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ANSYS ADVANTAGE MAGAZINE

January 2021

Freedom of Speed

By Steve Collie, Aerodynamics Coordinator, Emirates Team New Zealand, Auckland, New Zealand

 

To defend the Cup in 2021, the team is again relying on the combination of Ansys and in-house simulation tools for efficient assessment of large design spaces as it seeks to design a foiling monohull that balances speed and maneuverability.



Over the past quarter-century, America’s Cup racing yachts have undergone sweeping shifts in their form and function to comply with evolving design rules. During that same period, Emirates Team New Zealand has pioneered and extended its use of Ansys simulation software in its design process to become a premier racing syndicate. To defend the Cup in 2021, the team is again relying on the combination of Ansys and in-house simulation tools for efficient assessment of large design spaces as it seeks to design a foiling monohull that balances speed and maneuverability.

For the first America’s Cup, in 1851, the winning yacht was the schooner America, which defeated 14 other boats in a single race around the Isle of Wight in the U.K. that lasted more than 10 hours. America bested the British ships with a radical design for the time, validating technological advancements as a key to victory in the race. Over the next 132 years, fleet races gave way to match races, and the U.S. syndicate successfully defended 24 straight times before finally being defeated in 1983 by their challengers from Australia, who sailed with a technologically advanced winged-keel design. Since then, the last 10 America’s Cup events have seen a challenging team victorious five times. Emirates Team New Zealand reclaimed the “Auld Mug” trophy in 2017 in Bermuda by defeating entrants from the U.K., Japan, France and Sweden before vanquishing the U.S. team in the final best-of-13 series.

Scheduled for March 2021, the 36th America’s Cup will launch in Auckland’s Waitematā Harbor as Emirates Team New Zealand sails as the Defender for the first time since 2003. The boat design rules have evolved greatly over the past three decades, from the International America’s Cup Class (IACC) monohulls (1992–2007), to the wingsail catamarans (2010–2017), to the current AC75 class that features foiling monohulls and a return to soft sails. Throughout this period, Emirates Team New Zealand emerged as a consistent contender for the Cup — with consecutive victories in 1995 and 2000 — and has been at the vanguard of incorporating modeling and simulation tools into its design process.

The performance of the boat at different points of sail is evaluated with the help of Ansys CFX. The performance of the boat at different points of sail is evaluated with the help of Ansys CFX.

 

An Early Differentiator

In the late 1990s, the team used simulation, including Ansys Fluent for computational fluid dynamics (CFD) analysis, though mainly to supplement wind-tunnel testing and full-scale testing. A lot of the problems the designers tackled then were in evaluating different turbulence models and meshing strategies in an era when compute resources were a limiting factor. To determine whether design “A” was better than design “B,” the design was first tested in a wind tunnel and then on the water. The team would spend weeks benchmarking one boat against the other, sailing for hours side by side. This repetition was required to get a significant result as conditions on one area of water could be very different from another area only a few hundred meters away.

The team discovered one advantage of using CFD was the 3D visualization of the simulation predictions for the movement of air and water over different hull and sail shapes. This helped Emirates Team New Zealand learn more about aerodynamics for sailing conditions that would be difficult to achieve in a wind tunnel. Any single, small design change may not have been responsible for the Emirates Team New Zealand victories, as the skill and experience of the crew are still paramount to adapt to the real-world conditions. However, when a few seconds is all the difference in winning vs. losing, the addition of simulation allowed the team to predict the cumulative effects of many small changes before building the final race boat. The designers acknowledged the need for simulation as being a vital component to winning the Cup.

ETNZ engineers use Ansys Composite PrepPost for modeling the detailed composite layup in various parts  of the boat, such as the top of the mast shown here. ETNZ engineers use Ansys Composite PrepPost for modeling the detailed composite layup in various parts of the boat, such as the top of the mast shown here.

 

Widespread Adoption and New Design Rules

As more racing syndicates added the power of simulation to their design arsenals, the Cup changed hands twice more over the next four events, and the 82-ft (25-m) IACC designs gave way to foiling multihulled craft with rigid wingsails. Design rules limiting wind tunnel testing and full-scale testing further increased the team’s reliance on simulation; therefore, the team incorporated structural analysis software, including Ansys Mechanical and Ansys Composite PrepPost (ACP) into its modeling toolbox.

Where softsail monohulled crafts early in the IACC era were limited to an average speed of about 10 knots (11.5 mph), the combination of new hull concepts, lightweight materials and the wingsail over several Cup campaigns radically elevated race speeds to about 40 knots (46 mph). The multihulled catamarans could now essentially fly above the surface on a thin hydrofoil, known as a daggerboard, as the additional wind power transferred into much higher forward momentum. Races that once took hours in an IACC boat now would be completed in 30 minutes or less with a foiling craft. Previous engineering challenges to minimize wave-induced drag on the hull were replaced with designing structures to handle the massive forces generated by the wingsail and ensure that the sailors could control the crafts when foiling at such speeds. Specifically, the team needed a wing that could withstand large deformations in its ribs, spars and flaps. New courses set closer to the shore to be visible to large crowds of spectators required more sailing maneuvers, which further altered the design.

To achieve the balance of boat performance vs. controllability and maneuverability, Emirates Team New Zealand combined its large design space of Ansys structural and CFD models with its traditional velocity prediction program (VPP) into an in-house tool named Gomboc. Also known to the team as “the simulator,” Gomboc enabled real-time simulation of different sailing conditions for each variation of the wing and hull design being studied. For more complicated calculations, it could pick results from an existing library or response surface. Ansys CFX was used to analyze boat and sail configurations in different conditions to determine the complete aerodynamic forces and moments.

Downwind sails used to be developed with scaled models in a wind tunnel, but computational fluid dynamics has replaced wind tunnel and tow tank testing at ETNZ. Credit: University of Auckland, Twisted Flow Wind Tunnel / Burns Fallow, 2002 Downwind sails used to be developed with scaled models in a wind tunnel, but computational fluid dynamics has replaced wind tunnel and tow tank testing at ETNZ. Credit: University of Auckland, Twisted Flow Wind Tunnel / Burns Fallow, 2002
Time-consuming wind tunnel tests have  been replaced with simulation. Time-consuming wind tunnel tests have been replaced with simulation.

 

These conditions could include different angles of attack or different attitudes of the boat such as yaw, heel or pitch. From the aerodynamic results, the simulator could find the force and moment equilibrium and predict the speed.

The simulator started off more like a video game played on a keyboard, though with its backbone in physics. Over time, the human interface included multiple screens, virtual reality headsets, and a lot of the same equipment that sailors use on the yacht — including steering wheels and control devices. Having the sailors get feedback from the simulator in real time turned into an efficient way of answering their questions, and of working out what questions to ask. This feedback gave the team confidence about what the performance would be for a given design before ever getting out on the water. They could then benchmark their own on-water performance against the simulator’s predictions. Refined further by sensor data from experimental testing, Emirates Team New Zealand’s experience of virtual sailing in the simulator helped to provide a needed edge against the competing syndicates. Emirates Team New Zealand overcame a close loss at the 2013 Cup finals in San Francisco to take a dominant victory in 2017 in Bermuda.

Engineers look at surface pressures and air velocities with simulation to improve lift and reduce drag. Engineers look at surface pressures and air velocities with simulation to improve lift and reduce drag.

 

The Victor Keeps the Foils

After 10 years dominated by wingsails and multihulls, design rules were published in early 2018 for the 2021 Cup. The rules specify a monohull with soft sails, but with canted T-wing hydrofoils on both sides. Each team is limited to two full-sized 75-ft (23-m) boats, but may also build test craft up to 39 ft (12 m) long for additional on-water testing. The team only had a few months to design Boat 1, so fully relied on simulation to compare the performance of different foils, hulls and sail setups.

For the foil design, the rules require adherence to specific geometric and mass constraints. Emirates Team New Zealand and its competitors have been evaluating different foil shapes to accommodate a wide range of different speed conditions. For example, slender foils might work better at higher speeds but require adding ballast to conform to the mass rules. The foils have hydraulic and electronic control systems that power flaps analogous to airplane flaps to adjust vertical hydrodynamic force. Emirates Team New Zealand is using both Ansys CFX and Ansys Mechanical to model the complex relationship between the hydrodynamics and the structural mechanics involved in the foils and control systems.

Beyond the foil design, Emirates Team New Zealand is fully invested in analyzing almost every structural component of the boat. In the 2017 campaign, the designers used Mechanical primarily for coupled fluid-structure interaction (FSI) analysis of the wingsail with CFX. For the 2021 campaign, individual components are modeled with Mechanical, and there is also a global composite model of the hull and deck in ACP. These components must fulfill different strength or stiffness requirements based on rig loads, slamming loads and hydrodynamics loads, while still balancing between weight and performance.

With the return of soft sails and limited time for full-scale testing, the analysis of sail shapes is also a top concern. As sails are trimmable in a seemingly infinite number of ways, the designers have run more than 100,000 simulations of different sail shapes using CFX. Aerodynamic analysis of the hull is more important than ever because the hull will spend much of its time foiling, so the team also evaluated about 1,000 different hull geometries. By parameterizing the hulls and sails, they could efficiently generate very large design matrices that they could then use in the simulator.

Maneuvering Toward the Defense

The automation of the simulation process with Ansys software empowered the designers to accomplish in one week what once would have taken six months. Ansys simulation software allowed the New Zealand team to innovate faster. For example, a simple structural model was often used to verify an initial concept that was based on past experience. The simple model was then refined to take more loading effects into account. No idea was a bad idea because simulation enabled the team to explore lots of different concepts and test them relatively cheaply.

As Emirates Team New Zealand validates its designs on the water, both Boat 1 (christened as Te Aihe — Maori for “The Dolphin”) and the team’s 12-m test boat (Te Kahu — “The Hawk”) have sensors built into the masts because mast bending affects the sail shape. The sensor data then provides information about the compressive strain that the sailors can use as they test loading and unloading the rig to change the sail shape to get more speed for given wind conditions.

The intelligence gathered from the modeling and testing work of Te Aihe and Te Kahu will be critical as Emirates Team New Zealand designs and builds Boat 2, which will be the team’s primary racing yacht. This comparison between the live monitoring and simulation of many design variations is giving the team the understanding, and ultimately the confidence, to push the boundaries needed to defend the Cup once again.

Emirates Team New Zealand is supported by Ansys Channel Partner LEAP Australia and composites design specialists from Ansys.

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