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  • - THE ELEVENTH CHESAPEAKE SAILING YACHT SYMPOSIUM

    SPLASH Free-Surface Flow Code Methodology for Hydrodynamic Design and Analysis of IACC Yachts Bruce S. Rosen, South Bay Simulations, Inc., Babylon, New York, USA

    Joseph P. Laiosa, South Bay Simulations, Inc., Babylon, New York, USA

    Warren H. Davis, South Bay Simulations, Inc., Babylon, New York, USA

    David Stavetski, South Bay Simulations, Inc., Babylon, New York, USA

    ABSTRACT

    A unique free-surface flow methodology and its application to design and analysis of IACC yachts are discussed. Numerical aspects of the inviscid panel code and details of the free-surface boundary condition are included, along with enhancements developed specifically for the '92 America's Cup defense. Extensive code validation using wind tunnel and towing tank experimental data address several areas of interest to the yacht designer. Lift and

    induced drag at zero Froude number are studied via a series

    of isolated fin/bulb/winglet appendages. An isolated surface piercing foil is used to evaluate simple lift/free-

    surface interactions. For complete IACC yacht models, upright wave resistance is investigated, as well as lift and induced drag at heel and yaw. The excellent correlation obtained for these cases demonstrates the value of this linear free-surface methodology for use in designing high performance sailing yachts.

    NOMENCLATURE

    a, b Linear function coefficients relating free- surface sources and doublets

    Fr

    g UB u.v.w

    U,W Tr x,y,z

    ex

    µ

    Interior potential at control point of panel i

    induced by unit doublet on panel j Interior potential at control point of panel i

    induced by unit source on panel j

    Lift coefficient

    Drag coefficient

    Hydrodynamic pressure coefficient

    Derivative w.r.t. arclength along a free-surface

    streamline Froude number, Fr= UBI (g L)ll2, where

    L = characteristic length Acceleration of gravity, 32.174 ft/sec2 Boat speed

    Cartesian velocity components normalized with free stream velocity Cartesian contravariant velocity components Effective span or reduced draft Cartesian coordinate system fixed with undisturbed free-surface Yaw angle Incompressible flow perturbation potential Doublet singularity strength

    35

    cr Source singularity strength ~.~.n Local panel coordinate system

    ri free-surface elevation

    subscripts:

    Influenced panel Influencing panel

    norm Component normal to panel o Zero Froude number solution ro Free stream conditions

    BACKGROUND

    Computerized flow simulations have come to play

    an important role in the design of high performance sailing yachts. This is particularly true for America's Cup

    campaigns, as syndicates strive to put the best sailors on the

    fastest boats. One of the more successful efforts in the

    field of Computational Fluid Dynamics (CFD) has been the

    development of the SPLASH free-surface flow code, and

    its use for hydrodynamic design and analysis of Twelve

    Meter and IACC yachts.

    Naval architects must use their experience to

    integrate design information from a variety of sources.

    More classical techniques such as wind tunnel, towing tank and full scale testing will naturally continue to play major roles in the design process. CFD is merely one more tool in

    the designer's arsenal. It is useful not only for engineering prediction of overall performance characteristics, but also for research study where the detailed flow information

    generated can provide greater insight into the underlying

    physics.

    Over the years, the SPLASH free-surface code has proven to be a robust and reliable method for computing 3-D hydrodynamic flows about a variety of submerged and surface piercing shapes. For sailing yachts at heel and yaw, the combination of a displacement hull with side force generating appendages (keel, rudder, ballast bulb, winglets, etc.) results in a strong interaction between free-surface and lifting flow components. The ability to treat these highly

    coupled flows makes the code ideally suited for a variety of

    yacht design applications.

    The SPLASH linear free-surface code was originally developed and successfully applied during the

  • design of the Cup-winning yacht Stars & Stripes '87 (Ref. 1). A companion geometry package AGGPAN) also

    evolved, for automated modeling of Tw1 Ive Meter's with winged keels. Actual design applications : ncluded keel and winglet planform selection for im proved upwind performance, and winglet alignment (twii t and camber) in the presence of the free-surface to mi1 timize resistance downwind.

    Code application during the '87 campaign concentrated primarily on lift and indl ced drag design issues. However, when the races were lver it was clear that the linear free-surface calculatio ts yielded very reasonable predictions for upright wave resistance, accu- rately distinguishing differences betw1 en Liberty and Australia JI and the three individually mique Stars and Stripes Twelve Meter's ('85, '86, and '8T. In addition, the use of SPLASH methodology to treat nonlinear free-surface flows had been demonstnted, on a 2-D submerged vortex test case.

    Subsequent to the '87 races, c ide development continued, although with less emphasis 'm sailing yachts. Wave patterns about a Navy fleet tugboat were calculated,

    without knowledge of the experime ital results, for inclusion by David Taylor Research Cen :er in a compara- tive assessment of several numerical c )des. This study concluded that SPLASH (FLOP AN code Ref. 2) gave the best predictions for the details of the free-surface disturbance in the region within one st ip's beam of the model. In addition it was concluded th 1t the predictions showed free-surface details evident in e 'periments which were, at best, hinted at by the predictions from the better of the other programs. SPLASH was als< · selected for an Office of Naval Research sponsored !ffort to be the "inviscid" half of an inviscid/viscous int !ractive approach for calculating ship boundary layers anc wakes including the interaction with the free-surface (R1 :f. 3). This work was conducted using the well known Wii ;ley and Series 60 hulls, and is now being extended to stl dy the effects of yaw and to evaluate its utility for sailing ~ achts.

    With the selection of the new International America's Cup Class for the '92 races in San Diego, a major code development effort was undertaken to extend free-surface methodology for treatment of IACC yachts. A large portion of this effort was supported by and coordi- nated with the Partnership for America's Cup Technology (PACT). Specific engineering studies were also funded by Team Dennis Conner, Inc. (TDCI). The sections which follow give an overview of linear free-surface methodology, describe some of the enhancements developed specifically for the '92 America's Cup defense, and summarize results from various PACT and TDCI

    design and analysis studies.

    OVERVIEW OF COMPUTATIONAL METHOD

    Since 1985, the SPLASH free-surface code has been undergoing continuous development, by aerospace industry engineers specializing in CFD methods for

    aerodynamic design and analysis of aircraft. Many of the basic concepts and numerical algorithms employed in the code are well known and widely available throughout the

    aerospace industry.

    SPLASH can be characterized as an inviscid panel code, employing simple source and doublet singularities to represent both yacht and free-surface. The same basic approach is widely used for aircraft at subsonic speeds (Ref. 4). The unique free-surface boundary condition couples a Dawson-type upwind finite-difference operator (Ref. 5) with the basic (solid surface) Morino-type internal zero-perturbation formulation.

    For linear free-surface calculations, panels are placed on the undisturbed (flat) free-surface. The zero Froude number flow is computed first, by treating the free-surface panels as solid and fixed. The nonzero Froude number free-surface flow is then computed, using a linear free-surface boundary condition derived by formulating the problem as a small perturbation to the zero Froude number solution (Ref. 5).

    FIG. 1 SPL ~SH Free-Surface Panelization for IACC Yacht at 200 Heel

    36

  • . . . . . . . . -- - . . --- . · .... -·

    FIG. 2 SPLASH Model Panelization for IACC Yacht at 20° Heel

    Basic Flow Code Theozy convecting the shed vorticity downstream, along a panelized representation of the wake.

    The underlying assumption is that of incompressible potential flow, for which the governing equations reduce to Laplace's equation for the perturbation potential:

    (1)

    where cp is the perturbation potential, and the Cartesian

    flow velocities are given by

    (2)

    As shown in Figs. 1 and 2, panels are distributed over the surface of the yacht and over a finite portion of the free-surface surrounding the yacht. Constant source and constant doublet singularities of unknown strength are placed on each panel. Each singularity individually satisfies the flow equation (Eq. 1). When a well-posed set of boundary conditions is also applied, a unique combination of singularities, and hence the corresponding flow solution, can be determined.

    For the situation at hand, internal and external boundary conditions are applied at control points at the cent

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