Numerical simulation framework of bird-inspired ornithopter in forward flight

Abstract

This study presents an efficient numerical simulation framework for analyzing the flight dynamics of bird-inspired ornithopters in forward flight. The framework integrates a modified Unsteady Vortex Lattice Method (UVLM) with a Multi-Flexible-Body Dynamics (MFBD) model to simulate the Fluid-Structure Interaction (FSI) that occurs during the flapping flight. The UVLM is enhanced with a Pseudo-Leading-Edge Vortex (PLEV) model and an Adaptive Wake-Shedding (AWS) scheme to address limitations related to the leading edge vortex and spanwise wake-shedding. Additionally, the structural model of the ornithopter's flexible main wing is modeled using a modal-based reduced-order model generated through component mode synthesis. The framework is validated through wind tunnel tests on rigid and flexible wing models, demonstrating errors of <10 % in predicting mean lift and thrust forces. The ORNithopter Integrated Simulation Program (ORNISP), developed as a MATLAB App, is utilized to perform a flight dynamic simulation under free-flight conditions. The trim conditions for a forward flight of an ornithopter prototype named KRoFalcon (KAIST Robotic Falcon) are estimated. The simulation results show errors within 7 % for flight speed and angle of attack compared to flight test data. Additionally, the simulation results under free-flight and restricted degrees of freedom conditions are compared, and it shows that the flight simulation with restricted heaving and pitching can overestimate the aerodynamic forces. The proposed FSI simulation framework shows more efficient computational time than the FSI simulation using computational fluid dynamics and structural dynamics solvers, ensuring sufficient fidelity in aerodynamic force estimation.