Hull Forms and Propulsors: An Overview
Wavelengths: An Employee's Digest of Events and Issues (NAVSEA Carderock)
By Michael J. Davis and Dr. Joseph J. Gorski, reprinted from NSWCCD Technical Digest
Navy requirements for new missions; greater efficiency; lower signatures; and lower cost ships, submarines, and other vehicles have perpetuated the need for new and improved hull forms and propulsors. NSWCCD's engineers and scientists apply their expertise in hull form and propulsor technology, computational fluid dynamics (CFD) and simulation technologies, and the Division's world-renowned facilities to support these requirements.
The cost, efficiency, and flexibility of CFD and simulation technologies are extremely attractive to the design community, when compared to relatively expensive and time-consuming scale models and scale/full-size tests. On the other hand, CFD and other simulation tools vary widely in capability and accuracy, thus requiring verification, validation, and/or calibration. Division personnel have extensive experience with the development and use of these tools, as well as with model- and full-scale trials using today's advanced technologies. The wealth of data acquired from these efforts, coupled with the Division's unique facilities provides an ideal environment to improve current tools and ship systems and to support new hull form and propulsor designs.
Early U.S. Navy shipwrights often built wood models to visualize how a hull would look and detailed wooden structural models to aid the shipwrights as they built the ship. Benjamin Franklin conducted rudimentary ship model resistance tests with a simple gravity tow model basin. Admiral David W. Taylor was instrumental in institutionalizing a scientific approach to ship and aircraft model testing for the Navy. The Carderock Division's comprehensive hydromechanics testing facilities owe their origin to his vision and influence.
The Division's facilities, whose origins are found in the Experimental Model Basin constructed at the Washington Navy Yard in 1898, have been used to conduct experiments to improve the naval architect's range of knowledge and to support individual ship designs. The Division has provided hydromechanics research, development, and design support for many ship, submarine, and craft hull forms and their propulsors.
Though not obvious to the untrained observer, ship and submarine hulls and propulsors (especially those of Navy vessels) have changed significantly over the past 50 years. Evolution in the hull and propulsors arena is continual, as Division technology enables improvements. Future changes will occur with changing mission requirements that evolve from new operational arenas (e.g., littoral regions), new weapon threats, and advances in sensors. These evolving mission requirements may require radical changes to hull forms and propulsors. Current Navy concept designs such as tumble-home hull forms and integrated propulsor/hull designs, and waterjets, which are the propulsor of choice for the high-speed ferry community, are examples of such potential significant changes. These mission-oriented design variations will benefit from new design support technology under development at Carderock Division.
The Division has been at the forefront of the development and application of CFD for naval hydrodynamics. With improvements in computer technology and subsequently more accurate and timely flow predictions, a new paradigm for design and analysis of naval vehicles has emerged over the last decade. Still, the combination of experiments and computation (neither of which can provide a complete answer alone) is the best approach to develop new and out-of-the-box designs. Computations provide a complete "picture" of a flow field, which can help one understand the physics involved and any ongoing interactions. However, CFD methods are not always accurate because of modeling deficiencies and computer limitations. Consequently, experiments must determine detailed measurements in critical areas and provide validation and verification of the computations and the final design.
CFD, Simulation and Model Testing
Simulations for the naval architecture design community, as well as other design communities that work with complex systems, allow design options to be evaluated without the need for physical model tests or large-scale demonstrators. This is desirable from a cost, time, and risk standpoint, and many efforts are underway to reach this goal. The Navy's hydro-mechanics community is striving to use CFD and other simulations to support the design of new hull forms and propulsors.
The state-of-the-art and accuracy of these models vary widely. Ideally, a simulation could give predictions with only vehicle geometry input and a time history of environmental conditions, ordered speed, and control surface settings. At their current stage of development, some CFD models assume calm water, constant speed, and fixed-control surface settings. Some of the current motion simulation models require coefficients, generally determined only from model experiments, to examine maneuvering characteristics. These computational tools, though still under development, have been used to support the design of future ships and submarines.
The least costly method that provides the desired accuracy generally determines the approach to use to support a design or investigate an alternative. If available, a CFD or other simulation code that will provide the accuracy required by the designer will be used. Often, suitable codes are not available or are in the verification/validation stage. Depending on their capabilities, available codes may be used in conjunction with experimental results. For specific designs or alternatives, a physical model tested in the Division's facilities, in conjunction with applicable CFD and simulation models, frequently offers the necessary information to the designer in less time, for less money, and with greater accuracy than any available simulation code used alone.
Verification, Validation, and Calibration
A modeling process requires accurate CFD and simulation results. Consequently, recent efforts have focused on the verification and validation of computational tools. Verification and validation estimates the accuracy of the numerical process compared to the actual physical process being modeled. Verification ensures that the equations are solved correctly by the computational method, often accomplished with analytic benchmarks.
Validation assesses the uncertainty of modeling by using benchmark experimental data and estimating the sign and magnitude of the modeling error when conditions permit. The experimental benchmark data must be of high quality and acquired under well-controlled conditions to ensure that the model is accurate and is properly validated.
It is difficult to obtain true values for the physical process that can then be used to compare with predicted or measured values. The easy answer would be to measure the true physical value on the full-scale vehicle. In practice, the predicted values are most often for a vessel that has not yet been built. Also, it is difficult to make accurate full-scale measurements that account for all of the environmental forces acting on the ship or submarine; i.e., current, wind, and waves. Generally, the nature of these forces varies with time and spatial location.
The Division's hydromechanics test facilities were designed to address the problem of predicting the performance of vehicles not yet built and controlling and measuring the environmental conditions acting on the test vehicle. However, examination of data in some of the categories shows variability in model-scale data even under controlled conditions. The user of the data must be able to determine when variability can be reduced with additional effort or a different approach. In some cases, variability may be acceptable or unavoidable due to the nature of the phenomena being studied.
Verification and validation require that conditions be the same for each case being compared. Characteristics such as wave and wind direction, which can be precisely produced and controlled in a numerical simulation, are in reality quite random in nature. Therefore, it can be difficult to compare computed and full-scale or model-scale results unless extremely good measurements have been made of the environmental forces acting on the vehicle. The ability to make these measurements and decisions comes only from hands-on experience under the guidance of personnel with specific expertise.
Often, a simulation will correctly predict the trends in motions or forces resulting from a hull form or propulsor design change but will not give the absolute value of the motion or force. In many cases, model-scale data can be used to calibrate the simulation prediction and verify the trends. Then, the simulation can be used over a range of similar hull forms or in conjunction with model testing to reduce the number of test conditions that must be examined. Again, the experience and judgment of the user is critical to determine if the simulation process is accurate enough for the required use and if the simulation is being configured and applied correctly.
The U.S. Navy and maritime communities have a continuing need for hydro-mechanics research and development to support improvements in ship and vehicle performance and total operating cost, and to meet changing mission requirements. Changes in mission requirements may require radical changes to hull forms and propulsors.
There are challenges in model- and full-scale testing and in CFD and simulation development, calibration, verification, and validation. Knowledge and experience is necessary to control and judge the accuracy of the benchmarks being used for comparison and to understand what is needed to improve the accuracy of the CFD or simulation process. Numerical simulations with the capabilities and accuracy the Navy requires to support design work will require significant additional development, verification, and validation. The accuracy required for validation may require new model test or full-scale trial approaches, which in some cases may have been considered too expensive in the past. The experienced workforce at Carderock Division, combined with its comprehensive hydromechanics facilities, provide the most beneficial environment available for these efforts.
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