Wesley Wilson

Simulation based design optimization of waterjet propelled Delft catamaran

The present work focuses on the application of simulation-based design for the resistance optimization of waterjet propelled Delft catamaran, using integrated computational and experimental fluid dynamics. A variable physics/variable fidelity approach was implemented wherein the objective function was evaluated using both low fidelity potential flow solvers with a simplified CFD waterjet model and high fidelity RANS solvers with discretized duct flow calculations. Both solvers were verified and validated with data for the original hull. The particle swarm optimizer was used for single speed optimization at Fr = 0.5, and genetic algorithms were used for multi speed optimization at Fr = 0.3, 0.5 and 0.7. The variable physics/variable fidelity approach was compared with high fidelity approach for the bare-hull shape optimization and it showed an overall CPU time reduction of 54% and converged to the same optimal design at Fr = 0.5. The multi-speed optimization showed design improvement at Fr = 0.5 and 0.7, but not at Fr = 0.3 since the design variables were obtained based on sensitivity analysis at Fr = 0.5. High fidelity simulation results for the optimized barehull geometry indicated 4% reduction in resistance and the optimized waterjet equipped geometry indicated 11% reduction in effective pump power required at self-propulsion. Verification was performed for the optimized hull form and its reduction in powering will be validated in forthcoming experimental campaign.

Hydrodynamic Shape Optimization for Naval Vehicles

This effort is focused on hydrodynamic shape optimization with application to naval vehicles. At present, this is mainly applied to US Navy surface combatant hull forms and related systems; however, this approach could also be applied to other marine applications. In addition, this effort is aimed at assessing the current use of hydrodynamic analysis tools as a part of the ship design process and the need to perform proper validation of the tools to provide confidence in their use.

Virtual Model Basin for Resistance, Maneuvering, and Seakeeping: A RANS CFD Based Approach

A Virtual Model Basin (VMB) is developed based on a Computational Fluid Dynamics (CFD) approach to solving the Reynolds Averaged Navier-Stokes (RANS) equations along with the Volume of Fluid (VOF) method for predicting the free surface. The primary objective of this work is to develop methodologies for the VMB and to demonstrate the capabilities for a generic multi-hull ship geometry. The VMB is used to simulate various model basin tests for steady resistance, maneuvering and seakeeping. For a generic catamaran hull configuration, the methodologies are used for solving these problems and the results are discussed in this paper. VMB results are compared with the results of a benchmarked potential flow theory method for calm water resistance.© 2007 ASME

HPCMP CREATE-SH Integrated Hydrodynamic Design Environment

The HPCMP CREATE-SH Integrated Hydrodynamic Design Environment (IHDE) is a workbench-like desktop application that integrates a suite of hull form design and analysis tools allowing a user to execute round-trip evaluations of hydrodynamic performance, including visualization, in a simplified and timely manner. Ship hull designers are able to assess ship performance in areas of resistance, seakeeping, hydrodynamic loads, and operability for several different mission types. The development plan calls for additional capabilities related to maneuvering performance and multi-objective optimization. The IHDE also includes an analysis tool validation engine, which provides the user with validation information by leveraging historical model test data for comparisons. The advantages of the IHDE include automated analysis preparation, automated grid generation, and integrated visualization. The IHDE has been used to support several Navy design studies and this paper describes the capabilities of IHDE and shows examples of typical workflow processes and sample analysis results.

The Increasing Use of Visualization in Ship Hydrodynamics

Flow field visualization is an important part of the study of fluid dynamics and ship hydrodynamics. The field of computational fluid dynamics has provided an unprecedented ability to explore the hydrodynamics of marine vehicles through visualization. Many examples of this exist in the literature for steady flow field situations. However, unsteady visualization provides both challenges and opportunities to extract meaningful physical insight and information from computational simulations. This paper discusses some of these issues along with approaches being pursued to obtain adequate flow field information using remote high performance computing resources as well as concurrent visualization using local resources. A number of examples of flow field computations being pursued are discussed including: cavity flow, ship roll motions, trailing edge vortex shedding, ballast water exchange and crashback

Characteristics of Fuel Droplets Discharged From a Compensated Fuel/Ballast Tank

Fuel droplets, formed by the interaction of fuel plumes with a water/fuel interface, can be discharged during the refueling of water-filled compensated fuel/ballast tanks. Motivated by increasingly stringent environmental regulations, a study was initiated to understand the physical mechanisms involved in the formation and transport of fuel droplets by complex immiscible flows inside a model tank. In particular, optical measurements were made of the size distribution of fuel droplets in water discharged from a three-bay model of a compensated fuel/ballast tank. The volumetric fuel concentration of discharge from the tank was inferred from measurements of droplet size and number Flow visualizations inside the model were coupled to optical measurements of fuel droplets at the tank outlet to show that the presence of fuel in the discharged water was correlated to the formation of fuel plumes within the water-filled tank. The size distribution of fuel droplets at the tank exit is found to differ from the size distribution reported for the generation zone (near the fuel plumes) inside the tank. Thus, the advection of fuel droplets from the generation zone to the tank outlet is shown to affect the characteristics of discharged fuel droplets. The transport process specifically prevents large-diameter droplets from reaching the tank exit. Buoyancy tends to cause larger fuel droplets generated within the tank to rise and separate out of the flow before they can be discharged. The buoyancy time, τ b (D), relative to the characteristic advection time, τ a , of fuel droplets is a key parameter in predicting the fate of fuel droplets. The influence of buoyancy on the size distribution of discharged droplets was found to be modeled reasonably well by a Butterworth filter that depends on the ratio of timescales τ a /τ b (D). This model, which relates the size distribution of discharged droplets to generated droplets, is found to produce the correct qualitative behavior that larger fuel droplets are discharged when the fuel plumes move closer to the tank exit, i.e., for decreasing advection time τ a .

ENTRAINMENT CORRELATIONS BASED ON A FUEL/WATER STRATIFIED SHEAR FLOW

The purpose of this work was twofold: first, to develop correlations for the entrainment of small fuel droplets into water in a stratified fuel/water shear flow; second, to implement the correlations in a CFD code and validate it with experimental effluent fuel concentration data. It is assumed that the droplets act as passive scalars and are advected far from their generation regions where they may cause fuel contamination problems far down-stream. This work relied upon extensive experimental data obtained from a stably stratified shear flow: droplet number, droplet PDF, fluid fraction and velocity field data. The droplet data was expressed as a nondimensional entrainment velocity (E) for the volume flux of fuel due to small droplets. The fluid fraction and velocity fields at the interface were expressed in terms of Richardson numbers (Ri). It was found that E = Ce Ri−n where n = 1 and Ce is a constant, gives a good fit for the two experimental velocity cases. The best correlation was implemented in a computational simulation of the stably stratified shear flow, and the results show that the simulation can predict the entrainment quite well. A second simulation was performed for a flow with energetic vertical buoyant jets (“buoyant flow events”) and stably stratified shear flows with very large Richardson numbers. In this case, the simulations underpredicted effluent fuel concentrations by two orders of magnitude. Ad hoc corrections to the entrainment correlations show marked improvements.Copyright