Contours Newsletter

PLOT OF THE MONTH

Dr. Warren Davis develops and applies computational fluid dynamics (CFD) methods for aerodynamic and hydrodynamic analysis of aircraft and boat designs. He was a design consultant to the AmericaOne syndicate, a contender for the America's Cup 2000 in Auckland, NZ. Dr. Davis was involved in the underwater design including the hull, and the keel, bulb, wing and rudder appendages.

The goal of the design team was to reduce the underwater hull and appendage drag, thus improving the boat's speed. Weather and water (waves) conditions vary for different sailing courses, so this boat was optimized for the course sailed in New Zealand.

Our Image of the Month is the result of a SPLASH analysis of an AmericaOne yacht sailing upwind at 9.5 knots, heeled over 25 degrees. SPLASH is a free-surface panel method (inviscid) CFD code developed by South Bay Simulations, Inc. The calculation was performed early in the design process for AmericaOne before the first boat began construction.

In the image, calculated surface pressure coefficients are displayed on the underwater appendages and contoured on the free water surface. The hull's shaded surface is cut at the waterline, thus only the surface pressure coefficients are visible underwater. The water surface contours show the pattern of waves generated by the moving hull. A complete yacht is visible in the animation with topsides hull and main and Genoa sails.

SPLASH is a numerical simulation of a tow tank test. A physical tow tank test consists of obtaining yacht forces for a matrix of sailing speeds, yacht heel and yaw, rudder and keel tab deflections. "Testing yacht models in towing tanks is in many ways more demanding than testing of aircraft in wind tunnels, particularly since the model is typically free to sink (up and down) or trim (bow up or bow down)." Traditional physical model tests in a towing tank are also very expensive and can easily eat away at the limited budget of any design team.

Numerical model tests may deal more directly with uncertainties and complications of predicting open water sailing. Numerical simulation can also yield more consistent results in less time than physical testing.

Recently there has been a bit of role reversal of CFD simulation over expensive physical testing. "SPLASH played a lead role in the 2000 hull design, with predictions accurate enough to be relied upon for realistic performance differences between designs, while physical testing was used periodically to back up those results." To produce optimal performance all these tools work best in complimentary roles.

Tow tank simulations require several hundred specific sailing points to be analyzed. An automated SPLASH analysis is done for each of those sailing points. Scripting software collects and sorts all of the individual results.

SPLASH produces, for each test point sailing condition, the corresponding flow solution and computed forces - integrated lift, drag, and vertical force - and associated moments about all three axes (balanced against the sail's contributions).

The scripting software then constructs Tecplot data files for the model test results. Tecplot layout files produce a set of 15 plots, cross-plotting all the force and motion data across the full range of variables - speed, heel, yaw, tab and rudder. This process is critical to assessing the quality of the data and to quickly compare performance results from other yacht designs.

Dr. Davis says, "It would be incredibly time consuming and tedious to check the full output of a SPLASH tow tank simulation. It would take several days. With Tecplot, I just glance through the output plots and get my answers almost immediately."

Excerpts from:

Rosen, B. S., Laiosa, J. P., Davis, W. H. Jr., "CFD Design Studies for America's Cup 2000," 18th Applied Aerodynamics Conference, Denver, CO, Aug. 2000. (Download 587 KB PDF paper)