Simulation and Evaluation of Marine Propeller Crashback Through Computational Fluid Dynamics

Crashback is a maneuver which occurs when a ship or submarine reverses its propeller while traveling forward, slowing or stopping the vessel. This results in unpredictable forces and moments that decrease control and maneuverability. This project utilized computational fluid dynamics (CFD) to model the fluid flow during crashback in hopes of determining the physical causes of the unsteady forces and moments that occur. At the Naval Surface Warfare Center in Carderock, MD, there are two CFD approaches being applied to crashback: a pure Large Eddy Simulation (LES) technique and CRUNCH, which is a hybrid of LES and Reynolds Averaged Navier-Stokes (RANS). The LES approach provides extremely detailed three-dimensional, transient turbulence results but, for now, is limited to an open propeller. CRUNCH can also provide turbulent flow data, but it can be applied to more complex geometries, such as a duct or submarine hull. For this research, results generated with the pure LES technique were utilized due to complications that arose from adapting the CRUNCH model to crashback. There were two distinct aspects to this research. First, the LES results were validated against data from experiments with similar advance ratios (a dimensionless parameter relating propeller rotational speed with axial flow velocity). Mean, root mean square, and standard deviation values of the thrust, torque, and side force from the LES code were compared with those from the experiments to ensure the magnitudes and variations in the resultant loads were similar to experimental data. Spectral analysis was also performed on the thrust, torque, and side force magnitudes and angle to determine whether the resultant oscillation frequencies of the LES results were comparable to the response frequencies found in the experimental data. Once the LES results were shown to be sufficiently accurate, analysis was performed to determine the physical cause of the unsteady forces. Several sets of animations were cre