Session: 06-05-04 Marine Hydrodynamics IV

Paper Number: 123747

123747 - Maneuvering Prediction of a Surface Combatant in Calm Water With Cfd-Informed Mathematical Model 

Maneuvering assessment is crucial for both design and operation of surface vessels. Numerical methods are often used for maneuvering predictions. A robust, accurate, and computationally efficient prediction tool is essential to meet fast-paced design cycles. The computational costs of the numerical methods typically increase with the prediction accuracy. Free-running Computational Fluid Dynamics (CFD) method has been shown to provide the most accurate solution to the problem but requires significant computational resources that currently precludes time efficient solutions. Mathematical models known as system-based (SB) methods are more practical for parametric investigation and optimization study at the early stage design. In addition, the control systems of unmanned surface vessels rely on mathematical models. Empirical methods are very fast and easy to use, but less attractive due to their low fidelity.

SB methods require approximation of forces and moments due to the ship motions as well as propeller and rudder actions. A system of equations governing the ship motions presented by Taylor series expansion with a rigid body assumption are solved in time to predict the ship motions. Two common types of mathematical models consists of the whole ship (WHS) and modular models. The Abkowitz model, which is the basis for the WHS model considers the hull, rudder and propeller as one rigid body. In contrary, the modular based model which was originally proposed by the Maneuvering Modeling Group (MMG) at the Japanese Towing Tank conference in the late 1970’s, separates the hydrodynamic forces/moments contributions of hull, propeller and rudders and account for the individual open water characteristics of these modules as well as their interactions. This feature could be critical for ships with strong hydrodynamic interactions between the main three components during a maneuver. The unique advantage of this approach is that it allows for independent design exploration of modules in an affordable and timely manner, which is a critical design feature.

Inputs to the SB methods are mainly a set of coefficients representing quasi-steady hydrodynamic properties of a ship, propeller and rudder parameters, as well as parameters estimating the interactions between modules in the case of modular models. The model inputs can be estimated by empirical formulas, captive or dynamic model tests, and/or system identification methods by utilizing free running maneuvering data. Advancement in computational power and improvements in numerical methods make CFD an alternative to captive/dynamic model tests with a lower cost. Accessing to sufficient computational resources allows for concurrent simulations of various operational conditions, partly compensates the low computational efficiency (slower than real time computation) of the viscous flow solvers. High-fidelity computations can also provide detail flow physics key to understand the underlying mechanism of the hull-propeller-rudder interaction phenomenon and hydrodynamic characteristics, which are not readily available in model test. In addition, CFD can be used to perform simulations at full-scale Reynolds numbers and environmental conditions and eliminate the scale effects inherent to model tests.     

This paper will assess a 4-DoF MMG model programmed to model surface ship maneuvering in calm water. The model will be utilized to predict 25^o and 35^o turning circle as well as 20-20 zig-zag maneuvers of an Office of Naval Research Tumblehome (ONRT). Captive CFD simulations will provide necessary inputs to the MMG model. The predicted maneuvering characteristics will be compared with the model test data and free running CFD analysis. The effect of uncertainty of two rudder inflow velocity parameters on the maneuvering predictions will be investigated, which have not been addressed in the public domain. The influence of propeller side force on the maneuvering prediction will be examined as well.

Presenting Author: Shawn Aram Naval Surface Warfare Center - Carderock Division

Presenting Author Biography: Dr. Shawn Aram is a mechanical engineer in the Simulations and Analysis Branch (Code 851) at Naval Surface Warfare Center – Carderock Division, where he covers projects focused on multi-fidelity hydrodynamic analysis of surface ships including resistance maneuvering seakeeping predictions. He holds a Ph.D. in Mechanical Engineering from Johns Hopkins University and has more than 20 years of experience in the field of Computational Fluid Dynamics (CFD). He has authored over 30 technical papers. He is a member of the ITTC30 Maneuvering Committee.

Authors:

Shawn Aram Naval Surface Warfare Center - Carderock Division
David Wundrow Naval Surface Warfare Center - Carderock Division