial applications

An experimental investigation of cavity flow

Of particular interest are the flow structure and dynamics associated with open shallow rectangular cavities at low Mach numbers for various length-to-depth ratios. At the Reynolds number investigated, it is the presence of convective instabilities through the process of feedback disturbance that gives rise to a globally unstable flowfield. Using an instrumental wing model with a cut-out an experimental investigation of a cavity flowfield exhibiting ‘fluid-dynamic’ phenomenon has been completed. A post-processing module for the PIV image data was constructed which optimised the data fidelity and accuracy while improving upon velocity spatial resolution. These improvements were necessary to capture the flow scales of interest and minimise the measurement error for the presentation of velocity, velocity-derivative and turbulent statistics. It is shown that the hydrodynamics that is intrinsic to the cavity flowfield at these inflow conditions organises the oscillation of small- and large-scale vortical structures. The impingent scenario at the downstream edge is seen to be crucially important to the cavity oscillation and during the mass addition phase a jet-edge is seen to form over the rear bulkhead and floor. In some instances this jet-like flow is observed to traverse the total internal perimeter of the cavity and interact with the shear layer at the leading edge of the cavity, this disturbs the normal growth of the shear layer and instigates an increase in fluctuation. The coexistence and interplay between a lower frequency mode dominant within the cavity zone and the shear layer mode is seen to shed large-scale eddies from the cavity. This modulation imposes a modification to the feedback signal strength such that two distinct states of the shear layer are noted. Concepts for the passive reduction of internal cavity fluctuation are successful although modifications to the shear layer unsteadiness are encountered; an increase in drag is implied

Volumetric velocimetry for fluid flows

In recent years, several techniques have been introduced that are capable of extracting 3D three-component velocity fields in fluid flows. Fast-paced developments in both hardware and processing algorithms have generated a diverse set of methods, with a growing range of applications in flow diagnostics. This has been further enriched by the increasingly marked trend of hybridization, in which the differences between techniques are fading. In this review, we carry out a survey of the prominent methods, including optical techniques and approaches based on medical imaging. An overview of each is given with an example of an application from the literature, while focusing on their respective strengths and challenges. A framework for the evaluation of velocimetry performance in terms of dynamic spatial range is discussed, along with technological trends and emerging strategies to exploit 3D data. While critical challenges still exist, these observations highlight how volumetric techniques are transforming experimental fluid mechanics, and that the possibilities they offer have just begun to be explored.SD was partially supported under Grant No. DPI2016-79401-R funded by the Spanish State Research Agency (SRA) and the European Regional Development Fund (ERDF). FC was partially supported by the U.S. National Science Foundation (Chemical, Bioengineering, Environmental, and Transport Systems, Grant No. 1453538)

Method of measuring the water jet diameter : Método de medida del chorro de agua

Abrasive Water Jet (AWJ) machining is one of the manufacturing technologies with highest market growth. It is a process in which material is removed from a work piece using mineral abrasive particles. Due to this principle, AWJ machining technology produces significant amount of waste products that consists mainly on the mineral abrasive. It is possible to cut softer materials with pure water jet (WJ), but efficiency is then reduced. In order to increase the efficiency of the WJ machining and to make the process more sustainable than AWJ, the Ice Jet (IJ) technology was developed. The IJ technology aims to supplement the currently used AWJ technology in several applications. Unfortunately addition of liquid nitrogen to a high-speed water jet can cause dispersion of the jet and consequently reduction of cutting efficiency. Previous experiments in the field of jet diameter measurement have been done by different research groups. In this thesis these methods and the results from the studies based on them, were described and analyzed. This state of the art analysis gave us the knowledge to begin the research on our new method. It also showed that due to possible implementation in industry applications, a quick, easy and affordable technique would be useful. For this reason the phenomenological analysis method to measure the diameter of the water jet was explored. The new method combines digital image acquisition and processing with digital manipulation of the photography. The main advantages of such method are that the equipment can be cheap and as the method is contactless, it doesn‟t get destroyed over time. The analysis of the jet can be automatized and the time of the analysis is short. This means that the method could be applied in the production line to measure the wear of the nozzles during the production cycle without causing any delays. This would increase the lifetime of the nozzles, decreasing the production costs and through online modification of the process parameters also its quality. During the experiments several process parameters such as water pressure, water temperature and diameter of the water nozzle were modified. In order to verify of the new measurement method, the results were compared with results from another method that has already been established before. Therefore the results from gained from this method served as the reference used to evaluate the new method. The reference method measures the jet diameter through the measurement of the jet force. The experimental setup was built in order to evaluate new measurement method and to compare the novel photographic method with the method which makes use of the load cell to measure the WJ diameter. Setup was built on the machine for ice jet experiments. To illuminate the jet, two high performance light emitting diodes (LED) were used and a consumer digital single lens reflex (DSLR) camera was used to capture the images. In order to make the image acquisition as constant as possible a microcontroller was used to control the LEDs and to trigger the DSLR. Simultaneously a measurement set of WJ force has been made with the load cell. The focus of this work has been on the photographic method. Measurements with load cell were used for validation of the novel method only. During the experiments two water nozzles were used, five different standoff distances, three different water pressures and three different water temperatures. Regarding photography two different shutter speeds were used throughout all the experiments. Preliminary several combinations of shutter speed and light intensity has been tested in order to obtain optimal experimental conditions. Evaluation of results was performed in the Matlab software package. More than one thousand images were processed by fitting a custom function to the specific rows of image. Function was Gaussian with additional quadratic, linear and constant term. Function was fitted to five image rows which correspond to five different standoff distances from the water nozzle. At the precisely same distances also radial dependence of WJ force was measured. It was found, that the method using digital camera and computer evaluation of image can yield good results which are in good correspondence with measurements of WJ diameter obtained by the load cell.Escuela Técnica Superior de Ingeniería IndustrialUniversidad Politécnica de Cartagen

Velocimetry-based pressure information for spray analysis – novel experimental, processing and evaluation strategies

In der vorliegenden Arbeit wurde der Spraytransport von komplexen Benzindirekteinspritzungssprays (GDI) mittels auf Geschwindigkeitmessung basierter Druckauswertung untersucht. Für diesen Zweck wurden neue Versuchs-, Verarbeitungs- und Auswertestrategien eingeführt, um eine Druckauswertung der Spray-induzierten Strömung zu befähigen und deren Möglichkeiten auszuweiten. Dies umfasst unter anderem ein statistisches Verfahren auf Basis der Unsteady Reynolds-Averaged Navier-Stokes (URANS) Gleichungen und Ensemble-Mittelung, welche die Druckauswertung transienter, statistisch stationärer Strömungen mittels konventioneller Particle Image Velocimetry (PIV) ermöglicht. Darüber hinaus wurde eine neuartige Technik namens Dual-Plane-Stereo-Astigmatismus (DPSA) entwickelt, die die Auswertung momentaner Druckfelder und damit die Analyse einzelner Einspritzereignisse unter Verwendung eines stereoskopischen Aufbaus und einer einzigen Lichtquelle ermöglicht. Abschließend wurde die Methode der Physics-Informed Neural Networks (PINNs) erfolgreich aus dem Bereich des Deep Learnings in die experimentelle Strömungsmechanik und Spray-Analyse übertragen. Das PINN-Verfahren weitet die Möglichkeiten der bisherigen auf Geschwindigkeitsmessung basierenden Druckauswertung aus und ermöglicht die Auswertung von bislang nicht auswertbaren Strömungsbereichen, sowohl in Raum und Zeit. Unter Verwendung der beschriebenen Methoden wurde die Wechselwirkung zwischen Spray und Umgebungsgasströmung für unterschiedliche Betriebsbedingungen und Sprayauslegungen untersucht. Es zeigte sich, dass der Impulsaustausch mit höherem Einspritzdruck, Gasdichte, Kraftstofftemperatur, größerer Relativgeschwindigkeit, Spray-Gas-Grenzfläche, Sprayexpansion und stärkerer Zerstäubung bzw. Flash-Boiling zunimmt. Als eine wesentliche Erkenntnis wurde festgestellt, dass die Ablenkung von Sprays bzw. das Phänomen der Strahl-zu-Strahl-Wechselwirkung und Spraykontraktion auf einen Nettoimpuls zurückzuführen ist, der auf einzelne Spraykeulen infolge von induzierten Druckkräften wirkt. In diesem Zusammenhang wurde das Vorhandensein eines Niederdruckgebiets im Zentrum von Mehrlochsprays experimentell bestätigt. Es wurde aufgezeigt, dass das Ausmaß der Strahl-zu-Strahl-Wechselwirkung und der Spraykontraktion durch eine enge Spritzlochanordnung und -ausrichtung, eine starke Zerstäubung und ein erhöhtes Tropfen-Folgeverhalten begünstigt wird

Application of non-invasive flow measurement techniques for quantitative analysis of a biomedical device

Researchers have extensively studied the techniques and applications of Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF). Developments in computing and imaging techniques have significantly aided the accuracy and efficiency techniques which are now commonly used to study velocity vectors, predicting acceleration and mixing profiles in modern fluid dynamics. This project focused on analyzing the flow field inside a MicroWell, a component of the assay kits that are processed by the Vitros ECi analyzer. The assay kits and analyzer are manufactured by Ortho Clinical Diagnostics, Rochester, NY. Reagents and samples are mixed in the MicroWell during various operations of the analyzer. This process was extensively examined and quantified using the said techniques. This study involved building a test rig that emulates the injection and mixing process of the actual system and allows access for extensive PIV and PLIF analysis as well as optimization and synchronization of both systems for accurate real-time results. Further, results obtained from both experiments were studied in conjunction and then quantified for an aid of future assay and Immunodiagnostics development