Optimisation and control of shear flows

Transition to turbulence and flow control are studied by means of numerical simulations for different simple shear flows. Linear and non-linear optimisation methods using the Lagrange multiplier technique are employed. In the linear framework as objective function the standard disturbance kinetic energy is chosen and the constraints involve the linearised Navier–Stokes equations. We consider both the optimal initial condition leading to the largest disturbance energy growth at finite times and the optimal time-periodic forcing leading to the largest asymptotic response for the case of the flat plate boundary layer excluding the leading edge. The optimal disturbances for spanwise wavelengths of the order of the boundary layer thickness are streamwise vortices exploiting the lift-up mechanism to create streaks. For long spanwise wavelengths it is the Orr mechanism combined with the amplification of oblique wave packets that is responsible for the disturbance growth. Also linear optimal disturbances are computed around a leading edge and the effect of the geometry is considered. It is found that two-dimentional disturbances originating upstream, relative to the leading edge of the plate are inefficient at generating a viable disturbance, while three dimentional disturbances are more amplified. In the non-linear framework a new approach using ideas from non-equilibrium thermodynamics is developed. We determine the initial condition on the laminar/turbulent boundary closest to the laminar state. Starting from the general evolution criterion of non-equilibrium systems we propose a method to optimise the route to the statistically steady turbulent state, i.e. the state characterised by the largest entropy production. This is the first time information from the fully turbulent state is included in the optimisation procedure. The method is applied to plane Couette flow. We show that the optimal initial condition is localised in space for realistic flow domains, while the disturbance visits bent streaks before breakdown. Feedback control is applied to the bypass-transition scenario with high levels of free-stream turbulence. The flow is the flat-plate boundary layer. In this scenario low frequency perturbations enter the boundary layer and streamwise elongated disturbances emerge due to non-modal growth. The so-called streaky structures are growing in amplitude until they reach high enough energy levels and break down into turbulent spots via their secondary instability. When control is applied in the form of wall blowing and suction, the growth of the streaks is delayed, which implies a delay of the whole transition process. Additionally, a comparison with experimental work is performed demonstrating a remarkable agreement in the disturbance attenuation once the differences between the numerical and experimental setup are reduced. Open-loop control with wall travelling waves by means of blowing and suction is applied to a separating boundary layer. For downstream travelling waves we obtain a mitigation of the separation of the boundary layer while for upstream travelling waves a significant delay in the transition location accompanied by a modest reduction of the separated region.QC 2011051

Optimisation and control of boundary layer flows

Both optimal disturbances and optimal control are studied by means of numerical simulations for the case of the flat-plate boundary-layer flow. The optimisation method is the Lagrange multiplier technique where the objective function is the kinetic energy of the flow perturbations and the constraints involve the linearised Navier–Stokes equations. We consider both the optimal initial condition leading to the largest growth at finite times and the optimal time-periodic forcing leading to the largest asymptotic response. The optimal disturbances for spanwise wavelengths of the order of the boundary layer thickness are streamwise vortices exploiting the lift-up mechanism to create streaks. For long spanwise wavelengths it is the Orr mechanism combined with the amplification of oblique wave packets that is responsible for the disturbance growth. Control is applied to the bypass-transition scenario with high levels of free-stream turbulence. In this scenario low frequency perturbations enter the boundary layer and streamwise elongated disturbances emerge due to the non-modal growth. These so-called streaks are growing in amplitude until they reach high enough energy levels and breakdown into turbulent spots via their secondary instability. When control is applied in the form of wall blowing and suction, within the region that it is active, the growth of the streaks is delayed, which implies a delay of the whole transition process. Additionally, a comparison with experimental work is performed demonstrating a remarkable agreement in the disturbance attenuation once the differences between the numerical and experimental setup are reduced.  

Optimisation and control of shear flows

Transition to turbulence and flow control are studied by means of numerical simulations for different simple shear flows. Linear and non-linear optimisation methods using the Lagrange multiplier technique are employed. In the linear framework as objective function the standard disturbance kinetic energy is chosen and the constraints involve the linearised Navier–Stokes equations. We consider both the optimal initial condition leading to the largest disturbance energy growth at finite times and the optimal time-periodic forcing leading to the largest asymptotic response for the case of the flat plate boundary layer excluding the leading edge. The optimal disturbances for spanwise wavelengths of the order of the boundary layer thickness are streamwise vortices exploiting the lift-up mechanism to create streaks. For long spanwise wavelengths it is the Orr mechanism combined with the amplification of oblique wave packets that is responsible for the disturbance growth. Also linear optimal disturbances are computed around a leading edge and the effect of the geometry is considered. It is found that two-dimentional disturbances originating upstream, relative to the leading edge of the plate are inefficient at generating a viable disturbance, while three dimentional disturbances are more amplified. In the non-linear framework a new approach using ideas from non-equilibrium thermodynamics is developed. We determine the initial condition on the laminar/turbulent boundary closest to the laminar state. Starting from the general evolution criterion of non-equilibrium systems we propose a method to optimise the route to the statistically steady turbulent state, i.e. the state characterised by the largest entropy production. This is the first time information from the fully turbulent state is included in the optimisation procedure. The method is applied to plane Couette flow. We show that the optimal initial condition is localised in space for realistic flow domains, while the disturbance visits bent streaks before breakdown. Feedback control is applied to the bypass-transition scenario with high levels of free-stream turbulence. The flow is the flat-plate boundary layer. In this scenario low frequency perturbations enter the boundary layer and streamwise elongated disturbances emerge due to non-modal growth. The so-called streaky structures are growing in amplitude until they reach high enough energy levels and break down into turbulent spots via their secondary instability. When control is applied in the form of wall blowing and suction, the growth of the streaks is delayed, which implies a delay of the whole transition process. Additionally, a comparison with experimental work is performed demonstrating a remarkable agreement in the disturbance attenuation once the differences between the numerical and experimental setup are reduced. Open-loop control with wall travelling waves by means of blowing and suction is applied to a separating boundary layer. For downstream travelling waves we obtain a mitigation of the separation of the boundary layer while for upstream travelling waves a significant delay in the transition location accompanied by a modest reduction of the separated region.QC 2011051

Euro-Atlantic relations

Η παρούσα πτυχιακή εργασία εστιάζει στις Ευρωατλαντικές σχέσεις και εκπονήθηκε στα πλαίσια της ολοκλήρωσης των σπουδών μου στο τμήμα Πολιτικής Επιστήμης του Πανεπιστημίου Κρήτης. Οι διεθνείς σχέσεις μου κέντρισαν το ενδιαφέρον από την πρώτη φορά που το διδάχτηκα σαν μάθημα στο τμήμα και έτσι έλαβα την απόφαση να κάνω την εργασία αυτή και να διεξάγω μία δική μου έρευνα. Σκοπός μου κατά την διάρκεια της συγγραφής αποτελούσε τόσο η καλύτερη κάλυψη του θέματος που θα μπορούσα να κάνω όσο και η μεγαλύτερη απλούστευση του για την πιο γρήγορη κατανόηση της πτυχιακής εργασίας από τους αναγνώστες. Η εξωτερική πολιτική αποτελεί τον κύριο πυλώνα όπως μαρτυρεί ο τίτλος και οι Ευρωατλαντικές σχέσεις αποτελούν ένα σύνθετο και πολύχρονο φαινόμενο που έχουν απασχολήσει και προβληματίσει πολλούς επιστήμονες, για αυτόν τον λόγο παρουσιάζουν τεράστιο ενδιαφέρον τα γεγονότα και οι παράγοντες διαμόρφωσης αυτών των σχέσεων. Ευελπιστώ το περιεχόμενο της εργασίας αυτής να είναι ουσιώδες και να ανταποκρίνεται σε κάθε είδους απαίτηση των αναγνωστών.This dissertation focuses on Euro-Atlantic relations and was prepared in the context of the completion of my studies at the Department of Political Science of the University of Crete. I was interested in international relations since I was first taught it as a subject in the department and so I decided to do this work and do my own research. My goal during the writing process was both to cover the issue as best as possible I could and to simplify it to make it easier for readers to understand the dissertation. Foreign policy is the main pillar, as the title suggests, and Euro-Atlantic relations are a complex and long-standing phenomenon that has preoccupied and troubled many scientists. I hope the content of this work is substantial and meets all kinds of readers requirements

Optimal disturbances above and upstream a flat plate with an elliptic leading edge

Adjoint-based iterative methods are employed in order to compute linear optimal disturbances in a spatially growing boundary layer around an elliptic leading edge. The Lagrangian approach is used where an objective function is chosen and constraints are assigned. The optimisation problem is solved using power iterations combined with a matrix-free formulation, where the state is marched forward in time with a standard DNS solver and backward with the adjoint solver until a chosen convergence criterion is fulfilled. We consider the global and the upstream localised optimal initial condition leading to the largest possible energy amplification at time T. We found that the twodimensional initial condition with the largest potential for growth is a Tolmien-Schlichting-like wave packet that includes the Orr mechanism and is located inside the boundary layer, downstream of the leading edge. Three-dimensional disturbances induce streaks by the lift-up mechanism. Localised optimal initial condition enables us to better study the effects of the leading edge; with this approach we propose a new method to study receptivity. Two-dimensional upstream disturbances, are inefficient at triggering an unstable eigenmode. The three-dimensional disturbances instead induce elongated streamwise streaks; both the global and upstream localised disturbances give significant growth. This advocates for high receptivity to three-dimensional disturbances.QC 2011051

Minimal transition thresholds in plane Couette flow

International audienceSubcritical transition to turbulence requires finite-amplitude perturbations. Using anonlinear optimisation technique in a periodic computational domain, we identify theperturbations of plane Couette flow transitioning with least initial kinetic energy forRe ≤ 3000.We suggest a new scaling law Ec = O(Re−2.7) for the energy threshold vs.the Reynolds number, in quantitative agreement with experimental estimates for pipeflow. The route to turbulence associated with such spatially localised perturbationsis analysed in detail for Re = 1500. Several known mechanisms are found to occurone after the other: Orr mechanism, oblique wave interaction, lift-up, streak bending,streak breakdown, and spanwise spreading. The phenomenon of streak breakdown isanalysed in terms of leading finite-time Lyapunov exponents of the associated edgetrajectory

Ten years of co-management in Greek protected areas: an evaluation

The present study attempts an evaluation of the first co-management framework that has been adopted and implemented in Greece over the last 10 years for the management of its protected areas. To get insight as to how efficient it has been, we evaluate the performance and outcomes of the 28 management authorities of the protected areas of the country that substantiate it and the conditions under which they have been operating. The study involved a large part of the Greek conservation community. It was conducted via a questionnaire dealing with issues of financing and administration, environmental management and guarding, and connection with the local community. For co-management, it is essential that the actors involved undertake their share of responsibility, which should be clearly defined and delimited; this was not usually the case. Decentralization of the power of management requires capacity building locally and active involvement of the local community; these have been addressed and achieved only to a limited degree. Most importantly, the support and commitment to conservation of state actors were often missing. Funding discontinuities, delays in responding to needs associated with biodiversity monitoring and protection and inefficient guarding were the major problems resulting from the inadequacies detected. Despite its weaknesses, the co-management framework contributed considerably to the conservation of environmental values of Greece. We propose improvements and measures that can substantially increase its overall effectiveness towards nature conservation. However, under the severe crisis that the country has been facing, its future is currently unknown