STUDY EXPLAINING THE FUNDAMENTAL DIFFERENCES BETWEEN SURFACE AND SUBMERGED WAKES USING COMPUTATIONAL FLUID DYNAMICS
Open Access
- Author:
- Wolfgang, Christopher William
- Area of Honors:
- Mechanical Engineering
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisors:
- Dr. Eric Paterson, Thesis Supervisor
Dr. Eric Paterson, Thesis Supervisor
James Gordon Brasseur, Thesis Honors Advisor - Keywords:
- Computational Fluid Dynamics
Free Surface Wake
Submerged Wake vs. Surface Wake
OpenFOAM
Fluid Mechanics - Abstract:
- In 1992 David Sheres and Walter Munk compiled a summary of an unpublished experiment from the early 1950's in the field of fluid mechanics. John Isaacs, a world-renowned oceanographer at the Scripps Institute of Oceanography, completed an experiment in 1952 to study the differences in wake profiles along the ocean's surface between a submarine and surface ship. He had predicted that the velocity profile of the submerged case would have the same shape as the surface ship and would only differ by the maximum velocity of the displaced flow. However during the experiment it was determined that the velocity field for the submarine had a velocity gradient in the same direction as the submarine's direction of passage, directly contrasting Isaacs' original prediction. Unable to determine the cause of the large discrepancy between his prediction and the experimental data he aborted the test without publishing his results. It had been believed that the submarine would generate a wake almost identical to that of the surface ship due to similar propulsion characteristics of the two vessels. The results of the experiment however revealed that a fundamental difference between submerged and surface propulsion and their interactions with the free surface is poorly understood. The fact that no one has looked further into the cause of the expected results in the 59 years since the original experiment illustrates that this remains a poorly understood phenomenon in the scientific community. This thesis reopened the Isaacs experiment in the hope of uncovering and finally documenting a fundamental difference on how surface and submerged propulsion affects generated surface currents. The Isaacs experiment was recreated in this study using the OpenFOAM computational fluid dynamics (CFD) system with body-force approximations to replicate ship self-propulsion. The results of the OpenFOAM simulations revealed that the accelerated flow caused by the propellers in the submerged case never interacted with the free surface, and the velocity profile along the free surface was in response to the pressure distribution over the submarine's hull in the same direction as the sub's direction of travel. In the case of the surface ship,the ship's propellers accelerated the flow immediately below and at the free surface away from the stern of the ship, causing a visible displacement of the free surface opposite in direction to that of the ship's travel. It is concluded that the velocity profile from the submerged case was generated entirely by the submarine's pressure field and not in the opposite direction due to the propeller acceleration as Isaacs had expected. In contrast the surface ship generates a localized velocity profile opposite to the ship's direction of travel due o the acceleration of the free surface immediately behind the ship from the propellers.