|Assessment of Experiments and Prediction Methods for Naval Ships Maneuvering in Waves|
|Applied Vehicle Technology|
maneuvering, motions in waves, Naval platform, seakeeping
Course keeping (CK) and maneuvering in waves (MIW) is required for many ship operations such as replenishment at sea, ship-ship/platform interactions, and navigation in confined water and/or littoral zones. Furthermore, ships should have enough maneuverability in adverse weather conditions to ensure the safety of the ship, its crew and payload. Commercial and military ships must meet International Maritime Organization (IMO) Guidelines and NATO Standardization Agreements. Commercial interests are also increasing due to recent IMO Guidelines for energy efficiency and minimum power requirements. As with calm water maneuvering, the analysis requires free running conditions, which challenges both experimental and prediction method capability in comparison to the status for captive condition resistance & propulsion, seakeeping and maneuvering analysis. Additionally, although some criteria are available for controllability or maneuverability in heavy weather, there is ample room for interpretation and accurate prediction methods are needed in order to be able to better understand the relevant physics and to define clear and unambiguous performance criteria.
AVT-161 Assessment of Stability and Control Prediction Methods for NATO Air & Sea Vehicles, AVT-216 Evaluation of Prediction Methods for Ship Maneuvering and Control and AVT-280 Evaluation of Prediction Methods for Ship Performance in Heavy Weather, projects focusing on prediction of ship motions, were successful in assessing and advancing predictive methods for CK, but only limited attention was given to MIW. Experiments are few and not systematic in terms of separation of wave length vs. steepness and initial heading and wave phasing effects. Most methods are system based using ad hoc mathematical models that combine current seakeeping and calm water maneuvering approaches, which only provide mostly qualitative agreement with the experiments. CFD shows promise for prediction of wave drift distance but has large errors for wave drift angle.
AVT-183 Reliable Prediction of Separated Flow Onset and Progression for Air and Sea Vehicles and AVT-253 Assessment of Prediction Methods for Large Amplitude Dynamic Maneuvers for Naval Vehicles, projects focusing on the detailed flow field around ships in captive maneuvering motion, were successful in assessing and advancing prediction methods for the onset and progression of 3D steady/unsteady vortex separation, which showed the importance of accurate prediction of cross flow separation for predicting onset and the need for improved turbulence models for accurate prediction of progression. The experiments included local flow measurements using TPIV, which was instrumental in the assessment process.
As a follow-on activity of AVT-280 and building on the lessons learned from AVT-280 and AVT-253, a follow-on activity is proposed for assessment of experiments and prediction methods for naval vehicles MIW. The focus will be on assessment and understanding of physics of the ship motions during MIW and development of both experimental and prediction capabilities. A working hypothesis is that accurate prediction of MIW requires accurate modeling/prediction of the unsteady flow around the ship, and in particular smooth surface cross-flow and/or sharp-edge/bluff-body separations along with their interactions with the appendages. The work will therefore include local flow analysis of unsteady propeller and rudder inflow and visualization of onset and progression of 3D steady/unsteady vortex separation. The outcome will also provide insight into unsteady turbulent flow during MIW, which can be used in future prediction of acoustic signatures during MIW.
The scope of the proposed activity is an assessment of experiments and prediction methods for MIW, including issues of stability, control and propulsion performance. Available and required towing tank and wave basin experiments will be identified for benchmark validation test cases. Experimental conditions will focus on effects of the ship speed (including extreme weather low-speed critical condition), wave length and steepness, initial heading and wave phasing and irregular waves. Validation data will include both global (6DOF trajectories, propulsion performance, appendage/rudder/control surface forces and moments) and local (wave elevations and flow field) variables. Simulation codes will cover system based, potential flow and CFD. Simulations will guide the experiments and once validated will fill in sparse data. Verification and validation procedures will take into consideration both the comparison error E=D-S (where D and S are the experimental and simulation values, respectively) and validation uncertainty, i.e., root sum square of numerical and experimental uncertainties. Recommendations will be provided as to best practices for current simulation methods as well as directions for future research. Synergy and shared experience will be documented in the final report.
Physics of ship motions when maneuvering in waves, combined with smooth surface cross-flow and/or sharp-edge/bluff-body separations along with hull/propulsor/control-surface/wave/air-water-interface interactions for MIW.