Towards high-fidelity aero-servo-elasticity analysis

Abstract

The development of a high-fidelity fluid-structure interaction tool for the simulation of aircraft flight dynamics in the subsonic flow regime is presented. The tool combines a high-fidelity large-eddy simulation code with an immersed boundary method, a multibody solver and a loose coupling scheme between fluid and solid. The development of the code is motivated by the need to accurately and efficiently simulate aircraft flight dynamics at off-design conditions, such as in separated flow states. The accurate simulations are necessary for the continuous optimization of current aircraft designs and the development of novel aircraft concepts. The developments presented in this thesis focus on three fields of the fluid-structure interaction problem. The modeling of solid geometries in the fluid solver through an immersed boundary method, the coupling of the fluid and solid domain through a loose coupling scheme, and the development of a multi-body solver for the simulation of aircrafts and their components. An extensive literature review is presented on these three fields of the fluid-structure interaction problem. The literature review is conducted to select appropriate methods for the final solver. Based on this review, a ghost-cell approach is selected for the immersed boundary method of the solver. A loosely coupled serial staggered procedure is selected to couple the fluid and solid domains in the solver. The floating frame of reference approach is selected for the derivation of the multi-body solver and a Newmark time integration method is selected for the integration of the equations of motion. The mathematical formulation of the selected methods is presented. Novel approaches are derived for the immersed boundary method and multi-body solver. A hybrid-cell treatment is derived to reduce spurious numerical oscillations in flow fields with moving geometries. Further, the integration of wall-modeling approaches into the ghost-cell immersed boundary method is presented. A controller approach based on the time integration scheme of the multi-body solver is derived. The controller allows user-prescribed dynamic positions and orientations of constraints and bodies. The developed fluid-structure interaction solver has been rigorously verified and validated for multiple test cases. Simulations of an oscillating cylinder and moving airfoil are presented for the verification of the hybrid-cell treatment. Results with hybrid-cell treatment enabled and disabled demonstrate the effectiveness of the developed approach v vi Summary in the suppression of spurious oscillations in the flow field. Simulations of a single and double pendulum are presented to verify the implementation of the multi-body equations of motion and constraints. The validation of the developed solver is performed with reference numerical and experimental results. Results of the laminar flow around an in-line oscillating cylinder are in excellent agreement with available numerical and experimental reference results. Simulations of the dynamic stall problem of helicopter blade sections are presented. The results show that the solver accurately predicts the flow features of the dynamic stall problem. Discrepancies between the present code and the available numerical and experimental results are attributed to an insufficient modeling and resolution of the near-wall flow field. Last but not least, flutter of a two degree of freedom NACA0012 airfoil is simulated. The developed solver accurately predicts the presence of stable, limit cycle and flutter regions. Discrepancies were found in the response frequencies between the results of the present code and available numerical and experimental results. The discrepancies are caused by insufficient resolution of the near-wall flow field. The verification and validation simulations proof the effectiveness of the derived methods and the correct implementation. The validation results further show that the solver accurately and efficiently predicts the flow field of complex flows and fluid-structure interaction problems.