Body Force Propeller Model for Unsteady Surge Motion

Abstract

Accurately modelling a self-propelled vessel in a large amplitude seaway with CFD is very expensive and practically out of reach. The expense is due to the very small numerical time-step required for the propeller rotation and the large mesh size. A method for accurately modelling a propeller while reducing computational cost is desirable. This paper describes the first step towards developing a body force propeller model for unsteady conditions. The purpose of this study is to train a semi-empirical algorithm to accurately prescribe the unsteady body force to model the propeller. The MOERI Container Ship propeller is analyzed with RANS CFD. Open water test data is compared to the RANS CFD results of a steady Moving Reference Frame approach. Harmonic surge is applied to a transient rotating mesh model in open water and the behind condition.

Multi-Degree of Freedom Propeller Force Models Based on a Neural Network and Regression

“…The NMRI scale KCS propeller is examined in both open water and in the behind condition. The CFD used is very similar to that used by Knight & Maki [9,17], with the only difference of using an inlet condition on the sides of the domain when there is oblique flow. The OpenFOAM solver pimpleDyMFoam is used with the sliding mesh technique used for the unsteady rotating propeller simulations.…”

“…Therefore, a double body approximation is used. The mesh used in this study correlates to the coarse grid used by Knight & Maki [9,17]. The goal is for the model to be as accurate as the method that it is trained with.…”

“…It uses one unsteady CFD simulation of the propeller operating in the behind condition. It also uses steady state open water propeller coefficients which are extended into the behind condition in accordance with [9,17]. Knight & Maki used the thrust identity [19] to extend steady state open water K T and K Q into K T and K Q coefficients for the propeller operating in the behind condition if the same propeller is run in the behind condition at a single U and n. The KCS propeller has been simulated using a Moving Reference Frame approach at different J values in open water and a rotating propeller simulation was performed for the propeller operating in the behind condition with constant U and n at one speed to determine the Taylor Wake fraction w using the thrust identity [19].…”

“…Knight & Maki used the thrust identity [19] to extend steady state open water K T and K Q into K T and K Q coefficients for the propeller operating in the behind condition if the same propeller is run in the behind condition at a single U and n. The KCS propeller has been simulated using a Moving Reference Frame approach at different J values in open water and a rotating propeller simulation was performed for the propeller operating in the behind condition with constant U and n at one speed to determine the Taylor Wake fraction w using the thrust identity [19]. This parameter has been found in [9,17]. Therefore, the behind condition case is trained with the results of one unsteady behind condition propeller simulation and the open water K T and K Q values extended into the behind condition using the thrust identity.…”

“…The propeller examined is the National Maritime Research Institute (NMRI) scale Kriso Container Ship (KCS) propeller. Details of the CFD setup as well as the derivation for a semi-empirical propeller force method can be found in the authors' previous work [9,17]. This paper examines both a nonlinear regression model and a neural network that are trained and tested on unsteady RANS CFD results to determine the three-degree of freedom force of the propeller in open water and the behind condition.…”