Development of virtual and augmented reality methodologies for the building and the interactive interrogation of digital twins

The increasing complexity of modern engineering systems and the digital transformation promoted by Industry 4.0 and Industry 5.0 have accelerated the development of Digital Twins as key technologies for design, monitoring, simulation, and decision support. At the same time, advances in Extended Reality technologies have created new opportunities for intuitive humanmachine interaction through immersive visualization and natural user interfaces. This thesis investigates the convergence of these technological domains by proposing an integrated methodological framework for the construction and interactive interrogation of immersive digital twins. The research combines numerical simulation, real-time interaction, and immersive visualization to extend the role of digital twins beyond static digital representations, turning them into interactive engineering tools that support analysis, design, training, and maintenance. The proposed framework covers the entire development process, from digital twin modelling and real-time object tracking to interaction techniques, immersive geometric modelling, virtual assembly, industrial validation, and real-time physical simulation. A key contribution of this work is the integration of multibody dynamics, reduced order modelling, and finite element analysis within extended reality environments. This combination enables physically consistent real-time simulations while keeping the computational cost suitable for interactive applications. The research also introduces an immersive modelling approach based on subdivision surfaces, allowing users to create and modify geometries directly in augmented reality using hand gestures. Finally, the work presents a reproducible methodology for evaluating virtual reality hardware in industrial scenarios through virtual assembly case studies. The proposed approach provides objective metrics for assessing usability, performance, and ergonomics, and demonstrates its applicability through an industrial virtual assembly example. The proposed methodologies are validated through a series of engineering case studies, including interactive multibody simulations, reduced-order structural models, immersive prosthesis training, and augmented reality visualization of structural quantities on physical automotive components. These applications demonstrate the versatility of the proposed framework across different engineering domains and confirm its ability to combine numerical simulation, real-time interaction, and immersive visualization within a unied environment. Overall, this research presents different frameworks for immersive digital twins, showing how extended reality can become an effective interface for interacting with high-fidelity engineering models rather than serving solely as a visualization tool. By integrating simulation, spatial computing, and natural user interaction, the proposed approach broadens the role of digital twins in engineering practice and provides a basis for future developments involving articial intelligence, multiphysics simulation, and increasingly interactive cyber-physical systems.