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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
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.
a, b Linear function coefficients relating free- surface sources and doublets
g UB u.v.w
U,W Tr x,y,z
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
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
cr Source singularity strength ~.~.n Local panel coordinate system
ri free-surface elevation
Influenced panel Influencing panel
norm Component normal to panel o Zero Froude number solution ro Free stream conditions
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
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
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
. . . . . . . . -- - . . --- . · .... -·
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:
where cp is the perturbation potential, and the Cartesian
flow velocities are given by
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