On electromechanical-fluid-elastic coupled modeling: application to mechanism-free ornithopters and moored submerged ducted turbines
Creators: name: Shan, Xin
Publisher: name: No Publisher Supplied
Year: 2024

description: Electromechanical-fluid-elastic systems involve fluid-structure interactions and electromechanical coupling between structural, fluid, and electrical domains. The applications of electromechanical-fluid-elastic systems are ubiquitous, e.g., the piezoelectric material-based mechanism-free ornithopters in air and moored-submerged hydrokinetic turbines in tidal/riverine flows. This dissertation develops a systematic framework for deriving physics-based models for electromechanical-fluid-elastic systems to promote the understanding of coupling and dynamics of electromechanical systems and improve the system design and control performance. The framework addresses the deficiency of accurate and computationally efficient models. The physics-based models preserve physical insights and interpretations of the systems. To demonstrate the utility of the framework, the methods in the proposed framework are employed for unique applications. Various conceptualization, modeling, system identification, solution, and analysis methods are applied to a novel mechanism-free ornithopter concept, and a novel moored-submerged hydrokinetic turbine concept. A so-called solid-state (i.e., mechanism-free) ornithopter can be achieved by induced-strain actuators such as piezoelectric material-based devices – more specifically piezo composite devices. Such ornithopters with induced-strain actuated wings eliminate the need for electromagnetic motors and conventional power transmission mechanisms, potentially saving weight and energy consumption, improving durability, and reducing overall mechanical complexity. In this work, a novel mechanism-free ornithopter model is derived considering (1) two-way fluid-structure interaction, (2) body-wing coupling, (3) wing bend-twist coupling, and (4) electromechanical coupling. The piezoelectric material-based mechanism-free ornithopter concept is idealized by a rigid ornithopter body and piezocomposite wings. The surge, heave, and pitch degrees of freedom are considered. The structural, fluid, and circuit domains are modeled by the Rayleigh-Ritz method, empirical state space Theodorsen function, and transformer representation, respectively. The piezocomposite wing assumed shapes in the Rayleigh-Ritz method are coupled to the state space fluid domain representation. The fully coupled system model is in state space form, which facilitates the application of parametric analyses in the preliminary design stage, multi-disciplinary design optimization, and the analyses of stability and controllability. To demonstrate the utility of the proposed model, parametric analyses are presented that show the effects of critical design variables, including ornithopter body inertia, piezocomposite device positions and orientations, excitation voltage frequency, etc. Hydrokinetic energy is largely untapped since the expense to install, operate, monitor, maintain, repair, and recover hydrokinetic turbines in aquatic environments leads to a high levelized cost of energy. The multi-disciplinary design methods based on coupled aerodynamics/hydrodynamics, structural dynamics, control systems, power electronics, etc., are proposed to reduce the levelized cost of energy. The so-called moored-submerged hydrokinetic turbine consists of a ducted turbine array, mooring cable, and electrical load. The ducted turbine array, mooring cable, and generators are modeled by rigid body kinetics, finite element method, and first-order permanent magnet synchronous generator model, respectively. The hydrodynamics of the ducted turbine unit is modeled by the equivalent circuit-axial-momentum theory-blade element momentum method. The lumped-parameter multi-physics model is analytical, which keeps the physical insight. The physical parameters are identified from high-fidelity simulations or experiments. Compared to high-fidelity (e.g., finite element or distributed parameter) approaches, the lumped parameter model is computationally low-cost, which allows for rapid design optimization within a very large multi-physics design domain. The model also facilitates parametric investigations for preliminary design and sensitivity analysis, control synthesis to enhance power extraction and dynamic stability, and rotor design. Case studies, parametric investigations, control for enhancing power extraction, and dynamic stability are discussed to illustrate the utility of the proposed model. The models and the unified framework presented in this dissertation provide a fundamental understanding of the design space through parametric investigations. Design optimizations, performance and control analyses are demonstrated for all critical operating points.