“…This instability is reinforced as the wake develops downstream, also as a result of the interaction with the hub vortex (see for more details Posa et al. 2019 b , 2022). Figure 17.Power spectral density of the hydrodynamic pressure coefficient at probes placed at the streamwise coordinates ( a ) , ( b ) , ( c ) and ( d ) : comparison between radial locations at and .…”
Section: Resultsmentioning
confidence: 98%
“…Applications in the field of marine propulsion were recently reported by Posa et al. (2019 b ), Posa, Broglia & Balaras (2019 a , 2020 a , b , 2021, 2022) and Posa & Broglia (2021, 2022).…”
The acoustic analogy is adopted to characterise the signature of a seven-bladed submarine propeller, relying on a high-fidelity large-eddy simulation, performed on a computational grid consisting of 840 million points. Results demonstrate that the nonlinear terms of the Ffowcs-Williams and Hawkings equation quickly become dominant moving away from the propeller along the direction of its wake development. While the linear terms experience a decay moving downstream, the nonlinear terms grow in the near wake, as a result of the development of wake instability. In particular, this growth affects frequencies lower than the blade frequency. Therefore, the acoustic signature of the propeller is mainly tonal in the near field only, due to the thickness and loading components of noise from the surface of the propeller and the periodic perturbation caused by its tip vortices. They develop instability at a faster rate, compared with the hub vortex, triggering the process of energy cascade towards higher frequencies and contributing in this way to broadband noise.
“…This instability is reinforced as the wake develops downstream, also as a result of the interaction with the hub vortex (see for more details Posa et al. 2019 b , 2022). Figure 17.Power spectral density of the hydrodynamic pressure coefficient at probes placed at the streamwise coordinates ( a ) , ( b ) , ( c ) and ( d ) : comparison between radial locations at and .…”
Section: Resultsmentioning
confidence: 98%
“…Applications in the field of marine propulsion were recently reported by Posa et al. (2019 b ), Posa, Broglia & Balaras (2019 a , 2020 a , b , 2021, 2022) and Posa & Broglia (2021, 2022).…”
The acoustic analogy is adopted to characterise the signature of a seven-bladed submarine propeller, relying on a high-fidelity large-eddy simulation, performed on a computational grid consisting of 840 million points. Results demonstrate that the nonlinear terms of the Ffowcs-Williams and Hawkings equation quickly become dominant moving away from the propeller along the direction of its wake development. While the linear terms experience a decay moving downstream, the nonlinear terms grow in the near wake, as a result of the development of wake instability. In particular, this growth affects frequencies lower than the blade frequency. Therefore, the acoustic signature of the propeller is mainly tonal in the near field only, due to the thickness and loading components of noise from the surface of the propeller and the periodic perturbation caused by its tip vortices. They develop instability at a faster rate, compared with the hub vortex, triggering the process of energy cascade towards higher frequencies and contributing in this way to broadband noise.
“…This choice has no effect on the blade loads, but it is able to influence the wake dynamics, due to the significant signature on it by the hub vortex (Posa et al. 2019, 2022 b ). In the experiments, although a downstream shaft does not allow investigating the effect by the hub vortex on the wake dynamics, it makes easier the generation of the clean, uniform inflow conditions of the open-water configuration.…”
Section: Set-upmentioning
confidence: 99%
“…(2020), Wang et al. (2021), Zhang, Ding & Liang (2021) and Posa, Broglia & Balaras (2022 b ). However, those works focused on conventional propellers, with no winglets at the tips of their blades.…”
Section: Introductionmentioning
confidence: 99%
“…For instance, DES computations were conducted by , Muscari, Di Mascio & Verzicco (2013), Guilmineau et al (2015), Gong et al (2018), Viitanen et al (2018) and Sun et al (2020). LES was instead utilized by Balaras, Schroeder & Posa (2015), Kumar & Mahesh (2017), Posa et al (2019), Asnaghi, Svennberg & Bensow (2020), Liao et al (2020), Wang et al (2021), Zhang, Ding & Liang (2021 and Posa, Broglia & Balaras (2022b). However, those works focused on conventional propellers, with no winglets at the tips of their blades.…”
The tip vortices shed by two marine propellers are studied, relying on large-eddy simulation, using a cylindrical grid consisting of 5 billion points. A tip-loaded design, featuring winglets at the tips of its blades, is compared against a conventional one at the design advance coefficient and a model-scale Reynolds number equal to 432 000. The tip-loaded propeller achieves improved performance, but produces also more intense tip vortices. The propeller with winglets actually generates two vortices from the tip of each blade, originating at the edge of each winglet and at the junction between the winglets and the blades. They merge at a short distance downstream, within a diameter from the propeller plane. The helical vortices originating from this merging process experience a slower instability, in comparison with the tip vortices in the wake of the conventional propeller, persisting further downstream, due to the weaker shear with the wakes shed by the following blades. The results of the simulations highlight that splitting the single tip vortex of a conventional propeller into two smaller vortices by means of winglets does not imply necessarily the generation of weaker vortices and lower negative peaks of pressure at their core: the geometry of the winglets needs to be carefully optimized to achieve this target.
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