CFD performance prediction simulation for hull-form ...

Journal of

J Mar Sci Technol (1997) 2:257-267

Marine Science

and Technology >SNAJ 1997

CFD performance prediction simulation for hull-form design of sailing boats HlDEAKI MlYATA1, HlROMICHI AKIMOTO2, and FUMIYA HIROSHIMA3 1Department of Naval Architecture and Ocean Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113. Japan 2Department of Applied Mathematics and Physics, Tottori University, 4-101 Koyama-Minami, Tottori 680, Japan 'Mitsubishi Electric. Co., 325 Kamimachiya, Kamakura 247, Japan

Abstract: A new simulation method is developed for the de sign of sailing boats. A time-accurate, finite-volume method is combined with the equations of motion, and the hydrodynamic properties of sailing boats are predicted. The hull con figuration is designed by a CAD system, and a boundary-fitted grid system is generated for the finite-volume method. Six components of forces and moments are derived by integrating surface pressure and tangential stress, and input into the equations of motion. The translational or rotational motions obtained are represented by the deformation of the grid sys tem. This is repeated in a time-marching procedure. This system is applied to the prediction of both steady and un steady sailing performance of boats. The degree of accuracy is examined, and two examples of performance simulation are presented.

Key words: finite-volume method, sailing boat, moving coor dinate, ship wave, equation of motion

Introduction

One important point which is common to almost all computational fluid dynamics (CFD) simulations of ship flow is that they realize the flow field of a ship under the same conditions as with a tank experiment, i.e., the ship model is usually fixed to the grid system as if it was restricted by experimental equipment. In other words, most of the CFD simulations of ships are flow computations for a steady, fixed body. After the appearance of an important work by Rosenfeld and Kwak,1 the treatment of moving body boundaries

has

been

introduced

into finite-volume

Address correspondece to: H. Miyata Received for publication on May 30, 1997; accepted on Aug. 25, 1997

methods for practical purposes.2 On the other hand, the moving technique for a free surface has a long history, and a variety of techniques have been developed for particular problems. Therefore CFD techniques for a moving body and boundary can easily be applied to a freely moving ship. In this paper, the finite-volume method WISDAMVII is developed in the framework of a 0-0-type struc tured grid system with a moving body and free-surface boundaries. This CFD technique is combined with the equations of motion in each time step of time marching, so that the total performance of a sailing boat is realized in the computer. In the next section the procedure of designing a sail ing boat, especially one of International America's Cup Class (IACC), is described. In the following section the simulation method is explained, and its verification is given in the section after that by comparing two existing IACC class boats. Two relatively simple cases of devel opment procedure are then exemplified for both steady and unsteady motions.

Design procedure The most popular method of hull-form design with the help of CFD simulation is to use a CFD code as a flow simulator. The hull form is designed by a CAD system, the hydrostatic properties are examined, and then the design is transferred to the grid generator for the prepa ration of CFD simulation. Through a course of CFD simulation the pressure distribution, wave contours, vortex structures, and resultant forces and moments are

obtained. By comparison between candidate hull forms, the most appropriate modifications are considered with the aid of heuristic knowledge reinforced by the cumu lative results of full-scale and model ship experiments. The modified hull form is again designed and the CFD simulation is repeated.

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H. Miyata et al.: Hull-form design of sailing boats

For a sailing boat, for which the upwind sailing per formance is of significant importance, not only informa

simulation, together with those of three other elements

of a sailing boat which are currently derived from em pirical equations. All movement of the sailing boat is represented by the deformation of the structured grid system. In the simulation of forced oscillatory motion, the mode of motion is given to the hull, and in the simulation of the maneuvering motion the conditions of sails, keel, and rudder can be varied by the automatic control system, so that actual maneuvering motion can

tion about the flow fields, forces, and moments, but also the sailing attitude of the boat in relation to the actions

of sails, keel, and rudder are better derived by simula tion, since balance in the three rotational directions is of

critical importance.

This complicated problem can be resolved by com bining the time-marching CFD simulation with the solu tion of the equations of motion. The CFD performance simulation method for sailing boats is called PPS (per formance prediction simulation), and a block diagram is shown in Fig. 1. The finite-volume method WISDAMVII described in the next section is employed to solve the Navier-Stokes equation in a time-marching manner for a moving body and free-surface boundaries. The equations of motion are solved at each time level by use

be realized.

The procedure of designing a racing boat is shown in Fig. 2, which shows a four-stage design circuit. In the first stage, judgement is dependent on the hydrostatic properties and wave resistance values in an upright po sition estimated by MichelTs approximation and/or the panel method. In the second stage, the PPS is used mainly to predict the boat's performance in steady up wind sailing, and a polar diagram can be generated for

of the forces and moments of the hull derived from the

Empirical Formula Sail Keel

CFD simulation of the hull Force

Equation of Motion

Moment

with 6 degrees of freedom

Rudder

WISDAM-VH

Moment

Motion of the ship Simulation of static balance

Heave pitch Roll

Resistance property

Change of position grid deformation

Fig. 1. Composition of performance pre

Yaw

diction simulation

Sail modeling

CAD

Hull modeling

Appendage modeling

FEM

CFD

Lifting surface theory

Lifting surface theory

Hydrostatic Property Wave Resistance

calculated by Michell's Integral & Panel Method

Static PPS

Dynamic PPS

Wave Resistance , Ship Attitude

Dynamic stability , Tacking Simulation

Towing Tank Test Upright Test Leeway Heel Test etc

Fig. 2. Procedure of hull-form develop ment for IACC-class racing yacht

H. Miyata et al.: Hull-form design of sailing boats

259

possible candidate hull forms. In the third stage, un steady motion associated with sailing maneuvers is examined by the unsteady PPS system. Finally, some important relations of the predicted performance are verified by a tank test. The final determination of the best hull form can only

be made by a race simulation, because the speed and other performance characteristics depend entirely on the expected conditions of wind and waves. The most important pieces of information needed for the race simulation are the polar diagrams obtained at the sec ond stage of the design circuit. However, the race simu lation is beyond the scope of this paper. A much greater number of candidate hull forms can be tested by the PPS system compared with the classical way of hull-form development, which mostly relies on the tank test. The advance of computer technology pro motes the use of such a design procedure by perfor mance prediction simulation.

Performance prediction simulation method Finite-volume method for moving boundaries It is assumed that the flow interaction between hull,

sails, keel, and rudder is small, so the CFD simulation

method is developed only for the hull. The forces and moments generated by sails, keel, and rudder shown in Fig. 3 are taken from empirical equations or other numerical methods.

The 0-0-type grid system is employed (Fig. 4) be cause it can cope with the hull configuration of a racing boat with a moving free surface. The variables are de fined at the center of each cell of the grid system, and differencing is done with respect to the volume and surface areas of a cell.

All degrees of freedom of motion are represented by the deformation of the grid system, and then the

Navier-Stokes equation and the continuity equation are formulated, taking into consideration the time variation of the control volume. Denote the control volume as

V(f), the enclosing surface as S(f), the velocity vector as it. the moving velocity vector as v, and the stress tensor as 7", where / is the unit matrix and P is the normalized pressure without hydrostatic component. Then the governing equations are

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