Understanding the flow in stirred vessels can be useful for a wide number of industrial applications, like in mining, chemical and pharmaceutical processes. Remodeling and redesigning these processes may have a significant impact on the overall design characteristics, affecting directly product quality and maintenance costs. In most cases the flow around the rotating impeller blades interacting with stationary baffles can cause rapid changes of the flow characteristics, which lead to high levels of turbulence and higher shear rates. The flow is anisotropic and inhomogeneous over the entire volume. A better understanding and a detailed documentation of the turbulent flow field is needed in order to design stirred tanks that can meet the required operation conditions. This thesis describes efforts for accurate estimation of the velocity distribution and the turbulent characteristics (vorticity, turbulent kinetic energy, dissipation rate) in a cylindrical vessel agitated by a Rushton turbine (a disk with six flat blades) and in a tank typical of flotation cells. Results from simulations using FLUENT (a commercial CFD package) are compared with Time Resolved Digital Particle Image Velocimetry (DPIV) for baseline configurations in order to validate and verify the fidelity of the computations. Different turbulence models are used in this study in order to determine the most appropriate for the prediction of turbulent properties. Subsequently a parametric analysis of the flow characteristics as a function of the clearance height of the impeller from the vessel floor is performed for the Rushton tank as well as the flotation cell. Results are presented for both configurations along planes normal or parallel to the impeller axis, displaying velocity vector fields and contour plots of vorticity turbulent dissipation and others. Special attention is focused in the neighborhood of the impeller region and the radial jet generated there. This flow in this neighborhood involves even larger gradients and dissipation levels in tanks equipped with stators. The present results present useful information for the design of the stirring tanks and flotation cells, and provide some guidance on the use of the present tool in generating numerical solutions for such complex flow fields. / Master of Science

BACKGROUND: Understanding the hemodynamics of the total cavopulmonary connection (TCPC) may lead to further optimization of the connection design and surgical planning, which in turn may lead to improved surgical outcome. While most experimental and numerical investigations have mainly focused on somewhat simplified geometries, the investigation of the flow field of true TCPC configurations is necessary for a true understanding. METHODS: This study details a manufacturing methodology yielding more accurate in vitro models that would provide a better understanding of the TCPC hemodynamics and adequate data for the validation of anatomical CFD simulations. This approach is illustrated on two different TCPC templates: an intra-atrial TCPC with a single superior vena cava (SVC) and a bilateral SVC with an extra-cardiac conduit. Power loss, flow visualization, digital particle image velocimetry (DPIV) flow measurements as well as computational fluid dynamics simulations are performed to characterize the anatomic flow structure. Additional parametric glass models of the TCPC were manufactured to help understand the fluid dynamics of the anatomical models and support the computational model validation effort. RESULTS/CONCLUSIONS: Both anatomic configurations revealed very different fluid dynamics underlining once again the need for at least one comprehensive experimental campaign per TCPC template for a good understanding of the flow phenomena. The absence of caval offset in the anatomical intra-atrial model resulted in important flow turbulence, which was enhanced by the large connection area and yielded high pressure drops and power losses. On the other hand, the bilateral SVC, which featured a smooth extra-cardiac conduit and wider vessels, led to power losses that were one order of magnitude lower than those of the anatomic intra-atrial model and a smooth flow field with lower levels of instability. The simplified glass models demonstrated that the diameter of the connecting vessels and of the pulmonary arteries in particular, was a parameter of prime importance. Finally, this study also reports on a combined experimental and numerical validation methodology, suggesting a cautious approach for the straightforward use of available CFD tools and pointing out the need for developing high resolution CFD techniques specifically tailored to tackle the complexities of cardiovascular flows.

Η διδακτορική διατριβή αφορά την πειραματική μελέτη του ισόθερμου ροϊκού πεδίου που αναπτύσσεται λόγω της αλληλεπίδρασης μιας περιδινούμενης δέσμης με μια εξωτερική παράλληλη ροή. Η συγκεκριμένη μελέτη έχει άμεση αναφορά σε διατάξεις καύσης (στροβιλοκινητήρες, φούρνους υαλουργίας, καυστήρες ντίζελ σε πλοία) και ως στόχο έχει την εμβάθυνση της κατανόησης των φυσικών μηχανισμών που αναπτύσσονται στο μέσο και τυρβώδες ροϊκό πεδίο αλλά και στη βελτίωση της διαδικασίας μίξης μεταξύ καυσίμου και οξειδωτικού μέσω του χαρακτηρισμού του τρισδιάστατου διατμητικού στρώματος που δημιουργείται λόγω της αλληλεπίδρασης τους. Αναλυτικότερα, στα πλαίσια της παρούσης διδακτορικής διατριβής μελετώνται διαφορετικές συνθήκες αλληλεπίδρασης (λόγος παροχής μαζών) μεταξύ μιας εσωτερικής περιδινούμενης δέσμης εκροής και μια εξωτερικής ομοαξονικής ροής. Η δημιουργία της περιδίνησης βασίζεται στην εφαπτομενική έγχυση ρευστού, μια τεχνική της οποίας τα ιδιαίτερα χαρακτηριστικά έχουν μελετηθεί ελάχιστα στη μέχρι τώρα βιβλιογραφία. Το υπό μελέτη ροϊκό πεδίο παρουσιάζει ιδιαίτερο ενδιαφέρον τόσο ως προς την πολυπλοκότητα του όσο και ως προς τη διερεύνηση παραμέτρων που επηρεάζουν την απόδοση συστημάτων καύσης. Το κύριο σκέλος αποτελεσμάτων που παρουσιάζεται στην παρούσα διατριβή αφορά πειραματικές μετρήσεις οι οποίες πραγματοποιήθηκαν με τη μέθοδο Ταχυμετρίας Απεικόνισης Σωματιδίων (Digital Particle Image Velocimetry). Παρ’ όλα αυτά, στα πλαίσια του σχεδιασμού της πειραματικής διάταξης και της αρχικής αξιολόγησης του ροϊκού πεδίου, παρουσιάζονται αποτελέσματα τα οποία προέκυψαν με τη χρήση εμπορικού κώδικα Υπολογιστικής Ρευστοδυναμικής (CFD-ACE+) και με την πειραματική μέθοδο Ανεμομετρίας Θερμού Νήματος διάταξης Χ (HWA-X probe). Τα αποτελέσματα που προέκυψαν κατά πρώτα στάδια υλοποίησης της διδακτορικής διατριβής συσχετίζονται με αυτά της Ταχυμετρίας Απεικόνισης Σωματιδίων, επιτρέποντας είτε την αξιολόγηση των διάφορων τυρβωδών μοντέλων και διαφορικών σχημάτων των εξισώσεων κίνησης ή την επιβεβαίωση των συμπερασμάτων μέσω της σύγκρισης των αποτελεσμάτων των δύο πειραματικών μεθόδων. Στα πλαίσια της διδακτορικής διατριβής γίνεται αναφορά και σύγκριση με παρόμοιες διατάξεις ομοαξονικών ροών, ενώ προκύπτουν νέα στοιχεία σχετικά με τα κριτήρια ομοιότητας ομοαξονικών ροών με περιδίνηση και τους φυσικούς μηχανισμούς που αναπτύσσονται στο τρισδιάστατο στρώμα μίξης που διαμορφώνεται. Ιδιαίτερη έμφαση δίδεται στη σύνδεση της τοπολογίας του πεδίου ανακυκλοφορίας (φυσαλίδα ανακυκλοφορίας, στροβιλιζόμενος δακτύλιος) με τη δυναμική του μέσου και τυρβώδους ροϊκού πεδίου αλλά και στην επίδραση της μεταβολής του αριθμού Rossby στην ίδια τη φυσαλίδα ανακυκλοφορίας. Από την ανάλυση επίδρασης της φυσαλίδας ανακυκλοφορίας στο ροϊκό πεδίο προκύπτει η τοπολογία των ζωνών υψηλής μίξης μεταξύ των δύο ροών αλλά και αυτών που χαρακτηρίζονται από υψηλές τιμές τυρβώδους κινητικής ενέργειας όπως και από υψηλά επίπεδα ανακυκλοφορίας. Η μελέτη των χαρακτηριστικών του τρισδιάστατου διατμητικού στρώματος καταδεικνύει την ισχυρή αλληλεπίδραση του διαμήκους με το αζιμουθιακό διατμητικό στρώμα και παρέχει σημαντική πληροφορία ως προς την εξέλιξη της μίξης μεταξύ της περιδινούμενης δέσμης εκροής και της εξωτερικής ομοαξονικής ροής. Για την καλλίτερη κατανόηση της διαδικασίας μίξης εισαγάγεται ο συντελεστής διάχυσης στροφορμής (λ) ο οποίος παρέχει σημαντική πληροφορία ως προς την εξέλιξη της περιδινούμενης δέσμης εκροής σε σχέση με τις συνθήκες εισαγωγής του ροϊκού πεδίου. Τέλος, επιχειρείται η αναπαράσταση του τρισδιάστατου ροϊκού πεδίου από την επαλληλία των αποτελεσμάτων στο διαμήκες και εγκάρσιο επίπεδο μετρήσεων, όπου απεικονίζεται η τοπολογία της φυσαλίδας ανακυκλοφορίας. Από τη μελέτη των αποτελεσμάτων προκύπτει ότι η κλασική προσέγγιση ταξινόμησης ροών με περιδίνηση σύμφωνα με το βαθμό στροβιλισμού δεν επαρκεί για σύνθετες ροές όπως αυτή που εξετάζεται στην παρούσα διατριβή. Για αυτό το λόγο, προτείνεται ένας νέος αδιάστατος αριθμός (αριθμός Rossby) ο οποίος σχετίζει το πεδίο πιέσεων που δημιουργείται λόγω της συμπαράσυρσης της εσωτερικής δέσμης από την εξωτερική με αυτό που οφείλεται στην περιδίνηση της εσωτερικής δέσμης εκροής. Η εισαγωγή του τροποποιημένου αριθμού Rossby βασίστηκε στην ήδη υπάρχουσα βιβλιογραφία (σύγκριση δυνάμεων αδράνειας με τις δυνάμεις επιτάχυνσης Coriolis) ως προς την επιλογή των κλιμάκων ταχύτητας, παρ’ όλα αυτά διαφοροποιείται μιας και στην ήδη υπάρχουσα θεωρία δεν υπάρχουν αναφορές σε ομοαξονικές ροές. / In this work the isothermal flow field generated by the interaction of an internal swirling jet with an external parallel flow is experimentally investigated with the use of 2D Digital Particle Image Velocimetry. Swirl is produced through tangential injection of air. Parametric change of inlet flow rates (constant tangential injection with change of annular flow and vice versa) is being considered in order to study the mean and turbulent flow field. Coaxial swirling jets are widely used in combustion systems as they enhance fuel and oxidant mixing and flame stabilization. Amongst well known features of introducing swirl in jet flows (increase of jet growth, entrainment and decay), highly swirling jets have been studied in combustion configurations as they impose radial and axial pressure gradients generating an internal toroidal recirculation zone, a phenomenon known as “vortex breakdown”. The complex structure of vortex breakdown has been a challenging issue for experimentalists over the past few decades emphasizing on its effect on aerodynamic and mixing attributes of combustion flow fields. Focusing on the study of coaxial swirling jets, rather limited data has been presented up to now, regarding the topology and turbulent attributes of the flow field created by coaxial jets with inner and/or outer swirl. Following previous work on coaxial swirling jets with inner or outer swirl and coaxial jets without swirl which lead to recirculation, a sufficient need for a deeper understanding of the physical mechanisms developing in such complex flow fields, comes up. This Thesis stands as an attempt to present the main features of such a complex flow field, which results from the interaction of a typical swirling jet undergoing “vortex breakdown” with an outer annular flow with “back step flow” characteristics. An analysis of the mean and turbulent flow statistics is presented, correlating flow field mechanisms with the three dimensional shear layer characteristics and the topology of the recirculating flow field (recirculation bubble, vortex ring). Research on vortex breakdown phenomena has led to a parallel research on the critical parameters that could determine whether vortex breakdown will occur. The definition of non-dimensional parameters (Swirl/ Rossby number etc), mainly based on the correlation of axial and azimuthal velocities or momenta, has been an issue of scientific interest that has often led to different approaches and criteria for vortex breakdown prediction. Additionally, it is seen through literature review that predicting vortex breakdown is not by itself adequate to characterize the mean and turbulent features of the recirculating flow field. In the case of coaxial jets, with or without swirl, previous studies have shown that the flow field created is strongly affected not only by the velocity or mass flow ratio of the jets but also by the absolute values of the jets’ velocities or the velocity jump between the two streams. For the case of coaxial swirling jets it is apparent that the interaction between the shear layers (mainly azimuthal and axial) is the key to understand the features of such a complex flow field. Through the similarity study conducted within this Thesis, a modified Rossby number is proposed as a parameter sufficient to describe the flow field’s trends. The modified Rossby number correlates the pressure drop due to fluid entrainment to that due to the rotation of the inner swirling jet. Presentation of the experimental results breaks down into two main sections; the first one where the effect of inlet conditions on the recirculating flow field mean and turbulent characteristics is discussed and the second one dealing with the interaction between the azimuthal and longitudinal shear layer. Through the analysis of the recirculation bubble effect on flow field attributes, emphasis is given into the characterization of intense mixing and turbulence regions. Additionally, the interaction between the azimuthal and longitudinal shear layer is studied through a similarity approach, utilizing boundary layer non-dimensional scales. Finally, mixing between the two flows is studied in terms of angular momentum diffusion by introducing a non dimensional parameter (λ). Results show a global effect of the proposed Rossby number on the flow field attributes, such as the recirculation bubble length and flow characteristics and the mixing of the two flows.

Mechanical heart valve (MHV) flows are characterized by high shear stress, regions of recirculation, and high levels of turbulent fluctuations. It is well known that these flow conditions are hostile to blood constituents, which could lead to thromboembolism. In the ongoing effort to reduce long-term complications and morbidity, it is imperative that we better understand the flow characteristics of the natural valve as well as that of the mechanical valve. In this study, we overcome many of the limitations imposed by other measurement techniques by employing a powerful, high-speed Time-Resolved Digital Particle Image Velocimetry (TRDPIV) system to map the flow field. We compare the flows downstream from a St. Jude Medical bileaflet MHV, a porcine mitral valve (MV), and a combination of both valves to simulate the technique of chordal preservation. Instantaneous velocity fields and vorticity maps are presented, which provide detailed information about the development of the flow. Time-averaged velocity, vorticity, and turbulent kinetic energy measurements are also discussed. Asynchronous leaflet behavior was observed in all cases involving the mechanical valve. Extensive vortex formation and propagation are present distal to the MHV, which leads to high levels of jet dispersion. The porcine mitral jet exhibits lateral oscillatory behavior, but it does not disperse like the MHV. In the MHV/porcine combination system, the native tissue limits vortex propagation and jet dispersion. The results presented provide insight on the hemodynamic characteristics of natural and MHVs, reveal the detrimental character of asynchronous leaflet opening, document the mechanism of vortex formation and interaction distal to the valve, and illustrate the importance of chordal preservation. These results may improve MHV replacement clinical practice and/or motivate and aid the design of MHVs that better mimic natural mitral flow patterns. / Master of Science

This project offers a multidisciplinary approach towards the acquisition, analysis and visualization of experimental data that pertain to cardiovascular applications. First and foremost, the capabilities of our Time-Resolved Digital Particle Image Velocimetry (TRDPIV) system were improved, allowing near-wall wall TRDPIV on compliant, dynamically moving boundaries. As a result, false flow-field vectors due to reflections from the boundary walls were eliminated, and allowing measurement of wall shear stress, wall shear rate, and oscillating shear index within as little as fifty microns of the boundary. Similar in-vitro measurements have not been reported to date by any other group. Second, an immersive, virtual environment (VE) was developed for the investigation and analysis of vortical, spatio-temporally developing flows with complex fluid-structure interactions. This VE was used to study flows in the cardiovascular system, particularly for flow through mechanical heart valves and inside the heart left ventricle (LV). The simulation provides three-dimensional (3-D) visualization of in-vitro heart flow mechanics, allowing global, volumetric flow analysis, and a useful environment for comparison with in-vivo MRI velocimetry data. 3-D glyphs (symbols representing informational parameters) are used to visually represent the flow parameters in the form of an ellipse attached to a cone, where the ellipse represents a second-order Reynolds stress tensor, and the cone represents the velocity magnitude and direction at a particular point in space, and the color corresponds to an out-of-plane vorticity. This new system has a major advantage over conventional 2-D systems in that it successfully doubles the number of visualized parameters, and allows for visualization of a time-dependent series of flow data in the Virginia Tech CAVETM immersive VE. The user controls his/her viewpoint, and can thus navigate through the simulation and view the flow field from any perspective in the immersive VE. Finally, an edge detection algorithm was developed to determine the inner and outer myocardial boundaries, and from this information calculate the local thickness distribution of the myocardium and a myocardial area approximation. This information is important in validating our in-vitro system, and is integral to the evaluation and diagnosis of congestive heart disease and its progression. / Master of Science

Active pneumatic flow control methods as applied to aerospace applications have shown noteworthy improvements in lift compared to traditional means. The General Aviation Circulation Control (GACC) concept currently under investigation at NASA's Langley Research Center (LaRC) is an attempt at addressing some of the fundamental obstacles related to the successful development and implementation of such techniques. The primary focus of research in the field of high lift pneumatic devices is to investigate ways of obtaining significant improvements in the lift coefficient without resorting to moving surfaces. Though it has been demonstrated that the lift coefficient can be amplified in a variety of ways, the chosen method for the current work is via enhanced circulation stemming from a trailing edge Coanda jet. A secondary objective is to reduce the amount energy expenditure used in these pneumatic techniques by implementing time-variant flow. This paper describes experimental observations of the flow behavior at the trailing edge of a modified water tunnel based supercritical airfoil model that exploits both steady and pulsed Coanda driven circulation control. A total of 10 sets of data, excluding a baseline case of no Coanda jet, were sampled with five cases each for steady and pulsed flow, the latter at a reduced frequency, f+, of 1. Two cases of equal momentum coefficient but with varying forced frequencies were isolated for further study in an attempt to accurately compare the resultant flow dynamics of each method. All measurements were taken at a zero-lift angle of attack by means of a non-invasive time accurate flow visualization technique (DPIV). Vorticity behavior was investigated using Tecplot® and a MATLAB® program was developed to quantify the Strouhal Number of time-averaged velocity fluctuations moving aft of the Coanda surface for each case. / Master of Science

A series of laboratory experiments were conducted at three fetches of 4.8, 8.8 and 12.4 m, and at six wind speeds ranging from 4.1 to 9.6 m/s at each fetch in a wind-wave-current research facility. In addition, five surfactant-influenced experiments were conducted at concentrations ranging from 0.1 to 5.0 ppm at a wind speed of 7.9 m/s and a fetch of 4.8 m. The goals were to examine the momentum transfer and to characterize the turbulent flow structure beneath wind waves, and to investigate the relationship between wind waves and the gas transfer rate at the air-water interface. Digital particle image velocimetry (DPIV) was used to measure two-dimensional instantaneous velocity fields beneath the wind waves. The friction velocities and roughness lengths of the coupled boundary layers were used to characterize the flow regime and momentum transfer. The air-side flows were found to be aerodynamically rough and the water-side flows were found to be in transition and then become hydrodynamically smooth as wind speed increased. Airflow separation from the crests of breaking waves may be responsible for making the air-side boundary layer rougher and water-side boundary layer smoother. Momentum transfer was studied by examining the partitioning of the wind stress into the viscous tangential stress and wave-induced stress. It was found that the wave steepness was the most important wind-wave property that controls the momentum transfer in the coupled boundary layers. Two distinct layers were observed in the near-surface turbulence in the presence of a surfactant and three layers in clean water. In the surfactant-influenced experiments, the energy dissipation rate decayed as zeta^(-0.3) in the upper layer and in the lower layer energy dissipation rate decayed as zeta^(-1.0) similar to a wall-layer. For clean experiments, the energy dissipation rate could be scaled using the depth, friction velocity, wave height and phase speed as proposed by Terray et al. (1996) provided that layer based friction velocities were used. In the upper layer, the near-surface turbulence was dominated by wave-induced motions and the dissipation rates decayed as zeta^(-0.2) at all fetches. Below this in the transition layer turbulence was generated by both wave-induced motions and shear currents and the dissipation rate decayed as zeta^(-2.0) at a fetch of 4.8 m. However, at fetches of 8.8 and 12.4 m, the dissipation rate decayed at two different rates; as zeta^(-2.0) in the upper region and as zeta^(-4.0) in the lower region. In the third layer, the dissipation rate decayed as zeta^(-1.0) similar to a wall-layer at a fetch of 4.8 m. Four empirical relationships commonly used to predict the gas transfer rate were evaluated using laboratory measurements. The gas transfer rate was found to correlate most closely with the total mean square wave slope and varied linearly with this parameter. The three other parameterizations using wind speed, wind friction velocity and energy dissipation did not correlate as well. / Water Resources Engineering

No description available.

Digital Particle Image Velocimetry (DPIV) is a planar measurement technique to measure the velocity within a fluid by correlating the motion of flow tracers over a sequence of images recorded with a camera-laser system. Sophisticated digital processing algorithms are required to provide a high enough accuracy for quantitative DPIV results. This study explores the potential of a variety of cross-correlation filters to improve the accuracy and robustness of the DPIV estimation. These techniques incorporate the use of the Phase Transform (PHAT) Generalized Cross Correlation (GCC) filter applied to the image cross-correlation. The use of spatial windowing is subsequently examined and shown to be ideally suited for the use of phase correlation estimators, due to their invariance to the loss of correlation effects. The Robust Phase Correlation (RPC) estimator is introduced, with the coupled use of the phase correlation and spatial windowing. The RPC estimator additionally incorporates the use of a spectral filter designed from an analytical decomposition of the DPIV Signal-to-Noise Ratio (SNR). This estimator is validated in a variety of artificial image simulations, the JPIV standard image project, and experimental images, which indicate reductions in error on the order of 50% when correlating low SNR images. Two variations of the RPC estimator are also introduced, the Gaussian Transformed Phase Correlation (GTPC): designed to optimize the subpixel interpolation, and the Spectral Phase Correlation (SPC): estimates the image shift directly from the phase content of the correlation. While these estimators are designed for DPIV, the methodology described here provides a universal framework for digital signal correlation analysis, which could be extended to a variety of other systems. / Master of Science