Instabilities of Joined-Wing Aircraft using Accelerated Aeroelastic Simulations

Abstract

Several unmanned aircraft systems (UASs) have been and are being designed with long, thin flexible wings, such as the joined-wing SensorCraft, to enhance the operational capabilities. However, due to the long slender wings, these systems are susceptible to aeroelastic instabilities, such as flutter. Thus, there is a need for addressing nonlinear aeroelasticity and handling instabilities and post-instability behavior. Nonlinear aeroelastic models can be quite computationally expensive. In this dissertation, a nonlinear aeroelastic computational model is developed for the joined-wing SensorCraft and simulations are carried out in a co-simulation framework.

The aeroelastic model is composed of an unsteady vortex lattice method (UVLM) based aerodynamic model and a finite element based structural model for the joined-wing SensorCraft. Through computational cost profiling of the aeroelastic model, it is determined that the aerodynamic processes are the most computationally expensive. This means that the focus of the attempts to accelerate aeroelastic computations should be on the aerodynamic computations. Specifically, computations of the field point velocities are found to increase the computational workload as the wake grows over time.

In this dissertation, the fast multipole method (FMM) algorithm has been integrated with the UVLM based aerodynamic model to reduce the computational workload of evaluating the wake velocities. Furthermore, an aeroelastic computational model for the joined-wing SensorCraft has been developed by using the accelerated aerodynamic model and a structural model. Flutter boundaries for various structural health conditions have been determined with respect to parameters such as freestream speed, freestream direction, and freestream density.

In terms of contributions, this is the first effort in which the speedup capabilities of FMM accelerated vortex methods have been carried out and used in nonlinear, unsteady UVLM based schemes. Also, computational studies on nonlinear aeroelastic behavior of joined-wing aircraft have been carried out to examine dynamic instabilities and the effects of structural degradation on these instabilities. Although the joined-wing SensorCraft has been used as an illustrative application, it is believed that the present work can be relevant to many other UASs. In addition, the aeroelastic computations can be useful for integration for data-driven dynamic application systems meant for UAS decision making applications.