Advanced computational fluid dynamics methods used to tackle hydromechanics problems

New designs for faster, more maneuverable surface and sub-surface marine vehicles have emerged as important priorities for the US Navy with value in various mission scenarios. Dr. Len Imas of the Department of Civil, Environmental and Ocean Engineering at Stevens Institute of Technology is a specialist in development and application of computational fluid dynamics (CFD) methods to marine hydromechanics problems. He has recently been awarded two grants from the Office of Naval Research (ONR) to (i) improve the understanding of planing hull hydrodynamics and (ii) quantify uncertainty in modeling of external flows involving cavitation.

“The engineering of more agile and efficient ships can provide tremendous advantages for Naval and civilian vessels, giving them enhanced capabilities to maintain control at high speeds in severe conditions,” says Dr. Michael Bruno, Dean of the Charles V. Schaefer, Jr. School of Engineering and Science. “Continued innovation requires profound understanding of the various forces acting upon a ship in water, and experts in computational dynamics like Dr. Imas are providing critical insights.”

According to Dr. Alan Blumberg, Director of the Davidson Laboratory, “Len’s innovative work in computational marine hydrodynamics is crucial to providing understanding that will enable engineers to work with more realistic analysis tools.”

Computational Modeling of Planing Hulls

Dr. Imas’ first grant concerns numerical simulation and model validation to data from sea trials of prototype ultra-deep vee, planing hulls. Planing boats have hulls that take advantage of hydrodynamic lift. At low speeds, buoyancy is the dominant upward force opposing gravity that allows a vessel to float. Once a vessel with a planing hull achieves enough speed, the water pushes against it with a high enough pressure resulting in a force known as hydrodynamic lift. When such a lift force is the dominant upward force, the role buoyancy plays in keeping the hull afloat is reduced. As speeds increase, the vessel rises further in the water and minimizes resistance, enabling rapid and efficient travel through calm water. One problem with this design has been that a substantial seaway can make it difficult to maintain directional stability, so a planing boat may need to operate more slowly in rough conditions. The deep vee hull is a variation on this design with a wide, V-shaped rear converging at a forward point. This design helps balance the concerns of drag reduction and handling capability, with a front that “cuts” through the waves in order to maintain control and minimize drag, as well as a flatter rear section that enables planing.

Although the deep vee design has provided substantial benefits, it still has its disadvantages, one of which being that it takes more power from the engine to achieve planing. According to Dr. Alan Blumberg, Director of the Davidson Laboratory, “Researchers must improve the understanding of the relationship between hull geometry and the hydrodynamic behavior of ultra-deep vee hulls in order to help drive further design innovation. Dr. Imas’ methodology of combining computational hydrodynamics with data collected from and prototype sea trials could be crucial to the improved understanding of fluid dynamics around unconventional high speed surface craft concepts based on this important design.”

Quantifying Uncertainty to Provide More Realistic Simulation

The objective of Dr. Imas’ second ONR grant is to develop and evaluate a numerical uncertainty quantification tool for Navier-Stokes based simulations using an algorithm based on non-intrusive polynomial chaos, with an initial application to modeling of flows around partially-cavitating hydrofoils. Uncertainty quantification becomes a necessity due to the fact that the underlying analysis and simulation tools employed in marine hydrodynamics are typically applied with a unique set of input data and model variables, whereas realistic operating conditions are a superposition of numerous uncertainties under which the marine system of interest operates. For example, when performing a 3-D viscous flow analysis, there is uncertainty in the definition of the boundary conditions representing the operational environment; in the discrepancy between the CAD geometry and the real geometry resulting from the manufacturing tolerances and assembly process; in the true deformed geometry of the components being analyzed, etc. In addition, modeling uncertainties exist due to, but are not limited to, turbulence treatment, multiphase model formulation, numerical discretization schemes, grid dependency, etc. This leads to a global uncertainty on the results of the analysis, on which design decisions ultimately have to be made. The ability to quantify the impact of these uncertainties on the predicted hydromechanics of the marine vehicle and to account for those uncertainties in the design process is crucial for reliable performance analysis and risk management.

Len Imas, Associate Professor of Ocean Engineering, holds a BS and MEng. in Aeronautical Engineering from Rensselaer Polytechnic Institute, and a PhD. in Numerical Hydrodynamics from Massachusetts Institute of Technology . His interests are in basic and applied research involving hydrodynamics and computational fluid mechanics related to topics ranging from nonlinear free-surface turbulent flows, to vortex induced vibrations, to high-speed surface and sub-surface marine vehicles, and to high-performance racing yachts. His specialization is in the development and utilization of computational fluid mechanics and optimization methods in design analysis applications involving marine hydrodynamics and low speed aerodynamics.

The Davidson Laboratory was founded in 1935 and remains one of the world’s leading facilities for naval architecture research. The laboratory's renowned towing tank complex is 320 feet long, 16 feet wide and 8 feet deep. With recently upgraded instrumentation, glass walls for viewing and photography, and public access improvements, the facility is vital to the Laboratory's contributions to fundamental and applied research in ship design, hydrodynamics and ocean engineering.

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