Abstract Summary
The present work is concerned with the dynamic instabilities arising when a circular cylindrical shell containing a dense fluid is subjected to a seismic excitation of harmonic type. Typical Fluid Structure Interaction studies focused on the interactions between elastic structures and fluids are focused on inviscid or Newtonian fluids (compressible or incompressible). However, the Nature shows many examples where the Newtonian fluid models cannot be considered, e.g. blood, blood plasma, oil and many other examples are non-natural products of common use: toothpaste, paint, shampoo, melted butter, starch suspensions, corn starch. For such reason here the focus are non-Newtonian fluids and their interactions with vibrating structures. An intensive experimental campaign is aimed to understand the complex interactions arising from the interaction between the elastic vibrating structure and the fluid. The structure under investigation is a polymeric thin circular cylindrical shell clamped at the bottom to a shaking table, through a vibration table adapter, the top of shell is closed with a heavy rigid disk, which imposes a rigid body motion to the top, preserves the circular shape to the top and generates strong inertia forces due to the seismic excitation. Experiments are carried out with and without fluid, which is a mixture of water and corn starch flour, also known as oobleck. The dynamic scenario of the system is analyzed for different excitation levels and frequencies and different fluid levels: empty, partially, and full-filled. Modal testing is carried out in order to identify modal shapes and natural frequencies and quantify changes in the system modal properties vs fluid filling. The system dynamic instabilities are analyzed by exciting the system with a high amplitude base oscillation that pumps high energy into the system; the excitation consists in a stepped sine sweep procedure, which spans a frequency region where several natural frequencies are present and strong resonance phenomena can take place; different excitation levels have been considered in order to induce phase transitions in the fluid. The onset of complex dynamics has been detected using Fourier spectra and bifurcation diagrams of the Poincaré maps: when the fluid-solid transition takes place, the entangled non-Newtonian fluid rheology results in a complex dynamic scenario: period-doubling cascades, quasiperiodic and chaotic responses have been observed.
