Stephen Barr, Roger Birkbeck, Jeremy ShipmanCombustion Research and Flow Technology, Inc.
In the extreme sport of Big Wave Surfing, surfers ride specially-designed surfboards known as Guns or Rhino Chasers. Designed for speed and stability, the big wave board designs allow the surfer to drop down the face of the wave, and generate enough speed during the drive to stay in front and not be consumed by the crashing monster wave. To the elite big wave surfer, extra speed and stability are crucial parameters of the surfboard design that allow them to catch that monster wave, ride it….and ultimately survive it!
While most board-shapers focus their attention on the hydrodynamic aspects of the board, little attention has been paid to its aerodynamic characteristics. Considering that only about 25 percent of the board surface area is in contact with the water, the remaining 75 percent is exposed to aerodynamics as the surfer attempts to make the drop and race the crashing wave to the bottom, reaching speeds of up to 50 mph in the process.
Working with one of the world’s top big-wave board shapers, engineers at CRAFT Tech have applied computational fluid dynamics (CFD) within a design-optimization process that employs genetic algorithms to evolve the aerodynamic design of the board. By using the automated meshing tools provided by Cadence, we can explore a large design space to create an aerodynamically optimized leading edge design for the board that increases speed and stability for the surfer, which can mean the difference between riding the wave and being consumed by it!
In this article, we will explore the design framework that allowed the exploration of over 100 design iterations, resulting in a higher speed, lower drag, big wave surfboard design. Performing aerodynamic testing of the optimized board shape, compared to a conventional baseline board using a home-grown test fixture, bore promising results. The completed prototype big wave board concept now awaits the return of the big swells to the Northern Hemisphere, so that the concept can be evaluated in the big surf.
Big waves are classified as any wave greater than 20 feet (6.2 meters) in height from crest to trough, and can form in only a handful of locations around the globe when the weather
conditions and ocean currents are just right. Some of the more famous big wave surf locations include: Peahi/JAWS (Hawaii), Mavericks (California), Teapuhoo (Tahiti), and Nazare (Portugal), to name a few.
Due of the vast amounts of energy contained in these monster waves, big-wave surfing is an extremely dangerous sport, populated by only a handful of the most experienced, ultra-elite surfers. Similarly, Guns and Rhino-Chasers are shaped by only the most experienced, elite board shapers. Literally, a surfer’s life depends on the integrity of the board they ride (among other things). Guns differ greatly from other surfboards and are characterized as being much more massive: longer (typically 8-12 feet), and thicker, with distinct narrowing at the nose and tail. Guns are designed primarily for speed, stability, and strength.
Big waves can characteristically differ from one another based on many factors, including the topography of the sea floor, swell conditions, and surface winds. Consequently, big-wave surfers typically ride different boards for different waves. Figure 2 shows Greg Long’s ‘Quiver of Big Wave Guns’ that he uses during the World Surf League’s Big Wave World Tour competition.
Most surfboard shapers focus their attention on how the water interacts with and flows around the board, (i.e. the hydrodynamics). Ironically, only about 25 percent of the board’s surface is in contact with the water, while the remaining 75 percent is affected by the air. Big wave surfing differs in that both hydrodynamics and aerodynamics can affect how a board and surfer perform. Big wave surfers begin by dropping down the face of the wave to gain speed. Many times, this is literally a controlled free-fall. At the base of the wave, the surfer engages the board in the drive phase, turning the board at an angle to the wave direction and using the side edge of the board, the rail, to generate tremendous speed. Depending on the wave speed and the drive angle, surfers can reach speeds upwards of 50 mph. As the surfer’s speed increases, the aerodynamic effects become more and more prevalent. During the run, the surfer and board maintain a positive angle-of-attack to prevent the nose from dipping into the wave, causing a catastrophic failure (better known as a wipeout). Simultaneously, the surfer must not allow the angle-of-attack to grow too much, causing the board to loft and creating another mode of wipeout.
Speed and stability are key to the success and survival of the big-wave surfer. Hydrodynamic drag (pressure drag and skin friction) constitutes the majority of the overall drag force experienced by the surfer. However, as the surfer’s speed increases, the aerodynamic drag forces become more prevalent. With the nose of the board sticking out into the freestream air, and at an angle-of-attack, the air must maneuver around the nose of the board. As the air works its way around the rails of the board, the local airspeed increases and the pressure drops. This airflow detaches from the surface of the board, causing a low pressure “bubble” to form on the leeward side of the board (i.e. the top surface behind the nose). This separation bubble is a major source of aerodynamic drag. Essentially, the high-pressure air will always push against the underside of the board; however, with the presence of a low-pressure separation bubble, the upward force is compounded by a suction effect (see Figure 3).
There is another negative aspect of the separation bubble’s presence that may affect the performance of the board and surfer. The separation of the flow around the rails of the board creates an unsteady flow environment. The two unsteady flow separation points set up a communication path with each other, and begin to laterally oscillate. This unsteady aerodynamic environment is managed by the surfer and kept under control, but taxes the surfer’s overall performance. If left unmanaged, this oscillation can grow and lead to loss of control.
Although there are many aspects of big-wave surfing that can impact a surfer’s performance, there are typically two primary modes of catastrophic failure. In the first mode, the nose of the board catches air, causing the board to loft and pull out from under the feet of the surfer. In the second, the nose of the board catches an edge in the water, and the board is torn from underneath the feet of the surfer. The concept we are presenting in this effort addresses both modes of failure.
The concept developed in this effort involves the design of an aerodynamic modification to the top leading edge of the surf board. This fairing modification reduces the separation bubble that can form on a standard board, thereby reducing drag and increasing stability. The project provided an ideal opportunity for applying the design optimization framework developed at CRAFT Tech. This framework implements multi-physics solvers within an automated design loop. This includes geometry modification, grid generation, physics solvers including CFD, and genetic algorithms for generating new design variables that lead to the next iteration of design testing in the loop.
An illustration of the design optimization loop for the surfboard project is shown in Figure 4, along with a depiction of the modified board concept that was generated by the optimization study. OpenCascade was chosen to generate the geometry, based on a set of input design variables. It is an open-source set of C++ CAD libraries, capable of generating complex surface shapes, able to perform Boolean operations, and supports IGES and STL export formats. In this case, there are four input design variables used to define a single spline curve along the top center of the board, which is used to parameterize the surfboard geometry (Figure 5). Point 1 is fixed to the front of the board. Point 2 is constrained by the tangent vector from the bottom surface at Point 1, giving it a single degree of freedom. Point 3 is constrained by the tangent vector of the top surface at Point 4, also giving it a single degree of freedom. Point 4 defines the fairing length, and is restricted to no more than half of the total board length. The board shape is thus defined entirely by the baseline shape and these four points.
In the next step of the design loop, the geometry is handed off to Pointwise to generate the computational grid. It is important that the grid generation operate robustly, and without user intervention in order to work within the optimization framework. Therefore, Glyph scripts were used to run Pointwise in an automated manner, taking care of functions such as geometry import, surface and volume mesh creation, boundary condition specification, and solver export. Pointwise’s anisotropic tetrahedral extrusion (T-Rex) features were used to create the boundary layer volume mesh for each design iteration. This ensured that a high-quality boundary layer and volume mesh could be generated for the range of board shapes produced by modifying the design parameters. An illustration of one of the unstructured Pointwise grids created for the surfboard simulations is shown in Figure 6, along with the excerpt of Glyph script that generates the volume mesh.
The next step in the design loop involves exporting the grid from Pointwise to CRAFT Tech’s unstructured Navier-Stokes CFD solver, CRUNCH CFD, to calculate the air flow over the modified surfboard shape and compute the resulting drag force on the board. In a design optimization loop, any number of physical models can come into play in determining the success of the current generation of design evolution. In this case, a single metric - the drag force on the surfboard model - was extracted from the CFD solution to evaluate which generations of design modifications should proceed.
Dakota, an optimization toolkit developed by Sandia National Labs, was used in the design framework to evaluate the objective function – drag reduction – and generate new design variables using a genetic algorithm. In this case, we chose the single objective genetic algorithm (SOGA) optimizer from the Dakota toolkit. Each population contained five designs, with the best design from each population chosen for the next generation of design iterations. In all, the optimization progressed through 20 design generations resulting in the evaluation of 100 surfboard shape modifications.
The results of the optimization are illustrated in Figure 7, showing the progression of the baseline surfboard to the modified design, with a drag reduction of about 13 percent. Figure 8 shows a comparison of pressure contours in the centerline plane, demonstrating the modified aerodynamics between the baseline and modified board shapes that resulted in the lower drag.
Subsequent to the design optimization process, we wanted to boost confidence of our concept with some experimental test data. The design of the experiment involved a comparison of the aerodynamic drag force on the optimized board shape with a conventional baseline board. Two test articles were fabricated (one conventional baseline and one modified board) by a nearby surfboard supply company, Greenlight Surfboard Supply. Greenlight is an engineering-based designer and manufacturer of buoyant foam cores, resins, fiberglass, tools, and accessories for watercraft construction.
A rudimentary test fixture was fabricated that allowed the surfboard test articles to be mounted to the front of a vehicle (see Figure 9). This test fixture extended the test article in front of the vehicle to ensure undisturbed flow around the board. The test article was mounted to a near-frictionless sliding platform in order to isolate the aerodynamic drag component from the lift component. A crude data acquisition system was devised and mounted to the platform which simultaneously recorded drag force in respect to air speed.
Each test article was mounted to the test fixture at a fixed angle-of-attack of 15 degrees.
Testing was performed at an old abandoned dragstrip, now used as a private runway. Several test runs were taken, each run gradually accelerating from zero to 50 mph. The results of the testing and clearly show the expected drag reduction of the concept (see Figure 10).
Both the baseline and the modified board behaved similarly at lower speeds, however as speed increased, the optimized board began to experience much lower aerodynamic drag than the conventional baseline surfboard. As the airspeed increased toward the upper limit of 50 mph, so did the difference in aerodynamic drag. This difference in aerodynamic drag is indicative of a significant reduction, or elimination of the separation bubble behind the nose of the board.
The final phase of this effort involves evaluation of the concept in big wave surfing. Although many of the professional surfers on the Big Wave World Tour ride Guns shaped by Chris Christenson, there are two who work particularly close with Chris, namely Greg Long and Ian Walsh. Collectively, these three represent our test and evaluation team. Chris provided a 3-D CAD file of one of his proven Gun designs for CRAFT Tech to modify with the aerodynamic fairing - now affectionately referred to as “ORCA” (Optimization, Reduction, and Control of Aerodynamics). A modified CAD file was provided to Chris, who proceeded to fabricate and shape the prototype board with the ORCA nose fairing. This was no trivial task at his end, due to the additional mass at the nose of the board. Modifications were needed to balance and compensate for the board’s altered mass properties. Additional strength was also needed, which drove the use of carbon fiber composites. The finished prototype board, as seen in Figure 11, debuted at this year’s Boardroom International Surfboard Show.
Unfortunately, the big-wave season for 2017 has wound down in the northern hemisphere, and the waves have moved South. Testing and evaluation of the prototype board will likely begin sometime in the Fall when the swells return to the north. Testing and evaluation of the ORCA prototype board will likely be at one of the handful of known big-wave locations, such as: JAWS, Mavericks, or Todos Santos. We are all keeping our fingers crossed and wishing the evaluation team the very best of luck and safe surfing.