Active control of turbulent convective heat transfer with plasma actuators

We study an array of streamwise-oriented Dielectric Barrier Discharge (DBD) plasma actuators as an active control technique in turbulent flows. The analysis aims at elucidating the mechanism of interaction between the structures induced by the DBD-plasma actuators and the convective heat transfer process in a fully developed turbulent boundary layer. The employed flush-mounted DBD-plasma actuator array generates pairs of counter-rotating, stationary, streamwise vortices. The full three-dimensional, velocity field is measured with stereoscopic PIV and convective heat transfer at the wall is assessed by infrared thermography. The plasma actuator forcing diverts the main flow, yielding a low-momentum region that grows in the streamwise direction. The suction effect promoted on top of the exposed electrodes confines the vortices in the spanwise direction. Eventually, the pair of streamwise vortices locally reduces the convective heat transfer with a persistence of several outer lengthscales downstream of the actuation.Rodrigo Castellanos, Stefano Discetti and Andrea Ianiro have been supported by the project ARTURO, ref. PID2019-109717RB-I00/AEI/10.13039/501100011033, funded by the Spanish State Research Agency. Theodoros Michelis and Marios Kotsonis are supported by the European Research Council under StG project GloWing (#803082)

Power Distribution Solutions for Large Hadron Collider Upgrades

In view of the Large Hadrons Collider experiments upgrade, where the inner front-end electronics will require higher supply power, a new power distribution scheme has to be designed. This thesis presents a new and more efficient scheme based on step-down DC-DC converters, which will be placed close to front-end electronics where it will be exposed to intense radiation (250 Mrad) and magnetic field (4Tesla). Other than tolerance to these fields, additional constrains are small volume and mass and low noise emission not to affect the sensitive front-end electronics. Since no commercial component can meet these requirements, an Application Specific Integrated Circuit (ASIC) DC/DC converter has been developed and is presented in this work. The chosen converter topology is the buck because it presents the best compromise in terms of efficiency and low mass among the different compared topologies. Radiation hardness can be achieved with appropriate choice of the semiconductor technology combined with layout modifications. Magnetic field tolerance is attained with the use of an air core inductor whose shape can be toroidal for limiting the magnetic field emissions. All the ASIC building blocks are presented in this thesis likewise their design methodology: their schemes and the transistors sizing are discussed as well as the AC analysis to grant the stability of the output voltage over the selected range of frequencies. Three different prototypes had been integrated in the two chosen semiconductor technologies (mainstream and backup). Tests showing high efficiency (up to 85%) and the required level of radiation hardness are presented and discussed

Linear and Nonlinear Dynamics of a Turbine Blade in Presence of an Underplatform Damper with Friction

In this thesis, a numerical simulation of the linear behavior of a turbine blade subjected to a rotation with large displacements has been carried out by the use of a specialized code. A finite element formulation of the problem has been considered in order to to develop a realist model and examine the vibration of the blade undergoing a rigid body rotation. In or-der to achieve this goal, the dynamics of the multibody rotor blade system has been studied by the use of the Floating Frame of Reference Formulation [1] applied to the case of a de-formable body, the blade, connected to a rigid body, which represents a rotor rotating at a constant angular velocity. The second part of the thesis focuses on the non-linear dynamics of a system consisting of two blades with a underplatform damper between them. Turbine blades frequently reach the resonance conditions due to the high speed of rotation in working environment with the consequence of an increased amplitude of vibration which can cause fatigue failure of the structure. The presence of an underplatform damper can help to dissipate a part of the en-ergy at resonance through frictional contact between the blades platform and the damper, increasing the fatigue life of the blades

On knots and dynamics in games

We extend Kohlberg and Mertens' (1986) structure theorem concerning the Nash equilibrium correspondence to show that its graph is not only homeomorphic to the underlying space of games but that it is also unknotted. This is then shown to have some basic consequences for dynamics whose rest points are Nash equilibria.

On the indices of zeros of nash fields.

Given a game and a dynamics on the space of strategies it is possible to associate to any component of Nash equilibria, an integer, this is the index, see Ritzberger (1994). This number gives useful information on the equilibrium set and in particular on its stability properties under the given dynamics. We prove that indices of components always coincide with their local degrees for the projection map from the Nash equilibrium correspondence to the underlying space of games, so that essentially all dynamics have the same indices. This implies that in many cases the asymptotic properties of equilibria do not depend on the choice of dynamics, a question often debated in recent litterature. In particular many equilibria are asymptotically unstable for any dynamics. Thus the result establishes a further link between the theory of learning and evolutionary dynamics, the theory of equilibrium refinements and the geometry of Nash equilibria.The proof holds for very general situations that include not only any number of players and strategies but also general equilibrium settings and games with a continuum of pure strategies such as Shapley-Shubik type games, this case will be studied in a forthcoming paper.

Regulated Resonant Switched-Capacitor Point-of-Load Converter Architecture and Modeling

This article presents a novel control system for a regulated resonant switched-capacitor point-of-load converter, together with its steady-state and small-signal analyses. The output voltage is regulated through a combined frequency/phase shift on-chip controller that operates, thanks to a zero-crossing detector for the tank current. By using such a control technique, voltage regulation can be achieved for a wide range of conversion ratios. Different operation modes are adopted to maximize the efficiency at every load condition. A prototype based on an ASIC developed in a 130-nm CMOS technology has been designed, which achieves the 2.5 to 0.8-1.35 V conversion and supplies up to 3 A. The external resonant tank is composed of a 12 nH air-core inductor and a 4.7 μF capacitor in a 0805 package, achieving high power density. The converter efficiency is demonstrated to be similar to that of an integrated buck converter designed for the same application, whereas the inductor size is reduced by a factor of eight

A Reliability and Efficiency Optimization System for Hard-Switching DC/DC Converters

The Large Hadron Collider experiments at CERN will use power distribution schemes relying on integrated buck DCIDC converters. Due to the radiation-hardness requirements, the devices used for the development of such converters will have a voltage rating which is close to the converters' input voltage. The voltage spikes generated during the hard-switching operation can affect the reliability of such low-voltage MOSFETs. A fixed and sufficiently small gate driver current for the high-side switch could be used to guarantee the reliable operation even in the worst-case conditions in terms of input voltage, output current, temperature and process variations. Nevertheless, this would result in a suboptimal efficiency in all the other working conditions. This work presents an integrated system than monitors in real-time the voltage stress, and adjusts the gate driver current to achieve maximum efficiency in all conditions, while ensuring compliance with the reliability specifications. A buck converter including the voltage peak detector and an adjustable gate driver current has been designed in a 130 nm technology, demonstrating the functionality of the voltage stress monitoring system