No description available.

No description available.

The quality of the flow over a 75 degree-sweep delta wing was documented for steady angles of attack and during dynamic maneuvers with and without the use of two control surfaces. The three-dimensional velocity field over a delta wing at a steady angle of attack of 38 degrees and Reynolds number of 72,000 was mapped out using laser-Doppler velocimetry over one side of the wing. The three-dimensional streamline and vortex line distributions were visualized. Isosurfaces of vorticity, planar distributions of helicity and all three vorticity components, and the indicator of the stability of the core were studied and compared to see which indicated breakdown first. Visualization of the streamlines and vortex lines near the core of the vortex indicate that the core has a strong inviscid character, and hence Reynolds number independence, upstream of breakdown, with viscous effects becoming more important downstream of the breakdown location. The effect of cavity flaps on the flow over a delta wing was documented for steady angles of attack in the range 28 degrees to 42 degrees by flow visualization and surface pressure measurements at a Reynolds number of 470,000 and 1,000,000, respectfully. It was found that the cavity flaps postpone the occurrence of vortex breakdown to higher angles of attack than can be realized by the basic delta wing. The effect of continuously deployed cavity flaps during a dynamic pitch-up maneuver of a delta wing on the surface pressure distribution were recorded for a reduced frequency of 0.0089 and a Reynolds number of 1,300,000. The effect of deploying a set of cavity flaps <u>during</u> a dynamic pitch-up maneuver on the surface pressure distribution was recorded for a reduced frequency of 0.0089 and a Reynolds number of 1,300,000 and 187,000. The active deployment of the cavity flaps was shown to have a short-lived beneficial effect on the surface pressure distribution. The effect on the surface pressure distribution of the varying the reduced frequency at constant Reynolds number for a plain delta wing was documented in the reduced frequency range of 0.0089 to 0.0267. The effect of the active deployment of an apex flap <u>during</u> a pitch-up maneuver on the surface pressure distribution at Reynolds numbers of 532,000, 1,000,000, and 1,390,000 were documented with reduced frequencies of 0.0053 to 0.0114 with flap deployment locations in the range of 21° to 36° . The apex flap deployment was found to have a beneficial effect on the surface pressure distribution during the maneuver and in the post-stall regime after the maneuver is completed. / Ph. D.

Swirling flows abound in nature and numerous engineering applications. Under conditions which are not completely understood, the swirling cores could undergo a sudden enlargement of their vortex core, leading to a ’vortex breakdown’. The physics of vortex breakdown and strategies to control it have been active areas of research for nearly half a century. There are many competing theories of vortex breakdown in the literature; broadly, these are surmised on similarities to flow separation, hydrodynamic instability or transition from a supercritical to a subcritical state. However, a rational criterion for vortex breakdown continues to be elusive. One of the most well known criteria in the literature is the one due to Brown and Lopez (1990) based on an inviscid vortex dynamics model which suggests that the helix angle of the velocity vector should enclose the helix angle of the vorticity vector. However it appears that this only suggests that the stream surface would diverge and not necessarily constitute a condition for breakdown. In this work, we propose a new criterion based on helicity (scalar product of velocity and vorticity vectors) for characterizing breakdown since it has fundamental topological interpretations relating to change in linkages of vortex lines. In particular, it is suggested that the breakdown location corresponds to the location where helicity becomes zero. We study the problem of vortex breakdown in a cylindrical container with a rotating top lid in order to clarify and elucidate our hypothesis. We present results from Direct Numerical Simulation of this problem for three different Reynolds numbers and evaluate the utility of our proposed helicity criterion. Our studies indicate that helicity is indeed a better choice for characterizing vortex breakdown.

This work addresses the issue of transverse combustion instabilities in annular gas turbine combustor geometries. While modern low-emissions combustion strategies have made great strides in reducing the production of toxic emissions in aircraft engines and power generation gas turbines, combustion instability remains one of the foremost technical challenges in the development of next generation combustor technology. To that end, this work investigates the response of a swirling flow and swirl-stabilized flame to a transverse acoustic field is using a variety of high-speed laser techniques, especially high-speed particle image velocimetry (PIV) for detailed velocity measurements of this highly unsteady flow phenomenon. A description of the velocity-coupled transverse instability mechanism is explained with companion measurements describing each of the velocity disturbance pathways. Dependence on acoustic frequency, amplitude, and field symmetry is discussed. Significant emphasis is placed on the response of a swirling flow field to a transverse acoustic field. Details of the dynamics of the vortex breakdown bubble and the shear layers are explained using a wide variety of measurements for both non-reacting and reacting flow cases. This thesis concludes with an overview of the impact of this work and suggestions for future research in this area.

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.

Large-scale unsteady flow structures play an influential role in the dynamics of many practical flows, such as those found in gas turbine combustion chambers. This thesis is concerned primarily with large-scale unsteady structures that arise due to self-sustained hydrodynamic oscillations, also known as global hydrodynamic instability. Direct numerical simulation (DNS) of the Navier--Stokes equations in the low Mach number limit is used to obtain a steady base flow, and the most unstable direct and adjoint global modes. These are combined, using a structural sensitivity framework, to identify the region of the flow and the feedback mechanisms that are responsible for causing the global instability. Using a Lagrangian framework, the direct and adjoint global modes are also used to identify the regions of the flow where steady and unsteady control, such as a drag force or heat input, can suppress or promote the global instability. These tools are used to study a variety of reacting and non-reacting flows to build an understanding of the physical mechanisms that are responsible for global hydrodynamic instability in swirling diffusion flames. In a non-swirling lifted jet diffusion flame, two modes of global instability are found. The first mode is a high-frequency mode caused by the instability of the low-density jet shear layer in the premixing zone. The second mode is a low-frequency mode caused by an instability of the outer shear layer of the flame. Two types of swirling diffusion flames with vortex breakdown bubbles are considered. They show qualitatively similar behaviour to the lifted jet diffusion flames. The first type of flame is unstable to a low-frequency mode, with wavemaker located at the flame base. The second type of flame is unstable to a high-frequency mode, with wavemaker located at the upstream edge of the vortex breakdown bubble. Feedback from density perturbations is found to have a strong influence on the unstable modes in the reacting flows. The wavemaker of the high-frequency mode in the reacting flows is very similar to its non-reacting counterpart. The low-frequency mode, however, is only observed in the reacting flows. The presence of reaction increases the influence of changes in the base flow mixture fraction profiles on the eigenmode. This increased influence acts through the heat release term. These results emphasize the possibility that non-reacting simulations and experiments may not always capture the important instability mechanisms of reacting flows, and highlight the importance of including heat release terms in stability analyses of reacting flows.

No description available.

The aerodynamics of wings with moderately swept wings continues to be a challenging and important problem due to the current and future use in military aircraft. And yet, there is very little work devoted to the understanding of the aerodynamics of such wings. The problem is that such wings may be able to sustain attached flow next to broken-down delta-wing vortices, or stall like two-dimensional wings, while shedding vortices with generators parallel to their leading edge. To address this situation we studied the flow field over diamond-shaped planforms and sharp-edged finite wings. Possible mechanisms for flow control were identified and tested. We explored the aerodynamics of swept leading edges with no control. We presented velocity and vorticity distributions along planes normal and parallel to the free stream for wings with diamond shaped planform and sharp leading edges. We also presented pressure distributions over the suction side of the wing. Results indicated that in the inboard part of the wing, an attached vortex can be sustained, reminiscent of delta-wing type of a tip vortex, but further in the outboard region 2-D stall dominated even at 13° AOA and total stall at 21° AOA. To explore the unsteady flow field and the effectiveness of leading-edge control of the flow over a diamond-planform wing at 13° AOA, we employed Particle Image Velocimetry (PIV) at a Reynolds number of 43,000 in a water tunnel. Our results indicated that two-D-like vortices were periodically generated and shed. At the same time, an underline feature of the flow, a leading edge vortex was periodically activated, penetrating the separated flow, eventually emerging downstream of the trailing edge of the wing. To study the motion and its control at higher Reynolds numbers, namely 1.3 x 106 we conducted experiments in a wind tunnel. Three control mechanisms were employed, an oscillating mini-flap, a pulsed jet and spanwise continuous blowing. A finite wing with parallel leading and trailing edges and a rectangular tip was swept by 0°, 20°, and 40° and the pulsed jet employed as is control mechanism. A wing with a diamond-shaped-planform, with a leading edge sweep of 42°, was tested with the mini-flap. Surface pressure distributions were obtained and the control flow results were contrasted with the no-control cases. Our results indicated flow control was very effective at 20° sweep, but less so at 40° or 42°. It was found that steady spanwise blowing is much more effective at the higher sweep angle. / Ph. D.

This thesis describes an investigation on the blowout phenomenon in gas turbine combustors. The combustor primarily used for this study was a swirl- and dump-stabilized, atmospheric pressure device, which did not exhibit dynamic combustion instabilities. The first part of the thesis work concentrated on finding a sensing methodology to be able to predict the onset of approach of combustor blowout using optical methods. Temporary extinction-reignition events that occurred prior to blowout were found to be precursor events to blowout. A threshold based method was developed to identify these events in the time-resolved sensor output. The number and the average length of each event were found to increase as the LBO limit (fuel-air ratio) is approached. This behavior is used to predict the proximity to lean blowout. In the second part of this study, the blowout sensor was incorporated into a control system that monitored the approach of blowout and then actuated an alternate mechanism to stabilize the combustor near blowout. Enhanced stabilization was achieved by redirecting a part of the main fuel to a central preinjection pilot injection. The sensing methodology, without modification, was effective for the combustor with pilot stabilization. An event based control algorithm for controlling the combustor from blowing out was also developed in this study. The control system was proven to stabilize the combustor even when the combustor loading was rapidly changed. The final part of this study focused on understanding the physical mechanisms behind the precursor events. High speed movies of flame chemiluminescence and laser sheet scattering from oil droplets seeded into the reactants were analyzed to explain the physical processes that cause the extinction and the reignition of the combustor during a precursor event. A physical model for coupling of the fluid dynamics of vortex breakdown and combustion during precursor and blowout events is proposed. This model of blowout phenomenon, along with the sensing and control strategies developed in this study could enable the gas turbine combustor designers to design combustors with wider operability regimes. This could have significant payoffs in terms of reduction in NOx emissions from the combustor.