An unsteady aerodynamics reduced-order modeling method for maneuvering, flexible flight vehicles
Hiller, Brett R.
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Accurate aerodynamic predictions remain a cornerstone of the aircraft design process due to their significance in determining the performance and stability and control (S&C) characteristics of aircraft. Traditional flight dynamics modeling has historically relied on the use of quasi-steady stability derivatives, often calculated using simplified linear aerodynamic methods. These models are inherently incapable of predicting the nonlinear, unsteady aerodynamics encountered by modern flexible aircraft. Inaccurate predictions of such phenomena can lead to suboptimal vehicle performance and/or inaccurate control law design, potentially leading to loss of control of the vehicle. Recent advances in digital computing have motivated interest towards Virtual Flight Simulation (VFS), for which multidisciplinary numerical simulations are used to evaluate an aircraft’s performance and S&C characteristics at full-scale flight Reynolds and Mach numbers. Despite their demonstrated feasibility, these simulations often require thousands of computational hours, limiting their adoption and widespread application for aircraft design and analysis. Reduced-order modeling is viewed as a key enabler for the viable application of VFS methods. Reduced-order models (ROMs) are mathematical models aimed at accurately predicting the fundamental dynamics of a system at a computational cost much less than what is required in solving the original governing equations. These methods approximate the full-order numerical simulations of a system by extracting and reconstructing pertinent dynamic data solutions, without any limiting physical model assumptions. Significant progress in reduced-order modeling has been made over the past two decades with the development of a variety of reduced-order models for efficient unsteady aerodynamic predictions. However, most unsteady aerodynamic ROMs have been primarily used for either predicting the aerodynamic response of rigid maneuvering vehicles or identifying aeroelastic instabilities, such as a flutter. Multidisciplinary ROMs for virtual flight simulations remain a desirable, yet relatively unexplored area of research. The objective of this dissertation was to develop a ROM capable of providing quantitatively accurate, yet computationally efficient predictions of the nonlinear, unsteady aerodynamics encountered by maneuvering flexible flight vehicles. Indicial response theory is a nonintrusive ROM approach, which characterizes a system’s dynamics through identification of the system response due to numerically simulated unit step changes in a system’s inputs. A CFD-based dynamic modal aeroelastic analysis was proposed for identification of indicial responses with respect to the vehicle motion parameters. Linear and nonlinear indicial response ROMs, based on the mathematical principle of convolution, were extended to predict the unsteady aerodynamic response of flexible flight vehicles subject to arbitrary vehicle maneuvers. The aeroelastic ROM is tested through comparisons to dynamic aeroelastic simulations of NASA’s X-56A Multi-Utility Technology Testbed (MUTT) aircraft performing a series of harmonic forced oscillations and a right turn flight test maneuver. Once identified, the aeroelastic indicial response ROM method is shown to accurately capture the entire frequency spectrum of the system, resolving the unsteady aerodynamic response of flexible flight vehicles at any feasible motion rate. Quantified assessments of the computational cost and accuracy of the aeroelastic ROM demonstrate rapidly increasing performance benefits relative to high fidelity aeroelastic simulations as multiple maneuver are considered. Furthermore, traditional stability-derivative models are efficiently extracted from aeroelastic indicial responses and are shown to accurately predict slowly-varying maneuvers with reduced computational costs. For more dynamic motions, the ROM is shown to accurately resolve the complex fluid-structure interactions present for maneuvering, flexible flight vehicles that are incapable of being modeled by quasisteady models.