-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 free­surface sources and doublets

Fr

g UB u.v.w

U,W Tr x,y,z

ex <I>

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, accu­rately 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 compara­tive 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 coordi­nated 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 center of each panel. Lifting bodies are also treated, by

37

The internal boundary condition selected is that of

zero perturbation potential which, when applied to all panels, corresponds to specifying the flow inside the body and on the "other side" of the free-surface to be freestream flow. In matrix form

(3)

where Aij and Bij are the potentials at the interior control

point of panel i induced by unit doublets and sources, ~

and crj , respectively, on panel j.

It follows from the internal zero-perturbation formulation that the velocities on the body and free-surface are given by the local source strength and the local surface

gradient of doublet strength

Note the use of metrics corresponding to the transformation

from local panel ~. ~. and n computational coordinates to

Cartesian coordinates. As shown in Fig. 3, ~ and ~

represent arclength distance from control point i along each of the in-plane panel directions, and n represents arclength distance in the direction normal to the panel.

For the zero Froude number sc ,ution, the external boundary condition imposed on both ya :ht and free-surface panels is that of zero normal velocity

- - an an an v norm = v * n = u ax + v ay 1 w az = 0 (5)

which, together with Eq. 4, allows the ;ource strengths to be determined a priori

so that Eq. 3 becomes a linear system < f equations for the unknown doublet strengths

(7)

Finally, velocities on the yacht and "fl 1t" free-surface are computed using Eq. 4.

Linear Free-Surface Boundarv Conditio1 1

The linear free-surface boun lary condition is derived by formulating the problem as i small perturbation to the zero Froude number solution. 1 he exact nonlinear free-surface boundary condition can be i xpressed as

where

and

011 v as = (U2 + y2 + w2)112

(10)

Here, Tl is the free-surface elevation, Cp is the hydrodynamic pressure coefficient, UB is the freestream velocity, g is the acceleration due to gravity, and (a tas) denotes a derivative taken along a free-surface streamline.

Expressing the free-surface solution as a small perturbation to the zero Froude number solution, denoted by ( )0 , and retaining only first order terms, results in the

following approximation for the pressure coefficient

(11)

Applied to the zero Froude number solution, Eqs. 4 and 5 can be used to show that this form of the pressure coefficient depends only on the unknown doublet strengths, and is in fact independent of the unknown free-surface source strengths.

The free-surface streamline gradient terms in Eq. 8 are also approximated by their values along the zero Froude number streamlines

Cartesian Coordinates x,y,andz

Local Panel Coordinates i;, ~.and n

(12)

FIG. 3 Coordinate Systems for SPLASH Free-Surface Code

38

and

v (13)

The pressure gradient term is further split into contributions from each of the in-plane coordinates (Ref. 6)

(14)

where the contravariant velocities are found from the Cartesian velocities and the free-surface grid metrics

(15)

Combining Eqs. 13 and 14 with Eq. 8 gives the final form of the linear free-surface boundary condition, as a perturbation to the zero Froude number solution

This equation is then applied at free-surface panel control points in lieu of the usual solid surface boundary condition, Eq. 5.

To ensure that waves propagate in the downstream direction only, streamwise first derivatives of pressure in the free-surface boundary condition are constructed using upwind panel-to-panel finite-difference operators based on Lagrange polynomials (Dawson in Ref. 5 applied a similar approach, but to the second derivative of potential). Far field boundary conditions are also required, to avoid reflection of waves from the edge of the finite free-surface panel model. These are imposed by increasing panel size and reducing the number of points in the upwind difference operator as the outer edge of the model is approached.

In essence, Eq. 16 relates each unknown source strength (which appears only in the last term) to a linear combination of unknown doublet strengths, i.e.

(17)

where the summation in k is local, involving only those panels required to construct pressure coefficient and pressure gradient finite-difference stencils.

For the nonzero Froude number free-surface solution, the external solid surface boundary condition for panels on the surface of the yacht, (Eq. 6), and the external linear free-surface boundary condition for panels on the free-surface, (Eq. 17), are combined with the internal

39

zero-perturbation boundary condition, Eq. 3. In this instance, the result is the following linear matrix system of

equations for the unknown doublet strengths

I. Aij µj +I. Bij I. bjk µk = -I. Bij 0-j -I. Bij aj (18) BOAT FREE BOAT FREE

+ SURFACE SURFACE

FREE SURFACE

Finally, velocities on the yacht and free-surface are computed, again using Eq. 4.

SPLASH Code Development for the '92 America's Cup Defense

After the '87 races, SPLASH was still only a modification to the aerodynamic panel code described in Ref. 4. For the '92 races, a new code was developed, from the ground up. This provided a solid basis for more advanced study of free-surface physics and numerics, and reduced the computer resources required to process the types of calculations needed for engineering predictions.

The new SPLASH code has no restrictions on model size (number of panels). Input data format has been greatly simplified .. Several in-core direct and accelerated iterative matrix solvers are available. A new technique was developed to accurately transfer computed flow solutions from panel control points to panel comer points, for output to a TECPLOT plot file (Ref. 7), or for use in an enhanced algorithm for conducting velocity scans and tracing streamline trajectories. Even the basic panel-to-panel aerodynamic influence coefficient computation kernels have been reformulated, to streamline calculations and to avoid several recently discovered numerical anomalies. An average of about 2100 panels was used for most of the calculations to be presented, with typical execution times of 130 seconds on a CRAY YMP or 700 seconds on an IBM RS/6000 workstation.

Other code developments were geared towards providing a more realistic and more accurate flow simulation. A new non-wall-sided free-surface boundary condition was implemented. This consists of additional logic to allow panel-to-panel velocity and pressure gradient stencils to cross the hull/free-surface waterline intersection. This helps stabilize calculations for panel edges which are not orthogonal at the waterline, and for hull shapes with overhang and flare, where free-surface streamlines may in fact flow under the hull at the bow or emerge from under the hull at the stem. Coding was also added to allow the panel-to-panel free-surface pressure gradient stencil to cross the keel and rudder wakes. In the new code, wake panel geometry is organized the same as yacht and free-surface panel geometry. The greater control over wake geometry which results is exploited to model coplanar keel and rudder combinations. A new boundary condition is also available for robust modelling of separated flow base regions. This is used to model bulbs with finite base areas.

Two versions of a new automated geometry package, ACCPAN, were also developed, to treat IACC yachts and to take advantage of the capabilities of the new

SPLASH code. The first version utilil !S algebraic grid generation techniques to panel the hull and free-surface. The second version utilizes elliptic grid generation techniques (Ref. 8). The elliptic equi lions are solved directly on the hull database definit .on and on the undisturbed free-surface. The secon j version also incorporates previous stand-alone pre grams used for iterative sink-and-trim calculations.

Both versions of the geometry p tckage model the keel and rudder wakes as a coplanar sys em. As shown in Figs. 1 - 3, the upper portion of the keel wake stops at the rudder leading edge, while the lower por ion passes below the tip of the rudder. The rudder sheds its own wake as well. Wake lines are constrained to lie i1 t the center-plane of the yacht, and are approximated (Ref. 9) by a blending of streamlines derived from slender body theory as applied separately to the hull and to the bulb. Spa iwise spacings on the keel and rudder are adjusted accordingly. Trim tab and rudder deflections are treated using a well-validated non-zero normal velocity "transpir 1tion" boundary condition, rather than actual panel det lections. For the problems to be addressed, more so Jhisticated wake streamline or surface deflection model : are difficult to justify.

Sink and trim are determined b: iteration. Given initial values for sink and trim, calcul ttions proceed as follows: 1) a panel model is generated: 2) the SPLASH code is executed; 3) the resulting hydrod: ·namic forces and moments are used to determine new sinJ and trim values, by balancing the hydrostatic and hydro· lynamic terms in the vertical force and pitching mm 1ent equilibrium equations (provision is also made for iser-supplied sail forces or trim weights). These three ~ teps are repeated until sink and trim are satisfactorily com !rged. Two to six iterations are usually required, dependin : on the accuracy of the initial values as well as on boat spe ~d.

RESULTS

Applications have been taken dire :tly from analyses performed in support of P ACT's techno ogy development

SPLASH Towing Sink&Trim Code Tank

------- Fixed

--- 0 Free

6 10 12 14 16

Speed (kts)

FIG. 4 SPLASH Upright Wave Resist nee Correlation for PACT Baseline Hull with PACT Keel

40

effort and Team Dennis Connor's quest to defend the America's Cup. Selected configurations consist of isolated components, such as finlbulb/winglet appendages and a surface piercing foil, as well as full IACC yachts. Examples presented clearly show how SPLASH can be used to provide the yacht designer with the simultaneous calculation of effects such as upright wave resistance, lift (side force) and lift-induced drag (lifting efficiency or reduced draft) at heel and yaw. Unless stated otherwise, all calculations were performed with the configuration free to sink and trim, and with sail forces applied.

Upright Resistance

In most cases, SPLASH upright wave resistance calculations were obtained without prior knowledge of towing tank results. This served to provide an additional means of verifying code accuracy and robustness. Computational analysis of all configurations was performed using rudder off models, while tests were conducted rudder on. High density half-plane panel models took advantage of the centerline symmetry for the upright

"' g "' o; '(ii

"' a:

SPLASH Towing Hull Code Tank

------- 0 Baseline

--- 0 Heavy

6 8 10 12 14 16

Speed (kts)

FIG. 5 SPLASH Upright Wave Resistance Correlation for PACT Baseline and Heavy Displacement Hulls with PACT Keel

SPLASH Towing Appendage Code Tank

------- 0 PACT Keel

--- 0 Fin/Bulb

6 8 10 12 14 16

Speed (kts)

FIG. 6 SPLASH Upright Wave Resistance Correlation for PACT Baseline Hull with PACT Keel and Fin/Bulb Appendages

SPLASH Towing Code Tank

------- D

--- 0

Displacement

Light

Heavy

' ' '

Speed -

FIG. 7 SPLASH Upright Wave Resistance

Q) 0 c:

"' o; ·u; Q)

a:

Correlation for TDCI Baseline Hull with PACT Keel at Light and Heavy Displacements

SPLASH Towing Appendage Code Tank

------- D PACT Keel

--- 0 Fin/Bulb

' '

Speed -

FIG. 8 SPLASH Upright Wave Resistance Correlation for TDCI Baseline Hull with PACT Keel and Fin/Bulb Appendages

condition. Experimental wave drag results used to compare with computational predictions were obtained using a post­test data analysis procedure. Final tank residuary drag levels were determined by subtracting estimates for skin friction, parasite drag (i.e. boundary layer studs) and component interference effects from the total measured carriage drag. Tank derived residuary drag levels are

therefore strongly influenced by the accuracy of these estimates as well as the accuracy of the testing procedures.

A series of generic IACC hull forms was tested by PACT to develop a new data base for use by all American defense syndicates. These configurations provided the initial means of assessing the upright wave resistance prediction capability of the SPLASH free-surface method. In Figure 4, calculations for the first PACT hull/keel model are presented. This configuration consisted of a baseline hull with a zero sweep leading edge inverse tapered keel Predictions for fixed as well as free to sink and trim conditions are compared to towing tank data. As boat speed increases, the free to sink and trim calculations correctly trend with the wave resistance data, particularly for boat

41

~ c:

"' o; ·u; Q)

a:

Q) 0 c:

"' o; ·u; Q)

a:

SPLASH Towing Code Tank

------- D

--- 0

Hull

Baseline

Narrow

' ' ,

' '

Speed -

FIG. 9 SPLASH Upright Wave Resistance Correlation for TDCI Baseline and Narrow Hulls with Fin/Bulb Appendage

SPLASH Towing Hull

Code Tank

------- D Baseline

--- 0 High-Speed

' '

Speed -

FIG. 10 SPLASH Upright Wave Resistance Correlation for TDCI Baseline and High-Speeds Hulls with Fin/Bulb Appendage

speeds above 9 knots. Below 9 knots the predictions indicate a smooth variation in drag, whereas the data shows

a slight "bump" which peaks just under 8 knots. The excellent agreement at high speeds verifies the sink and

. trim methodology.

Unlike the 12 meter class, the new IACC rule offers the yacht designer greater latitude in developing a competitive sailboat. PACT studies included the effect of yacht displacement, which can have a major impact on performance. Calculations are presented in Figure 5 for the original baseline hull and for a heavy displacement hull of the same length. SPLASH correctly predicts the higher resistance of the heavy hull form. At very high speeds the code tends to under predict the heavy boat upright wave resistance.

SPLASH predictions were also correlated to PACT towing tank data for several fin/ballast-bulb appendages. A typical example is given in Figure 6. Results for the fin/bulb configuration are quite similar to the baseline keel calculations. SPLASH correctly predicts not only the

fin/bulb wave resistance increment, but ' lso the initial boat speed where drag levels for the two conf gurations begin to diverge.

Engineering studies were funded by TDCI as part of their Stars & Stripes '92 America's 1 ~up design effort. Calculations supplemented other desi ;n efforts which included wind tunnel, towing tank and f eld tests. Figure 7 illustrates how accurately the diffr rences between a light and heavy displacement boat are r :solved. As in the case of the PACT configurations, the le ;ation where drag levels for the models begin to diverge J rom each other is also correctly predicted.

The TDCI baseline hull was te~ ted with both the PACT keel and a fin/bulb append lge. Calculations obtained for these configurations are cc mpared with tank data in Figure 8. Results indicate the pro 1er drag increment due to the presence of the bulb throughm t the speed range.

Comparison with data for th' : TDCI baseline configuration and for a narrow hull, I oth fitted with a fin/bulb appendage, is presented in FiJ . 9. This narrow hull, a parametric variant of the baseli 1e, was one of a series of candidate hull forms for TD~ :rs second IACC yacht. Predictions show the subtle < ecrease in wave resistance for the narrow hull, as boat spt ed increases.

A high speed hull form was deve oped by the TDCI design team using a numerical optimizi lion scheme. This procedure was based on a method more approximate than SPLASH, but extended to include an automated design option. Optimization was performed fa · minimum calm­water wave resistance at a particulru boat speed. As intended, the optimized configuration h~ less upright wave

81

81B1W11 81 B1W21

resistance than the baseline hull at high speeds (Fig 10). SPLASH predictions capture all aspects of this design, including the low speed nonlinear drag penalty.

Isolated Appendages

Calculation of IACC yacht lift induced drag can be greatly affected by component interference and the presence of the free-surface. In order to accurately determine code prediction capabilities in this area, tests needed to be conducted to isolate certain sailboat components from each other and from the free surface. In the fall of '91 such a test was conducted by Boeing/PACT at the University of Washington Aerodynamics Laboratory (UW AL) wind tunnel test facility. Several isolated appendage configurations, consisting of various bulbs with and without winglets, mounted on an unswept, untapered fin were analyzed. Nominal test conditions consisted of a Mach number of 0.21 and a Reynold's number of 1.35 million. Post-test computational analyses, without a free­surface, were performed using both SPLASH and PAN­AlR (Boeing program A502, ref. 10).

Calculations were performed for the isolated fin (Sl), fin with ballast-bulb (SlBl), fin/bulb with small span winglets (SlBlWl.1), fin/bulb with large span winglets (S1BlW2.l), and fin/beavertail-bulb with aft mounted large span winglets (S1B2W2.3). For comparison purposes shaded surface representations of these configurations are given in Figure 11. A typical panel model consisted of approximately 4,400 surface panels and 750 wake panels (Fig. 12). Predicted surface pressure contours for the small span winglet case is presented in Fig. 13.

81B2W23

FIG. 11 Boeing/PACT Appendage Models

42

FIG. 12 SPLASH Panelization for Boeing/PACT Isolated Appendage

0

I UWAL Wind Tunnel (Boeing) SPLASH Code A502 Code (Boeing)

51 51B1 51B1W1.1 51B1W2.1 51B2W2.3

Configuration

FIG. 14 SPLASH Effective Draft Correlation for Boeing/PACT Isolated Appendage Models

60.--------------------~

I UWAL Wind Tunnel (Boeing) SPLASH Code A502 Code (Boeing)

c 40 e iil "' e 30 0..

Si 20 c:

c'.'l

~ 10

.'.3

0 51 51B1 51B1W1.1 S1B1W2.1 51B2W2.3

Configuration

FIG. 16 SPLASH Lateral Center of Pressure Correlation for Boeing/PACT Isolated Appendage Models

43

1.0

0.9 0.8

0.7

0.6

0.5

0.4

0.3 0.2

0.1 0.0

·0.1

-0.2 -03 -0.4

-0.5 , .... ,., -0.6

-1.2

-1.3

-1.4

FIG. 13 SPLASH Surface Pressures for Boeing/PACT Isolated Appendage Model, Suction Side at

a.=40

0.10 ,...-----------------------,

~ 0.08 e °' Q) "t>

~ 0.06

~ ~ 0.04 2: a 5 0.02

0.00

I UWAL Wind Tunnel (Boeing) SPLASH Code A502 Code (Boeing)

51 5181 51B1W1.1 51B1W2.1 51B2W2.3

Configuration

FIG. 15 SPLASH Lift-Curve Slope Correlation for Boeing/PACT Isolated Appendage Models

35.----------~-----------,

l:! _g 30 <.:>

1 25

i 20

£ 15 0

~ 10 c'.'l l:! _g 5 (.)

0

I UWAL Wind Tunnel (Boeing) SPLASH Code A502 Code (Boeing)

51 5181 51B1W1.1 51B1W2.1 51B2W2.3

.Configuration

FIG. 17 SPLASH Chordwise Center of Pressure Correlation for Boeing/PACT Isolated Appendage Models

Effective draft (Tr) comparisc 1s between both prediction methods and the wind tunnel 1 ata can be seen in Fig. 14. Agreement is excellent for all m :thods and models except for the aft mounted winglet cor figuration. In this case the data indicates a slightly lower ~ r than both PAN­AIR and SPLASH. The lower experim( 11tal reduced draft might be attributed to viscous effects, which would be more pronounced in the aft bulb region. l redicted lift curve slope levels are nearly identical to the e: perimental values as shown in Fig. 15. Lateral (vertical) an I chordwise center of pressure locations are presented in Figs. 16 and 17 respectively. Surprisingly the com1 utational results consistently indicate a lower lateral cent1 r of pressure than the wind tunnel data. Chordwise C( 11ter of pressure predictions are in fair agreement with the experimental results

Surface Piercing Foil

An isolated surface piercing fr ll was tested by PACT to provide insight into the flow fo ld experienced by rudders and in anticipation of novel IAC ::: design concepts (i.e. twin foil or tandem rudders). TestinE was performed at the David Taylor Research Center (D1 RC) towing tank facility. Foil geometric characteristics \\-:re selected to be representative of rudders/foils typically found on present IACC configurations. Tests were conduc :ed at boat speeds

o.

cJ' a; 0

~ ·;;; Q> a:

FIG. 18 SPLASH Free-Surface Pru elization for Isolated Surface Piercing I oil

•

,/<>--.___ 'o, ---o~~~ s_H_·_:~"~-~~---_,./'_,.,· ---------.'(>., ---- SPu SH Code

06-~~~~~~~~~~ ....... ====-=<.>--=~~-v 0 2 4 6 8

Speed (kts)

FIG. 20 SPLASH Upright Wave F esistance Correlation for PACT Iso ated Surface Piercing Foil

10

44

ranging from 2 to 10 knots and yaw angles of 0°, 2°, 4°and 8°. Some selected data was obtained at yaw angles of -4°. No boundary layer tripping devices (i.e. studs) were installed.

Computational flow simulations were obtained at zero heel and for boat speeds ranging from 0 to 10 knots. Unlike a full IACC yacht, these speeds correspond to much higher values of Froude number (0 < Fr < 2.5) when based on foil chord length. This required that the free-surface grid outer boundary extend much further away from the foil (Fig. 18). For ease of modeling, this distance was set equal to standard limits set for the complete yacht. The total surface grid system consisted of the usual 0-grid topology for the free-surface with 1,512 panels (Fig. 18), a foil model with 882 surface panels, and 756 wake panels. (Fig. 19)

Initially, comparisons revealed inconsistencies between SPLASH predictions and tank data. Those predictions included a "handbook" skin friction calculation assuming fully laminar flow. In subsequent analyses, a fully turbulent estimate was added to the inviscid SPLASH results. As can be seen in Fig. 20, the data trends very well, lying between the two predictions. This indicates the flow probably starts off laminar at the foil leading edge and transitions to turbulent further downstream. Foil total lift

FIG. 19 SPLASH Model Panelization for Isolated Surface Piercing Foil

--------0-, __ CI=4' •

- --Ii{---- -- --~ - --- - --·-- --- --

-----~~----<>--------------.-----·-·-·----.------------· .... -·--·--·-·-·

2 4 6 8

Speed (kts)

10

FIG. 21 SPLASH Lift Correlation for PACT Isolated Surface Piercing Foil

•

•

• Towing

/Tank

"SPLASH Code

_c:=!~--0--------~--------~--------~--------

-·-·-~-2~.---<>-·-·-·-·--·-·-+-·-·-·-·-·-·-·-+-·-·-·-·-·- ·-·-+-·-·-------·-· 0

0 2 4 6 8 10

Speed (kts)

FIG. 22 SPLASH Drag Due to Lift Correlation for PACT

Isolated Surface Piercing Foil

u ==

Fr= o Knots

0.0

2 Knots

O.fi

4 Knots 1.0

__________________ i:'!Y~~I-~r~~ ________________ _

Speed (kts)

FIG. 23 SPLASH Effective Draft Correlation for PACT

Isolated Surface Piercing Foil

6 Knots 1.5

1 O Knots 2.5

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3 0.2

0.1

0.0

-0.1

-0.2

·0.3 -0.4

·0.5

-0.6

·0.7

-0.8

-0.9

-1.0

FIG. 24 SPLASH Surface Pressures for PACT Isolated Surface Piercing Foil, Suction Side at a=40

(side force) coefficient and drag-due-lift are accurately

predicted at most speeds and yaw angles (Figures 21 and

22). Discrepancies in drag exist primarily at the higher yaw

angle. The differences in drag levels do not adversely

affect the prediction of reduced draft (FiE. 23). SPLASH

calculations are slightly lower, but trend very well with

boat speed. This agreement is particularly surprising

considering that the linearized SPLASH formulation may

encounter difficulties at these extremely high Froude

numbers.

Computed suction side pressure contours for the

isolated foil at various speeds at 4 degreei~ yaw are shown

in Fig. 24. At zero Froude number the endplating at the

free-surface is very evident. At 2 kno1s an extremely

45

nonlinear lift/free-surface interaction is predicted. As speed

increases further, the pressures approach the infinite Froude

number situation, with almost no endplate effect

Heeled and Yawed Analyses

Computational analyses of complete IACC yachts at

heel and yaw demonstrate the ability of SPLASH to treat

the complex interaction of sailboat components in the

presence of the free-surface. Predictions were obtained for

the PACT baseline hull with the fin/bulb and rudder

appendages. Calculations were performed at yaw angles of

0° & 4° for the 0° heel condition, and 0°, 2° & 4° yaw at

20° heel.

Overall, SPLASH side force prec ictions agree well with towing tank data (Fig.25), with cor iputational results indicating slightly higher lift levels. A~ :eement for drag­due-to-lift is not as good (Fig. 26) Assessing code accuracy for drag due to lift can be difl icult. Data scatter and the number of numerical calcu ations limit the conclusions that can be drawn. Predic ed reduced draft levels at both heel conditions (Fig. 27 a 1d 28) show only fair agreement with the experimental v Llues. Rudder off comparisons, not presented, exhibit betu r correlation with data.

Subsequent to these calculation ., during another study, it was determined that reduced dn ft calculations are

~ 0

al 0 :0 "O

"' a:

O Heel=<J', Yaw=4°

SPLASH Code - Open Symb< s Towing Tank-Closed Symbc s

-<> •••• r.+. •

0 o- - - - -- - - - - - -•--j:J-.--llJ..i--... -=i-•-lltl • • Heel..20°, Yaw=<J' ""'

0 2 4 6 8 10 12 14 16

Speed (kts)

FIG. 25 SPLASH Side Force Correl ttion for PACT Baseline Hull with Fin/Bult Appendage, Rudder On

·- . . .. - ----8>------~·~.-c..:-:::!:•~-.. ~-~ :j_.____ -· -+- ·--- ... -.-

SPLASH Towing Appendage Code Tank

-0-- ---11--· PACT Keel

--<>- ---+--· Fin/Bulb

0 ~0---2--4 ___ 6 ___ 8 __ 1_0_ -1-2--14--1-'6

Speed (kts)

FIG. 27 SPLASH Reduced Draft Correl 1tion for PACT Baseline Hull with Fin/Bulb AI pendage, Rudder On, o0 Heel

46

more sensitive than had been previously thought. Fig. 29 shows a rudder off drag polar with computed points at several yaw angles, and the quadratic curve fits which result from including some or all of the computed points. Only when the negative yaw angle is included in the curve fit is the "bucket", and hence the polar shape and effective draft, properly captured. This effect is shown across a range of speeds in Fig. 30_

Computed yacht and free-surface pressure contours are shown in Figs. 31 and 32. In Fig. 31 the continuity of pressure across the waterline is very evident, while in Fig. 32 the keel suction pressures are seen to propagate onto the free-surface.

"' ::; s "' :0 0

Heel=D', YaW=4'

SPLASH Code - Open Symbols Towing Tank - Closed Symbols

v------------ .-.--·- T Heel..20', Yaw=4' • T

T T T T

~ . Ci Heel=20: ~~'!::?:-- __ - - - - - - --~ :;._- - - --6.

~ 0

al 0 :0 "O

"' a:

A----- A 0--D- - - -0- -

o o------------•--.i-.--'b-- • • • ~-itl • I Heel=20', Yaw=<J0

• •

0 2 4 6 8 10 12 14 16

Speed (kts)

FIG. 26 SPLASH Drag Due to Side Force Correlation for PACT Baseline Hull with Fin/Bulb Appendage, Rudder On

. ·----SPLASH Towing Appendage

Code Tank

-0-- ---11--· PACT Keel

--<>- ---+--· Fin/Bulb

4 6 8 10 14 16

Speed (kts)

FIG. 28 SPLASH Reduced Draft Correlation for PACT Baseline Hull with Fin/Bulb Appendage, Rudder On, 200 Heel

D Computed Point

----- Frttooa.2°,4°YawPoints

-·-·-·-·-·-·· Frt to 0°, 2°, 4°, 6° Yaw Points

--- Frtto-2°,0°,2°,4eivawPoints

----------··· F"rt to -20. 0°, 2°. 4°, 6° Yaw Points

X "Origin" of Quadratic Curve Fit

0

Lift-

FIG. 29 SPLASH Drag Polar Prediction f x PACT Baseline Hull with PACT Keel, F.udder Off at

\ \ J )

I (

12 Knots, 200 Heel

I I I

\ I

16

14

Computed Yaw Points Used for Quadratic Curve Fit to Drag Polar

--0-- 0',2°,4°

-----b,---·· 0°, 2'. 4'. 6°

g ---0- -2',0'.2°.4° .t:: e 0

"' > ·u ~

--·--'iJ'·---· ·2', 0°, 2'. 4°, 6°

12

10

8 6 8 10 12

Speed (kts)

FIG. 30 SPLASH Effective Draft Prediction for PACT Baseline Hull with PACT Keel, Rudder Off at

200 Heel

\

\

I

I I

/ I

/

I /

:.::#

·:~;::" ..

cp 0.26

0.24

0.22

0.20

0.18

0.16

0.14

0.12

0.10

0.08

0.06

0.04

0.02

-0.00

-0.02

-0.04

-0.06

-0.08

-0.10

-0.12

-0.14

-0.16

-0.18

-0.20

FIG. 31 SPLASH Pressure Contours for IACC Yacht at 9 Knots, 200 Heel: Bottom View

CONCLUSIONS

Linear SPLASH free-surface methodology was

found to accurately capture critical details ·in wave resistance and lift and induced drag characteristics for a variety of appendage and complete yacht models. The examples presented illustrate the value o: the code for design and analysis of IACC yachts. For upright wave resistance in particular, linear SPLASH results are unequalled by any other available method.

Differences between the computed predictions and

47

experimental data can be attributed, at least partially, to

viscous effects; for example the usual tendencies to

overpredict lift and effective draft and to underpredict

induced drag. Nonlinear free-surface effects are also a

consideration, particularly for the complete yacht models. These may arise through the nonlinear free-surface hydrodynamics and through the resulting geometric and hydrostatic nonlinearities. For example, accounting for variable wetted areas was found to be important for deriving tank residuary drag levels. Viscous and nonlinear effects are sure to be a subject of study for many years to come.

ACKNOWLEDGMENTS

The PACT and TDCI studies described herein would not have been possible without the cooperation of many people and organizations. The authors would especially like to thank the following for their efforts: John Marshall, PACT General Manager; Jim Gretzky, PACT Technical Coordinator; Chris Todter, TDCI Technical Coordinator; Bill Trenkle, TDCI General Manager; Bruce Nelson, Nelson/Marek Yacht Design: Dave Pedrick and T.J. Perotti, Pedrick Yacht Designs; Jim 1"eeters, Sparkman & Stephens; Frank Debord, Scientific Marine Services; Carl Scragg, SAIC; Ed Tinoco and Paul Bogataj, Boeing Aerospace; and Claudio Fassardi, Artec Offshore Corporation.

Additional support was also contributed by the following individuals and organizations: Kent Misegades, CRAY Research (CRAY computer resources); Milt Thrasher, IBM (IBM RS/6000 workstations); and Mike Peery, Amtec Engineering (TECPLOT !:raphics and flow visualization software). All results presented were computed on CRAY supercomputers or on an IBM workstation. All figures presented were generated using TECPLOT.

REFERENCES

1. Boppe, C.W., Rosen, B.S., Laiosa, J.P., and Chance, B., Jr., "Stars and Stripes '87: Computational Flow Simulations for Hydrodynamic

J '

I

I

) )

2.

3.

4.

5.

6.

7.

Design," the Eighth Chesapeake Sailing Yacht Symposium, Annapolis, MD, 1987.

Lindenmuth, W.T., Ratcliffe, T.J., and Reed, A.M., "Comparative Accuracy of Numerical Kelvin Wake Code Predictions - Wake Off," DTRC/SHD-1260-01, May 1988.

Tahara, Y., Stern, F. and Rosen, B., "An Interactive Approach for Calculating Ship Boundary Layers and Wakes for Nonzero Froude Number," Journal of Computational Physics, Vol. 98, No. 1, Jan. 1992, pp. 33-53.

Maskew, B., "PROGRAM VSAERO: A Computer Program for Calculating the Non-linear Aerodynamic Characteristics of Arbitrary Configurations," NASA-CR-166476, Dec. 1982.

Dawson, C.W., "A Practical Computer Method for Solving Ship- Wave Problems," in The Proceedings. 2nd International Conference on Numerical Ship Hydrodynamics, Berkeley, CA, Sept. 1977.

Jameson, A., "Iterative Solution of Transonic Flows over Airfoils and Wings, Including Flows at Mach 1," Communications on Pure and Applied Mathematics, Vol. 27, 1974, pp. 283-309.

"TECPLOT: Version 5 User's Manual," Amtec Engineering, Inc., Bellevue, WA, 1992.

0.26

0.24

0.22

0.20

0.18

0.16

0.14

0.12

0.10

0.08

0.06

0.04

0.02

·B.{JO ·"0°.02

- - - - _---,... ~~"'--~~~------------~··- - - - ·0.08

-0.10

·0.12

-0.14

-0.16

-0.18

-0.20

FIG. 32 SPLASH Pressure Contours for IACC Yacht at 9 Knots, 200 Heel: Windward View, Keel Suction Side

48

8. Steinbrenner, J.P., Chawner, J.R., and Fouts, C.L.,

"The GRIDGEN 3D Multiple Block Grid

Generation System; Vol. I: Fi 1al Report,"

WRDC-TR-90-3022, July. 1990.

9. Rosen, B.S., "Method to Predict External Store

Carriage Characteristics at Transc nic Speeds,"

NASA-CR-4170, Aug. 1988.

10. Magnus, A.E., and Epton, MA, "FAN AIR - A

Computer Program for Prediction Subsonic or

Supersonic Linear Potential Flows A t>out Arbitrary

Configurations Using a Higher order Panel

Method,", Vol. I - Theory Document (Version 1.0),

NASA-CR-3251, 1980.

. ..

49