Steep water waves may be responsible for damages to offshore structures, as inducing a high-frequency resonant response, commonly known as ringing, found to occur in conjunction with a peak in the load timeseries, named secondary load cycle (SLC), whose causes are still not properly known. In this thesis, an experimental study of the forces generated upon flow separation and vortex formation behind a bottom-hinged, vertical slender cylinder forced by steep waves, both breaking and non-breaking, is presented. An innovative and complex laboratory setup was arranged, this combining the use of optical measurement technique (Particle Image Velocimetry, PIV) for the investigation of the flow downstream the cylinder over four horizontal planes parallel to the bottom at different elevations with the recording of synchronized measurements both of the force acting on the cylinder and of the incoming wave elevation. PIV results showed the occurrence of flow separation and the formation of vortices for many of the breaking waves cases and for all the non-breaking waves, but with a completely different fashion. A correspondence between the SLC and the vortical structures has been found: vortex formation starts just after the wave crest has passed, at a stage corresponding to about one quarter of the wave period after the main load peak, where a second peak occurs i.e. secondary load cycle. The occurrence of a SLC has been identified by some synthetic parameters such as the Froude number Fr>0.6, the dimensionless wavenumber kR≥0.1 and the wave slope kη≥0.25, these falling within the range of limits provided by the experiences of Chaplin et al. (1997), Grue and Huseby (2002), Suja-Thauvin et al. (2017), Riise et al. (2018). A correlation between the vortex-shedding-induced force and the SLC was found, but such contribution is not the only one to the SLC. Generated vortices measure (20-30)% of the cylinder diameter at most, in disagreement with the larger size, about the cylinder diameter, of the vortices observed in the CFD simulations by Paulsen et al. (2014) and Kristiansen and Faltinsen (2017). The SLC occurrence is found to coincide with the ringing response, governed by free surface and flow separation effects, according with Riise et al. (2018).
In ambiente marino offshore, le onde ripide sono responsabili di episodi di danneggiamento delle strutture tanto quanto gli eventi più estremi. Un tipico fenomeno riconducibile alle onde ripide è il ringing, una risposta risonante ad alta frequenza che è stata spesso osservato in concomitanza alla presenza di un picco nella serie temporale del carico, denominato secondary load cycle (SLC), le cui cause devono essere ancora chiarite. In questa tesi, si presenta uno studio sperimentale volto all’analisi delle forze agenti, della separazione del flusso e della formazione di vortici a valle di un cilindro verticale snello, incernierato alla base, investito da onde ripide, frangenti e non, con lo scopo di chiarire la loro relazione. Si è utilizzata una innovativa e complessa configurazione di prova, che combina l’impiego di tecniche di misurazione ottica (Particle Image Velocimetry, PIV) per l’indagine del flusso a valle del cilindro su quattro piani orizzontali a diverse quote dal fondo alla registrazione sincronizzata di misure della forza agente sul cilindro e di elevazione dell’onda incidente. I risultati hanno mostrato che la separazione del flusso e la formazione di vortici si verifica per molte delle onde frangenti utilizzate e per le onde non frangenti, seppure in una forma completamente diversa. La generazione dei vortici è stata osservata subito dopo il passaggio della cresta dell’onda, a circa un quarto del periodo dell’onda, dove compare un secondo picco di carico, ovvero il SLC. La presenza del SLC è stata osservata per numeri di Froude Fr>0.6, lunghezza d’onda adimensionale kR≥0.1 e ripidità kη≥0.25, valori che ricadono all’interno dei range forniti da precedenti esperienze (Chaplin et al., 1997; Grue and Huseby, 2002; Suja-Thauvin et al., 2017; Riise et al., 2018). È stata trovata una correlazione tra la forza indotta dalla generazione dei vortici e il SLC, tuttavia la formazione dei vortici non può spiegare da sola il fenomeno. La dimensione massima dei vortici osservati è il (20-30)% del diametro del cilindro; questo risulta in disaccordo coi vortici di dimensione circa pari al diametro del cilindro osservati nelle simulazioni numeriche di Paulsen et al. (2014) e Kristiansen and Faltinsen (2017). Infine, la presenza del SLC è stata riscontrata con il ringing, indotto da effetti dovuti alla superficie libera ed alla separazione del flusso, secondo Riise et al. (2018).
Dynamics induced by Steep Waves at a Vertical Slender Cylinder in Deep Waters: Laboratory Experiments / Antolloni, Giulia. - (2019 Mar 27).
Dynamics induced by Steep Waves at a Vertical Slender Cylinder in Deep Waters: Laboratory Experiments
ANTOLLONI, GIULIA
2019-03-27
Abstract
Steep water waves may be responsible for damages to offshore structures, as inducing a high-frequency resonant response, commonly known as ringing, found to occur in conjunction with a peak in the load timeseries, named secondary load cycle (SLC), whose causes are still not properly known. In this thesis, an experimental study of the forces generated upon flow separation and vortex formation behind a bottom-hinged, vertical slender cylinder forced by steep waves, both breaking and non-breaking, is presented. An innovative and complex laboratory setup was arranged, this combining the use of optical measurement technique (Particle Image Velocimetry, PIV) for the investigation of the flow downstream the cylinder over four horizontal planes parallel to the bottom at different elevations with the recording of synchronized measurements both of the force acting on the cylinder and of the incoming wave elevation. PIV results showed the occurrence of flow separation and the formation of vortices for many of the breaking waves cases and for all the non-breaking waves, but with a completely different fashion. A correspondence between the SLC and the vortical structures has been found: vortex formation starts just after the wave crest has passed, at a stage corresponding to about one quarter of the wave period after the main load peak, where a second peak occurs i.e. secondary load cycle. The occurrence of a SLC has been identified by some synthetic parameters such as the Froude number Fr>0.6, the dimensionless wavenumber kR≥0.1 and the wave slope kη≥0.25, these falling within the range of limits provided by the experiences of Chaplin et al. (1997), Grue and Huseby (2002), Suja-Thauvin et al. (2017), Riise et al. (2018). A correlation between the vortex-shedding-induced force and the SLC was found, but such contribution is not the only one to the SLC. Generated vortices measure (20-30)% of the cylinder diameter at most, in disagreement with the larger size, about the cylinder diameter, of the vortices observed in the CFD simulations by Paulsen et al. (2014) and Kristiansen and Faltinsen (2017). The SLC occurrence is found to coincide with the ringing response, governed by free surface and flow separation effects, according with Riise et al. (2018).File | Dimensione | Formato | |
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