Non-linear structural phenomena influencing flutter performance
Time: Fri 2026-05-29 10.15
Location: Konferensrum Freja, Teknikringen 8
Language: Swedish
Subject area: Aerospace Engineering
Doctoral student: Anders Ellmo , Flyg- och rymdteknik, marina system och rörelsemekanik
Opponent: Technical Doctor Mats Dalenbring,
Supervisor: Professor Ulf Ringertz, Flyg- och rymdteknik, marina system och rörelsemekanik
QC260506
Abstract
This thesis explores the effect of local non-linear phenomena on a structure that can otherwise be described linearly with a high degree of accuracy, with the focus on fuel sloshing in external stores. While it is possible to make high fidelity simulations of these phenomena, they are complex enough that it is not possible in a production environment where thousands of analyses are made.
Experimental modal analysis of multiple sets of composite wings show manufacturing induced asymmetries, with a significant frequency shift observed in the first bending mode between the left and right wings of one set. These variations caused nominal symmetric and anti-symmetric mode shapes to shift into single wing dominated modes. Tests using a wing-fuel tank system demonstrated that liquid-filled configurations exhibit distinct dynamic behaviors compared to rigid-mass equivalents. At a 50% fill level, a store sway structural mode present in the dry configuration was found to dissipate in the wet configuration. Additionally, the liquid-filled stores were subject to frequency shifts in torsional modes and an increase in overall structural damping. Excitation using robotic motion was evaluated using the same sloshing tank attached to a six-degrees-of-freedom industrial robot, with a focus on achieving chaotic fluid motion. It is shown to be a valid alternative to traditional excitation schemes.
Numerical simulations using individually updated finite element models showed a substantial variation in critical flutter speeds. Configurations utilizing liquid-filled tanks demonstrated higher critical flutter velocities than rigid-filled counterparts due to increased frequency separation between fundamental wing bending and torsion modes. The results indicate that linear models approximating fuel as a frozen mass can lead to an underestimation of critical dynamic pressure. Experimental validation remain essential for ensuring the robustness of analytical flutter predictions.