Flutter analysis of open rotors

LAUREA MAGISTRALEQuesto lavoro di tesi è volto a dimostrare l’importanza dell’analisi aeroelastica, e.g. predizione del flutter, nell’analisi preliminare di Contra Rotating Open Rotor (CROR). A tal fine, il sistema strutturale è modellato con i programmi ad elementi finiti NASTRAN® e ANSYS®. Viene condoto un confronto delle frequenze proprie e delle forme modali usando sia modelli ad elementi solidi (tetraedri) sia ad elementi di piastra. Il sistema aerodinamico è modellato con il solutore a potenziale a volumi finiti ST . Il programma risolve correnti transoniche non stazionarie grazie ad un’approssimazione indipendente per i campi della densità e del potenziale di velocità. Questo è anche in grado di risolvere correnti non isentropiche grazie ad una nuova forma della condizione di Kutta [53–55]. È stata sviluppata una tecnica ad hoc per simulare le correnti attorno a due corpi portanti in moto relativo (cioè lo stadio anteriore e posteriore). Le analisi di flutter sono effetuate con il metodo K-E(fficiente) (già usato nel toolbox aeroelastico di NASTRAN®) e con un metodo non lineare di inseguimento degli autovalori in modo da tracciare i diagrammi V - g e V -w. La validazione della tecnica proposta per l’analisi aeroelastica di Single Rotating Propfan è ampiamente supportata dal confronto dei risultati ottenuti con i dati sperimentali e numerici presenti in letteratura e con quelli ottenuti dal solutore euleriano AeroX [44, 58]. Non è stato possibile validare le tecniche per il CROR poichè non sono presenti dati in letteratura e non sono reperibili altri strumenti di analisi. Parole chiave: aeroelasticità, contra-rotating open rotor, regime transonico, flutter, aerodinamica a potenziale.The thesis is aimed to show the importance of aeroelastic analysis, e.g. flutter prediction, in the preliminary design of Contra Rotating Open Rotor (CROR). To this end, the structural sub-system is modelled with the Finite-Element (FE) solvers NASTRAN® and ANSYS®. A comparison of natural frequencies and mode shapes is performed using both tetrahedral and shell element models. The aerodynamic subsystem is modelled with the Finite-Volume (FV) full potential solver ST . The solver model unsteady transonic flows by means of an independent approximation of the density and velocity potential fields. It can also model non-isentropic flows thanks to a new form of Kutta condition [53–55]. An ad hoc technique is developed for simulating the flow field around two lifting bodies with relative motion, i.e. the front and rear propeller stages. The flutter analyses are carried out using the so-called K-E(fficient) method (also used in NASTRAN® aeroelastic toolbox) and a root-tracking non-linear method for computing the V - g and V -w diagrams. The effectiveness of the proposed aeroelastic analysis for Single Rotating Propfan (SRP) is successfully assessed by tackling a set of realistic dynamic problems and by comparing the results with reference experimental and numerical data available in literature and with the results obtained by the Euler flow solver AeroX [44, 58]. The results of the CROR cannot be validated because there are neither data available in the literature nor other alternative procedures. Key words: aeroelasticity, contra-rotating open rotor, transonic flow, flutter, full potential

Prediction of Helicopter Rotor Loads and Fatigue Damage Evaluation with Neural Networks

In recent years, machine learning algorithms have experienced rapid advancement, driven by the exponential growth of data availability and computational capabilities. Among these algorithms, artificial neural networks stand out as one of the most renowned and effective classes, possessing the ability to discern relationships within data. In this study, we harness neural networks to deduce the relationship between flight mechanics parameters and resulting loads in an articulated rotor configuration. The accuracy of these algorithms hinges closely on the quality of the dataset used for training. Given that rotor loads manifest as time-periodic signals with precise harmonic content, we train dedicated neural networks to predict each harmonic individually. Subsequently, the load time history is reconstructed post hoc by amalgamating predictions from each individual network. Various network architectures are explored, and a sensitivity analysis is conducted on hyper-parameters to determine the optimal configuration for this specific application. Moreover, these predictions serve as input for a fatigue damage calculation algorithm

Assessment and Optimization of Dynamic Stall Semi-empirical Model for Pitching Aerofoils

Dynamic stall is a phenomenon affecting aerofoils in unsteady flows which is particularly relevant in the rotary-wing field. Semi-empirical models are simplified tools to simulate this phenomenon, especially during preliminary design phases and for aeroelastic assessments. However, they need a large number of tuning parameters to provide reliable estimations of unsteady airloads. To face this problem, a parameter identification procedure based on sequential resolutions of optimization problems using a genetic algorithm is developed and it is applied to the state-space formulation of a modified version of the so-called "Second Generation” Leishman-Beddoes model. The effects of the optimal parameters on the model prediction capabilities are discussed and the variability of the parameters with reduced frequency is studied. The estimations of the unsteady airloads obtained by applying the optimization of parameters show a great improvement in the correlation of the experimental data if compared to the predictions obtained by using the parameters provided in the literature, especially for pitching moments where the negative peaks are very well described. These improvements justify the need for optimization to set the parameters

Flutter analysis of propfan-open rotors

This work aims at showing the importance of flutter predictions in the preliminary design of propfan–open rotors. Both single rotating propellers and contrarotating open rotors are investigated. The related structural subsystems are modeled through finite element analyses; the aerodynamic subsystems exploit a finite volume, full potential formulation suitable for unsteady transonic flows. An ad hoc technique is developed for simulating the flowfield around a contrarotating open rotor configuration. The effectiveness of the proposed aeroelastic analysis is successfully assessed for single propellers through comparisons with reference numerical and experimental data available in the literature, as well as against Euler flow-based solutions. The results for contrarotating open rotors cannot be validated because of the lack of corresponding open-literature data