Hydrodynamic Analysis of a Triple Thruster Unit Via a BEM/RANS Interactive Method

Abstract

In this paper, a boundary element method (BEM) is used to predict the unsteady performance of ducted propellers in open water and ship behind conditions. The model propeller adopted includes the non-axisymmetric duct appendages (e.g., gearbox, brackets, and vertical structure connected to the hub), which induce severe shedding vortices on the propeller plane. This study thus investigates the effects of separation from the duct appendages as well as the upstream hull on the unsteady ducted propeller performance under different loading conditions. To improve the accuracy of a potential flow solver for highly viscous problems with separated flow near a blunt body, the present method is coupled with a viscous Reynolds-Averaged Navier-Stokes (RANS) solver. The former solves the ducted propeller problem to produce the propeller-induced flow field and body forces, with which the latter solves the total flow field based on a finite volume method. This approach is implemented in an iterative manner until the predicted 3D effective wake on the propeller surface becomes fully converged. An automated interface is developed to facilitate this process. A complete analysis of the propeller performance (i.e., predicted effective wake, flow-field, unsteady forces, and circulations on the blade) is presented at various operating conditions to investigate how the flow field inside and outside the nozzle is influenced by the viscous interaction among the incoming flow, propeller, its appendages, and upstream hull. For the sake of validation, the predicted results are compared with experimental measurements and results from unsteady full-blown RANS simulations. The presented results show satisfactory agreement among the results from different approaches, which makes the BEM/RANS coupling scheme adequate and computationally efficient for practical applications.