The understanding of sloshing dynamics has gained specific interest from aircraft manufacturers in their attempt to reduce design loads of flexible wing structures carrying liquid (fuel). For it is essential the development of modeling capabilities that can describe the resulting wing loads from flying in atmospheric gusts, turbulence, and landing impacts. This objective could be achieved if a proper characterization of the damping properties of the fluid-structure system is carried out. From the experimental investigation point of view, the sloshing mechanism depends on how the dynamic energy is transferred and dissipated between the mechanical and the fluid systems. Several tests have been already carried out to estimate the damping properties of a fluid-structure system excited by harmonic loadings. However, such a periodic energy exchange between the systems is not accurately describing the damping mechanism occurring in a complex dynamic system as a flexible wing structure in real operating conditions. The objective of the research is the experimental identification of the damping properties of a wing-like structure carrying a tank filled with fluid simulating the sloshing behavior through the use of Operational Modal Analysis (OMA) experimental techniques and the direct measurement of the dynamic loading at the structure/tank interface for the estimate of the dissipating force. Results will serve the SLOshing Wing Dynamics (SLOWD) European funded project for the accuracy assessment. The experimental investigation will take advantage of the environmental testing facility available at the Department of Mechanical and Aerospace Engineering of the University of Rome "La Sapienza". The sensitivity of the damping estimates to the type, direction, and level of excitation as well as to the different filling levels will be studied.
The ground of this research project is the capability to experimentally characterize the dynamic behavior of a complex system, such as the sloshing one, based on experimental techniques already developed by the proponents. The key points are the development of new methodologies for the identification of the damping ratios aimed at improving the accuracy of its estimate that is generally really coarse due to the lack of the understanding of the mechanism of energy dissipation. Thus, the experimental findings will enable accurate calculations, to date still not available, of the fuel slosh - structural interaction and resulting damping that require the fully coupled simulation of a violent fuel slosh involving an incompressible fluid and compressible gas, non-linear structural deflections of the wing-tank geometry, and strongly coupled fluid-structure interaction between tank walls and fuel slosh. Indeed, the single-discipline state-of-the-art allows acceptable predictions in fluid & structural dynamic testing, computational fluid dynamics (CFD), structural dynamics, multi-physics coupling, reduced-order modeling (ROM) and multi-disciplinary optimization (MDO). The novelty of the experimental findings will contribute to the advancement of the modeling capabilities towards a fully developed digital twin representation of a system, thus improving the reliability, accuracy of the concurrent design, and reducing the cost needed to develop the final prototype. Such academic goals are strongly connected to the actual needs of the European industry. Indeed, the capabilities enabled by this research activity, together with those that will be achieved at the end of the SLOWD project, will allow the design a new civil aircraft wings characterized by high flexibility, thanks to the vibration alleviation effect induced by the sloshing fuel within the aircraft wing. It is worth highlighting that the actual design criteria are not including the fuel sloshing vibrations yet, therefore this research activity could bring a dollop of novelty in the aircraft design criteria aimed increasing not only the competitiveness of the aeronautical industry, but also the naval, see for example possible effects of the fuel sloshing in fast vessels, and more generally the industrial engineering when dealing with fuel-structure coupling. specifically for this research activity, the following impacts are expected:
1) Improving the accuracy of the sloshing models
2) Improving the accuracy of the estimates of the dissipation of energy in operational dynamic systems
This last point is of paramount importance when designing the aircraft flight tests as requested by the civil aviation authority for the flight certification.