Non-linearity aspects of vortex induced vibrations of stayed cable

In slender structures and their components, the occurrence of vortex-induced vibrations can lead to fatigue of fixed elements, loosening of connections, or even failure and drop-out at joints. Vortex-induced vibrations (VIV) represent a major issue for slender structures such as stayed cables, where large oscillations may affect serviceability and fatigue life. This Master Thesis focuses on the nonlinear aspects of VIV and investigates the role of surface modifications, in particular with helical wires of different relative diameters (d/D), as passive suppression devices. The experimental campaign was carried out in a wind tunnel using smooth cylinders and cylinders equipped with helical wires of different diameters. From these tests, the aerodynamic force coefficient was extracted as a function of oscillation amplitude. This coefficient, confirmed with free-response data, provides an applicable aerodynamic input that can be combined with the Scruton number of a target structure to predict its response amplitude. Unlike conventional VIV prediction methods, this framework does not rely on same geometric similarity. It enables prediction even for structures with different cross-sectional formats, making it directly applicable to practical engineering problems. The results show that while aerodynamic force coefficient-based predictions are little bit underestimate free-response amplitudes, the method remains consistent with experimental trends. The adoption of non-dimensional parameters, especially the diameter ratio d/D, reinforces the generality of the findings and facilitates comparison with other studies. Counter maps further demonstrate how increasing helical diameter progressively dissipate energy and suppresses unstable regions of positive aerodynamic damping: for d/D=0.0133, instability is partially reduced, while for d/D=0.0444, the unstable region is reduced and for d/D=0.0667 it is almost eliminated. This confirms their efficiency in perturbing vortex coherence. These results validate that helical wires can reduce vortex-induced vibrations and highlight the role of amplitude and aerodynamic effects in cable dynamics. Overall, the study establishes a framework that connects wind tunnel experiments with the practical engineering applications, to predict VIV amplitudes in real structures.

Nelle strutture sottili, le vibrazioni indotte da distacco di vortici (VIV) possono causare fenomeni di fatica, degrado di collegamenti bullonati e, più in generale, cedimenti nei giunti. Le VIV rappresentano un problema significativo per cavi sospesi e strutture snelle, nelle quali grandi oscillazioni influenzano funzionalità e durata a fatica. Questa tesi si concentra sugli aspetti non lineari delle VIV e sul ruolo dei fili elicoidali di diverso diametro relativo (d/D) come dispositivi passivi di soppressione. La sperimentazione in galleria del vento ha coinvolto cilindri lisci e con fili elicoidali. Da questi test è stato ricavato il coefficiente di forza aerodinamica in funzione dell’ampiezza di oscillazione, confermato dai dati di risposta libera, utile per prevedere l’ampiezza di risposta di una struttura tramite il numero di Scruton. Questo approccio consente previsioni anche per strutture con diverse sezioni trasversali, a differenza dei metodi convenzionali. I risultati mostrano che, sebbene le previsioni possano leggermente sottostimare le ampiezze di risposta, restano coerenti con le tendenze sperimentali. L’uso di parametri adimensionali, in particolare d/D, facilita confronti con altri studi. Mappe contrarie evidenziano come l’aumento del diametro elicoidale dissipi energia e riduca le regioni instabili: parzialmente per d/D=0,0133, più significativamente per d/D=0,0444, quasi completamente per d/D=0,0667. Questi risultati confermano che i fili elicoidali riducono le VIV e sottolineano il ruolo dell’ampiezza e degli effetti aerodinamici nella dinamica dei cavi, collegando esperimenti in galleria del vento ad applicazioni ingegneristiche pratiche.