The Transmission Control Protocol (TCP) is the dominant transport protocol in today?s Internet. The original design of TCP left congestion control open to future designers. Short of implementing changes to the TCP stack on the end-nodes themselves, Internet Service Providers have employed several techniques to be able to operate their network equipment efficiently. These techniques amount to shaping traffic to reduce cost and improve overall customer satisfaction. <br /><br /> The method that gives maximum control when performing traffic shaping is using an inline traffic shaper. An inline traffic shaper sits in the middle of any flow, allowing packets to pass through it and, with policy-limited freedom, inspects and modifies all packets as it pleases. However, a number of practical issues such as hardware reliability or ISP policy, may prevent such a solution from being employed. For example, an ISP that does not fully trust the quality of the traffic shaper would not want such a product to be placed in-line with its equipment, as it places a significant threat to its business. What is required in such cases is third-party rate control. <br /><br /> Formally defined, a third-party rate controller is one that can see all traffic and inject new traffic into the network, but cannot remove or modify existing network packets. Given these restrictions, we present and study a technique to control TCP flows, namely triple-ACK duplication. The triple-ACK algorithm allows significant capabilities to a third-party traffic shaper. We provide an analytical justification for why this technique works under ideal conditions and demonstrate via simulation the bandwidth reduction achieved. When judiciously applied, the triple-ACK duplication technique produces minimal badput, while producing significant reductions in bandwidth consumption under ideal conditions. Based on a brief study, we show that our algorithm is able to selectively throttle one flow while allowing another to gain in bandwidth.

In this thesis, we consider optimal control problems arising in high-speed integrated communication networks with Quality of Service (QOS) constraints. Integrated networks are expected to carry a large variety of traffic sources with widely varying traffic characteristics and performance requirements. Broadly, the traffic sources fall into two categories: (a) real-time sources with specified performance criteria, like small end to end delay and loss probability (sources of this type are referred to as Type 1 sources below), and (b) sources that do not have stringent performance criteria and do not demand performance guarantees from the network - the so-called Best Effort Type sources (these are referred to as Type 2 sources below). From the network's point of view, Type 2 sources are much more "controllable" than Type 1 sources, in the sense that the Type 2 sources can be dynamically slowed down, stopped or speeded up depending on traffic congestion in the network, while for Type 1 sources, the only control action available in case of congestion is packet dropping. Carrying sources of both types in the same network concurrently while meeting the performance objectives of Type 1 sources is a challenge and raises the question of equitable sharing of resources. The objective is to carry as much Type 2 traffic as possible without sacrificing the performance requirements of Type 1 traffic. We consider simple models that capture this situation. Consider a network node through which two connections pass, one each of Types 1 and 2. One would like to maximize the throughput of the Type 2 connection while ensuring that the Type 1 connection's performance objectives are met. This can be set up as a constrained optimization problem that, however, is very hard to solve. We introduce a parameter b that represents the "cost" of buffer occupancy by Type 2 traffic. Since buffer space is limited and shared, a queued Type 2 packet means that a buffer position is not available for storing a Type 1 packet; to discourage the Type 2 connection from hogging the buffer, the cost parameter b is introduced, while a reward for each Type 2 packet coming into the buffer encourages the Type 2 connection to transmit at a high rate. Using standard on-off models for the Type 1 sources, we show how values can be assigned to the parameter b; the value depends on the characteristics of the Type 1 connection passing through the node, i.e., whether it is a Variable Bit Rate (VBR) video connection or a Continuous Bit Rate (CBR) connection etc. Our approach gives concrete networking significance to the parameter b, which has long been considered as an abstract parameter in reward-penalty formulations of flow control problems (for example, [Stidham '85]). Having seen how to assign values to b, we focus on the Type 2 connection next. Since Type 2 connections do not have strict performance requirements, it is possible to defer transmitting a Type 2 packet, if the conditions downstream so warrant. This leads to the question: what is the "best" transmission policy for Type 2 packets? Decisions to transmit or not must be based on congestion conditions downstream; however, the network state that is available at any instant gives information that is old, since feedback latency is an inherent feature of high speed networks. Thus the problem is to identify the best transmission policy under delayed feedback information. We study this problem in the framework of Markov Decision Theory. With appropriate assumptions on the arrivals, service times and scheduling discipline at a network node, we formulate our problem as a Partially Observable Controlled Markov Chain (PO-CMC). We then give an equivalent formulation of the problem in terms of a Completely Observable Controlled Markov Chain (CO-CMC) that is easier to deal with., Using Dynamic Programming and Value Iteration, we identify structural properties of an optimal transmission policy when the delay in obtaining feedback information is one time slot. For both discounted and average cost criteria, we show that the optimal policy has a two-threshold structure, with the threshold on the observed queue length depending, on whether a Type 2 packet was transmitted in the last slot or not. For an observation delay k > 2, the Value Iteration technique does not yield results. We use the structure of the problem to provide computable upper and lower bounds to the optimal value function. A study of these bounds yields information about the structure of the optimal policy for this problem. We show that for appropriate values of the parameters of the problem, depending on the number of transmissions in the last k steps, there is an "upper cut off" number which is a value such that if the observed queue length is greater than or equal to this number, the optimal action is to not transmit. Since the number of transmissions in the last k steps is between 0 and A: both inclusive, we have a stack of (k+1) upper cut off values. We conjecture that these (k + l) values axe thresholds and the optimal policy for this problem has a (k + l)-threshold structure. So far it has been assumed that the parameters of the problem are known at the transmission control point. In reality, this is usually not known and changes over time. Thus, one needs an adaptive transmission policy that keeps track of and adjusts to changing network conditions. We show that the information structure in our problem admits a simple adaptive policy that performs reasonably well in a quasi-static traffic environment. Up to this point, the models we have studied correspond to a single hop in a virtual connection. We consider the multiple hop problem next. A basic matter of interest here is whether one should have end to end or hop by hop controls. We develop a sample path approach to answer this question. It turns out that depending on the relative values of the b parameter in the transmitting node and its downstream neighbour, sometimes end to end controls are preferable while at other times hop by hop controls are preferable. Finally, we consider a routing problem in a high speed network where feedback information is delayed, as usual. As before, we formulate the problem in the framework of Markov Decision Theory and apply Value Iteration to deduce structural properties of an optimal control policy. We show that for both discounted and average cost criteria, the optimal policy for an observation delay of one slot is Join the Shortest Expected Queue (JSEQ) - a natural and intuitively satisfactory extension of the well-known Join the Shortest Queue (JSQ) policy that is optimal when there is no feedback delay (see, for example, [Weber 78]). However, for an observation delay of more than one slot, we show that the JSEQ policy is not optimal. Determining the structure of the optimal policy for a delay k>2 appears to be very difficult using the Value Iteration approach; we explore some likely policies by simulation.

Die vorwiegend experimentelle Arbeit befasst sich mit der systematischen Untersuchung von Parametervariationen bei der aktiven Strömungskontrolle mit elektromagnetischen Kräften. An einer angestellten Platte und einem NACA0015-Profil wurde die saugseitige abgelöste Strömung durch das Einbringen einer periodischen wandparallelen Lorentzkraft an der Vorderkante beeinflusst und experimentell mittels zeitaufgelöster Particle Image Velocimetry (PIV) untersucht. Dabei wurde für verschiedene Anstellwinkel und Reynoldszahlen die Frequenz der Anregung, deren Impulseintrag und der zeitliche Kraftverlauf variiert. Strömungsmechanische Untersuchungen experimenteller und numerischer Natur wurden für eine elektrochemische Zelle und den Fall der Elektrolyse an Millieelektroden unter dem Einfluss externer Magnetfelder durchgeführt. Die Übereinstimmung der gemessenen und berechneten Geschwindigkeitsfelder war dabei sehr gut. Entgegen der Annahme, dass im Falle homogener Magnetfelder keine Strömungen induziert werden, konnte nachgewiesen werden, dass durch die lokale Krümmung der elektrischen Feldlinien in Elektrodennähe starke Lorentzkräfte generiert werden. Dies führt zu sehr komplexen Primär-und Sekundärströmungen. Die gleichen Effekte bewirken ebenfalls in der Nähe von Millieelektroden starke Lorentzkräfte in homogenen magnetischen Feldern. Die experimentellen Beobachtungen an Millieelektroden von Leventis et. al (2005), welche zum Beweis der Konzentrationsgradientenkraft herangezogen wurden, konnten alle auf das Wirken lokaler Lorentzkräfte zurückgeführt werden. Der experimentelle Nachweis der Konzentrationsgradientenkraft steht damit weiterhin aus. Zur Messung der Konzentrationen in elektrochemischen Systemen wurde erstmals das Hintergrundschlierenverfahren angewendet. Dieses Verfahren erlaubt die Bestimmung der räumlichen Konzentrationsgradienten mit erheblich weniger messtechnischen Aufwand gegenüber spektroskopischen Methoden und der Schlierentechnik.

<p>In this thesis, controllers for power oscillation damping, transient stability improvement and power flow control by means of a Controlled Series Compensator (CSC) and and a Dynamic Power Flow Controller (DPFC) are proposed. These devices belong to the group of power system components referred to as Flexible AC Transmission System (FACTS) devices. The developed controllers use only quantities measured locally at the FACTS device as inputs, thereby avoiding the risk of interrupted communications associated with the use of remote signals for control.</p><p>For power systems with one dominating, poorly damped inter-area power oscillation mode, it is shown that a simple generic system model can be used as a basis for damping- and power flow control design. The model for control of CSC includes two synchronous machine models representing the two grid areas participating in the oscillation and three reactance variables, representing the interconnecting transmission lines and the FACTS device. The model for control of DPFC is of the same type but it also includes the phase shift of the internal phase-shifting transformer of the DPFC.</p><p>The key parameters of the generic grid models are adaptively set during the controller operation by estimation from the step responses in the FACTS line power to the changes in the line series reactance inserted by the FACTS device. The power oscillation damping controller is based on a time-discrete, non-linear approach which aims to damp the power oscillations and set the desired power flow on the FACTS line by means of two step changes in the line reactance separated in time by half an oscillation cycle.</p><p>A verification of the proposed controllers was done by means of digital simulations using power system models of different complexities. The CSC and DPFC controllers were shown to significantly improve the small-signal- and transient stability in one four-machine system of a type commonly used to study inter-area oscillations. The CSC controller was also tested for 18 different contingencies in a 23-machine system, resulting in an improvement in both the system transient stability and the damping of the critical oscillation mode. </p>

No description available.

Lab-on-a-chip (LOC) technology has blossomed into a major new technology fundamentally influencing the sciences of life and nature. From a systemic point of view however, microfluidics is still in its infancy. Here, we present the concept of a microfluidic central processing unit (CPU) which shows remarkable similarities to early electronic Von Neumann microprocessors. It combines both control and execution units and, moreover, the complete power supply on a single chip and introduces the decision-making ability regarding chemical information into fluidic integrated circuits (ICs). As a consequence of this system concept, the ICs process chemical information completely in a self-controlled manner and energetically self-sustaining. The ICs are fabricated by layer-by-layer deposition of several overlapping layers based on different intrinsically active polymers. As examples we present two microchips carrying out long-term monitoring of critical parameters by around-the-clock sampling. / Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

This thesis studies different aspects of Flexible AC Transmission System (FACTS) devices which are used to improve the power transfer capability and increase the controllability in electric power systems. In the thesis, different aspects on the usage and control of Dynamic Power Flow Controllers (DPFC) and related FACTS devices are studied. The DPFC is a combination of a Phase Shifting Transformer (PST) and a Thyristor Switched Series Capacitor (TSSC)/Thyristor Switched Series Reactor (TSSR). The thesis proposes and studies a new method, the Ideal Phase-Shifter (IPS) method, for selection and rating of Power Flow Controllers (PFC) in a power grid. The IPS method, which is based on steady-state calculations, is proposed as a first step in the design process for a PFC. The method uses the Power controller plane, introduced by Brochu et al in 1999. The IPS method extends the usage of decoupling methods in the Power controller plane to a power system of arbitrary size. The IPS method was in the thesis used to compare the ratings of different PFC:s required to improve the power transfer capability in two test systems. The studied devices were here the PST, the TSSC/TSSR and the DPFC. The thesis treats control of ideal Controlled Series Capacitors (CSC), TCSC, TSSC/TSSR, and DPFC. The goals of the FACTS controllers which are developed are Power Oscillation Damping (POD), fast power flow control, and transient stability improvement in the power system. New adaptive control strategies for POD and power flow control are proposed and studied in different models of power systems by time-domain simulations. A strategy for transient stability improvement is also proposed and studied. Additionally, different methods for study of Subsynchronous Resonance (SSR), which is associated with series compensation in power systems, are investigated. Here, four of the most common methods for frequency scanning to determine the electrical damping of subsynchronous oscillations in a power grid are studied. The study reveals significant differences of the electrical damping estimates of the studied standard methods when applied to a four-machine test system. / QC 20110819

Ce travail présente les résultats de recherches sur le contrôle d'écoulement dans les liquides diélectriques. L'objectif est d'étudier les potentialités du contrôle d'écoulement au moyen d'actionneurs ElectroHydroDynamiques. La 1re partie de cette thèse est notamment consacrée à l'étude bibliographique générale du contrôle d'écoulement et des techniques disponibles pour la mesure de vitesse de fluide. La méthode PIV est choisie pour caractériser l'écoulement de panache chargé. Cependant la présence d'un champ électrique intense dans un liquide diélectrique remet potentiellement en question l'hypothèse selon laquelle les traceurs suivent fidèlement les mouvements du liquide. Des études théorique et expérimentale permettent de préciser les conditions d'un traceur idéal et de choisir le meilleur type d'ensemencement pour l'huile de silicone. La 2nde partie de ce travail est consacrée à l'étude du contrôle d'écoulement sur un profil d'aile NACA0015 à ultra-bas Reynolds (Re < 5000). Une étude bibliographique présente les stratégies de contrôle d'écoulement autour de profil d'aile ainsi que les types d'actionneurs EHD appliqués aux liquides diélectriques. L'écoulement naturel en champ de vitesse moyen puis instationnaire est caractérisé et comparé à l'écoulement contrôlé. Le calcul de la force à partir d'un bilan de quantité de mouvement (Navier-Stokes), permet d'estimer les efforts hydrodynamiques appliqués par le fluide sur le profil immergé. Des polaires de portance et de traînée sont obtenues et permettent de quantifier l'efficacité de l'actionneur EHD. Enfin, les mécanismes de contrôle sont précisés et mettent en lumière les potentiels et les limites de l'actionneur. / This work presents results of research on flow control in a dielectric liquid. The aim is to demonstrate our ability to control flow by means of ElectroHydroDynamic actuation. The first part of this PhD thesis is dedicated to a general overview of flow control and the methods available for measuring fluid velocity. The PIV method is selected to charaterize the flow of a charged plume. However, the presence of a high electric field in the dielectric liquid might bring into question the validity of using PIV, which is based on the fact that tracers accurately follow fluid movement. Theoretical and experimental studies were performed to find the proper conditions for using an ideal tracer that guarantees the accuracy of velocity measurements. This part enables us to choose the best seeding particle in silicone oil. The second part of this work is devoted to the study of flow control on a NACA0015 wing profile at ultra-low Reynolds numbers (Re < 5000). A bibliographic study presents strategies of flow control around wing profiles and in addition deals with different EHD actuators for dielectric liquids. Mean velocity fields and unsteady velocity fields of baseline flow are characterized and compared to controlled flow. The calculation of force based on the conservation of momentum (Navier-Stokes equations) enables us to estimate the hydrodynamic stresses applied by the fluid to the immersed profile. Lift and drag polarities are obtained to quantify the efficiency of the EHD actuator. Finally, the mechanisms of control are clarified and highlight the potential and limits of the EHD actuator for flow control applications.

Le contrôle actif d’écoulement est actuellement étudié dans le but d’améliorer les performances aérodynamiques des véhicules aériens ou terrestres. La diminution de la traînée permettrait de réduire la consommation de carburants fossiles et donc l’émission des gaz à effet de serre des véhicules. Les actionneurs fluidiques sont utilisés comme dispositifs de contrôle depuis une quinzaine d’année. Le contrôle par jet synthétique semble être le plus adapté à une application sur un véhicule de série dans la mesure où l’actionneur ne doit pas être alimenté en fluide. Le travail présenté dans cette thèse combine l’expérimentation physique et la simulation numérique. Elle s’intéresse tout particulièrement au contrôle de l’écoulement autour d’un corps de Ahmed à culot droit à l’aide d’un actionneur de type jet synthétique. Les essais en soufflerie ont été essentiellement utilisés pour caractériser l’écoulement autour du corps de Ahmed et dans son sillage. L’écoulement autour de cette géométrie simplifiée de véhicule terrestre a été caractérisé par des pesées aérodynamiques, des mesures de pressions pariétales, des acquisitions des fluctuations de vitesse par anémomètrie à fil chaud et des mesures de champs de vitesse par Vélocimétrie par Images de Particules. Les grandeurs moyennes et instationnaires de l’écoulement ont ainsi pu être caractérisées. Les simulations numériques à l’aide du code de calcul elsA ont ensuite été réalisées sur une configuration similaire. Les résultats des simulations de l’écoulement non contrôlé ont été confrontés aux résultats expérimentaux. Dans le but d’agir sur la traînée, le contrôle à l’aide d’un actionneur de type jet synthétique a été réalisé sur la même géométrie. Les paramètres de contrôle tels que la quantité de mouvement, la fréquence d’actionnement et l’orientation des jets synthétiques ont été testés numériquement. Le contrôle à l’aide des paramètres testés, a entrainé une augmentation de la traînée qui est due à une réduction de la longueur de la zone de recirculation associée à une diminution de la pression pariétale au niveau du culot de la maquette. Il ressort de ce travail que le contrôle par jet synthétique à basse fréquence orienté selon le sens principal de l’écoulement semble être une voie à explorer. / Active flow control is currently studied in order to improve aerial or ground vehicle aerodynamics. Diminishing aerodynamic drag leads to a reduction of fuel consumption and so in greenhouse gas emissions of vehicles. Fluidic actuators have been used as control devices for about fifteen years. Considering the fact that the actuator does not need external fluid supply system, synthetic jet control seems to be the most suitable solution that can fit on production vehicles. This thesis combines experimental tests and numerical simulations. It tackles with the flow control around a square back Ahmed body with synthetic jet actuator. Wind tunnel tests have essentially been used to characterize the flow around and in the wake of the Ahmed body. Flow around this simplified geometry of ground vehicle has been characterized using hot wire anemometry, flush mounted pressure taps and two components Particular Image Velocimetry. The steady and unsteady features of the wake flow have thus been characterized. Simulations of this flow have been performed with the computation code elsA. Results of the simulations of the natural flow around the square back Ahmed body have been compared to experimental results. With a view to modifying the drag, flow control thanks to a synthetic jet actuator has been tested on a square back Ahmed body. Parameters of the flow control, such as momentum coefficient, actuation frequency and orientation of the synthetic jet have been numerically investigated. Results show a decrease of the circulation length leading to a diminution of the base pressure and hence to an increase of the drag. Flow control by using a low frequency with slots oriented along the mainstream seems to be a path to explore.

Les oscillateurs fluidiques qui peuvent générer des excitations périodiques sont des actionneurs très prometteurs pour des applications de contrôle actif des écoulements. Les oscillations sont en effet complètement auto-induites et produites en l'absence de parties mobiles ce qui rend ces actionneurs très intéressants en termes de fiabilité et de robustesse. Ce travail de thèse avait pour objectif principal d'identifier les mécanismes physiques qui contrôlent la dynamique de fonctionnement de ce type d'oscillateurs fluidiques et de proposer des lignes directrices pour la conception d'oscillateurs dont les performances soient adaptées aux applications de contrôle d'écoulements envisagées. L'analyse expérimentale de plusieurs prototypes couplée à des simulations numériques a permis de mettre en évidence que le mécanisme de basculement du jet dans ce type d'oscillateurs est contrôlé par les gradients de pression existants au niveau de deux parties critiques de ces actionneurs. A partir de cette analyse, une relation simple a été établie permettant d'estimer la fréquence des oscillations. Deux méthodes de synchronisation, permettant le contrôle du déphasage entre les actionneurs, ont été proposées et validées expérimentalement ainsi qu'à l'aide de simulations numériques. Une matrice de micro­ oscillateurs fluidiques a été conçue, fabriquée et finalement intégrée sur une rampe installée en soufflerie. L'analyse expérimentale de son efficacité pour le contrôle de l'écoulement séparé a mis en évidence un gain important par rapport aux résultats obtenus lors de travaux précédents sur des écoulements de paroi similaires à l'aide d'autres types d'actionneurs fluidiques. / Fluidic oscillators which can generate periodic excitations are very promising for active flow control applications, due to their reliability and robustness, as their internal flow oscillation is totally self-induced and self-sustained. The main objective of this work is to identify the underlying mechanisms controlling the dynamics of this kind of fluidic oscillator and to propose guiding lines for the design of oscillators. Experimental analysis of several oscillator prototypes and associated numerical simulations have permitted to explain that the jet switching in this kind of oscillator is controlled by pressure gradients in two critical parts of the device. From these analyses, a simple function has been proposed to estimate the oscillation frequency. Two synchronization methods, allowing the control of the phase lag between the actuators, have been proposed and validated experimentally and by numerical simulations. An array of micro-fluidic oscillators has then been designed and tested on a ramp separated flow, showing much higher efficiency compared to other kind of fluidic actuators tested on similar wall flows in previous studies.