The dynamics of the tip vortices shed by a tip-loaded propeller with winglets

Abstract

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.

Anisotropy of turbulence at the core of the tip and hub vortices shed by a marine propeller

“…2015; Posa et al. 2019, 2022 a ; Posa 2022 b ). More details on the overall methodology can be found in the works by Balaras (2004) and Yang & Balaras (2006).…”

“…However, the resolution of the computational grid and in turn the range of scales that LES is able to explicitly resolve to accurately reproduce the mechanism of wake instability has been pushed even forward in the works by Balaras, Schroeder & Posa (2015), Kumar & Mahesh (2017), Posa et al. (2019), Posa, Broglia & Balaras (2022 a ) and Posa (2022 b ).…”

“…(2019, 2022 a ) revealed the importance of the interaction between the tip vortices and the wake shed by the following blades in promoting the instability of the former, accelerated at higher rotational speeds by their decreasing pitch, shifting the streamwise location of this interaction closer to the propeller plane. More recently, Posa (2022 b ) utilized a grid of 5 billion points to simulate both conventional and tip-loaded propellers at design working conditions, to compare the development of their wakes and in particular their tip vortices. The LES computations revealed that, despite the use of pressure side winglets at the end of the tip-loaded blades, splitting the tip vortices into two smaller helical structures, tip loading still resulted in more intense tip vortices, in comparison with the conventional blade design.…”

“…While vorticity at their core was found almost proportional to the rotational speed of the propeller, the growth of both turbulence maxima and pressure minima, which are potential sources of cavitation phenomena, was verified to be faster than linear. In addition, in the earlier work by Posa (2022 b ) the wake development of the same tip-loaded propeller, including a downstream shaft, was compared against that of a conventional propeller without winglets, to assess the ability of winglets of reducing the intensity of the tip vortices, despite the higher load at the outer radii of the propeller blades.…”

“…However, the LES studies currently available in the literature typically rely on computational grids consisting of O(10 7 ) points, similar to those for the DES computations reported above, and are usually targeted at analysing the process of instability of the wake system of marine propellers and the cavitation phenomena occurring within the large coherent structures they shed (Liefvendahl 2010;Liefvendahl, Felli & Troëng 2010;Asnaghi, Svennberg & Bensow 2018a,b, 2020aHu et al 2019a;Zhu & Gao 2019;Ahmed, Croaker & Doolan 2020;Asnaghi et al 2020b;Long et al 2020;Kimmerl, Mertes & Abdel-Maksoud 2021a,b;Wang et al 2021bWang et al , 2022aWang, Liu & Wu 2022d). However, the resolution of the computational grid and in turn the range of scales that LES is able to explicitly resolve to accurately reproduce the mechanism of wake instability has been pushed even forward in the works by Balaras, Schroeder & Posa (2015), Kumar & Mahesh (2017), Posa et al (2019), Posa, Broglia & Balaras (2022a) and Posa (2022b). Kumar & Mahesh (2017) adopted an unstructured grid consisting of 181 million hexahedral cells to conduct wall-resolved computations and analyse in detail the development and eventual instability of the wake shed by the five-bladed DTMB 4381 propeller at the design working condition, using a body-fitted approach.…”