Simulation and Evaluation of Marine Propeller Crashback Through Computational Fluid Dynamics

Released on by Matthew Paul Shearer
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  • Simulation and Evaluation of Marine Propeller Crashback Through Computational Fluid Dynamics Book Detail

  • Author : Matthew Paul Shearer
  • Release Date : 2007
  • Publisher :
  • Genre : Computational fluid dynamics
  • Pages : 65
  • ISBN 13 :
  • File Size : 5,5 MB
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Simulation and Evaluation of Marine Propeller Crashback Through Computational Fluid Dynamics by Matthew Paul Shearer PDF Summary

Book Description: 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 created which enabled many different aspects of the flow to be observed simultaneously. These animations were created for forty revolutions, spanning nearly 3.5 seconds. These animations yielded a wealth of insight into the flow field generated by crashback. They showed many different aspects of the flow field that have never been seen before, illustrating the relationships between flow field characteristics and measurable quantities, such as side force. Several theories proposed by other crashback research, such as the influence of the ring vortex, were evaluated through the visualization analysis. First, animations were created by taking constant radial cuts of all five propeller blades, displaying blade cross sections and visualizing the flow field over all the blades simultaneously. The observations provided by these animations prompted the creation of animations that displayed both the location of vortices within the flow (including the ring vortex) and the pressure distribution over the blade surfaces. These two animations did not reveal any direct relationship between vortex locations (including the ring vortex) or the instantaneous pressure distribution and the side force. There was, however, a strong relationship between the pressure fluctuations on the blade surfaces, particularly near the blade roots, and the side force direction. There is no known cause for these fluctuations at this time, but it is theorized that they are due to vortices either ingested into the propeller plane or generated near the blade root, as suggested by the blade sections animations. This research should provide a strong foundation in the computational modeling of crashback.

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