Modélisation numérique d’actionneurs plasma pour le contrôle d’écoulement

Abstract

As aerodynamic flow control still remains one of the top subjects of research in the aerospace scientific world, new ways to perform such a control are being constantly studied. Microwave plasma discharges have been proposed as a mean of a non-intrusive flow control method based on the creation of hot spots of air(via the creation of plasma discharges) which can eventually interact with the external flow and modify it is attributes in a beneficial way to the aerodynamic coefficients of the body of interest. Moreover, other types of plasma actuators, based on the momentum addition in the flow instead of heat, have been proven capable of positively modifying the flow aerodynamic features. Nevertheless, the development and optimization of such actuators, require further understanding of the basic multi-scale physics involved. Experiments can provide such information, but they turn to be costly and incapable of capturing the complex interactions in small space and time scales. In this thesis, we are interested in the numerical modeling of plasma flow control actuators, in order to understand deeper their nature and applicability. Firstly, we present the theory behind aerodynamic flow control and plasma physics along with a literature review of the plasma actuators’ physics, applicability and effects. Then, numerical simulations performed using adapted techniques for each different actuator, reveal interesting features of their physics and complex nature, on the way to a deeper understanding and possible optimization of their performance in aerodynamic flow control applications. Three types of plasma actuators are considered: Microwave Plasma Discharges (MPD), the Dielectric Barrier Discharge (DBD)and the Plasma Synthetic Jet (PSJ). Concerning microwave discharges, the objectives are more fundamental than the other types of actuators. Numerical simulations can provide the basic information on the physics and complex interaction between the EM waves the plasma and the gas, in order to understand the plasma creation, evolution and energy balance. Flow control applications of such discharges, as an energy deposition method, have been already quite well documented in the literature. In order to reduce the computational cost, a novel implicit coupling of the Maxwell equations with the momentum transfer equation for electrons has been developed which with the addition of a simplified model of plasma-fluid equations have enabled three-dimensional simulations in time domain. The microwave breakdown and evolution of the plasma due to the electromagnetic waves has been studied numerically, in order to quantify the energy absorbed by the plasma and transferred to neutral molecules as gas heating. Coupling of the EM-plasma model with an Euler based solver accounting for real gas effects, have revealed interesting features of the complex interaction between the plasma itself and the pressure shock waves formed due to the intense gas heating during the plasma breakdown and evolution. Concerning the two other groups of actuators, the goals of this thesis is more applied as we are interested mostly on modeling their operation and consequent momentum production. For the PSJ actuator, the numerical solver consists of three coupled numerical models: One describing the plasma formation between both electrodes in an axisymmetrical configuration, one describing the electrical supply by an external generator, and a last one that focuses on the air’s heating in the cylindrical cavity where the plasma is formed, and the actual operation of the actuator. It uses the energy distribution computed by the first sub-model as source term and calculates the jet’s flow mass rate, momentum and energy, exiting the cavity at high speeds. An external CFD solver is used to integrate the calculated momentum source term into a global model of a flat plate flow, taking into account turbulent effects. Lastly, the DBD actuator model, developed in DTIM/ONERA, Toulouse, consists of solving the momentum equations for a set of simplified species and the Poisson’s equation. The calculated body force produced by the DBD actuator is being compared with experimental results and the induced wall-jet flow due to this force is modeled in a CFD solver. A similar model developed in LAPLACE/UPS has been used to perform parametric studies on the influence of turbulence - studying different turbulent models in a RANS approach - as well as more applied simulations for flow control purposes, including laminar-turbulent transition over a flat plate and lift coefficient modification on an airfoil due to a leading edge DBD actuator. These studies offer new perspectives in the understanding and the optimization of plasma actuators for flow control purposes.

Καθώς ο αεροδυναμικός έλεγχος ροής εξακολουθεί να παραμένει ένα από τα κορυφαία θέματα έρευνας στην αεροδιαστημική επιστήμη, νέοι τρόποι για την εκτέλεση ενός τέτοιου ελέγχου μελετώνται συνεχώς. Σε αυτή τη διατριβή, μας ενδιαφέρει η αριθμητική μοντελοποίηση ενεργοποιητών πλάσματος για τον έλεγχο της αεροδυναμκής ροής, προκειμένου να κατανοήσουμε βαθύτερα τη φύση και τη δυνατότητα εφαρμογής τους. Αρχικά, παρουσιάζουμε τη θεωρία πίσω από τον έλεγχο αεροδυναμικής ροής και της φυσική πλάσματος μαζί με βιβλιογραφική ανασκόπηση. Στη συνέχεια, πραγματοποιήθηκαν αριθμητικές προσομοιώσεις χρησιμοποιώντας προσαρμοσμένα μοντέλα και τεχνικές για κάθε διαφορετικό ενεργοποιητή, οι οποίες αποκαλύπτουν ενδιαφέροντα χαρακτηριστικά της φυσικής και της πολύπλοκης φύσης τους, στο δρόμο προς μια βαθύτερη κατανόηση καιπιθανή βελτιστοποίηση της απόδοσής τους. Τρεις τύποι πλάσματος μελετήθηκαν: Εκκένωση πλάσματος μέσω μικροκυμάτων , εκκένωση πλάσματος διηλεκτρικού φραγμού (DBD)και συνθετικοί πίδακες πλάσματος. Όσον αφορά τις εκκενώσεις μικροκυμάτων, οι στόχοι είναι πιο θεμελιώδεις από τους άλλους τύπους ενεργοποιητών. Αυτές οι μελέτες προσφέρουν νέες προοπτικές στην κατανόηση και τη βελτιστοποίηση ενεργοποιητών πλάσματος για σκοπούς ελέγχου ροής.