Intakes are inlet structures where fluid is accelerated to a certain flow velocity to provide required amount of water into a hydraulic system. Intake size and geometry affects the formation of flow patterns, which can be influential for hydraulic performance of the whole system. An experimental study is conducted by measuring velocity field in the hydraulic model of the head pond of a hydropower plant to investigate vortex formation. Vortex strength based on potential flow theory is calculated from the measured velocity field. It was shown that vortex strength increases with the submergence Froude number. The free surface flow in the head pond is simulated using Flow-3D software. Vortex strength calculations are repeated using the computational velocity distributions and compared to experimentally obtained values. Similar computations were carried on with some idealized pond geometries such as rectangular and circular.

Η παρούσα διδακτορική διατριβή με τίτλο «Αλληλεπίδραση Ομόρροπα Περιστρεφόμενων Στροβίλων Απορρέματος» αφορά την πειραματική μελέτη του ροϊκού πεδίου της αλληλεπίδρασης δύο ομόρροπα περιστρεφόμενων στροβίλων που δημιουργούνται από επιφάνειες άντωσης. Η μελέτη αυτού του φαινομένου έχει καταστεί ιδιαίτερα σημαντική κυρίως τις τελευταίες δεκαετίες, με αφορμή ατυχήματα αεροσκαφών τα οποία αποδόθηκαν στη δράση του στροβίλου απορρέματος. Η διερεύνηση του μηχανισμού αλληλεπίδρασης και συγχώνευσης του συστήματος δύο στροβίλων έχει αποτελέσει αντικείμενο ενδιαφέροντος για πολλούς ερευνητές σε όλο τον κόσμο. Παρόλα αυτά, ακόμη και σήμερα η γνώση και κατανόηση τέτοιων προβλημάτων δεν ανταποκρίνεται στις σύγχρονες απαιτήσεις. Η ερευνητική δουλεία που έχει γίνει έχει σαν στόχο να συμβάλει στην εξέλιξη της επιστημονικής γνώσης γύρω από το πολύ σημαντικό πρόβλημα του στροβίλου απορρέματος. Η ερευνητική εργασία είναι πειραματική και περιλαμβάνει μετρήσεις του ροϊκού πεδίου πίσω από διαφορική πτέρυγα, τύπου NACA 0030, τοποθετημένη σε ροή αεροσήραγγας ανοικτού κυκλώματος. Σκοπός της μελέτης αυτής είναι η διερεύνηση της δομής και δυναμικής εξέλιξης του τρισδιάστατου πεδίου ταχύτητας-στροβιλότητας του παραγόμενου ζεύγους ομόρροπα περιστρεφόμενων στροβίλων κατά τη διάρκεια της αλληλεπίδρασης τους και της φυσικής διαδικασίας συγχώνευσης που ακολουθεί ως τον τελικό σχηματισμό ενός γραμμικού στροβίλου. Η τεχνική μέτρησης που χρησιμοποιήθηκε είναι η "Ανεμομετρία Θερμού Σύρματος" υπό σταθερή θερμοκρασία (constant temperature hot wire anemometry) με χρήση αισθητήρων δύο (X-probe), τεσσάρων και δώδεκα συρμάτων (multi sensors). Η παρούσα πειραματική μελέτη, στην οποία για πρώτη φορά μετρούνται ταυτόχρονα τα μέσα και στατιστικά χαρακτηριστικά των τρισδιάστατων πεδίων ταχύτητας-στροβιλότητας σε ροή αυτού του τύπου, επιχειρεί να καλύψει ορισμένα από τα επιστημονικά κενά που υπάρχουν. Το ροϊκό πεδίο αξίζει την ιδιαίτερη προσοχή, δεδομένου ότι σημαντικοί ανταγωνιστικοί μηχανισμοί εμφανίζονται να επιβάλλουν το δομικό σχηματισμό του πεδίου του ζεύγους στροβίλων και της δυναμικής διαμήκους εξέλιξής του, συμπεριλαμβανομένης και της "διάρκειας ζωής" τους. Το συγκεκριμένο σχήμα έχει σημαντικό ενδιαφέρον επίσης από άποψη εφαρμογών, με δεδομένη την επιδίωξη της μείωσης της επικινδυνότητας του στροβίλου απορρέματος, καθώς οι ομόρροπα περιστρεφόμενοι στρόβιλοι σχηματίζονται, μερικές φορές πολύ κοντά και σε άλλες θέσεις, εκτός από το άκρο πτέρυγας, όπως για παράδειγμα από τα flaps και την άτρακτο και γενικά από όλες τις επιφάνειες άντωσης του αεροσκάφους. / Experimental evidence is reported regarding the structure of the three-dimensional mean and fluctuating velocity and vorticity fields of a turbulent corotating vortex pair. The presented results constitute part of ongoing research on vortex wakes aiming at contributing to the understanding of trailing vortex interaction dynamics and turbulence structure. The flow field under study is the result of interaction of the pair of co-rotating tip vortices formed by a split wing configuration, consisting of two half wings of equal length l = 24.5cm arranged at equal and opposite angles of attack,  =  8 degrees. The airfoil profile of the wings is that of a NACA0030 with cord length c = 10cm. The wing arrangement is placed at the entrance of the test section of a low turbulence subsonic wind tunnel, of dimensions 30cm  50cm  300cm. In the near wake region, simultaneous measurements of the three-dimensional vector fields of velocity and vorticity in the corotating vortex pair were conducted using an in-house designed and constructed 12-hotwire sensors vorticity probe. The probe consists of three closely separated orthogonal 4–wire velocity sensor arrays, measuring simultaneously the three–dimensional velocity vector at three closely spaced locations on a cross plane of the flow field. This configuration makes possible the estimation of spatial velocity derivatives by means of a forward difference scheme of first order accuracy. The probe was calibrated in-situ in the core region of a round jet rotatable about the pitch and yaw directions. Based on preliminary visualization experiments the cross plane at x/c=0.3 (near wake) has been selected as representative of the vortex pair formation. The evolution of the vortex pair interaction (far wake region) was recorded by a 4-hotwire sensor, capable of measuring simultaneously the three-dimensional velocity vector of the flow filed. After shedding the two vortices are swept along the streamwise direction. The cores initially move away from each other. The rotational velocity field around each core induces a rotational velocity to the other vortex and thus both vortex cores are spiraling around each other, developing a braid of two vortices and deforming the external flow field in the downstream direction. Gradually the interaction flow field links both vortices together until the final merging and the formation of a new stable linear vortex. In the near wake location, the flow field is dictated by the pressure distribution established by the flow around the wings, mobilizing large masses of air and leading to the roll up of fluid sheets. Fluid streams penetrating between the wings collide, creating on the cross plane flow a stagnation point and an ΄΄impermeable΄΄ line joining the two vortex centres. Along this line fluid is directed towards the two vortices, expanding their cores and increasing their separation distance. This feeding process generates a dipole of opposite sign streamwise mean vorticity within each vortex. The rotational flow within the vortices obligates an adverse streamwise pressure gradient leading to a significant streamwise velocity deficit characterizing the vortices. As vortices start to interact, the two cores lose their symmetry and obtain an elliptical formation. The corotating vortex pair is observed to merge at about 0.6 orbit periods and at a downstream distance of 7c from wing tips. Visualization experiments show that the instantaneous flow field of the vortices preserves at all times the structural characteristics of the mean flow field. The vortices are continuously formed close to the wing tips and the fluctuating flow field is the result of changes in the intensity of the formation (e.g. larger or smaller vortex core size) or changes in the position of the cores (wandering) which both should be attributed to secondary attenuating flow instabilities. In this sense the relation of the turbulent field to the mean field is significantly different from cases with no identifiable coherent flow structures (e.g. grid turbulence) or flow fields in which the successive presence of distinct structures result in an idealized but never present mean flow structure.

This thesis details an in investigation of two problems arising during the testing of a jet engine in a test cell, namely the formation and ingestion of vortices and the generation and propagation of infrasound. Investigation involved the use of computational fluid dynamic as well as analytical tools. The author extended the work of previous researchers by investigating the effect when a suction inlet in surrounded by four walls, (as it is in a test cell). A previously suspected but not documented small region of unsteady vortex was discovered to lie between the steady vortex and no vortex regions. The preferential attachment of the vortex, when formed, to a particular surface was investigated and a low velocity region near that surface has been proven as a possible cause. A cell bypass ratio > 90% was found to be necessary to avoid the formation of vortices in typical situations. Parametric studies (conducted cetaris paribus) on four different geometries and flow parameters were also conducted to determine how they affected the vortex formation threshold. Boundary layer thickness on the vortex attachment surface, upstream vorticity, size of suction inlet was found to have a direct relationship with probability of vortex formation whereas Reynolds number of flow was found to have an inverse relationship. Three hypotheses regarding the generation and propagation of infrasound in test cells were analysed. The first hypothesis states that the fluctuating of flow within the test cell led to a periodic fluctuation of pressure. The second hypothesis predicts a change in flow conditions can leads to a change in the acoustic reflection characteristics of the blast basket perforates. The final hypothesis proposes that changing engine location and size of augmenter, can lead to a reduction in the slip velocity between the engine exhaust jet and the cell bypass flow thus reducing the engine jet noise. The first hypothesis has been disproved using CFD techniques, although the results are as yet inconclusive. The second and third hypotheses have been proven to be potentially feasible techniques to be employed in the future. The changes proposed in the final hypothesis are shown to reduce the engine jet noise by up to 5 dB.

The results of an experimental study of phenomena which occur in the flow of a viscous fluid in closed domains with discrete symmetries are presented. The purpose is to investigate the role which ideas from low-dimensional dynamical systems have to play in describing qualitative changes that take place with variation of the governing parameters. Such a descriptive framework already exists for the case of the Taylor-Couette system, where the domain possesses a continuous azimuthal symmetry group. The present investigation is aimed at establishing the typicality of previously reported behaviour under progressive reductions of azimuthal symmetry. In the first investigations, the fixed outer circular cylinder of the standard system is replaced with one of square cross-section. Thus there is now discrete Ζ₄ symmetry in the azimuthal direction. Knowledge of the two-dimensional flow field is used to establish the nature of the steady three-dimensional motion equivalent to Taylor vortex flow. It is shown that similar bifurcation sequences exist in both standard and square systems for the case of very small aspect ratio where a single Taylor cell is formed. This flow develops as the result of a bifurcation which breaks the Ζ₂ symmetry that is imposed on the annulus by two solid stationary ends. The study is then extended to consider time-dependent effects in the square system. Two different oscillatory single-cell flows are identified, and it is shown that each is the result of a Hopf bifurcation. Selection of a particular dynamic mode is found to depend on the aspect ratio of the system. A low-dimensional bifurcation structure is uncovered which connects the two modes in parameter space, and involves a novel type of steady single-cell flow. Finally, observations are reported of a nontrivial type of dynamical behaviour which bears strong resemblance to motion found in a circularly symmetric Taylor-Couette system that is related to the Šilnikov mechanism for finite-dimensional chaos. A second variant on the Taylor-Couette system is considered where the outer cylinder is shaped like a stadium. The effect is to reduce further the overall symmetry of the domain to a Ζ₂ × Ζ₂ group. The two-dimensional flow field is investigated using both numerical and experimental techniques. Time-dependent phenomena are then investigated in the three-dimensional flow over a relatively wide range of aspect ratio. It is found that a sequence of a Hopf bifurcation followed by period-doubling bifurcations exists up to a certain aspect ratio, beyond which there is an apparently sudden and reversible transition between regular and irregular dynamical behaviour. Although this transition is not of a low-dimensional nature, the experimental results suggest that it exists as the result of a coalescence of the bifurcations which are found at lower values of aspect ratio.

Over the last forty or so years interest in the study of wing-tip vortices has increased, primarily due to the introduction of larger passenger aircraft and their subsequent interaction with smaller aircraft. The vortices generated by these larger aircraft present a problem in two main areas; the wake hazard problem, where other aircraft can be subjected to the large tangential velocities of the vortex, and the interaction with ground based features of vortices created during landing and take-off. The first of these is particularly dangerous close to the ground when aircraft are in a high lift configuration at take-off and landing. As the vortices effectively scale with aircraft wing span, significant encounters between large vortices and smaller aircraft have been documented over the years. An example of one such documented wake vortex interaction incident can be found in Ogawa. In this study, the system of vortices are described as classical Batchelor vortices (or linear superpositions thereof) which are then subjected to small perturbations. By discretising the domain and solving for the eigenvalues of the system it is possible to ascertain the stability characteristics of the flow as a function of the system parameters which include the axial wave-number, the spacing of the vortices, their cross-flow decay rate and their axial strength. We first consider the inviscid instability of multiple tip vortices, an approximation which is valid in the limit of large Reynolds numbers. In this limit the stability of the flow is dominated by the axial component of the basic vortex flow. The governing equations of continuity and momentum are reduced to a second order partial differential equation (PDE). This equation is solved numerically to determine which vortex configurations produce the greatest instability growth rate. These results are extended to consider the effect of compressibility on the inviscid instability. Finally we consider the effects of viscosity on the stability of the full Batchelor similarity solution which results in a second order PDE in four dependent variables. The stability equations are solved both globally (for the entire eigenspectra) and locally (for a single eigenvalue in a pre-determined region) using codes that run in both serial and parallel form. The numerical methods are based on pseudospectral discretisation (Chebyshev polynomials for Cartesian and radial directions and Fourier for azimuthal) in the global scheme, the eigenvalues being recovered either with a QZ algorithm or a shift-and-invert Arnoldi algorithm. For the local scheme, fourth order centred finite-diffences are used in conjunction with an iterative eigenvalue recovery method. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1383207 / Thesis (Ph.D.) - University of Adelaide, School of Mathematical Sciences, 2010

Over the last forty or so years interest in the study of wing-tip vortices has increased, primarily due to the introduction of larger passenger aircraft and their subsequent interaction with smaller aircraft. The vortices generated by these larger aircraft present a problem in two main areas; the wake hazard problem, where other aircraft can be subjected to the large tangential velocities of the vortex, and the interaction with ground based features of vortices created during landing and take-off. The first of these is particularly dangerous close to the ground when aircraft are in a high lift configuration at take-off and landing. As the vortices effectively scale with aircraft wing span, significant encounters between large vortices and smaller aircraft have been documented over the years. An example of one such documented wake vortex interaction incident can be found in Ogawa. In this study, the system of vortices are described as classical Batchelor vortices (or linear superpositions thereof) which are then subjected to small perturbations. By discretising the domain and solving for the eigenvalues of the system it is possible to ascertain the stability characteristics of the flow as a function of the system parameters which include the axial wave-number, the spacing of the vortices, their cross-flow decay rate and their axial strength. We first consider the inviscid instability of multiple tip vortices, an approximation which is valid in the limit of large Reynolds numbers. In this limit the stability of the flow is dominated by the axial component of the basic vortex flow. The governing equations of continuity and momentum are reduced to a second order partial differential equation (PDE). This equation is solved numerically to determine which vortex configurations produce the greatest instability growth rate. These results are extended to consider the effect of compressibility on the inviscid instability. Finally we consider the effects of viscosity on the stability of the full Batchelor similarity solution which results in a second order PDE in four dependent variables. The stability equations are solved both globally (for the entire eigenspectra) and locally (for a single eigenvalue in a pre-determined region) using codes that run in both serial and parallel form. The numerical methods are based on pseudospectral discretisation (Chebyshev polynomials for Cartesian and radial directions and Fourier for azimuthal) in the global scheme, the eigenvalues being recovered either with a QZ algorithm or a shift-and-invert Arnoldi algorithm. For the local scheme, fourth order centred finite-diffences are used in conjunction with an iterative eigenvalue recovery method. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1383207 / Thesis (Ph.D.) - University of Adelaide, School of Mathematical Sciences, 2010

Thesis (Ph.D.); Submitted to the Department of Physics and Astronomy, Rice University, Houston, TX (US); 1 Sep 2004. / Published through the Information Bridge: DOE Scientific and Technical Information. "LA-14149-T" J. Koller. US DOE (US) 09/01/2004. Report is also available in paper and microfiche from NTIS.

Orientador: Rafael Zadorosny / Resumo: Nesse trabalho estudou-se a dinâmica e os fatores de formação de vórtices cinemáticos em supercondutores mesoscópicos sem gap (gapless). Os resultados foram comparados com aqueles de um sistema com gap (gap-like). Para tal, as simulações computacionais foram direcionadas para a solução das equações generalizadas de Ginzburg-Landau dependente do tempo (GTDGL). Primeiramente simulamos amostras homogêneas onde verificou-se que, no sistema gap-like, as correntes se concentram no centro da amostra, com isso, há formação de vórtices cinemáticos. Para criar artificialmente o acúmulo de correntes no centro da amostra, inserimos uma constrição e assim, obtivemos a formação de vórtices cinemáticos em supercondutores gapless. A dinâmica é sempre com um par se formando nas bordas da amostra e se aniquilando no centro. Nota-se que, além da formação de VAv (início do estado resistivo) ocorrer em valores distintos da densidade de corrente aplicada para as diferentes amostras, a corrente crítica apresenta uma pequena diferença entre as amostras gapless e gap-like. Vale ressaltar que parâmetros como o tamanho dos contatos elétricos e a constrição afetam a corrente crítica da amostra, bem como a velocidade média do vórtice cinemático. / Abstract: In this work we study the dynamics and the formation of kinematic vortices in gapless mesoscopic superconductors. The results were compared to those ones of a gap-like system. Then computational simulations were carried out to solve the Generalized Time-Dependent Ginzburg-Landau equations (GTDGL). Firtly, we simulated homogeneous gap-like and gapless samples where it was found that, in the rst one, the currents concentrate in the center of the sample and then, there was the formation of kinematic vortices. To arti cially promote the crowding of the currents in the center of the sample, we worked with samples a constriction. In this way, kinematic vortices in gapless superconductors were formed. The dynamic is always with a pair forming at the edges of the sample and annihilating in the center. It is noted that, besides the formation of a vortex (antivortex) (begin of resistive state) occurs at distinct values of the applied current density for diferent samples, the electric critical current presents a tiny di erence between gapless and gap-like samples. It is worth mentioning that parameters such as the size of electrical contacts and constriction a ect the critical current of the sample as well as the average velocity of the kinematic vortex. / Mestre

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Resumo: O interesse em conhecer o comportamento supercondutor tem sido cada vez maior nas últimas décadas. Na busca de melhores características supercondutoras, descobriu-se que amostras volumétricas apresentam características muito diferentes de amostras mesoscópicas (amostras com dimensões próximas dos comprimentos de penetração de London e coerência). Como exemplo, podemos citar a não formação de rede de Abrikosov, como consequência do efeito de confinamento (efeito associado às dimensões reduzidas da amostra) e também uma mudança considerável nos valores dos campos críticos. Neste trabalho foram resolvidas as equações de Ginzburg-Landau dependentes do tempo (TDGL), para fazer uma análise detalhada da dinâmica de vórtices em filmes finos mesoscópicos. Para revolvê-las, utilizamos o método das variáveis de ligação com invariância de calibre, adaptado para o algoritmo de diferenças finitas, utilizado para obter a densidade dos pares de Cooper e também curvas de magnetização. O estudo dessa dinâmica de vórtices, foi feito em três amostras com superfícies geométricas diferentes (côncova, convexa e rugosa). Observamos que na comparação entre as duas primeiras, há uma diferença considerável nos valores dos campos críticos, bem como no comportamento da magnetização comparado com um filme plano. Já para a amostra de superfície rugosa, observamos que existe uma competição entre o efeito de confinamento e a rugosidade em relação à configuração dos vórtices. Apresentamos também, uma tabela que mostra resumidamente os estados estacionários dos vórtices nas três amostras. / Abstract: The interest to investigate the investigate the behavior of a superconductor has grown in the last few decades. Having in mind to search for better superconducting characteristics, it has been found that bulk samples present characteristics much more different than mesoscopic samples (samples with dimensions of the same order of the same order of the London penetration length and the coherence length). As an example, we can mention the non-formation of an Abrikosov vortex lattice as a consequence of the confinement effect (effect associated with the reduced dimensions of the sample) and also considerable change in the critical field values. In the present work we solved the time dependent Ginzburg-Landau equation (TDGL), in order to make a detailed analysis of the vortex dynamics in mesoscopic thin films. To solve these equations, we have used the link variables method which is gauge invariant. From this, we obtain the Cooper pair density and the magnetization curves. The vortex dynamics was investigated for three different surfaces of the film (concave, convex, and irregular). We have observed that, with respect to the parabolic geometries, there is a considerable difference for the critical fields, as well as for the behavior of the magnetization compared to a flat film. On the other hand, for a sample with an irregular surface, we have seen that there is a competition between the confinement effect and rugosity with respect to vortex configurations. We also present a table which summarizes the vortex stationary states for the three topologies mentioned above. / Orientador: Paulo Noronha Lisboa Filho / Coorientador: Edson Sardella / Banca: Wilson Aires Ortiz / Banca: Clelio Clemente de Souza Silva / Mestre