Large-Scale Computations of Unsteady Forces on Marine Vehicles

Despite efforts to maintain streamlined shapes for minimal resistance and noise, US Navy marine vehicles oftentimes must have bluff body geometries or must operate in off-design modes. These situations produce separated flows that are unsteady. At large scales the flow unsteadiness may be the cause of structural concerns while the entire range of fluid dynamic scales, from large to small, may be the cause of unwanted hydro-acoustic radiated noise. Large-eddy simulations (LES) are a class of fluid flow solvers that resolve the energy-containing scales of attached and separated flows, potentially providing the velocity field forcing functions for structural and hydro-acoustics analyses. LES have strict temporal and spatial grid spacing requirements and in order to resolve the energy-containing turbulence scales at practical flow velocities, the problem sizes and run times can be quite impressive. The objective of this Challenge Project is to apply LES to several problems of urgent US Navy need that have heretofore been considered too large to solve in a timely fashion. The first of these problems occurs when a marine vehicle, translating forward, must stop suddenly. This entails reversing the rotation of the propeller in a maneuver called crashback. This generates the largest forces that a propeller will undergo in its lifetime, and therefore, prediction of the forces and determination of procedures to reduce the forces, are of utmost importance. Using LES we have been able to uncover and verify the physics of force generation. With this Challenge Project, methodologies are being developed for one-way coupling between fluid flow and structural calculations that entail 300,000+ hours of CPU time. Another problem that is being tackled with the help of the Challenge grant is flow about the Advanced SEAL Delivery System (ASDS). This is a challenging geometry as it has both complex geometry and is a high-Reynolds number attached flow. Thus, the grid quality and size are important issues to overcome. In this paper, we document simulations of bare-hull structured grids with 4 and 20 million cells. Results of our wall-resolved simulation show that the attached boundary layers behave as expected and therefore, give us confidence that we can correctly predict the occurrence and strength of stern flow separations.