Practical Ship Hydrodynamics

Practical Ship Hydrodynamics

Butterworth-Heinemann

Chapter One

Chapter Outline 1.1. Overview of Problems and Approaches 1 1.2. Model Tests — Similarity Laws 5 1.3. Full-Scale Trials 9 1.4. Numerical Approaches (Computational Fluid Dynamics) 10 1.4.1. Basic Equations 10 1.4.2. Basic CFD Techniques 16 1.4.3. Applications 17 1.4.4. Cost and Value Aspects of CFD 20 1.5. Viscous Flow Computations 24 1.5.1. Turbulence Models 24 1.5.2. Boundary Conditions 28 1.5.3. Free-Surface Treatment 31 1.5.4. Further Details 32 1.5.5. Multigrid Methods 33 1.5.6. Numerical Approximations 34 1.5.7. Grid Generation 36

Models now in tanks we tow. All of that to Froude we owe. Will computers, fast and new, Make us alter Euler's view? Marshall Tulin

1.1. Overview of Problems and Approaches

The prediction of ship hydrodynamic performance can be broken down into the general areas of:

• resistance and propulsion;

• seakeeping and ship vibrations;

• maneuvring.

Propeller flows and propeller design can be seen as a subtopic of resistance and propulsion, but it is so important and features special techniques that it is treated as a separate topic in its own right. Morgan and Lin (1998) give a good short introduction to the historical development of these techniques to the state of the art in the late 1990s.

The basic approaches can be roughly classified into:

• Empirical/statistical approaches. Design engineers need simple and reasonably accurate estimates, e.g. of the power requirements of a ship. Common approaches combine a rather simple physical model and regression analysis to determine required coefficients either from one parent ship or from a set of ships. The coefficients may be given in the form of constants, formulae, or curves. Because of the success with model testing, experimental series of hull forms have been developed for varying hull parameters. Extensive series were tested in the 1940s and the subsequent two decades. These series were created around a 'good' hull form as the parent form. The effect of essential hull parameters, e.g. block coefficient, was determined by systematic variations of these parameters. Because of the expense of model construction and testing, there are no recent comparable series tested of modern hull forms and the traditional ship series must be considered as outdated by now. Rather than using model tests, today computational fluid dynamics could be used to create data for systematic series varying certain parameters for a ship type (Harries and Tillig 2011). Once such a dedicated 'numerical hull series' is set up, designers can rapidly interpolate within such a database.

• Experimental approaches, either in model tests or in full-scale trials. The basic idea of model testing is to experiment with a scale model to extract information that can be scaled (transformed) to the full-scale ship. Despite continuing research and standardization efforts, a certain degree of empiricism is still necessary, particularly in the model-to-ship correlation, which is a method to enhance the prediction accuracy of ship resistance by empirical means. The total resistance can be decomposed in various ways. Traditionally, model basins tend to adopt approaches that seem most appropriate to their respective organization's corporate experience and accumulated databases. Unfortunately, this makes various approaches and related aggregated empirical data incompatible. Although there has been little change in the basic methodology of ship resistance since the days of Froude (1874), various aspects of the techniques have progressed. We now understand better the flow around three-dimensional, appended ships, especially the boundary layer effects. Also non-intrusive experimental techniques like laser-Doppler velocimetry (LDV) or particle image velocimetry (PIV) allow the measurement of the velocity field in the ship wake to improve propeller design. Another more recent experimental technique is wave pattern analysis to determine the wave-making resistance. In propulsion tests, measurements include towing speed and propeller quantities such as thrust, torque, and rpm. Normally, open-water tests on the propeller alone are run to aid the analysis process as certain coefficients are necessary for the propeller design. Strictly, open-water tests are not essential for power prediction. The model propeller is usually a stock propeller (taken from a large selection (= stock) of propellers) that approximates the actual design propeller. Propulsion tests determine important input parameters for the actual detailed propeller design, e.g. wake fraction and thrust deduction. The wake distribution, also needed for propeller design, is measured behind the ship model using pitot tubes or laser-Doppler velocimetry (LDV). For propeller design, measured nominal wakes (for the ship without a propeller) for the model must be transformed to effective wakes (for the ship with a working propeller) for the full-scale ship. While semi-empirical methods for this transformation apparently work well for most hull forms, for those with considerable flow separation at the stern, i.e. typically full hulls, there are significant scale effects on the wake between model and full scale. To some extent, computational fluid dynamics can help here in estimating the scale effects. Although the procedures for predicting full-scale resistance from model tests are well accepted, full-scale data available for validation purposes are extremely limited and difficult to obtain. The powering performance of a ship is validated by actual ship trials, ideally conducted in calm seas. The parameters usually measured are torque, rpm, and speed. Thrust is measured only as a special requirement because of the difficulty and extra expense involved in obtaining accurate thrust data. Whenever possible and appropriate, corrections are made for the effects of waves, current, wind, and shallow water. Since the 1990s, the Global Positioning System (GPS) and computer-based data acquisition systems have considerably increased the accuracy and economy of full-scale trials. The GPS has eliminated the need for 'measured miles' trials near the shore with the possible contamination of data due to shallow-water effects. Today trials are usually conducted far away from the shore. Model tests for seakeeping are often used only for validation purposes. However, for open-top container ships and ro-ro ships model tests are often performed as part of the regular design process, as IMO regulations require certain investigations for ship safety which may be documented using model tests. Most large model basins have a maneuvring model basin. The favored method to determine the coefficients for the equations of motion is through a planar motion mechanism. However, scaling the model test results to full scale using the coefficients derived in this manner is problematic, because vortex shedding and flow separation are not similar between model and full scale. Appendages generally make scaling more difficult. Also, maneuvering tests have been carried out with radio-controlled models in lakes and large reservoirs. These tests introduce additional scale effects, since the model propeller operates in a different self-propulsion point than the full-scale ship propeller. Despite these concerns, the maneuvering characteristics of ships seem generally to be predicted with sufficient accuracy by experimental approaches.

• Numerical approaches, either rather analytical or using computational fluid dynamics (CFD). For ship resistance and powering, CFD has become increasingly important and is now an indispensable part of the design process. Typically inviscid free-surface methods based on the boundary element approach are used to analyze the forebody, especially the interaction of bulbous bow and forward shoulder. Viscous flow codes focus on the aftbody or appendages. Flow codes modeling both viscosity and the wave-making are widely applied for flows involving breaking waves. CFD is still considered by industry as too inaccurate for resistance or power predictions. So far CFD has been used to gain insight into local flow details and derive recommendation on how to improve a given design or select a most promising candidate design for model testing. However, numerical power prediction with good accuracy has been demonstrated in research applications by 2010 for realistic ship geometries. It is expected to drift into industry applications within the next decade. For seakeeping, simple strip methods are used to analyze the seakeeping properties. These usually employ boundary element methods to solve a succession of two-dimensional problems and integrate the results into a quasi-three-dimensional result with usually good accuracy. For water impact problems (slamming and sloshing), free-surface RANSE methods are widely used in industry practice. Also, for problems involving green water on deck, free-surface RANSE methods have become the preferred tool in practice, replacing previously favored non-linear boundary element methods. A commonly used method to predict the turning and steering of a ship is to use equations of motions with experimentally determined coefficients. Once these coefficients are determined for a specific ship design — by model tests, estimated from similar ships, by empirically enhanced strip methods or CFD — the equations of motions are used to simulate the dynamic behavior of the ship. The form of the equations of motions is fairly standard for most hull designs. The predictions can be used, for example, to select rudder size and steering control systems, or to predict the turning characteristics of ships. As viscous CFD codes became more robust and efficient to use, the reliance on experimentally derived coefficients in the equations of motions has been reduced. In some industry applications, CFD has been used exclusively to compute maneuvering coefficients for ship simulators, for example.

Although a model of the final ship design is still tested in a towing tank, the testing sequence and content have changed significantly over time. Traditionally, unless the new ship design was close to an experimental series or a known parent ship, the design process incorporated many model tests. The process has been one of design, test, redesign, test, etc., sometimes involving more than ten models, each with slight variations. This is no longer feasible due to time-to-market requirements from shipowners and no longer necessary thanks to CFD developments. Combining CAD (computer-aided design) to generate new hull shapes in concert with CFD to analyze these hull shapes allows for rapid design explorations without model testing. With massive parallel computing and progress in optimization strategies (e.g. response surfaces), formal optimization of hulls, propellers, and appendages has drifted into industrial applications. CFD is increasingly used for the actual design of hull and propellers. Then often only the final design is actually tested to validate the intended performance features and to get a power prediction accepted in practice as highly accurate. As a consequence of this practice, model tests for shipyard customers have declined considerably since the 1980s. This was partially compensated for by more sophisticated and detailed tests funded from research projects to validate and calibrate CFD methods.

One of the biggest problems for predicting ship seakeeping is determining the nature of the sea: how to predict and model it, for both experimental and computational analyses. Many longterm predictions of the sea require a Fourier decomposition of the sea and ship responses with an inherent assumption that the sea and the responses are 'moderately small', while the physics of many seakeeping problems is highly non-linear. Nevertheless, seakeeping predictions are often considered to be less important or covered by empirical safety factors where losses of ships are shrugged off as 'acts of God', until they occur so often or involve such spectacular losses of life that safety factors and other regulations are adjusted to a stricter level. Seakeeping is largely not understood by shipowners and global 'sea margins' of, e.g., 15% to finely tuned (±1%) power predictions irrespective of the individual design are not uncommon.

(Continues...)

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Excerpted from Practical Ship Hydrodynamics by Volker Bertram Copyright © 2012 by Volker Bertram. Excerpted by permission of Butterworth-Heinemann. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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