DOI

10.17077/etd.5z2auqsl

Document Type

Dissertation

Date of Degree

Spring 2017

Access Restrictions

.

Degree Name

PhD (Doctor of Philosophy)

Degree In

Mechanical Engineering

First Advisor

Pablo M. Carrica

Second Advisor

J. Ezequiel Martin

First Committee Member

H.S. Udaykumar

Second Committee Member

James Buchholz

Third Committee Member

Casey Harwood

Fourth Committee Member

Juan Ezequiel Martin

Abstract

A computational fluid dynamics (CFD) methodology is presented to predict density stratified flows in the near-field of ships and submarines. The density is solved using a higher-order transport equation coupled with mass and momentum conservation. Turbulence is implemented with a k-ε/k-ω based Delayed Detached Eddy Simulation (DDES) approach, enabling explicit solution of larger energy-containing vortices in the wake. Validation tests are performed for a two-dimensional square cavity and the three-dimensional stratified flow past a sphere, showing good agreement with available data. The near-field flow of the self-propelled Research Vessel Athena advancing in a stably stratified fluid is studied, as well as the operation in stratified flow of the notional submarine Joubert BB2 also in self-propelled condition. The resulting density, velocity, pressure and turbulent quantities at the exit plane of the near-field computation contain a description of the relevant scales of the flow and can be used to compute the far-field stratified flow, including internal waves. The generation of internal waves is shown in the case of the submarine for two different conditions, one with the pycnocline located at the propeller centerline, and the second with the pycnocline located slightly below the submarine, concluding that distance to the pycnocline strongly affects the internal wave generation due to the presence of the vessel. It is also shown that, as in the case of surface waves, the generation of internal waves requires energy that results in an increase in resistance. For the case of the surface ship the near field wakes are mostly affected by the separation at the wet transom and propeller mixing. However, in the case of the underwater vessel, the disturbance of the background density profile by the presence of the submarine affects the near-field wakes. Finally, the dead-water phenomenon, which occurs at very low Froude numbers, is studied for R/V Athena. Though the dead water problem has been studied in the literature using potential flow methods, this thesis presents the first attempt at using computational fluid dynamics (CFD) to analyze the flow. Results show that, while CFD can reproduce trends observed in potential flow studies, viscous effects are significant in the wake and the friction coefficient.

Public Abstract

A stratified fluid is defined by its mean density variation with depth as a result of any change in salinity and/or temperature. Almost all fluids found in the environment are stratified except on very small scales, and stratified flows are prevalent in various natural environments such as oceans, lakes and estuaries. Therefore, the flow caused by moving objects in these media are of importance and have directed much attention to the subject of stably stratified flows. This thesis presents contributions to simulate stratified flow effects on moving surface ships and submarines. Two different geometries are considered, the research vessel Athena, a surface ship, and the Joubert BB2 geometry, a generic submarine. It is observed that the structure of the stratified flow is completely different from the non-stratified one and the presence of buoyancy forces due to stratification has a substantial effect on the flow development and mixing processes around both geometries and hence influences the flow pattern and results in different resistance forces. Also, when a ship sails slowly in stratified seas she loses steering power and speed and experiences a large change in resistance force. This resistance force can be as high as that of the ship moving two to three times faster in non-stratified flow, a phenomenon known as the dead-water effect. Though the dead water problem has been studied in the literature using potential flow methods, this thesis presents the first attempt at using computational fluid dynamics (CFD) to analyze the flow. Results show that CFD can reproduce trends observed in potential flow studies, and also account for significant viscous effects in the wake and the friction coefficient. Studying this phenomenon shows the design factors for manufacturing a ship with high level of efficiency will change.

Keywords

Computational fluid dynamics, Computational ship hydrodynamics, Dead water, Internal waves, Stratified flow

Pages

xiii, 141 pages

Bibliography

Includes bibliographical references (pages 136-141).

Copyright

Copyright © 2017 Mehdi Esmaeilpour

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