A generalised superelement formulation for fluid-conveying flexible multibody systems

This thesis contributes to the development of efficient and versatile modelling techniques for flexible multibody dynamic systems that may include fluid-conveying pipes within the floating frame of reference formulation. Flexible multibody systems are subject to geometric non-linearities due to the large rigid body motion in their components. The floating frame formulation is well suited to such systems, as it separates the gross rigid-body motion from the elastic deformation by introducing a floating frame for each component. Under the assumption of small elastic deformations, well-established model order reduction techniques can be applied at component level. This work expands the capabilities of floating frame of reference-based modelling approaches by addressing key challenges in inertia approximation, in generalisation of reference conditions and reduction methods in floating frame of reference-based superelements, and in integration of fluid-conveying pipe elements.

The methods developed in this thesis provide a foundation for analysing systems in which elastic deformations remain small relative to component dimensions, and where inertia effects due to the fluid flow play a significant role while the flow itself can be assumed as inviscid plug flow. The proposed methods offer a practical implementation, because the inertia forces can be computed using solely the consistent finite element mass matrix for elements with both translational and rotational degrees of freedom, supplemented by derivatives along the centreline for fluid-conveying pipes. Moreover, the systems can be modelled using superelements within the global boundary coordinate formulation framework, which is a superelement formulation that supports various reference conditions and reduction bases, enhancing both accuracy and computational efficiency.