ON THE WAKE STRUCTURES OF FLOW AROUND DEFORMING AND VIBRATING BODIES

Abstract: This thesis concerns the study of Fluid-Structure interaction phenomena among deforming and (non-deforming) vibrating objects under unsteady fluid fl ow exposures. This multi-physical phenomenon is widely encountered in real-life situations and therefore it is of significant importance to understand the underlying physics. The trend is that both industrial and research facilities aim for developing methods that treat this complex and multi-disciplinary problem with high accuracy and also sufficient efficiency. Time-domain simulations, that is the dominating prediction tool within the FSI-community although frequency-domain representation is still used to some extent, have been integrated with two different structural models that model the solid objects. For the vibrating rigid object an Immersed Boundary (IB) method based on the use of Cartesian mesh is used to represent the solid object by using momentum source that enforce the required boundary condition. The deforming object, on the other hand, is modelled by a three-dimensional Finite Element (FE) formulation based on collocated mesh formulation. An Arbitrary Lagrangian-Eulerian (ALE) formulation provides the deformation of the object which is solved in conjunction with the fl uid and solid solvers. Further, a partitioned FSI-approach based on strong coupling strategy assures for reliable flow and solid domain quantities. The time-dependent and unsteady fl uid flow is predicted based on Implicit Large Eddy Simulations (ILES) which becomes a prerequisite in order to resolve flow processes involving large-scale structures with sufficient accuracy. In particular, separation processes, vortex shedding and possibly vortex-pairing are flow phenomena of such kind that typically are encountered when the flow around an object enforces the body to be deformed or displaced, which in turn, alters the character of the flow structures. In the context of deformable- and rigid body motions, this study is especially focused in the near-wake (instantaneously) behaviour of fl ow structures in conjunction with dependency of wake topologies when the unsteady wake fl ow generates unsteady loading on the object. The variations in response dynamics of the objects are studied in parallel and a direct/indirect coupling is made in the object-wake dynamics in order to better understand the complex two-way FSI phenomenon. In the analyses, particular post-processing tools for the fl uid- flow, such as Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) have served as a toolbox. It is shown that very useful conclusions can be drawn and hence, attribute these findings to relevant mechanisms underlying the FSI problem.